JTC1/SC22/WG14
N794
Programming languages - C
Contents
1. Scope................................................... 1
2. Normative references.................................... 2
3. Definitions and conventions............................. 3
4. Compliance.............................................. 7
5. Environment............................................. 9
5.1 Conceptual models................................. 9
5.1.1 Translation environment.................. 9
5.1.2 Execution environments................... 12
5.2 Environmental considerations...................... 20
5.2.1 Character sets........................... 20
5.2.2 Character display semantics.............. 23
5.2.3 Signals and interrupts................... 24
5.2.4 Environmental limits..................... 24
6. Language................................................ 34
6.1 Lexical elements.................................. 34
6.1.1 Keywords................................. 36
6.1.2 Identifiers.............................. 37
6.1.3 Constants................................ 53
6.1.4 String literals.......................... 61
6.1.5 Operators................................ 63
6.1.6 Punctuators.............................. 64
6.1.7 Header names............................. 65
6.1.8 Preprocessing numbers.................... 66
6.1.9 Comments................................. 67
6.2 Conversions....................................... 68
6.2.1 Arithmetic operands...................... 68
6.2.2 Other operands........................... 73
6.3 Expressions....................................... 77
6.3.1 Primary expressions...................... 79
6.3.2 Postfix operators........................ 81
6.3.3 Unary operators.......................... 91
6.3.4 Cast operators........................... 96
6.3.5 Multiplicative operators................. 97
6.3.6 Additive operators....................... 98
6.3.7 Bitwise shift operators.................. 101
6.3.8 Relational operators..................... 102
6.3.9 Equality operators....................... 103
6.3.10 Bitwise AND operator..................... 104
6.3.11 Bitwise exclusive OR operator............ 105
6.3.12 Bitwise inclusive OR operator............ 105
6.3.13 Logical AND operator..................... 106
6.3.14 Logical OR operator...................... 106
6.3.15 Conditional operator..................... 107
6.3.16 Assignment operators..................... 109
6.3.17 Comma operator........................... 111
6.4 Constant expressions.............................. 113
6.5 Declarations...................................... 116
6.5.1 Storage-class specifiers................. 117
6.5.2 Type specifiers.......................... 119
6.5.3 Type qualifiers.......................... 129
6.5.4 Function specifiers...................... 135
6.5.5 Declarators.............................. 137
6.5.6 Type names............................... 147
6.5.7 Type definitions......................... 148
6.5.8 Initialization........................... 151
6.6 Statements........................................ 160
6.6.1 Labeled statements....................... 160
6.6.2 Compound statement, or block............. 161
6.6.3 Expression and null statements........... 162
6.6.4 Selection statements..................... 163
6.6.5 Iteration statements..................... 165
6.6.6 Jump statements.......................... 167
6.7 External definitions.............................. 171
6.7.1 Function definitions..................... 172
6.7.2 External object definitions.............. 175
6.8 Preprocessing directives.......................... 177
6.8.1 Conditional inclusion.................... 180
6.8.2 Source file inclusion.................... 182
6.8.3 Macro replacement........................ 184
6.8.4 Line control............................. 193
6.8.5 Error directive.......................... 194
6.8.6 Pragma directive......................... 194
6.8.7 Null directive........................... 195
6.8.8 Predefined macro names................... 195
6.8.9 Pragma operator.......................... 196
6.9 Future language directions........................ 198
6.9.1 Character escape sequences............... 198
6.9.2 Storage-class specifiers................. 198
6.9.3 Function declarators..................... 198
6.9.4 Function definitions..................... 198
6.9.5 Pragma directives........................ 198
7. Library................................................. 199
7.1 Introduction...................................... 199
7.1.1 Definitions of terms..................... 199
7.1.2 Standard headers......................... 200
7.1.3 Reserved identifiers..................... 202
7.1.4 Errors <errno.h>......................... 203
7.1.5 Limits <float.h> and <limits.h>.......... 204
7.1.6 Common definitions <stddef.h>............ 204
7.1.7 Boolean type and values <stdbool.h>...... 205
7.1.8 Use of library functions................. 206
7.2 Diagnostics <assert.h>............................ 209
7.2.1 Program diagnostics...................... 209
7.3 Character handling <ctype.h>...................... 211
7.3.1 Character testing functions.............. 211
7.3.2 Character case mapping functions......... 216
7.4 Integer types <inttypes.h>........................ 217
7.4.1 Typedef names for integer types.......... 218
7.4.2 Limits of specified-width integer
types.................................... 220
7.4.3 Macros for integer constants............. 223
7.4.4 Macros for format specifiers............. 224
7.4.5 Limits of other integer types............ 226
7.4.6 Conversion functions for greatest-width
integer types............................ 228
7.5 Localization <locale.h>........................... 230
7.5.1 Locale control........................... 231
7.5.2 Numeric formatting convention
inquiry.................................. 233
7.6 Floating-point environment <fenv.h>............... 237
7.6.1 The FENV_ACCESS pragma................... 239
7.6.2 Exceptions............................... 241
7.6.3 Rounding................................. 244
7.6.4 Environment.............................. 245
7.7 Mathematics <math.h>.............................. 248
7.7.1 Treatment of error conditions............ 251
7.7.2 The FP_CONTRACT pragma................... 252
7.7.3 Classification macros.................... 252
7.7.4 Trigonometric functions.................. 256
7.7.5 Hyperbolic functions..................... 259
7.7.6 Exponential and logarithmic
functions................................ 261
7.7.7 Power and absolute value functions....... 267
7.7.8 Error and gamma functions................ 270
7.7.9 Nearest integer functions................ 271
7.7.10 Remainder functions...................... 275
7.7.11 Manipulation functions................... 277
7.7.12 Maximum, minimum, and positive
difference functions..................... 279
7.7.13 Floating multiply-add.................... 281
7.7.14 Comparison macros........................ 281
7.8 Complex arithmetic <complex.h>.................... 285
7.8.1 The CX_LIMITED_RANGE pragma.............. 286
7.8.2 Complex functions........................ 287
7.9 Type-generic math <tgmath.h>...................... 297
7.9.1 Type-generic macros...................... 297
7.10 Nonlocal jumps <setjmp.h>......................... 301
7.10.1 Save calling environment................. 301
7.10.2 Restore calling environment.............. 302
7.11 Signal handling <signal.h>........................ 304
7.11.1 Specify signal handling.................. 305
7.11.2 Send signal.............................. 307
7.12 Variable arguments <stdarg.h>..................... 308
7.12.1 Variable argument list access
macros................................... 308
7.13 Input/output <stdio.h>............................ 313
7.13.1 Introduction............................. 313
7.13.2 Streams.................................. 315
7.13.3 Files.................................... 317
7.13.4 Operations on files...................... 320
7.13.5 File access functions.................... 323
7.13.6 Formatted input/output functions......... 328
7.13.7 Character input/output functions......... 354
7.13.8 Direct input/output functions............ 361
7.13.9 File positioning functions............... 362
7.13.10 Error-handling functions................. 365
7.14 General utilities <stdlib.h>...................... 368
7.14.1 String conversion functions.............. 369
7.14.2 Pseudo-random sequence generation
functions................................ 379
7.14.3 Memory management functions.............. 380
7.14.4 Communication with the environment....... 383
7.14.5 Searching and sorting utilities.......... 386
7.14.6 Integer arithmetic functions............. 388
7.14.7 Multibyte character functions............ 391
7.14.8 Multibyte string functions............... 394
7.15 String handling <string.h>........................ 396
7.15.1 String function conventions.............. 396
7.15.2 Copying functions........................ 396
7.15.3 Concatenation functions.................. 398
7.15.4 Comparison functions..................... 400
7.15.5 Search functions......................... 403
7.15.6 Miscellaneous functions.................. 407
7.16 Date and time <time.h>............................ 409
7.16.1 Components of time....................... 409
7.16.2 Time manipulation functions.............. 411
7.16.3 Time conversion functions................ 417
7.17 Alternative spellings <iso646.h>.................. 424
7.18 Wide-character classification and mapping
utilities <wctype.h>.............................. 425
7.18.1 Introduction............................. 425
7.18.2 Wide-character classification
utilities................................ 426
7.18.3 Wide-character mapping utilities......... 433
7.19 Extended multibyte and wide-character utilities
<wchar.h>......................................... 436
7.19.1 Introduction............................. 436
7.19.2 Formatted wide-character input/output
functions................................ 437
7.19.3 Wide-character input/output
functions................................ 459
7.19.4 General wide-string utilities............ 465
7.19.5 The wcsftime function.................... 486
7.19.6 The wcsfxtime function................... 487
7.19.7 Extended multibyte and wide-character
conversion utilities..................... 487
7.20 Future library directions......................... 496
7.20.1 Errors <errno.h>......................... 496
7.20.2 Character handling <ctype.h>............. 496
7.20.3 Integer types <inttypes.h>............... 496
7.20.4 Localization <locale.h>.................. 496
7.20.5 Signal handling <signal.h>............... 496
7.20.6 Input/output <stdio.h>................... 497
7.20.7 General utilities <stdlib.h>............. 497
7.20.8 Complex arithmetic <complex.h>........... 497
7.20.9 String handling <string.h>............... 497
7.20.10 Wide-character classification and
mapping utilities <wctype.h>............. 498
7.20.11 Extended multibyte and wide-character
utilities <wchar.h>...................... 498
A Bibliography............................................ 499
B Language syntax summary................................. 501
B.1 Lexical grammar................................... 501
B.2 Phrase structure grammar.......................... 507
B.3 Preprocessing directives.......................... 513
C Sequence points......................................... 515
D Library summary......................................... 516
D.1 Errors <errno.h>.................................. 516
D.2 Common definitions <stddef.h>..................... 516
D.3 Boolean type and values <stdbool.h>............... 516
D.4 Diagnostics <assert.h>............................ 516
D.5 Character handling <ctype.h>...................... 517
D.6 Integer types <inttypes.h>........................ 517
D.7 Floating-point environment <fenv.h>............... 523
D.8 Localization <locale.h>........................... 523
D.9 Mathematics <math.h>.............................. 524
D.10 Complex <complex.h>............................... 528
D.11 Type-generic math <tgmath.h>...................... 529
D.12 Nonlocal jumps <setjmp.h>......................... 530
D.13 Signal handling <signal.h>........................ 531
D.14 Variable arguments <stdarg.h>..................... 531
D.15 Input/output <stdio.h>............................ 531
D.16 General utilities <stdlib.h>...................... 534
D.17 String handling <string.h>........................ 536
D.18 Date and time <time.h>............................ 537
D.19 Alternative spellings <iso646.h>.................. 537
D.20 Wide-character classification and mapping
utilities <wctype.h>.............................. 538
D.21 Extended multibyte and wide-character utilities
<wchar.h>......................................... 538
E Implementation limits................................... 543
F IEC 559 floating-point arithmetic....................... 545
F.1 Introduction...................................... 545
F.2 Types............................................. 545
F.3 Operators and functions........................... 546
F.4 Floating to integer conversion.................... 548
F.5 Binary-decimal conversion......................... 549
F.6 Contracted expressions............................ 549
F.7 Environment....................................... 550
F.8 Optimization...................................... 553
F.9 <math.h>.......................................... 558
G IEC 559-compatible complex arithmetic................... 573
G.1 Introduction...................................... 573
G.2 Types............................................. 573
G.3 Conversions....................................... 573
G.4 Binary operators.................................. 574
G.5 <complex.h>....................................... 580
G.6 <tgmath.h>........................................ 588
H Language independent arithmetic......................... 590
H.1 Introduction...................................... 590
H.2 Types............................................. 590
H.3 Notification...................................... 594
I Universal character names for identifiers............... 597
J Common warnings......................................... 599
K Portability issues...................................... 601
K.1 Unspecified behavior.............................. 601
K.2 Undefined behavior................................ 604
K.3 Implementation-defined behavior................... 620
K.4 Locale-specific behavior.......................... 629
K.5 Common extensions................................. 630
Index....................................................... 633
page 1
1. Scope
[#1] This International Standard specifies the form and
establishes the interpretation of programs written in the C
programming language.1 It specifies
- the representation of C programs;
- the syntax and constraints of the C language;
- the semantic rules for interpreting C programs;
- the representation of input data to be processed by C
programs;
- the representation of output data produced by C
programs;
- the restrictions and limits imposed by a conforming
implementation of C.
[#2] This International Standard does not specify
- the mechanism by which C programs are transformed for
use by a data-processing system;
- the mechanism by which C programs are invoked for use
by a data-processing system;
- the mechanism by which input data are transformed for
use by a C program;
- the mechanism by which output data are transformed
after being produced by a C program;
- the size or complexity of a program and its data that
will exceed the capacity of any specific data-
processing system or the capacity of a particular
processor;
__________
1. This International Standard is designed to promote the
portability of C programs among a variety of data-
processing systems. It is intended for use by
implementors and programmers.
page 1 General
page 2
- all minimal requirements of a data-processing system
that is capable of supporting a conforming
implementation.
2. Normative references
[#1] The following standards contain provisions which,
through reference in this text, constitute provisions of
this International Standard. At the time of publication,
the editions indicated were valid. All standards are
subject to revision, and parties to agreements based on this
International Standard are encouraged to investigate the
possibility of applying the most recent editions of the
standards indicated below. Members of IEC and ISO maintain
registers of currently valid International Standards.
IEC 559:1993, Binary floating-point arithmetic for
microprocessor systems, second edition.
ISO 646:1983, Information processing - ISO 7-bit coded
character set for information interchange.
ISO/IEC 2382-1:1993, Information technology -
Vocabulary - Part 1: Fundamental terms.
ISO 4217:1987, Codes for the representation of
currencies and funds.
ISO 8601:1988, Data elements and interchange formats -
Information interchange - Representation of dates and
times.
ISO/IEC TR 10176, Information technology - Guidelines
for the preparation of programming language standards.
ISO/IEC 10646-1:1993, Information technology -
Universal Multiple-Octet Coded Character Set (UCS) -
Part 1: Architecture and Basic Multilingual Plane.
page 2 General
page 3
3. Definitions and conventions
[#1] In this International Standard, ``shall'' is to be
interpreted as a requirement on an implementation or on a
program; conversely, ``shall not'' is to be interpreted as a
prohibition.
[#2] For the purposes of this International Standard, the
following definitions apply. Other terms are defined where
they appear in italic type or being on the left side of a
syntax rule. Terms explicitly defined in this International
Standard are not to be presumed to refer implicitly to
similar terms defined elsewhere. Terms not defined in this
International Standard are to be interpreted according to
ISO 2382-1.
3.1 Alignment
[#1] A requirement that objects of a particular type be
located on storage boundaries with addresses that are
particular multiples of a byte address.
3.2 Argument
[#1] An expression in the comma-separated list bounded by
the parentheses in a function call expression, or a sequence
of preprocessing tokens in the comma-separated list bounded
by the parentheses in a function-like macro invocation.
Also known as ``actual argument'' or ``actual parameter.''
3.3 Bit
[#1] The unit of data storage in the execution environment
large enough to hold an object that may have one of two
values. It need not be possible to express the address of
each individual bit of an object.
page 3 General
page 4
3.4 Byte
[#1] The unit of data storage large enough to hold any
member of the basic character set of the execution
environment. It shall be possible to express the address of
each individual byte of an object uniquely. A byte is
composed of a contiguous sequence of bits, the number of
which is implementation-defined. The least significant bit
is called the low-order bit; the most significant bit is
called the high-order bit.
3.5 Character
[#1] A bit representation that fits in a byte. The
representation of each member of the basic character set in
both the source and execution environments shall fit in a
byte.
3.6 Constraints
[#1] Restrictions, both syntactic and semantic, by which the
exposition of language elements is to be interpreted.
3.7 Correctly rounded result
[#1] A representation in the result format that is nearest
in value, subject to the effective rounding mode, to what
the result would be given unlimited range and precision.
3.8 Diagnostic message
[#1] A message belonging to an implementation-defined subset
of the implementation's message output.
3.9 Forward references
[#1] References to later subclauses of this International
Standard that contain additional information relevant to
this subclause.
page 4 General
page 5
3.10 Implementation
[#1] A particular set of software, running in a particular
translation environment under particular control options,
that performs translation of programs for, and supports
execution of functions in, a particular execution
environment.
3.11 Implementation-defined behavior
[#1] Unspecified behavior where each implementation shall
document how the choice is made.
3.12 Implementation limits
[#1] Restrictions imposed upon programs by the
implementation.
3.13 Locale-specific behavior
[#1] Behavior that depends on local conventions of
nationality, culture, and language that each implementation
shall document.
3.14 Multibyte character
[#1] A sequence of one or more bytes representing a member
of the extended character set of either the source or the
execution environment. The extended character set is a
superset of the basic character set.
3.15 Object
[#1] A region of data storage in the execution environment,
the contents of which can represent values. Except for
bit-fields, objects are composed of contiguous sequences of
one or more bytes, the number, order, and encoding of which
are either explicitly specified or implementation-defined.
When referenced, an object may be interpreted as having a
particular type; see 6.2.2.1.
page 5 General
page 6
3.16 Parameter
[#1] An object declared as part of a function declaration or
definition that acquires a value on entry to the function,
or an identifier from the comma-separated list bounded by
the parentheses immediately following the macro name in a
function-like macro definition. Also known as ``formal
argument'' or ``formal parameter.''
3.17 Recommended practice
[#1] Sections so entitled contain specification that is
strongly recommended as being in keeping with the intent of
the standard, but that may be impractical for some
implementations.
3.18 Undefined behavior
[#1] Behavior, upon use of a nonportable or erroneous
program construct, of erroneous data, or of indeterminately
valued objects, for which this International Standard
imposes no requirements. Permissible undefined behavior
ranges from ignoring the situation completely with
unpredictable results, to behaving during translation or
program execution in a documented manner characteristic of
the environment (with or without the issuance of a
diagnostic message), to terminating a translation or
execution (with the issuance of a diagnostic message).
[#2] If a ``shall'' or ``shall not'' requirement that
appears outside of a constraint is violated, the behavior is
undefined. Undefined behavior is otherwise indicated in
this International Standard by the words ``undefined
behavior'' or by the omission of any explicit definition of
behavior. There is no difference in emphasis among these
three; they all describe ``behavior that is undefined.''
[#3] The implementation must successfully translate a given
program unless a syntax error is detected, a constraint is
violated, or it can determine that every possible execution
of that program would result in undefined behavior.
page 6 General
page 7
3.19 Unspecified behavior
[#1] Behavior where this International Standard provides two
or more possibilities and imposes no requirements on which
is chosen in any instance. A program that is correct in all
other aspects, operating on correct data, containing
unspecified behavior shall be a correct program and act in
accordance with subclause 5.1.2.3.
Examples
[#2]
1. An example of unspecified behavior is the order in
which the arguments to a function are evaluated.
2. An example of undefined behavior is the behavior on
integer overflow.
3. An example of implementation-defined behavior is the
propagation of the high-order bit when a signed
integer is shifted right.
4. An example of locale-specific behavior is whether the
islower function returns true for characters other
than the 26 lowercase Latin letters.
Forward references: bitwise shift operators (6.3.7),
expressions (6.3), function calls (6.3.2.2), the islower
function (7.3.1.7), localization (7.5).
4. Compliance
[#1] A strictly conforming program shall use only those
features of the language and library specified in this
International Standard.2 It shall not produce output
__________
2. This implies that a strictly conforming program can use
features in a conditionally normative annex provided the
use is conditioned by a #ifdef directive with the
conformance macro for the annex, as in
#ifdef __STDC_IEC_559__ /* FE_UPWARD defined */
/* ... */
fesetround(FE_UPWARD);
/* ... */
page 7 General
page 8
dependent on any unspecified, undefined, or implementation-
defined behavior, and shall not exceed any minimum
implementation limit.
[#2] The two forms of conforming implementation are hosted
and freestanding. A conforming hosted implementation shall
accept any strictly conforming program. A conforming
freestanding implementation shall accept any strictly
conforming program in which the use of the features
specified in the library clause (clause 7) is confined to
the contents of the standard headers <float.h>, <limits.h>,
<stdarg.h>, <stddef.h>, and <iso646.h>. A conforming
implementation may have extensions (including additional
library functions), provided they do not alter the behavior
of any strictly conforming program.3
[#3] A conforming program is one that is acceptable to a
conforming implementation.4
[#4] An implementation shall be accompanied by a document
that defines all implementation-defined characteristics and
all extensions.
Forward references: limits <float.h> and <limits.h>
(7.1.5), variable arguments <stdarg.h> (7.12), common
definitions <stddef.h> (7.1.6), alternate spellings
<iso646.h> (7.17).
____________________________________________________________
#endif
3. This implies that a conforming implementation reserves
no identifiers other than those explicitly reserved in
this International Standard.
4. Strictly conforming programs are intended to be
maximally portable among conforming implementations.
Conforming programs may depend upon nonportable features
of a conforming implementation.
page 8 General
page 9
5. Environment
[#1] An implementation translates C source files and
executes C programs in two data-processing-system
environments, which will be called the translation
environment and the execution environment in this
International Standard. Their characteristics define and
constrain the results of executing conforming C programs
constructed according to the syntactic and semantic rules
for conforming implementations.
Forward references: In the environment clause (clause 5),
only a few of many possible forward references have been
noted.
5.1 Conceptual models
5.1.1 Translation environment
5.1.1.1 Program structure
[#1] A C program need not all be translated at the same
time. The text of the program is kept in units called
source files, also known as preprocessing files, in this
International Standard. A source file together with all the
headers and source files included via the preprocessing
directive #include is known as a preprocessing translation
unit. After preprocessing, a preprocessing translation unit
is called a translation unit. Previously translated
translation units may be preserved individually or in
libraries. The separate translation units of a program
communicate by (for example) calls to functions whose
identifiers have external linkage, manipulation of objects
whose identifiers have external linkage, or manipulation of
data files. Translation units may be separately translated
and then later linked to produce an executable program.
Forward references: conditional inclusion (6.8.1), linkages
of identifiers (6.1.2.2), source file inclusion (6.8.2),
external definitions (6.7), preprocessing directives (6.8).
page 9 Environment
page 10
5.1.1.2 Translation phases
[#1] The precedence among the syntax rules of translation is
specified by the following phases.5
1. Physical source file multibyte characters are mapped
to the source character set (introducing new-line
characters for end-of-line indicators) if necessary.
Any multibyte source file character not in the basic
source character set is replaced by the universal-
character-name that designates that multibyte
character.6 Then, trigraph sequences are replaced by
corresponding single-character internal
representations.
2. Each instance of a backslash character immediately
followed by a newline character is deleted, splicing
physical source lines to form logical source lines.
Only the last backslash on any physical source line
shall be eligible for being part of such a splice. A
source file that is not empty shall end in a new-line
character, which shall not be immediately preceded by
a backslash character before any such splicing takes
place.
3. The source file is decomposed into preprocessing
tokens7 and sequences of white-space characters
__________
5. Implementations must behave as if these separate phases
occur, even though many are typically folded together in
practice.
6. The process of handling extended characters is specified
in terms of mapping to an encoding that uses only the
basic source character set, and, in the case of
character literals and strings, further mapping to the
execution character set. In practical terms, however,
any internal encoding may be used, so long as an actual
extended character encountered in the input, and the
same extended character expressed in the input as a
universal-character-name (i.e., using the \U or \u
notation), are handled equivalently.
7. As described in 6.1, the process of dividing a source
file's characters into preprocessing tokens is context-
dependent. For example, see the handling of < within a
#include preprocessing directive.
page 10 Environment
page 11
(including comments). A source file shall not end in
a partial preprocessing token or comment. Each
comment is replaced by one space character. New-line
characters are retained. Whether each nonempty
sequence of white-space characters other than new-line
is retained or replaced by one space character is
implementation-defined.
4. Preprocessing directives are executed, macro
invocations are expanded, and pragma unary operator
expressions are executed. If a character sequence
that matches the syntax of a universal-character-name
is produced by token concatenation (6.8.3.3), the
behavior is undefined. A #include preprocessing
directive causes the named header or source file to be
processed from phase 1 through phase 4, recursively.
All preprocessing directives are then deleted.
5. Each source character set member, escape sequence, and
universal-character-name in character constants and
string literals is converted to a member of the
execution character set.
6. Adjacent character string literal tokens are
concatenated and adjacent wide string literal tokens
are concatenated.
7. White-space characters separating tokens are no longer
significant. Each preprocessing token is converted
into a token. The resulting tokens are syntactically
and semantically analyzed and translated as a
translation unit.
8. All external object and function references are
resolved. Library components are linked to satisfy
external references to functions and objects not
defined in the current translation. All such
translator output is collected into a program image
which contains information needed for execution in its
execution environment.
Constraints
[#2] A universal-character-name shall not specify a
character short identifier in the range 0000 through 0020 or
007F through 009F inclusive. A universal-character-name
shall not designate a character in the basic source
character set.
page 11 Environment
page 12
Forward references: lexical elements (6.1), preprocessing
directives (6.8), trigraph sequences (5.2.1.1), external
definitions (6.7).
5.1.1.3 Diagnostics
[#1] A conforming implementation shall produce at least one
diagnostic message (identified in an implementation-defined
manner) if a preprocessing translation unit or translation
unit contains a violation of any syntax rule or constraint,
even if the behavior is also explicitly specified as
undefined or implementation-defined. Diagnostic messages
need not be produced in other circumstances.8
Examples
[#2] An implementation shall issue a diagnostic for the
translation unit:
char i;
int i;
because in those cases where wording in this International
Standard describes the behavior for a construct as being
both a constraint error and resulting in undefined behavior,
the constraint error shall be diagnosed.
5.1.2 Execution environments
[#1] Two execution environments are defined: freestanding
and hosted. In both cases, program startup occurs when a
designated C function is called by the execution
environment. All objects in static storage shall be
initialized (set to their initial values) before program
startup. The manner and timing of such initialization are
otherwise unspecified. Program termination returns control
to the execution environment.
__________
8. The intent is that an implementation should identify the
nature of, and where possible localize, each violation.
Of course, an implementation is free to produce any
number of diagnostics as long as a valid program is
still correctly translated. It may also successfully
translate an invalid program.
page 12 Environment
page 13
Forward references: initialization (6.5.8).
5.1.2.1 Freestanding environment
[#1] In a freestanding environment (in which C program
execution may take place without any benefit of an operating
system), the name and type of the function called at program
startup are implementation-defined. Any library facilities
available to a freestanding program are implementation-
defined.
[#2] The effect of program termination in a freestanding
environment is implementation-defined.
5.1.2.2 Hosted environment
[#1] A hosted environment need not be provided, but shall
conform to the following specifications if present.
5.1.2.2.1 Program startup
[#1] The function called at program startup is named main.
The implementation declares no prototype for this function.
It shall be defined with no parameters:
int main(void) { /* ... */ }
or with two parameters (referred to here as argc and argv,
though any names may be used, as they are local to the
function in which they are declared):
int main(int argc, char *argv[]) { /* ... */ }
or equivalent,9 or in some other implementation-defined
manner.
[#2] If they are defined, the parameters to the main
function shall obey the following constraints:
- The value of argc shall be nonnegative.
- argv[argc] shall be a null pointer.
__________
9. Thus, int can be replaced by a typedef name defined as
int, or the type of argv can be written as char ** argv,
and so on.
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- If the value of argc is greater than zero, the array
members argv[0] through argv[argc-1] inclusive shall
contain pointers to strings, which are given
implementation-defined values by the host environment
prior to program startup. The intent is to supply to
the program information determined prior to program
startup from elsewhere in the hosted environment. If
the host environment is not capable of supplying
strings with letters in both uppercase and lowercase,
the implementation shall ensure that the strings are
received in lowercase.
- If the value of argc is greater than zero, the string
pointed to by argv[0] represents the program name;
argv[0][0] shall be the null character if the program
name is not available from the host environment. If
the value of argc is greater than one, the strings
pointed to by argv[1] through argv[argc-1] represent
the program parameters.
- The parameters argc and argv and the strings pointed to
by the argv array shall be modifiable by the program,
and retain their last-stored values between program
startup and program termination.
5.1.2.2.2 Program execution
[#1] In a hosted environment, a program may use all the
functions, macros, type definitions, and objects described
in the library clause (clause 7).
5.1.2.2.3 Program termination
[#1] A return from the initial call to the main function is
equivalent to calling the exit function with the value
returned by the main function as its argument.10 If the }
that terminates the main function is reached, the
termination status returned to the host environment is
unspecified.
__________
10. In accordance with subclause 6.1.2.4, objects with
automatic storage duration declared in main will no
longer have storage guaranteed to be reserved in the
former case even where they would in the latter.
page 14 Environment
page 15
Forward references: definition of terms (7.1.1), the exit
function (7.14.4.3).
5.1.2.3 Program execution
[#1] The semantic descriptions in this International
Standard describe the behavior of an abstract machine in
which issues of optimization are irrelevant.
[#2] Accessing a volatile object, modifying an object,
modifying a file, or calling a function that does any of
those operations are all side effects,11 which are changes
in the state of the execution environment. Evaluation of an
expression may produce side effects. At certain specified
points in the execution sequence called sequence points, all
side effects of previous evaluations shall be complete and
no side effects of subsequent evaluations shall have taken
place.
[#3] In the abstract machine, all expressions are evaluated
as specified by the semantics. An actual implementation
need not evaluate part of an expression if it can deduce
that its value is not used and that no needed side effects
are produced (including any caused by calling a function or
accessing a volatile object).
[#4] When the processing of the abstract machine is
interrupted by receipt of a signal, only the values of
objects as of the previous sequence point may be relied on.
Objects that may be modified between the previous sequence
point and the next sequence point need not have received
their correct values yet.
[#5] An instance of each object with automatic storage
duration is associated with each entry into its block. Such
__________
11. The IEC 559 standard for binary floating-point
arithmetic requires certain status flags and control
modes, with user access. Floating-point operations
implicitly set the status flags; modes affect result
values of floating-point operations. Implementations
that support such floating-point state will need to
regard changes to it as side effects - see Annex F for
details. The floating-point environment library
<fenv.h> provides a programming facility for indicating
when these side effects matter, freeing the
implementations in other cases.
page 15 Environment
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an object exists and retains its last-stored value during
the execution of the block and while the block is suspended
(by a call of a function or receipt of a signal).
[#6] The least requirements on a conforming implementation
are:
- At sequence points, volatile objects are stable in the
sense that previous evaluations are complete and
subsequent evaluations have not yet occurred.
- At program termination, all data written into files
shall be identical to the result that execution of the
program according to the abstract semantics would have
produced.
- The input and output dynamics of interactive devices
shall take place as specified in 7.13.3. The intent of
these requirements is that unbuffered or line-buffered
output appear as soon as possible, to ensure that
prompting messages actually appear prior to a program
waiting for input.
[#7] What constitutes an interactive device is
implementation-defined.
[#8] More stringent correspondences between abstract and
actual semantics may be defined by each implementation.
Examples
1. An implementation might define a one-to-one
correspondence between abstract and actual semantics:
at every sequence point, the values of the actual
objects would agree with those specified by the
abstract semantics. The keyword volatile would then
be redundant.
Alternatively, an implementation might perform various
optimizations within each translation unit, such that
the actual semantics would agree with the abstract
semantics only when making function calls across
translation unit boundaries. In such an
implementation, at the time of each function entry and
function return where the calling function and the
called function are in different translation units,
the values of all externally linked objects and of all
objects accessible via pointers therein would agree
with the abstract semantics. Furthermore, at the time
of each such function entry the values of the
parameters of the called function and of all objects
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accessible via pointers therein would agree with the
abstract semantics. In this type of implementation,
objects referred to by interrupt service routines
activated by the signal function would require
explicit specification of volatile storage, as well as
other implementation-defined restrictions.
2. In executing the fragment
char c1, c2;
/* ... */
c1 = c1 + c2;
the ``integer promotions'' require that the abstract
machine promote the value of each variable to int size
and then add the two ints and truncate the sum.
Provided the addition of two chars can be done without
overflow, or with overflow wrapping silently to
produce the correct result, the actual execution need
only produce the same result, possibly omitting the
promotions.
3. Similarly, in the fragment
float f1, f2;
double d;
/* ... */
f1 = f2 * d;
the multiplication may be executed using single-
precision arithmetic if the implementation can
ascertain that the result would be the same as if it
were executed using double-precision arithmetic (for
example, if d were replaced by the constant 2.0, which
has type double).
4. Implementations employing wide registers must take
care to honor appropriate semantics. Values must be
independent of whether they are represented in a
register or in memory. For example, an implicit
spilling of a register must not alter the value.
Also, an explicit store and load must round to the
precision of the storage type. In particular, casts
and assignments must perform their specified
conversion: for the fragment
double d1, d2;
float f;
d1 = f = expression;
d2 = (float) expressions;
page 17 Environment
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the values assigned to d1 and d2 must have been
converted to float.
5. Rearrangement for floating-point expressions is often
restricted because of limitations in precision as well
as range. The implementation cannot generally apply
the mathematical associative rules for addition or
multiplication, nor the distributive rule, because of
roundoff error, even in the absence of overflow and
underflow. Likewise, implementations cannot generally
replace decimal constants in order to rearrange
expressions. In the following fragment,
rearrangements suggested by mathematical rules for
real numbers are often not valid. See Annex F.8.
double x, y, z;
/* ... */
x = (x * y) * z; // not equivalent to x *= y * z;
z = (x - y) + y ; // not equivalent to z = x;
z = x + x * y; // not equivalent to z = x * (1.0 + y);
y = x / 5.0; // not equivalent of y = x * 0.2;
6. To illustrate the grouping behavior of expressions, in
the following fragment
int a, b;
/* ... */
a = a + 32760 + b + 5;
the expression statement behaves exactly the same as
a = (((a + 32760) + b) + 5);
due to the associativity and precedence of these
operators. Thus, the result of the sum (a + 32760) is
next added to b, and that result is then added to 5
which results in the value assigned to a. On a
machine in which overflows produce an explicit trap
and in which the range of values representable by an
int is [-32768, + 32767], the implementation cannot
rewrite this expression as
a = ((a + b) + 32765);
since if the values for a and b were, respectively, -
32754 and - 15, the sum a + b would produce a trap
while the original expression would not; nor can the
expression be rewritten either as
page 18 Environment
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a = ((a + 32765) + b);
or
a = (a + (b + 32765));
since the values for a and b might have been,
respectively, 4 and -8 or -17 and 12. However on a
machine in which overflow silently generates some
value and where positive and negative overflows
cancel, the above expression statement can be
rewritten by the implementation in any of the above
ways because the same result will occur.
7. The grouping of an expression does not completely
determine its evaluation. In the following fragment
#include <stdio.h>
int sum;
char *p;
/* ... */
sum = sum * 10 - '0' + (*p++ = getchar());
the expression statement is grouped as if it were
written as
sum = (((sum * 10) - '0') + ((*(p++)) = (getchar())));
but the actual increment of p can occur at any time
between the previous sequence point and the next
sequence point (the ;), and the call to getchar can
occur at any point prior to the need of its returned
value.
Forward references: compound statement, or block (6.6.2),
expressions (6.3), files (7.13.3),
sequence points (6.6), the signal function (7.11),
type qualifiers (6.5.3).
page 19 Environment
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5.2 Environmental considerations
5.2.1 Character sets
[#1] Two sets of characters and their associated collating
sequences shall be defined: the set in which source files
are written, and the set interpreted in the execution
environment. The values of the members of the execution
character set are implementation-defined; any additional
members beyond those required by this subclause are locale-
specific.
[#2] In a character constant or string literal, members of
the execution character set shall be represented by
corresponding members of the source character set or by
escape sequences consisting of the backslash \ followed by
one or more characters. A byte with all bits set to 0,
called the null character, shall exist in the basic
execution character set; it is used to terminate a character
string.
[#3] Both the basic source and basic execution character
sets shall have at least the following members: the 26
uppercase letters of the Latin alphabet
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
the 26 lowercase letters of the Latin alphabet
a b c d e f g h i j k l m
n o p q r s t u v w x y z
the 10 decimal digits
0 1 2 3 4 5 6 7 8 9
the following 29 graphic characters
! " # % & ' ( ) * + , - . / :
; < = > ? [ \ ] ^ _ { | } ~
the space character, and control characters representing
horizontal tab, vertical tab, and form feed. In both the
source and execution basic character sets, the value of each
character after 0 in the above list of decimal digits shall
be one greater than the value of the previous. In source
files, there shall be some way of indicating the end of each
line of text; this International Standard treats such an
end-of-line indicator as if it were a single new-line
character. In the execution character set, there shall be
page 20 Environment
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control characters representing alert, backspace, carriage
return, and new line. If any other characters are
encountered in a source file (except in a character
constant, a string literal, a header name, a comment, or a
preprocessing token that is never converted to a token), the
behavior is undefined.
[#4] The universal-character-name construct provides a way
to name other characters.
hex-quad:
hexadecimal-digit hexadecimal-digit
hexadecimal-digit hexadecimal-digit
universal-character-name:
\u hex-quad
\U hex-quad hex-quad
[#5] The character designated by the universal-character-
name \Unnnnnnnn is that character whose character short
identifier is nnnnnnnn specified by ISO/IEC 10646-1; the
character designated by the universal-character-name \unnnn
is that character whose character short identifier is
0000nnnn specified by ISO/IEC 10646-1.
Forward references: identifiers (6.1.2), character
constants (6.1.3.4), preprocessing directives (6.8), string
literals (6.1.4), comments (6.1.9), string (7.1.1).
5.2.1.1 Trigraph sequences
[#1] All occurrences in a source file of the following
sequences of three characters (called trigraph sequences12)
are replaced with the corresponding single character.
__________
12. The trigraph sequences enable the input of characters
that are not defined in the Invariant Code Set as
described in ISO/IEC 646:1991, which is a subset of the
seven-bit ASCII code set.
page 21 Environment
page 22
??= #
??( [
??/ \
??) ]
??' ^
??< {
??! |
??> }
??- ~
No other trigraph sequences exist. Each ? that does not
begin one of the trigraphs listed above is not changed.
Examples
[#2] The following source line
printf("Eh???/n");
becomes (after replacement of the trigraph sequence ??/)
printf("Eh?\n");
5.2.1.2 Multibyte characters
[#1] The source character set may contain multibyte
characters, used to represent members of the extended
character set. The execution character set may also contain
multibyte characters, which need not have the same encoding
as for the source character set. For both character sets,
the following shall hold:
- The single-byte characters defined in 5.2.1 shall be
present.
- The presence, meaning, and representation of any
additional members is locale-specific.
- A multibyte character may have a state-dependent
encoding, wherein each sequence of multibyte characters
begins in an initial shift state and enters other
locale-specific shift states when specific multibyte
characters are encountered in the sequence. While in
the initial shift state, all single-byte characters
retain their usual interpretation and do not alter the
shift state. The interpretation for subsequent bytes
in the sequence is a function of the current shift
state.
- A byte with all bits zero shall be interpreted as a
null character independent of shift state.
page 22 Environment
page 23
- A byte with all bits zero shall not occur in the second
or subsequent bytes of a multibyte character.
[#2] For the source character set, the following shall hold:
- A comment, string literal, character constant, or
header name shall begin and end in the initial shift
state.
- A comment, string literal, character constant, or
header name shall consist of a sequence of valid
multibyte characters.
5.2.2 Character display semantics
[#1] The active position is that location on a display
device where the next character output by the fputc function
would appear. The intent of writing a printable character
(as defined by the isprint function) to a display device is
to display a graphic representation of that character at the
active position and then advance the active position to the
next position on the current line. The direction of writing
is locale-specific. If the active position is at the final
position of a line (if there is one), the behavior is
unspecified.
[#2] Alphabetic escape sequences representing nongraphic
characters in the execution character set are intended to
produce actions on display devices as follows:
\a (alert) Produces an audible or visible alert. The active
position shall not be changed.
\b (backspace) Moves the active position to the previous
position on the current line. If the active position is
at the initial position of a line, the behavior is
unspecified.
\f (form feed) Moves the active position to the initial
position at the start of the next logical page.
\n (new line) Moves the active position to the initial
position of the next line.
\r (carriage return) Moves the active position to the
initial position of the current line.
\t (horizontal tab) Moves the active position to the next
horizontal tabulation position on the current line. If
the active position is at or past the last defined
horizontal tabulation position, the behavior is
page 23 Environment
page 24
unspecified.
\v (vertical tab) Moves the active position to the initial
position of the next vertical tabulation position. If
the active position is at or past the last defined
vertical tabulation position, the behavior is
unspecified.
[#3] Each of these escape sequences shall produce a unique
implementation-defined value which can be stored in a single
char object. The external representations in a text file
need not be identical to the internal representations, and
are outside the scope of this International Standard.
Forward references: the isprint function (7.3.1.8), the
fputc function (7.13.7.3).
5.2.3 Signals and interrupts
[#1] Functions shall be implemented such that they may be
interrupted at any time by a signal, or may be called by a
signal handler, or both, with no alteration to earlier, but
still active, invocations' control flow (after the
interruption), function return values, or objects with
automatic storage duration. All such objects shall be
maintained outside the function image (the instructions that
comprise the executable representation of a function) on a
per-invocation basis.
5.2.4 Environmental limits
[#1] Both the translation and execution environments
constrain the implementation of language translators and
libraries. The following summarizes the environmental
limits on a conforming implementation.
5.2.4.1 Translation limits
[#1] The implementation shall be able to translate and
execute at least one program that contains at least one
instance of every one of the following limits:13
- 127 nesting levels of compound statements, iteration
statements, and selection statements
__________
13. Implementations should avoid imposing fixed translation
limits whenever possible.
page 24 Environment
page 25
- 63 nesting levels of conditional inclusion
- 12 pointer, array, and function declarators (in any
combinations) modifying an arithmetic, structure,
union, or incomplete type in a declaration
- 63 nesting levels of parenthesized declarators within a
full declarator
- 63 nesting levels of parenthesized expressions within a
full expression
- 63 significant initial characters in an internal
identifier or a macro name
- 31 significant initial characters in an external
identifier
- 4095 external identifiers in one translation unit
- 511 identifiers with block scope declared in one block
- 4095 macro identifiers simultaneously defined in one
preprocessing translation unit
- 127 parameters in one function definition
- 127 arguments in one function call
- 127 parameters in one macro definition
- 127 arguments in one macro invocation
- 4095 characters in a logical source line
- 4095 characters in a character string literal or wide
string literal (after concatenation)
- 65535 bytes in an object (in a hosted environment only)
- 15 nesting levels for #included files
- 1023 case labels for a switch statement (excluding
those for any nested switch statements)
- 1023 members in a single structure or union
- 1023 enumeration constants in a single enumeration
- 63 levels of nested structure or union definitions in a
single struct-declaration-list
page 25 Environment
page 26
5.2.4.2 Numerical limits
[#1] A conforming implementation shall document all the
limits specified in this subclause, which shall be specified
in the headers <limits.h> and <float.h>.
5.2.4.2.1 Sizes of integer types <limits.h>
[#1] The values given below shall be replaced by constant
expressions suitable for use in #if preprocessing
directives. Moreover, except for CHAR_BIT and MB_LEN_MAX,
the following shall be replaced by expressions that have the
same type as would an expression that is an object of the
corresponding type converted according to the integer
promotions. Their implementation-defined values shall be
equal or greater in magnitude (absolute value) to those
shown, with the same sign.
- number of bits for smallest object that is not a bit-
field (byte)
CHAR_BIT 8
- minimum value for an object of type signed char
SCHAR_MIN -127
- maximum value for an object of type signed char
SCHAR_MAX +127
- maximum value for an object of type unsigned char
UCHAR_MAX 255
- minimum value for an object of type char
CHAR_MIN see below
- maximum value for an object of type char
CHAR_MAX see below
- maximum number of bytes in a multibyte character, for
any supported locale
MB_LEN_MAX 1
- minimum value for an object of type short int
SHRT_MIN -32767
- maximum value for an object of type short int
SHRT_MAX +32767
- maximum value for an object of type unsigned short int
USHRT_MAX 65535
page 26 Environment
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- minimum value for an object of type int
INT_MIN -32767
- maximum value for an object of type int
INT_MAX +32767
- maximum value for an object of type unsigned int
UINT_MAX 65535
- minimum value for an object of type long int
LONG_MIN -2147483647
- maximum value for an object of type long int
LONG_MAX +2147483647
- maximum value for an object of type unsigned long int
ULONG_MAX 4294967295
- minimum value for an object of type long long int
LLONG_MIN -9223372036854775807
- maximum value for an object of type long long int
LLONG_MAX +9223372036854775807
- maximum value for an object of type unsigned long long
int
ULLONG_MAX 18446744073709551615
[#2] If the value of an object of type char is treated as a
signed integer when used in an expression, the value of
CHAR_MIN shall be the same as that of SCHAR_MIN and the
value of CHAR_MAX shall be the same as that of SCHAR_MAX.
Otherwise, the value of CHAR_MIN shall be 0 and the value of
CHAR_MAX shall be the same as that of UCHAR_MAX.14 The
value UCHAR_MAX+1 shall equal 2 raised to the power
CHAR_BIT.
5.2.4.2.2 Characteristics of floating types <float.h>
[#1] The characteristics of floating types are defined in
terms of a model that describes a representation of
floating-point numbers and values that provide information
about an implementation's floating-point arithmetic.15 The
__________
14. See 6.1.2.5.
15. The floating-point model is intended to clarify the
description of each floating-point characteristic and
page 27 Environment
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following parameters are used to define the model for each
floating-point type:
s sign (_1)
b base or radix of exponent representation (an integer > 1)
e exponent (an integer between a minimum e and a maximum e )
p precision (the number of base-b digits inithe significand) max
f nonnegative integers less than b (the significand digits)
k
[#2] A normalized floating-point number x (f > 0 if x / 0)
is defined by the following model: 1
e p -k
x = s x b x R fk x b , emin < e < emax
k=1
[#3] Floating types might include values that are not
normalized floating-point numbers, for example subnormal
numbers (x / 0, e = e , f = 0), infinities, and NaNs. A
NaN is an encodingmisignifying Not-a-Number. A quiet NaN
propagates through almost every arithmetic operation without
raising an exception; a signaling NaN generally raises an
exception when occurring as an arithmetic operand.16
[#4] All integer values in the <float.h> header, except
FLT_ROUNDS, shall be constant expressions suitable for use
in #if preprocessing directives; all floating values shall
be constant expressions. All except FLT_EVAL_METHOD,
FLT_RADIX, and FLT_ROUNDS have separate names for all three
floating-point types. The floating-point model
representation is provided for all values except
FLT_EVAL_METHOD and FLT_ROUNDS.
[#5] The rounding mode for floating-point addition is
characterized by the value of FLT_ROUNDS:17
____________________________________________________________
does not require the floating-point arithmetic of the
implementation to be identical.
16. IEC 559:1993 specifies quiet and signaling NaNs. For
implementations that do not support IEC 559:1993, the
terms quiet NaN and signaling NaN are intended to apply
to encodings with similar behavior.
17. Evaluation of FLT_ROUNDS correctly reflects any
execution-time change of rounding mode through the
function fesetround in <fenv.h>.
page 28 Environment
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-1 indeterminable
0 toward zero
1 to nearest
2 toward positive infinity
3 toward negative infinity
All other values for FLT_ROUNDS characterize
implementation-defined rounding behavior.
[#6] The values of operations with floating operands and
values subject to the usual arithmetic conversions and of
floating constants are evaluated to a format whose range and
precision may be greater than required by the type. The use
of evaluation formats is characterized by the value of
FLT_EVAL_METHOD:18
-1 indeterminable;
0 evaluate all operations and constants just to the range
and precision of the type;
1 evaluate operations and constants of type float and
double to the range and precision of the double type,
evaluate long double operations and constants to the
range and precision of the long double type;
2 evaluate all operations and constants to the range and
precision of the long double type.
All other negative values for FLT_EVAL_METHOD characterize
implementation-defined behavior.
[#7] The values given in the following list shall be
replaced by implementation-defined expressions that shall be
equal or greater in magnitude (absolute value) to those
shown, with the same sign:
- radix of exponent representation, b
FLT_RADIX 2
__________
18. The evaluation method determines evaluation formats of
expressions involving all floating types, not just real
types. For example, if FLT_EVAL_METHOD is 1, then the
product of two float complex operands is represented in
the double complex format, and its parts are evaluated
to double.
page 29 Environment
page 30
- number of base-FLT_RADIX digits in the floating-point
significand, p
FLT_MANT_DIG
DBL_MANT_DIG
LDBL_MANT_DIG
- number of decimal digits, q, such that any floating-
point number with q decimal digits can be rounded into
a floating-point number with p radix b digits and back
again without change to the q decimal digits,
(1 if b is a power of 10
| (p - 1) x log b | + |
| 10 | (0 otherwise
FLT_DIG 6
DBL_DIG 10
LDBL_DIG 10
- minimum negative integer such that FLT_RADIX raised to
that power minus 1 is a normalized floating-point
number, e
min
FLT_MIN_EXP
DBL_MIN_EXP
LDBL_MIN_EXP
- minimum negative integer such that 10 raised to that
power is in the range of normalized floating-point
| e -1 |
numbers, | log b min |
| 10 |
FLT_MIN_10_EXP -37
DBL_MIN_10_EXP -37
LDBL_MIN_10_EXP -37
- maximum integer such that FLT_RADIX raised to that
power minus 1 is a representable finite floating-point
number, e
max
FLT_MAX_EXP
DBL_MAX_EXP
LDBL_MAX_EXP
- maximum integer such that 10 raised to that power is in
the range of representable finite floating-point
| e |
numbers, | log ((1 - b-p) x b max) |
| 10 |
page 30 Environment
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FLT_MAX_10_EXP +37
DBL_MAX_10_EXP +37
LDBL_MAX_10_EXP +37
[#8] The values given in the following list shall be
replaced by implementation-defined expressions with values
that shall be equal to or greater than those shown:
- max-pum reemaxentable finite floating-point number, (1
- b ) x b
FLT_MAX 1E+37
DBL_MAX 1E+37
LDBL_MAX 1E+37
[#9] The values given in the following list shall be
replaced by implementation-defined expressions with values
that shall be equal to or less than those shown:
- the difference between 1 and the least value greater
than 1 that1-ps representable in the given floating
point type, b
FLT_EPSILON 1E-5
DBL_EPSILON 1E-9
LDBL_EPSILON 1E-9
- eini-1m normalized positive floating-point number,
b min
FLT_MIN 1E-37
DBL_MIN 1E-37
LDBL_MIN 1E-37
Examples
[#10]
1. The following describes an artificial floating-point
representation that meets the minimum requirements of
this International Standard, and the appropriate
values in a <float.h> header for type float:
e 6 -k
x = s x 16 x R fk x 16 , -31 < e < +32
k=1
page 31 Environment
page 32
FLT_RADIX 16
FLT_MANT_DIG 6
FLT_EPSILON 9.53674316E-07F
FLT_DIG 6
FLT_MIN_EXP -31
FLT_MIN 2.93873588E-39F
FLT_MIN_10_EXP -38
FLT_MAX_EXP +32
FLT_MAX 3.40282347E+38F
FLT_MAX_10_EXP +38
2. The following describes floating-point representations
that also meet the requirements for single-precision
and double-precision normalized numbers in IEC 559,19
and the appropriate values in a <float.h> header for
types float and double:
e 24 -k
xf = s x 2 x R fk x 2 , -125 < e < +128
k=1
e 53 -k
xd = s x 2 x R fk x 2 , -1021 < e < +1024
k=1
__________
19. The floating-point model in that standard sums powers of
b from zero, so the values of the exponent limits are
one less than shown here.
page 32 Environment
page 33
FLT_RADIX 2
FLT_MANT_DIG 24
FLT_EPSILON 1.19209290E-07F // decimal constant
FLT_EPSILON 0X1P-23F // hex constant
FLT_DIG 6
FLT_MIN_EXP -125
FLT_MIN 1.17549435E-38F // decimal constant
FLT_MIN 0X1P-126F // hex constant
FLT_MIN_10_EXP -37
FLT_MAX_EXP +128
FLT_MAX 3.40282347E+38F // decimal constant
FLT_MAX 0X1.fffffeP127F // hex constant
FLT_MAX_10_EXP +38
DBL_MANT_DIG 53
DBL_EPSILON 2.2204460492503131E-16 // decimal constant
DBL_EPSILON 0X1P-52 // hex constant
DBL_DIG 15
DBL_MIN_EXP -1021
DBL_MIN 2.2250738585072014E-308 // decimal constant
DBL_MIN 0X1P-1022 // hex constant
DBL_MIN_10_EXP -307
DBL_MAX_EXP +1024
DBL_MAX 1.7976931348623157E+308 // decimal constant
DBL_MAX 0X1.ffffffffffffeP1023 // hex constant
DBL_MAX_10_EXP +308
Forward references: conditional inclusion (6.8.1).
page 33 Environment
page 34
6. Language
[#1] In the syntax notation used in the language clause
(clause 6), syntactic categories (nonterminals) are
indicated by italic type, and literal words and character
set members (terminals) by bold type. A colon (:)
following a nonterminal introduces its definition.
Alternative definitions are listed on separate lines, except
when prefaced by the words ``one of.'' An optional symbol
is indicated by the suffix ``-opt,'' so that
{ expression-opt }
indicates an optional expression enclosed in braces.
6.1 Lexical elements
Syntax
[#1]
token:
keyword
identifier
constant
string-literal
operator
punctuator
preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
operator
punctuator
each non-white-space character that cannot be one of the above
Constraints
[#2] Each preprocessing token that is converted to a token
shall have the lexical form of a keyword, an identifier, a
constant, a string literal, an operator, or a punctuator.
page 34 Language
page 35
Semantics
[#3] A token is the minimal lexical element of the language
in translation phases 7 and 8. The categories of tokens
are: keywords, identifiers, constants, string literals,
operators, and punctuators. A preprocessing token is the
minimal lexical element of the language in translation
phases 3 through 6. The categories of preprocessing token
are: header names, identifiers, preprocessing numbers,
character constants, string literals, operators,
punctuators, and single non-white-space characters that do
not lexically match the other preprocessing token
categories. If a ' or a " character matches the last
category, the behavior is undefined. Preprocessing tokens
can be separated by white space; this consists of comments
(described later), or white-space characters (space,
horizontal tab, new-line, vertical tab, and form-feed), or
both. As described in 6.8, in certain circumstances during
translation phase 4, white space (or the absence thereof)
serves as more than preprocessing token separation. White
space may appear within a preprocessing token only as part
of a header name or between the quotation characters in a
character constant or string literal.
[#4] If the input stream has been parsed into preprocessing
tokens up to a given character, the next preprocessing token
is the longest sequence of characters that could constitute
a preprocessing token.
[#5] A header name preprocessing token is only recognized
within a #include preprocessing directive, and within such a
directive, a sequence of characters that could be either a
header name or a string literal is recognized as the former.
Examples
1. The program fragment 1Ex is parsed as a preprocessing
number token (one that is not a valid floating or
integer constant token), even though a parse as the
pair of preprocessing tokens 1 and Ex might produce a
valid expression (for example, if Ex were a macro
defined as +1). Similarly, the program fragment 1E1
is parsed as a preprocessing number (one that is a
valid floating constant token), whether or not E is a
macro name.
2. The program fragment x+++++y is parsed as x+++++y,
which violates a constraint on increment operators,
even though the parse x+++++y might yield a correct
expression.
page 35 Language
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Forward references: character constants (6.1.3.4), comments
(6.1.9), expressions (6.3), floating constants (6.1.3.1),
header names (6.1.7), macro replacement (6.8.3), postfix
increment and decrement operators (6.3.2.4), prefix
increment and decrement operators (6.3.3.1), preprocessing
directives (6.8), preprocessing numbers (6.1.8), string
literals (6.1.4).
6.1.1 Keywords
Syntax
[#1]
keyword: one of
auto break case char
complex const continue default
do double else enum
extern float for goto
if imaginary inline int
long register restrict return
short signed sizeof static
struct switch typedef union
unsigned void volatile while
Semantics
[#2] The token complex is reserved in translation units
where the header <complex.h> is included; the token
imaginary is reserved in translation units where both the
header <complex.h> is included and the macro _Imaginary_I is
defined; all other keyword tokens are reserved in all
translation units. When reserved, the above tokens
(entirely in lowercase) are keywords (in translation phases
7 and 8), and shall not be used otherwise. When the token
complex or imaginary is reserved, its use prior to the first
inclusion of the header <complex.h> results in undefined
behavior.
page 36 Language
page 37
6.1.2 Identifiers
Syntax
[#1]
identifier:
nondigit
identifier nondigit
identifier digit
nondigit: one of
universal-character-name
_ a b c d e f g h i j k l m
n o p q r s t u v w x y z
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z
digit: one of
0 1 2 3 4 5 6 7 8 9
Description
[#2] An identifier is a sequence of nondigit characters
(including the underscore _ and the lowercase and uppercase
letters) and digits. Each universal-character-name in an
identifier shall designate a character whose encoding in ISO
10646-1 falls into one of the ranges specified in Annex
H.20 The first character shall be a nondigit character.
Semantics
[#3] An identifier can denote an object, a function, or one
of the following entities that will be described later: a
tag or a member of a structure, union, or enumeration; a
typedef name; a label name; a macro name; or a macro
parameter. The same identifier can denote different
entities at different points in the program. A member of an
enumeration is called an enumeration constant. Macro names
__________
20. On systems in which linkers cannot accept extended
characters, an encoding of the universal-character-name
may be used in forming valid external identifiers. For
example, some otherwise unused character or sequence of
characters may be used to encode the \u in a universal-
character-name. Extended characters may produce a long
external identifier.
page 37 Language
page 38
and macro parameters are not considered further here,
because prior to the semantic phase of program translation
any occurrences of macro names in the source file are
replaced by the preprocessing token sequences that
constitute their macro definitions.
[#4] There is no specific limit on the maximum length of an
identifier.
[#5] When preprocessing tokens are converted to tokens
during translation phase 7, if a preprocessing token could
be converted to either a keyword or an identifier, it is
converted to a keyword.
Implementation limits
[#6] The implementation shall treat at least the first 63
characters of an internal name (a macro name or an
identifier that does not have external linkage) as
significant. The implementation may further restrict the
significance of an external name (an identifier that has
external linkage) to 31 characters. In both external and
internal names, lower-case and upper-case letters are
different. The number of significant characters in an
identifier is implementation-defined.
[#7] Any identifiers that differ in a significant character
are different identifiers. If two identifiers differ in a
nonsignificant character, the behavior is undefined.
Forward references: linkages of identifiers (6.1.2.2),
macro replacement (6.8.3).
6.1.2.1 Scopes of identifiers
[#1] For each different entity that an identifier
designates, the identifier is visible (i.e., can be used)
only within a region of program text called its scope.
Different entities designated by the same identifier either
have non-overlapping scopes, or are in different name
spaces. There are four kinds of scopes: function, file,
block, and function prototype. (A function prototype is a
declaration of a function that declares the types of its
parameters.)
[#2] A label name is the only kind of identifier that has
function scope. It can be used (in a goto statement)
anywhere in the function in which it appears, and is
declared implicitly by its syntactic appearance (followed by
a : and a statement). Label names shall be unique within a
function.
page 38 Language
page 39
[#3] Every other identifier has scope determined by the
placement of its declaration (in a declarator or type
specifier). If the declarator or type specifier that
declares the identifier appears outside of any block or list
of parameters, the identifier has file scope, which
terminates at the end of the translation unit. If the
declarator or type specifier that declares the identifier
appears inside a block or within the list of parameter
declarations in a function definition, the identifier has
block scope, which terminates at the } that closes the
associated block. If the declarator or type specifier that
declares the identifier appears within the list of parameter
declarations in a function prototype (not part of a function
definition), the identifier has function prototype scope,
which terminates at the end of the function declarator. If
an identifier designates two different entities in the same
name space, the scopes might overlap. If so, the scope of
one entity (the inner scope) will be a strict subset of the
scope of the other entity (the outer scope). Within the
inner scope, the identifier designates the entity declared
in the inner scope; the entity declared in the outer scope
is hidden (and not visible) within the inner scope.
[#4] Unless explicitly stated otherwise, where this
International Standard uses the term identifier to refer to
some entity (as opposed to the syntactic construct), it
refers to the entity in the relevant name space whose
declaration is visible at the point the identifier occurs.
[#5] Two identifiers have the same scope if and only if
their scopes terminate at the same point.
[#6] Structure, union, and enumeration tags have scope that
begins just after the appearance of the tag in a type
specifier that declares the tag. Each enumeration constant
has scope that begins just after the appearance of its
defining enumerator in an enumerator list. Any other
identifier has scope that begins just after the completion
of its declarator.
Forward references: compound statement, or block (6.6.2),
declarations (6.5), enumeration specifiers (6.5.2.2),
function calls (6.3.2.2), function declarators (including
prototypes) (6.5.5.3), function definitions (6.7.1), the
goto statement (6.6.6.1), labeled statements (6.6.1), name
spaces of identifiers (6.1.2.3), scope of macro definitions
(6.8.3.5), source file inclusion (6.8.2), tags (6.5.2.3),
type specifiers (6.5.2).
page 39 Language
page 40
6.1.2.2 Linkages of identifiers
[#1] An identifier declared in different scopes or in the
same scope more than once can be made to refer to the same
object or function by a process called linkage. There are
three kinds of linkage: external, internal, and none.
[#2] In the set of translation units and libraries that
constitutes an entire program, each instance of a particular
identifier with external linkage denotes the same object or
function. Within one translation unit, each instance of an
identifier with internal linkage denotes the same object or
function. Identifiers with no linkage denote unique
entities.
[#3] If the declaration of a file scope identifier for an
object or a function contains the storage-class specifier
static, the identifier has internal linkage.21
[#4] For an identifier declared with the storage-class
specifier extern in a scope in which a prior declaration of
that identifier is visible,22 if the prior declaration
specifies internal or external linkage, the linkage of the
identifier at the later declaration becomes the linkage
specified at the prior declaration. If no prior declaration
is visible, or if the prior declaration specifies no
linkage, then the identifier has external linkage.
[#5] If the declaration of an identifier for a function has
no storage-class specifier, its linkage is determined
exactly as if it were declared with the storage-class
specifier extern. If the declaration of an identifier for
an object has file scope and no storage-class specifier, its
linkage is external.
[#6] The following identifiers have no linkage: an
identifier declared to be anything other than an object or a
function; an identifier declared to be a function parameter;
a block scope identifier for an object declared without the
storage-class specifier extern.
__________
21. A function declaration can contain the storage-class
specifier static only if it is at file scope; see 6.5.1.
22. As specified in 6.1.2.1, the later declaration might
hide the prior declaration.
page 40 Language
page 41
[#7] If, within a translation unit, the same identifier
appears with both internal and external linkage, the
behavior is undefined.
Forward references: compound statement, or block (6.6.2),
declarations (6.5), expressions (6.3), external definitions
(6.7).
6.1.2.3 Name spaces of identifiers
[#1] If more than one declaration of a particular identifier
is visible at any point in a translation unit, the syntactic
context disambiguates uses that refer to different entities.
Thus, there are separate name spaces for various categories
of identifiers, as follows:
- label names (disambiguated by the syntax of the label
declaration and use);
- the tags of structures, unions, and enumerations
(disambiguated by following any23 of the keywords
struct, union, or enum);
- the members of structures or unions; each structure or
union has a separate name space for its members
(disambiguated by the type of the expression used to
access the member via the . or -> operator);
- all other identifiers, called ordinary identifiers
(declared in ordinary declarators or as enumeration
constants).
Forward references: enumeration specifiers (6.5.2.2),
labeled statements (6.6.1), structure and union specifiers
(6.5.2.1), structure and union members (6.3.2.3), tags
(6.5.2.3).
__________
23. There is only one name space for tags even though three
are possible.
page 41 Language
page 42
6.1.2.4 Storage durations of objects
[#1] An object has a storage duration that determines its
lifetime. There are three storage durations: static,
automatic, and allocated. Allocated storage is described in
7.14.3.
[#2] An object whose identifier is declared with external or
internal linkage, or with the storage-class specifier static
has static storage duration. For such an object, storage is
reserved and its stored value is initialized only once,
prior to program startup. The object exists, has a constant
address, and retains its last-stored value throughout the
execution of the entire program.24
[#3] An object whose identifier is declared with no linkage
and without the storage-class specifier static has automatic
storage duration. Storage is guaranteed to be reserved for
a new instance of such an object on each normal entry into
the block with which it is associated. If the block with
which the object is associated is entered by a jump from
outside the block to a labeled statement in the block or in
an enclosed block, then storage is guaranteed to be reserved
provided the object does not have a variable length array
type. If the object is variably modified and the block is
entered by a jump to a labeled statement, then the behavior
is undefined. If an initialization is specified for the
value stored in the object, it is performed on each normal
entry, but not if the block is entered by a jump to a
labeled statement beyond the declaration. A backwards jump
might cause the initializer to be evaluated more than once;
if so, a new value will be stored each time. Storage for
the object is no longer guaranteed to be reserved when
execution of the block ends in any way. (Entering an
enclosed block suspends but does not end execution of the
enclosing block. Calling a function suspends but does not
end execution of the block containing the call.) The value
of a pointer that referred to an object with automatic
storage duration that is no longer guaranteed to be reserved
__________
24. The term constant address means that two pointers to the
object constructed at possibly different times will
compare equal. The address may be different during two
different executions of the same program.
In the case of a volatile object, the last store may not
be explicit in the program.
page 42 Language
page 43
is indeterminate. During execution of the associated block,
the object has a constant address.
Forward references: compound statement, or block (6.6.2),
function calls (6.3.2.2), declarators (6.5.5), array
declarators (6.5.5.2), initialization (6.5.8).
6.1.2.5 Types
[#1] The meaning of a value stored in an object or returned
by a function is determined by the type of the expression
used to access it. (An identifier declared to be an object
is the simplest such expression; the type is specified in
the declaration of the identifier.) Types are partitioned
into object types (types that describe objects), function
types (types that describe functions), and incomplete types
(types that describe objects but lack information needed to
determine their sizes).
[#2] An object declared as type char is large enough to
store any member of the basic execution character set. If a
member of the required source character set enumerated in
5.2.1 is stored in a char object, its value is guaranteed to
be positive. If any other character is stored in a char
object, the resulting value is implementation-defined but
shall be within the range of values that can be represented
in that type.
[#3] There are five standard signed integer types,
designated as signed char, short int, int, long int, and
long long int. (These and other types may be designated in
several additional ways, as described in 6.5.2.) There may
also be implementation-defined extended signed integer
types.25 The standard and extended signed integer types are
collectively called just signed integer types.26
[#4] An object declared as type signed char occupies the
same amount of storage as a ``plain'' char object. A
``plain'' int object has the natural size suggested by the
architecture of the execution environment (large enough to
__________
25. Implementation-defined keywords must have the form of an
identifier reserved for any use as described in 7.1.3.
26. Therefore, any statement in this Standard about signed
integer types also applies to the extended signed
integer types.
page 43 Language
page 44
contain any value in the range INT_MIN to INT_MAX as defined
in the header <limits.h>).
[#5] For each of the signed integer types, there is a
corresponding (but different) unsigned integer type
(designated with the keyword unsigned) that uses the same
amount of storage (including sign information) and has the
same alignment requirements. The unsigned integer types
that correspond to the standard signed integer types are the
standard unsigned integer types. The unsigned integer types
that correspond to the extended signed integer types are the
extended unsigned integer types.
[#6] The extended signed integer types and extended unsigned
integer types are collectively called the extended integer
types.
[#7] For any two types with the same signedness and
different integer conversion rank, the range of values of
the type with smaller integer conversion rank is a subrange
of the values of the other type.
[#8] The range of nonnegative values of a signed integer
type is a subrange of the corresponding unsigned integer
type, and the representation of the same value in each type
is the same.27 A computation involving unsigned operands
can never overflow, because a result that cannot be
represented by the resulting unsigned integer type is
reduced modulo the number that is one greater than the
largest value that can be represented by the resulting
unsigned integer type.
[#9] There are three real floating types, designated as
float, double, and long double. The set of values of the
type float is a subset of the set of values of the type
double; the set of values of the type double is a subset of
the set of values of the type long double.
[#10] There are three complex types, designated as float
complex, double complex, and long double complex.28 The
__________
27. The same representation and alignment requirements are
meant to imply interchangeability as arguments to
functions, return values from functions, and members of
unions.
28. A specification for imaginary types is in informative
Annex G.
page 44 Language
page 45
real floating and complex types are collectively called the
floating types.
[#11] For each floating type there is a corresponding real
type, which is always a real floating type. For real
floating types, it is the same type. For complex types, it
is the type given by deleting the keyword complex from the
type name.
[#12] Each complex type has the same representation and
alignment requirements as an array type containing exactly
two elements of the corresponding real type; the first
element is equal to the real part, and the second element to
the imaginary part, of the complex number.
[#13] The type char, the signed and unsigned integer types,
and the floating types are collectively called the basic
types. Even if the implementation defines two or more basic
types to have the same representation, they are nevertheless
different types.29
[#14] The three types char, signed char, and unsigned char
are collectively called the character types. The
implementation shall define char to have the same range,
representation, and behavior as one of signed char and
unsigned char.30
[#15] An enumeration comprises a set of named integer
constant values. Each distinct enumeration constitutes a
different enumerated type.
[#16] The void type comprises an empty set of values; it is
an incomplete type that cannot be completed.
__________
29. An implementation may define new keywords that provide
alternative ways to designate a basic (or any other)
type. An alternate way to designed a basic type does
not violate the requirement that all basic types be
different. Implementation-defined keywords must have
the form of an identifier reserved for any use as
described in 7.1.3.
30. CHAR_MIN, defined in <limits.h>, will have one of the
values 0 or SCHAR_MIN, and this can be used to
distinguish the two options. Irrespective of the choice
made, char is a separate type from the other two, and it
not compatible with either.
page 45 Language
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[#17] Any number of derived types can be constructed from
the object, function, and incomplete types, as follows:
- An array type describes a contiguously allocated
nonempty set of objects with a particular member object
type, called the element type.31 Array types are
characterized by their element type and by the number
of elements in the array. An array type is said to be
derived from its element type, and if its element type
is T, the array type is sometimes called ``array of
T.'' The construction of an array type from an element
type is called ``array type derivation.''
- A structure type describes a sequentially allocated
nonempty set of member objects, each of which has an
optionally specified name and possibly distinct type.
- A union type describes an overlapping nonempty set of
member objects, each of which has an optionally
specified name and possibly distinct type.
- A function type describes a function with specified
return type. A function type is characterized by its
return type and the number and types of its parameters.
A function type is said to be derived from its return
type, and if its return type is T, the function type is
sometimes called ``function returning T.'' The
construction of a function type from a return type is
called ``function type derivation.''
- A pointer type may be derived from a function type, an
object type, or an incomplete type, called the
referenced type. A pointer type describes an object
whose value provides a reference to an entity of the
referenced type. A pointer type derived from the
referenced type T is sometimes called ``pointer to T.''
The construction of a pointer type from a referenced
type is called ``pointer type derivation.''
[#18] These methods of constructing derived types can be
applied recursively.
[#19] The type char, the signed and unsigned integer types,
and the enumerated types are collectively called integer
__________
31. Since object types do not include incomplete types, an
array of incomplete type cannot be constructed.
page 46 Language
page 47
types. The integer and real floating types are collectively
called real types.
[#20] Integer and floating types are collectively called
arithmetic types. Arithmetic types and pointer types are
collectively called scalar types. Array and structure types
are collectively called aggregate types.32
[#21] Each arithmetic type belongs to one type-domain. The
real type-domain comprises the real types. The complex
type-domain comprises the complex types.
[#22] An array type of unknown size is an incomplete type.
It is completed, for an identifier of that type, by
specifying the size in a later declaration (with internal or
external linkage). A structure or union type of unknown
content (as described in 6.5.2.3) is an incomplete type. It
is completed, for all declarations of that type, by
declaring the same structure or union tag with its defining
content later in the same scope.
[#23] Array, function, and pointer types are collectively
called derived declarator types. A declarator type
derivation from a type T is the construction of a derived
declarator type from T by the application of an array-type,
a function-type, or a pointer-type derivation to T.
[#24] A type is characterized by its type category, which is
either the outermost derivation of a derived type (as noted
above in the construction of derived types), or the type
itself if the type consists of no derived types.
[#25] Any type so far mentioned is an unqualified type.
Each unqualified type has several qualified versions of its
type,33 corresponding to the combinations of one, two, or
all three of the const, volatile, and restrict qualifiers.
The qualified or unqualified versions of a type are distinct
types that belong to the same type category and have the
same representation and alignment requirements.27 A derived
type is not qualified by the qualifiers (if any) of the type
from which it is derived.
__________
32. Note that aggregate type does not include union type
because an object with union type can only contain one
member at a time.
33. See 6.5.3 regarding qualified array and function types.
page 47 Language
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[#26] A pointer to void shall have the same representation
and alignment requirements as a pointer to a character type.
Similarly, pointers to qualified or unqualified versions of
compatible types shall have the same representation and
alignment requirements.27 All pointers to structure types
shall have the same representation and alignment
requirements as each other. All pointers to union types
shall have the same representation and alignment
requirements as each other. Pointers to other types need
not have the same representation or alignment requirements.
Examples
[#27]
1. The type designated as ``float *'' has type ``pointer
to float.'' Its type category is pointer, not a
floating type. The const-qualified version of this
type is designated as ``float * const'' whereas the
type designated as ``const float *'' is not a
qualified type - its type is ``pointer to const-
qualified float'' and is a pointer to a qualified
type.
2. The type designated as ``struct tag (*[5])(float)''
has type ``array of pointer to function returning
struct tag.'' The array has length five and the
function has a single parameter of type float. Its
type category is array.
Forward references: character constants (6.1.3.4),
compatible type and composite type (6.1.2.6), integer
conversion rank (6.2.1.1), declarations (6.5), tags
(6.5.2.3), type qualifiers (6.5.3).
6.1.2.6 Compatible type and composite type
[#1] Two types have compatible type if their types are the
same. Additional rules for determining whether two types
are compatible are described in 6.5.2 for type specifiers,
in 6.5.3 for type qualifiers, and in 6.5.5 for
declarators.34 Moreover, two structure, union, or
enumerated types declared in separate translation units are
compatible if their tags and members satisfy the following
requirements. If one is declared with a tag, the other
__________
34. Two types need not be identical to be compatible.
page 48 Language
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shall be declared with the same tag. If both are completed
types, then the following additional requirements apply:
there shall be a one-to-one correspondence between their
members such that each pair of corresponding members are
declared with compatible types, and such that if one member
of a corresponding pair is declared with a name, the other
member is declared with the same name. For two structures,
corresponding members shall be declared in the same order.
For two structures or unions, corresponding bit-fields shall
have the same widths. For two enumerations, corresponding
members shall have the same values.
[#2] All declarations that refer to the same object or
function shall have compatible type; otherwise, the behavior
is undefined.
[#3] A composite type can be constructed from two types that
are compatible; it is a type that is compatible with both of
the two types and satisfies the following conditions:
- If one type is an array of known constant size, the
composite type is an array of that size; otherwise, if
one type is a variable length array, the composite type
is that type.
- If only one type is a function type with a parameter
type list (a function prototype), the composite type is
a function prototype with the parameter type list.
- If both types are function types with parameter type
lists, the type of each parameter in the composite
parameter type list is the composite type of the
corresponding parameters.
These rules apply recursively to the types from which the
two types are derived.
[#4] For an identifier with internal or external linkage
declared in a scope in which a prior declaration of that
identifier is visible,35 if the prior declaration specifies
internal or external linkage, the type of the identifier at
the later declaration becomes the composite type.
__________
35. As specified in 6.1.2.1, the later declaration might
hide the prior declaration.
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Examples
[#5] Given the following two file scope declarations:
int f(int (*)(), double (*)[3]);
int f(int (*)(char *), double (*)[]);
The resulting composite type for the function is:
int f(int (*)(char *), double (*)[3]);
Forward references: declarators (6.5.5), enumeration
specifiers (6.5.2.2), structure and union specifiers
(6.5.2.1), type definitions (6.5.7), type qualifiers
(6.5.3), type specifiers (6.5.2).
6.1.2.7 Predefined identifiers
[#1] The following identifier shall be defined by the
implementation:
__func__ The name of the lexically-enclosing function.
Forward references: the identifier __func__ (6.3.1.1).
6.1.2.8 Representations of types
[#1] The representations of all types are unspecified except
as stated in this subclause.
6.1.2.8.1 General
[#1] Values of type unsigned char shall be represented using
a pure binary notation.36
[#2] When stored in objects of any other object type, values
of that type consist of n*CHAR_BIT bits, where n is the size
__________
36. A positional representation for integers that uses the
binary digits 0 and 1, in which the values represented
by successive bits are additive, begin with 1, and are
multiplied by successive integral powers of 2, except
perhaps the bit with the highest position. (Adapted
from the American National Dictionary for Information
Processing Systems.) A byte contains CHAR_BIT bits, and
the values of type unsigned char range from 0 to
2CHAR_BIT-1.
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of an object of that type, in bytes. The value may be
copied into an object of type unsigned char [n] (e.g., by
memcpy); the resulting set of bytes is called the object
representation of the value. Two values with the same
object representation shall compare equal, but values that
compare equal might have different object representations.
[#3] Certain object representations might not represent a
value of that type. If the stored value of an object has
such a representation and is accessed by an lvalue
expression that does not have character type, the behavior
is undefined. If such a representation is produced by a
side effect that modifies all or any part of the object by
an lvalue expression that does not have character type, the
behavior is undefined.37 Such representations are called
trap representations.
[#4] When a value is stored in an object of structure or
union type, including in a member object, the bytes of the
object representation that correspond to any padding bytes
take unspecified values.38 The values of padding bytes
shall not affect whether the value of such an object is a
trap representation. Those bits of a structure or union
object that are in the same byte as a bit-field member, but
are not part of that member, shall similarly not affect
whether the value of such an object is a trap
representation.
[#5] When a value is stored in a member of an object of
union type, the bytes of the object representation that do
not correspond to that member but do correspond to other
members take unspecified values, but the value of the union
object shall not thereby become a trap representation.
[#6] Where an operator is applied to a value which has more
than one object representation, which object representation
is used shall not affect the value of the result. Where a
value is stored in an object using a type that has more than
one object representation for that value, it is unspecified
__________
37. Thus an automatic variable can be initialized to a trap
representation without causing undefined behavior, but
the value of the variable cannot be used until a proper
value is stored in it.
38. Thus, for example, structure assignment may be
implemented element-at-a-time or via memcpy.
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which representation is used, but a trap representation
shall not be generated.
6.1.2.8.2 Integer types
[#1] For unsigned integer types other than unsigned char,
the bits of the object representation shall be divided into
two groups: value bits and padding bits (there need not be
any of the latter). If there are N value bits, each bit
shall represent a different power of 2 between 1 and 2N - 1,
so that objects of that type shall be capable of
representing values from 0 to 2N - 1 using a pure binary
representation; this shall be known as the value
representation. The values of any padding bits are
unspecified.39
[#2] For signed integer types, the bits of the object
representation shall be divided into three groups: value
bits, padding bits, and the sign bit. There need not be any
padding bits; there shall be exactly one sign bit. Each
bit that is a value bit shall have the same value as the
same bit in the object representation of the corresponding
unsigned type (if there are M value bits in the signed type
and N in the unsigned type, then M<N). If the sign bit is
zero, it shall not affect the resulting value. If the sign
bit is one, then the value shall be modified in one of the
following ways:
- the corresponding value with sign bit 0 is negated;
- the sign bit has the value -2N;
- the sign bit has the value 1 - 2N.
[#3] The values of any padding bits are unspecified.39 A
valid (non-trap) object representation of a signed integer
type where the sign bit is zero is a valid object
__________
39. Some combinations of padding bits might generate trap
representations, for example, if one padding bit is a
parity bit. Regardless, no arithmetic operation on
valid values can generate a trap representation other
than as part of an exception such as an overflow, and
this cannot occur with unsigned types. All other
combinations of padding bits are alternative object
representations of the value specified by the value
bits.
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representation of the corresponding unsigned type, and shall
represent the same value.
[#4] The precision of an integer type is the number of bits
it uses to represent values, excluding any sign and padding
bits. The width of an integer type is the same but
including any sign bit; thus for unsigned integer types the
two values are the same, while for signed integer types the
width is one greater than the precision.
6.1.3 Constants
Syntax
[#1]
constant:
floating-constant
integer-constant
enumeration-constant
character-constant
Constraints
[#2] The value of a constant shall be in the range of
representable values for its type.
Semantics
[#3] Each constant has a type, determined by its form and
value, as detailed later.
6.1.3.1 Floating constants
Syntax
[#1]
floating-constant:
decimal-floating-constant
hexadecimal-floating-constant
decimal-floating-constant:
fractional-constant exponent-part-opt floating-suffix-opt
digit-sequence exponent-part floating-suffix-opt
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hexadecimal-floating-constant:
0x hexadecimal-fractional-constant
binary-exponent-part floating-suffix-opt
0X hexadecimal-fractional-constant
binary-exponent-part floating-suffix-opt
0x hexadecimal-digit-sequence
binary-exponent-part floating-suffix-opt
0X hexadecimal-digit-sequence
binary-exponent-part floating-suffix-opt
fractional-constant:
digit-sequence-opt . digit-sequence
digit-sequence .
exponent-part:
e sign-opt digit-sequence
E sign-opt digit-sequence
sign: one of
+ -
digit-sequence:
digit
digit-sequence digit
hexadecimal-fractional-constant:
hexadecimal-digit-sequence-opt .
hexadecimal-digit-sequence
hexadecimal-digit-sequence .
binary-exponent-part:
p sign-opt digit-sequence
P sign-opt digit-sequence
hexadecimal-digit-sequence:
hexadecimal-digit
hexadecimal-digit-sequence hexadecimal-digit
hexadecimal-digit: one of
0 1 2 3 4 5 6 7 8 9
a b c d e f
A B C D E F
floating-suffix: one of
f l F L
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Description
[#2] A floating constant has a significand part that may be
followed by an exponent part and a suffix that specifies its
type. The components of the significand part may include a
digit sequence representing the whole-number part, followed
by a period (.), followed by a digit sequence representing
the fraction part. The components of the exponent part are
an e, E, p, or P followed by an exponent consisting of an
optionally signed digit sequence. Either the whole-number
part or the fraction part shall be present; for decimal
floating constants, either the period or the exponent part
shall be present.
Semantics
[#3] The significand part is interpreted as a (decimal or
hexadecimal) rational number; the digit sequence in the
exponent part is interpreted as a decimal integer. For
decimal floating constants, the exponent indicates the power
of 10 by which the significand part is to be scaled. For
hexadecimal floating constants, the exponent indicates the
power of 2 by which the significand part is to be scaled.
For decimal floating constants, and also for hexadecimal
floating constants when FLT_RADIX is not a power of 2, if
the scaled value is in the range of representable values
(for its type) the result is either the nearest
representable value, or the larger or smaller representable
value immediately adjacent to the nearest representable
value, chosen in an implementation-defined manner. For
hexadecimal floating constants, if FLT_RADIX is a power of 2
and the scaled value is in the range of representable values
(for its type), then the result of a hexadecimal floating
constant is correctly rounded.
[#4] An unsuffixed floating constant has type double. If
suffixed by the letter f or F, it has type float. If
suffixed by the letter l or L, it has type long double.
Recommended practice
[#5] The implementation produces a diagnostic message if a
hexadecimal constant cannot be represented exactly in its
evaluation format; the implementation then proceeds with the
translation of the program.
[#6] The translation-time conversion of floating constants
matches the execution-time conversion of character strings
by library functions, such as strtod, given matching inputs
suitable for both conversions, the same result format, and
default execution-time rounding.40
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6.1.3.2 Integer constants
Syntax
[#1]
integer-constant:
decimal-constant integer-suffix-opt
octal-constant integer-suffix-opt
hexadecimal-constant integer-suffix-opt
decimal-constant:
nonzero-digit
decimal-constant digit
octal-constant:
0
octal-constant octal-digit
hexadecimal-constant:
0x hexadecimal-digit
0X hexadecimal-digit
hexadecimal-constant hexadecimal-digit
nonzero-digit: one of
1 2 3 4 5 6 7 8 9
octal-digit: one of
0 1 2 3 4 5 6 7
integer-suffix:
unsigned-suffix long-suffix-opt
long-suffix unsigned-suffix-opt
unsigned-suffix long-long-suffix-opt
long-long-suffix unsigned-suffix-opt
unsigned-suffix: one of
u U
long-suffix: one of
l L
__________
40. The specification for the library functions recommends
more accurate conversion than required for floating
constants. See strtod (7.14.1.5).
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long-long-suffix: one of
ll LL
Description
[#2] An integer constant begins with a digit, but has no
period or exponent part. It may have a prefix that
specifies its base and a suffix that specifies its type.
[#3] A decimal constant begins with a nonzero digit and
consists of a sequence of decimal digits. An octal constant
consists of the prefix 0 optionally followed by a sequence
of the digits 0 through 7 only. A hexadecimal constant
consists of the prefix 0x or 0X followed by a sequence of
the decimal digits and the letters a (or A) through f (or F)
with values 10 through 15 respectively.
Semantics
[#4] The value of a decimal constant is computed base 10;
that of an octal constant, base 8; that of a hexadecimal
constant, base 16. The lexically first digit is the most
significant.
[#5] The type of an integer constant is the first of the
corresponding list in which its value can be represented.
Unsuffixed decimal: int, long int, long long int;
unsuffixed octal or hexadecimal: int, unsigned int, long
int, unsigned long int, long long int, unsigned long long
int; suffixed by the letter u or U: unsigned int, unsigned
long int, unsigned long long int; decimal suffixed by the
letter l or L: long int, long long int; octal or
hexadecimal suffixed by the letter l or L: long int,
unsigned long int, long long int, unsigned long long int;
suffixed by both the letters u or U and l or L: unsigned
long int, unsigned long long int; decimal suffixed by ll or
LL: long long int; octal or hexadecimal suffixed by the
letter ll or LL: long long int, unsigned long long int;
suffixed by both u or U and ll or LL: unsigned long long
int. If an integer constant can not be represented by any
type in its list, it may have an extended integer type, if
the extended integer type can represent its value. If all
of the types in the list for the constant are signed, the
extended integer type shall be signed. If all of the types
in the list for the constant are unsigned, the extended
integer type shall be unsigned. If the list contains both
signed and unsigned types, the extended integer type may be
signed or unsigned.
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6.1.3.3 Enumeration constants
Syntax
[#1]
enumeration-constant:
identifier
Semantics
[#2] An identifier declared as an enumeration constant has
type int.
Forward references: enumeration specifiers (6.5.2.2).
6.1.3.4 Character constants
Syntax
[#1]
character-constant:
'c-char-sequence'
L'c-char-sequence'
c-char-sequence:
c-char
c-char-sequence c-char
c-char:
any member of the source character set except
the single-quote ', backslash \, or new-line character
escape-sequence
universal-character-name
escape-sequence:
simple-escape-sequence
octal-escape-sequence
hexadecimal-escape-sequence
simple-escape-sequence: one of
\' \" \? \\
\a \b \f \n \r \t \v
octal-escape-sequence:
\ octal-digit
\ octal-digit octal-digit
\ octal-digit octal-digit octal-digit
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hexadecimal-escape-sequence:
\x hexadecimal-digit
hexadecimal-escape-sequence hexadecimal-digit
Description
[#2] An integer character constant is a sequence of one or
more multibyte characters enclosed in single-quotes, as in
'x' or 'ab'. A wide character constant is the same, except
prefixed by the letter L. With a few exceptions detailed
later, the elements of the sequence are any members of the
source character set; they are mapped in an implementation-
defined manner to members of the execution character set.
[#3] The single-quote ', the double-quote ", the question-
mark ?, the backslash \, and arbitrary integer values, are
representable according to the following table of escape
sequences:
single-quote ' \'
double-quote " \"
question-mark ? \?
backslash \ \\
octal integer \octal digits
hexadecimal integer \xhexadecimal digits
[#4] The double-quote " and question-mark ? are
representable either by themselves or by the escape
sequences \" and \?, respectively, but the single-quote '
and the backslash \ shall be represented, respectively, by
the escape sequences \' and \\.
[#5] The octal digits that follow the backslash in an octal
escape sequence are taken to be part of the construction of
a single character for an integer character constant or of a
single wide character for a wide character constant. The
numerical value of the octal integer so formed specifies the
value of the desired character or wide character.
[#6] The hexadecimal digits that follow the backslash and
the letter x in a hexadecimal escape sequence are taken to
be part of the construction of a single character for an
integer character constant or of a single wide character for
a wide character constant. The numerical value of the
hexadecimal integer so formed specifies the value of the
desired character or wide character.
[#7] Each octal or hexadecimal escape sequence is the
longest sequence of characters that can constitute the
escape sequence.
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[#8] In addition, certain nongraphic characters are
representable by escape sequences consisting of the
backslash \ followed by a lowercase letter: \a, \b, \f, \n,
\r, \t, and \v.41 If any other escape sequence is
encountered, the behavior is undefined.42
Constraints
[#9] The value of an octal or hexadecimal escape sequence
shall be in the range of representable values for the type
unsigned char for an integer character constant, or the
unsigned type corresponding to wchar_t for a wide character
constant.
Semantics
[#10] An integer character constant has type int. The value
of an integer character constant containing a single
character that maps to a member of the basic execution
character set is the numerical value of the representation
of the mapped character interpreted as an integer. The
value of an integer character constant containing more than
one character, or containing a character or escape sequence
not represented in the basic execution character set, is
implementation-defined. If an integer character constant
contains a single character or escape sequence, its value is
the one that results when an object with type char whose
value is that of the single character or escape sequence is
converted to type int.
[#11] A wide character constant has type wchar_t, an integer
type defined in the <stddef.h> header. The value of a wide
character constant containing a single multibyte character
that maps to a member of the extended execution character
set is the wide character (code) corresponding to that
multibyte character, as defined by the mbtowc function, with
an implementation-defined current locale. The value of a
wide character constant containing more than one multibyte
character, or containing a multibyte character or escape
sequence not represented in the extended execution character
set, is implementation-defined.
__________
41. The semantics of these characters were discussed in
5.2.2.
42. See ``future language directions'' (6.9.1).
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Examples
[#12]
1. The construction '\0' is commonly used to represent
the null character.
2. Consider implementations that use two's-complement
representation for integers and eight bits for objects
that have type char. In an implementation in which
type char has the same range of values as signed char,
the integer character constant '\xFF' has the value -
1; if type char has the same range of values as
unsigned char, the character constant '\xFF' has the
value +255 .
3. Even if eight bits are used for objects that have type
char, the construction '\x123' specifies an integer
character constant containing only one character,
since a hexadecimal escape sequence is terminated only
by a non-hexadecimal character. To specify an integer
character constant containing the two characters whose
values are '\x12' and '3', the construction '\0223'
may be used, since an octal escape sequence is
terminated after three octal digits. (The value of
this two-character integer character constant is
implementation-defined.)
4. Even if 12 or more bits are used for objects that have
type wchar_t, the construction L'\1234' specifies the
implementation-defined value that results from the
combination of the values 0123 and '4'.
Forward references: characters and integers (6.2.1.1),
common definitions <stddef.h> (7.1.6), the mbtowc function
(7.14.7.2).
6.1.4 String literals
Syntax
[#1]
string-literal:
"s-char-sequence-opt"
L"s-char-sequence-opt"
s-char-sequence:
s-char
s-char-sequence s-char
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s-char:
any member of the source character set except
the double-quote ", backslash \, or new-line character
escape-sequence
universal-character-name
Description
[#2] A character string literal is a sequence of zero or
more multibyte characters enclosed in double-quotes, as in
"xyz". A wide string literal is the same, except prefixed
by the letter L.
[#3] The same considerations apply to each element of the
sequence in a character string literal or a wide string
literal as if it were in an integer character constant or a
wide character constant, except that the single-quote ' is
representable either by itself or by the escape sequence \',
but the double-quote " shall be represented by the escape
sequence \".
Semantics
[#4] In translation phase 6, the multibyte character
sequences specified by any sequence of adjacent character
and wide string literal tokens are concatenated into a
single multibyte character sequence. If any of the tokens
are wide string literal tokens, the resulting multibyte
character sequence is treated as a wide string literal;
otherwise, it is treated as a character string literal.
[#5] In translation phase 7, a byte or code of value zero is
appended to each multibyte character sequence that results
from a string literal or literals.43 The multibyte
character sequence is then used to initialize an array of
static storage duration and length just sufficient to
contain the sequence. For character string literals, the
array elements have type char, and are initialized with the
individual bytes of the multibyte character sequence; for
wide string literals, the array elements have type wchar_t,
and are initialized with the sequence of wide characters
corresponding to the multibyte character sequence.
__________
43. A character string literal need not be a string (see
7.1.1), because a null character may be embedded in it
by a \0 escape sequence.
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[#6] These arrays need not be distinct provided their
elements have the appropriate values. If the program
attempts to modify such an array, the behavior is undefined.
Examples
[#7] This pair of adjacent character string literals
"\x12" "3"
produces a single character string literal containing the
two characters whose values are and '3', because escape
sequences are converted into single members of the execution
character set just prior to adjacent string literal
concatenation.
Forward references: common definitions <stddef.h> (7.1.6).
6.1.5 Operators
Syntax
[#1]
operator: one of
[ ] ( ) . ->
++ -- & * + - ~ ! sizeof
/ % << >> < > <= >= == != ^ | && ||
? :
= *= /= %= += -= <<= >>= &= ^= |=
, # ## <: :> %: %:%:
Constraints
[#2] The operators [ ], ( ), and ? : (independent of
spelling) shall occur in pairs, possibly separated by
expressions. The operators # and ## (also spelled %: and
%:%:, respectively) shall occur in macro-defining
preprocessing directives only.
Semantics
[#3] An operator specifies an operation to be performed (an
evaluation) that yields a value, or yields a designator, or
produces a side effect, or a combination thereof. An
operand is an entity on which an operator acts.
[#4] In all aspects of the language, these six tokens
<: :> <% %> %: %:%:
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behave, respectively, the same as these six tokens
[ ] { } # ##
except for their spelling.44
Forward references: expressions (6.3), macro replacement
(6.8.3).
6.1.6 Punctuators
Syntax
[#1]
punctuator: one of
[ ] ( ) { } * , : = ; ... #
<: :> <% %> %:
Constraints
[#2] The punctuators [ ], ( ), and { } (independent of
spelling) shall occur (after translation phase 4) in pairs,
possibly separated by expressions, declarations, or
statements. The punctuator # (also spelled %:) shall occur
in preprocessing directives only.
Semantics
[#3] A punctuator is a symbol that has independent syntactic
and semantic significance but does not specify an operation
to be performed that yields a value. Depending on context,
the same symbol may also represent an operator or part of an
operator.
Forward references: expressions (6.3), declarations (6.5),
preprocessing directives (6.8), statements (6.6).
__________
44. Thus [ and <: behave differently when ``stringized''
(see subclause 6.8.3.2), but can otherwise be freely
interchanged.
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6.1.7 Header names
Syntax
[#1]
header-name:
<h-char-sequence>
"q-char-sequence"
h-char-sequence:
h-char
h-char-sequence h-char
h-char:
any member of the source character set except
the new-line character and >
q-char-sequence:
q-char
q-char-sequence q-char
q-char:
any member of the source character set except
the new-line character and "
Semantics
[#2] The sequences in both forms of header names are mapped
in an implementation-defined manner to headers or external
source file names as specified in 6.8.2.
[#3] If the characters ', \, ", //, or /* occur in the
sequence between the < and > delimiters, the behavior is
undefined. Similarly, if the characters ', \, //, or /*
occur in the sequence between the " delimiters, the behavior
is undefined.45 A header name preprocessing token is
recognized only within a #include preprocessing directive.
__________
45. Thus, sequences of characters that resemble escape
sequences cause undefined behavior.
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Examples
[#4] The following sequence of characters:
0x3<1/a.h>1e2
#include <1/a.h>
#define const.member@$
forms the following sequence of preprocessing tokens (with
each individual preprocessing token delimited by a { on the
left and a } on the right).
{0x3}{<}{1}{/}{a}{.}{h}{>}{1e2}
{#}{include} {<1/a.h>}
{#}{define} {const}{.}{member}{@}{$}
Forward references: source file inclusion (6.8.2).
6.1.8 Preprocessing numbers
Syntax
[#1]
pp-number:
digit
. digit
pp-number digit
pp-number nondigit
pp-number e sign
pp-number E sign
pp-number p sign
pp-number P sign
pp-number .
Description
[#2] A preprocessing number begins with a digit optionally
preceded by a period (.) and may be followed by letters,
underscores, digits, periods, and e+, e-, E+, E-, p+, p-,
P+, or P- character sequences.
[#3] Preprocessing number tokens lexically include all
floating and integer constant tokens.
Semantics
[#4] A preprocessing number does not have type or a value;
it acquires both after a successful conversion (as part of
translation phase 7) to a floating constant token or an
integer constant token.
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6.1.9 Comments
[#1] Except within a character constant, a string literal,
or a comment, the characters /* introduce a comment. The
contents of a comment are examined only to identify
multibyte characters and to find the characters */ that
terminate it.46
[#2] Except within a character constant, a string literal,
or a comment, the characters // introduce a comment that
includes all multibyte characters up to, but not including,
the next new-line character. The contents of such a comment
are examined only to identify multibyte characters and to
find the terminating new-line character.
Examples
[#3]
"a//b" // four-character string literal
#include "//e" // undefined behavior
// */ // comment, not syntax error
f = g/**//h; // equivalent to f = g / h;
//\
i(); // part of a two-line comment
/\
/ j(); // part of a two-line comment
#define glue(x,y) x##y
glue(/,/) k(); // syntax error, not comment
/*//*/ l(); // equivalent to l();
m = n//**/o
+ p; // equivalent to m = n + p;
__________
46. Thus, /* ... */ comments do not nest.
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6.2 Conversions
[#1] Several operators convert operand values from one type
to another automatically. This subclause specifies the
result required from such an implicit conversion, as well as
those that result from a cast operation (an explicit
conversion). The list in 6.2.1.7 summarizes the conversions
performed by most ordinary operators; it is supplemented as
required by the discussion of each operator in 6.3.
[#2] Conversion of an operand value to a compatible type
causes no change to the value or the representation.
Forward references: cast operators (6.3.4).
6.2.1 Arithmetic operands
6.2.1.1 Characters and integers
[#1] Every integer type has an integer conversion rank
defined as follows:
- No two signed integer types shall have the same rank,
even if they have the same representation.
- The rank of a signed integer type shall be greater than
the rank of any signed integer type with less
precision.
- The rank of any standard signed integer type shall be
greater than the rank of any extended signed integer
type with the same precision.
- The rank of long long int shall be greater than the
rank of long int, which shall be greater than the rank
of int, which shall be greater than the rank of short
int, which shall be greater than the rank of signed
char.
- The rank of any unsigned integer type shall equal the
rank of the corresponding signed integer type.
- The rank of char shall equal the rank of signed char
and unsigned char.
- The rank of any enumerated type shall equal the rank of
the compatible integer type.
- The rank of any extended signed integer type relative
to another extended signed integer type with the same
precision is implementation-defined, but still subject
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to the other rules for determining the integer
conversion rank.
- For all integer types T1, T2, and T3, if T1 has greater
rank than T2 and T2 has greater rank than T3, then T1
has greater rank than T3.
[#2] The following may be used in an expression wherever an
int or unsigned int may be used.
- An object or expression with an integer type whose
integer conversion rank is less than the rank of int
and unsigned int.
- A bit-field of type int, signed int, or unsigned int.
[#3] If an int can represent all values of the original
type, the value is converted to an int; otherwise, it is
converted to an unsigned int. These are called the integer
promotions.47 All other types are unchanged by the integer
promotions.
[#4] The integer promotions preserve value including sign.
As discussed earlier, whether a ``plain'' char is treated as
signed is implementation-defined.
Forward references: enumeration specifiers (6.5.2.2),
structure and union specifiers (6.5.2.1).
6.2.1.2 Signed and unsigned integers
[#1] When a value with integer type is converted to another
integer type, if the value can be represented by the new
type, it is unchanged.
[#2] Otherwise, if the new type is unsigned, the value is
converted by repeatedly adding or subtracting one more than
the maximum value that can be represented in the new type
until the value is in the range of the new type.
__________
47. The integer promotions are applied only as part of the
usual arithmetic conversions, to certain argument
expressions, to the operands of the unary +, -, and ~
operators, and to both operands of the shift operators,
as specified by their respective subclauses.
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[#3] Otherwise, the new type is signed and the value cannot
be represented in it; the result is implementation-defined.
6.2.1.3 Real floating and integer
[#1] When a value of real floating type is converted to
integer type, the fractional part is discarded. If the
value of the integral part cannot be represented by the
integer type, the behavior is undefined.48
[#2] When a value of integer type is converted to real
floating type, if the value being converted is in the range
of values that can be represented but cannot be represented
exactly, the result is either the nearest higher or nearest
lower value, chosen in an implementation-defined manner. If
the value being converted is outside the range of values
that can be represented, the behavior is undefined.
6.2.1.4 Real floating types
[#1] When a float is promoted to double or long double, or a
double is promoted to long double, its value is unchanged.
[#2] When a double is demoted to float or a long double to
double or float, if the value being converted is outside the
range of values that can be represented, the behavior is
undefined. If the value being converted is in the range of
values that can be represented but cannot be represented
exactly, the result is either the nearest higher or nearest
lower value, chosen in an implementation-defined manner.
6.2.1.5 Complex types
[#1] When a value of complex type is converted to another
complex type, both the real and imaginary parts follow the
conversion rules for the corresponding real types.
__________
48. The remaindering operation performed when a value of
integer type is converted to unsigned type need not be
performed when a value of real floating type is
converted to unsigned type. Thus, the range of portable
real floating values is (-1,Utype_MAX+1).
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6.2.1.6 Real and complex
[#1] When a value of real type is converted to a complex
type, the real part of the complex result value is
determined by the rules of conversion to the corresponding
real type and the imaginary part of the complex result value
is a positive zero or an unsigned zero.
[#2] When a value of complex type is converted to a real
type, the imaginary part of the complex value is discarded
and the value of the real part is converted according to the
conversion rules for the corresponding real type.
6.2.1.7 Usual arithmetic conversions
[#1] Many operators that expect operands of arithmetic type
cause conversions and yield result types in a similar way.
The purpose is to determine a common real type for the
operands and result. For the specified operands, each
operand is converted, without change of type-domain, to a
type whose corresponding real type is the common real type.
Unless explicitly stated otherwise, the common real type is
also the corresponding real type of the result, whose type-
domain is determined by the operator. This pattern is called
the usual arithmetic conversions:
First, if the corresponding real type of either operand
is long double, the other operand is converted, without
change of type-domain, to a type whose corresponding
real type is long double.
Otherwise, if the corresponding real type of either
operand is double, the other operand is converted,
without change of type-domain, to a type whose
corresponding real type is double.
Otherwise, if the corresponding real type of either
operand is float, the other operand is converted,
without change of type-domain, to a type whose
corresponding real type is float.49
Otherwise, the integer promotions are performed on both
operands. Then the following rules are applied to the
__________
49. For example, addition of a double complex and a float
entails just the conversion of the float operand to
double (and yields a double complex result).
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promoted operands:
If both operands have the same type, then no
further conversion is needed.
Otherwise, if both operands have signed integer
types or both have unsigned integer types, the
operand with the type of lesser integer conversion
rank is converted to the type of the operand with
greater rank.
Otherwise, if the operand that has unsigned
integer type has rank greater or equal to the rank
of the type of the other operand, then the operand
with signed integer type is converted to the type
of the operand with unsigned integer type.
Otherwise, if the type of the operand with signed
integer type can represent all of the values of
the type of the operand with unsigned integer
type, then the operand with unsigned integer type
is converted to the type of the operand with
signed integer type.
Otherwise, both operands are converted to the
unsigned integer type corresponding to the type of
the operand with signed integer type.
[#2] The values of floating operands and of the results of
floating expressions may be represented in greater precision
and range than that required by the type; the types are not
changed thereby.50
__________
50. The cast and assignment operators still must perform
their specified conversions, as described in 6.2.1.3 and
6.2.1.4.
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6.2.2 Other operands
6.2.2.1 Lvalues and function designators
[#1] An lvalue is an expression (with an object type or an
incomplete type other than void) that designates an
object.51 When an object is said to have a particular type,
the type is specified by the lvalue used to designate the
object. A modifiable lvalue is an lvalue that does not have
array type, does not have an incomplete type, does not have
a const-qualified type, and if it is a structure or union,
does not have any member (including, recursively, any member
or element of all contained aggregates or unions) with a
const-qualified type.
[#2] Except when it is the operand of the sizeof operator,
the unary & operator, the ++ operator, the -- operator, or
the left operand of the . operator or an assignment
operator, an lvalue that does not have array type is
converted to the value stored in the designated object (and
is no longer an lvalue). If the lvalue has qualified type,
the value has the unqualified version of the type of the
lvalue; otherwise, the value has the type of the lvalue. If
the lvalue has an incomplete type and does not have array
type, the behavior is undefined.
[#3] Except when it is the operand of the sizeof operator or
the unary & operator, or is a character string literal used
to initialize an array of character type, or is a wide
string literal used to initialize an array with element type
compatible with wchar_t, an lvalue that has type ``array of
type'' is converted to an expression that has type ``pointer
to type'' that points to the initial element of the array
object and is not an lvalue. If the array object has
register storage class, the behavior is undefined.
__________
51. The name ``lvalue'' comes originally from the assignment
expression E1 = E2, in which the left operand E1 must be
a (modifiable) lvalue. It is perhaps better considered
as representing an object ``locator value.'' What is
sometimes called ``rvalue'' is in this International
Standard described as the ``value of an expression.''
An obvious example of an lvalue is an identifier of an
object. As a further example, if E is a unary
expression that is a pointer to an object, *E is an
lvalue that designates the object to which E points.
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[#4] A function designator is an expression that has
function type. Except when it is the operand of the sizeof
operator52 or the unary & operator, a function designator
with type ``function returning type'' is converted to an
expression that has type ``pointer to function returning
type.''
Forward references: address and indirection operators
(6.3.3.2), assignment operators (6.3.16), common definitions
<stddef.h> (7.1.6), initialization (6.5.8), postfix
increment and decrement operators (6.3.2.4), prefix
increment and decrement operators (6.3.3.1), the sizeof
operator (6.3.3.4), structure and union members (6.3.2.3).
6.2.2.2 void
[#1] The (nonexistent) value of a void expression (an
expression that has type void) shall not be used in any way,
and implicit or explicit conversions (except to void) shall
not be applied to such an expression. If an expression of
any other type occurs in a context where a void expression
is required, its value or designator is discarded. (A void
expression is evaluated for its side effects.)
6.2.2.3 Pointers
[#1] A pointer to void may be converted to or from a pointer
to any incomplete or object type. A pointer to any
incomplete or object type may be converted to a pointer to
void and back again; the result shall compare equal to the
original pointer.
[#2] For any qualifier q, a pointer to a non-q-qualified
type may be converted to a pointer to the q-qualified
version of the type; the values stored in the original and
converted pointers shall compare equal.
[#3] An integer constant expression with the value 0, or
such an expression cast to type void *, is called a null
pointer constant.53 If a null pointer constant is assigned
__________
52. Because this conversion does not occur, the operand of
the sizeof operator remains a function designator and
violates the constraint in 6.3.3.4.
53. The macro NULL is defined in <stddef.h> as a null
pointer constant; see 7.1.6.
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to or compared for equality to a pointer, the constant is
converted to a pointer of that type. Such a pointer, called
a null pointer, is guaranteed to compare unequal to a
pointer to any object or function.
[#4] Conversion of a null pointer to another pointer type
yields a null pointer of that type. Any two null pointers
shall compare equal.
[#5] An integer may be converted to any pointer type. The
result is implementation-defined, might not be properly
aligned, and might not point to an entity of the referenced
type.54
[#6] Any pointer type may be converted to an integer type;
the result is implementation-defined. If the result cannot
be represented in the integer type, the behavior is
undefined. The result need not be in the range of values of
any integer type.55
[#7] A pointer to an object or incomplete type may be
converted to a pointer to a different object or incomplete
type. If the resulting pointer is not correctly aligned56
for the pointed to type, the behavior is undefined.
Otherwise, when converted back again, the result shall
compare equal to the original pointer.
[#8] A pointer to a function of one type may be converted to
a pointer to a function of another type and back again; the
result shall compare equal to the original pointer. If a
converted pointer is used to call a function that has a type
that is not compatible with the type of the called function,
the behavior is undefined.
__________
54. The mapping functions for converting a pointer to an
integer or an integer to a pointer are intended to be
consistent with the addressing structure of the
execution environment.
55. Thus, if the conversion is to unsigned int but yields a
negative value, the behavior is undefined.
56. In general, the concept correctly aligned is transitive:
if a pointer to type A is correctly aligned for a
pointer to type B, which in turn is correctly aligned
for a pointer to type C, then a pointer to type A is
correctly aligned for a pointer to type C.
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Forward references: cast operators (6.3.4), equality
operators (6.3.9), simple assignment (6.3.16.1).
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6.3 Expressions
[#1] An expression is a sequence of operators and operands
that specifies computation of a value, or that designates an
object or a function, or that generates side effects, or
that performs a combination thereof.
[#2] Between the previous and next sequence point an object
shall have its stored value modified at most once by the
evaluation of an expression. Furthermore, the prior value
shall be accessed only to determine the value to be
stored.57
[#3] Except as indicated by the syntax58 or otherwise
specified later (for the function-call operator (), &&, ||,
?:, and comma operators), the order of evaluation of
subexpressions and the order in which side effects take
place are both unspecified.
__________
57. This paragraph renders undefined statement expressions
such as
i = ++i + 1;
while allowing
i = i + 1;
58. The syntax specifies the precedence of operators in the
evaluation of an expression, which is the same as the
order of the major subclauses of this subclause, highest
precedence first. Thus, for example, the expressions
allowed as the operands of the binary + operator (6.3.6)
shall be those expressions defined in 6.3.1 through
6.3.6. The exceptions are cast expressions (6.3.4) as
operands of unary operators (6.3.3), and an operand
contained between any of the following pairs of
operators: grouping parentheses () (6.3.1),
subscripting brackets [] (6.3.2.1), function-call
parentheses () (6.3.2.2), and the conditional operator
?: (6.3.15).
Within each major subclause, the operators have the same
precedence. Left- or right-associativity is indicated
in each subclause by the syntax for the expressions
discussed therein.
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[#4] Some operators (the unary operator ~, and the binary
operators <<, >>, &, ^, and |, collectively described as
bitwise operators) shall have operands that have integer
type. These operators return values that depend on the
internal representations of integers, and have
implementation-defined and undefined aspects for signed
types.
[#5] If an exception occurs during the evaluation of an
expression (that is, if the result is not mathematically
defined or not in the range of representable values for its
type), the behavior is undefined.
[#6] The effective type of an object for an access to its
stored value is the declared type of the object, if it has
one. If a value is stored into an object having no declared
type through an lvalue having a type that is not a character
type, then the type of the lvalue becomes the effective type
of the object for that access and for subsequent accesses
that do not modify the stored value. If a value is copied
into an object having no declared type using memcpy or
memmove, or is copied as an array of character type, then
the effective type of the modified object for that access
and for subsequent accesses that do not modify the value is
the effective type of the object from which the value is
copied, if it has one. For all other accesses to an object
having no declared type, the effective type of the object is
simply the type of the lvalue used for the access.
[#7] An object shall have its stored value accessed only by
an lvalue expression that has one of the following types:59
- a type compatible with the effective type of the
object,
- a qualified version of a type compatible with the
effective type of the object,
- a type that is the signed or unsigned type
corresponding to the effective type of the object,
- a type that is the signed or unsigned type
corresponding to a qualified version of the effective
__________
59. The intent of this list is to specify those
circumstances in which an object may or may not be
aliased.
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type of the object,
- an aggregate or union type that includes one of the
aforementioned types among its members (including,
recursively, a member of a subaggregate or contained
union), or
- a character type.
[#8] A floating expression may be contracted, that is,
evaluated as though it were an atomic operation, thereby
omitting rounding errors implied by the source code and the
expression evaluation method.60 The FP_CONTRACT pragma in
<math.h> provides a way to disallow contracted expressions.
Otherwise, whether and how expressions are contracted is
implementation-defined.61
6.3.1 Primary expressions
Syntax
[#1]
primary-expr:
identifier
constant
string-literal
( expression )
Semantics
[#2] An identifier is a primary expression, provided it has
been declared as designating an object (in which case it is
an lvalue) or a function (in which case it is a function
designator).62
__________
60. A contracted expression might also omit the raising of
floating-point exception flags.
61. This license is specifically intended to allow
implementations to exploit fast machine instructions
that combine multiple C operators. As contractions
potentially undermine predictability, and can even
decrease accuracy for containing expressions, their use
must be well-defined and clearly documented.
62. Thus, an undeclared identifier is a violation of the
syntax.
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[#3] A constant is a primary expression. Its type depends
on its form and value, as detailed in 6.1.3.
[#4] A string literal is a primary expression. It is an
lvalue with type as detailed in 6.1.4.
[#5] A parenthesized expression is a primary expression.
Its type and value are identical to those of the
unparenthesized expression. It is an lvalue, a function
designator, or a void expression if the unparenthesized
expression is, respectively, an lvalue, a function
designator, or a void expression.
Forward references: declarations (6.5).
6.3.1.1 The identifier __func__
Semantics
[#1] The identifier __func__ is implicitly declared by the
translator as if, immediately following the opening brace of
each function definition, the declaration
static const char __func__[] = "function-name";
appeared, where function-name is the name of the lexically-
enclosing function.63 This name is the unadorned name of
the function.
[#2] This name is encoded as if the implicit declaration had
been written in the source character set and then translated
into the execution character set as indicated in translation
phase 5.
Examples
[#3] Consider the code fragment:
__________
63. Note that since the name __func__ is reserved for any
use by the implementation (7.1.3), if any other
identifier is explicitly declared using the name
__func__, the behavior is undefined.
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#include <stdio.h>
void myfunc(void)
{
printf("%s\n", __func__);
/* ... */
}
Each time the function is called, it will print to the
standard output stream:
myfunc
6.3.2 Postfix operators
Syntax
[#1]
postfix-expr:
primary-expr
postfix-expr [ expression ]
postfix-expr ( argument-expression-list-opt )
postfix-expr . identifier
postfix-expr -> identifier
postfix-expr ++
postfix-expr --
( type-name ) { initializer-list }
( type-name ) { initializer-list , }
argument-expression-list:
assignment-expr
argument-expression-list , assignment-expr
6.3.2.1 Array subscripting
Constraints
[#1] One of the expressions shall have type ``pointer to
object type,'' the other expression shall have integer type,
and the result has type ``type.''
Semantics
[#2] A postfix expression followed by an expression in
square brackets [] is a subscripted designation of an
element of an array object. The definition of the subscript
operator [] is that E1[E2] is identical to (*(E1+(E2))).
Because of the conversion rules that apply to the binary +
operator, if E1 is an array object (equivalently, a pointer
to the initial element of an array object) and E2 is an
integer, E1[E2] designates the E2-th element of E1 (counting
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from zero).
[#3] Successive subscript operators designate an element of
a multidimensional array object. If E is an n-dimensional
array (n>2) with dimensions ixjx ... xk, then E (used as
other than an lvalue) is converted to a pointer to an (n-
1)-dimensional array with dimensions jx ... x k. If the
unary * operator is applied to this pointer explicitly, or
implicitly as a result of subscripting, the result is the
pointed-to (n - 1)-dimensional array, which itself is
converted into a pointer if used as other than an lvalue.
It follows from this that arrays are stored in row-major
order (last subscript varies fastest).
Examples
[#4] Consider the array object defined by the declaration
int x[3][5];
Here x is a 3x5 array of ints; more precisely, x is an array
of three element objects, each of which is an array of five
ints. In the expression x[i], which is equivalent to
(*(x+(i))), x is first converted to a pointer to the initial
array of five ints. Then i is adjusted according to the
type of x, which conceptually entails multiplying i by the
size of the object to which the pointer points, namely an
array of five int objects. The results are added and
indirection is applied to yield an array of five ints. When
used in the expression x[i][j], that in turn is converted to
a pointer to the first of the ints, so x[i][j] yields an
int.
Forward references: additive operators (6.3.6), address and
indirection operators (6.3.3.2), array declarators
(6.5.5.2).
6.3.2.2 Function calls
Constraints
[#1] The expression that denotes the called function64 shall
have type pointer to function returning void or returning an
object type other than an array type.
__________
64. Most often, this is the result of converting an
identifier that is a function designator.
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[#2] If the expression that denotes the called function has
a type that includes a prototype, the number of arguments
shall agree with the number of parameters. Each argument
shall have a type such that its value may be assigned to an
object with the unqualified version of the type of its
corresponding parameter.
Semantics
[#3] A postfix expression followed by parentheses ()
containing a possibly empty, comma-separated list of
expressions is a function call. The postfix expression
denotes the called function. The list of expressions
specifies the arguments to the function.
[#4] An argument may be an expression of any object type.
In preparing for the call to a function, the arguments are
evaluated, and each parameter is assigned the value of the
corresponding argument.65 If the expression that denotes
the called function has type pointer to function returning
an object type, the function call expression has the same
type as that object type, and has the value determined as
specified in 6.6.6.4. Otherwise, the function call has type
void.
[#5] If the expression that denotes the called function has
a type that does not include a prototype, the integer
promotions are performed on each argument, and arguments
that have type float are promoted to double. These are
called the default argument promotions. If the number of
arguments does not agree with the number of parameters, the
behavior is undefined. If the function is defined with a
type that does not include a prototype, and the types of the
arguments after promotion are not compatible with those of
the parameters after promotion, the behavior is undefined,
except for the following cases:
- one promoted type is a signed integer type, the other
promoted type is the corresponding unsigned integer
__________
65. A function may change the values of its parameters, but
these changes cannot affect the values of the arguments.
On the other hand, it is possible to pass a pointer to
an object, and the function may change the value of the
object pointed to. A parameter declared to have array
or function type is converted to a parameter with a
pointer type as described in 6.7.1.
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type, and the value is representable in both types;
- one type is pointer to void and the other is a pointer
to a character type.
[#6] If the function is defined with a type that includes a
prototype, and the types of the arguments after promotion
are not compatible with the types of the parameters, or if
the prototype ends with an ellipsis (, ...), the behavior is
undefined.
[#7] If the expression that denotes the called function has
a type that includes a prototype, the arguments are
implicitly converted, as if by assignment, to the types of
the corresponding parameters, taking the type of each
parameter to be the unqualified version of its declared
type. The ellipsis notation in a function prototype
declarator causes argument type conversion to stop after the
last declared parameter. The default argument promotions
are performed on trailing arguments.
[#8] If the function is defined with a type that is not
compatible with the type (of the expression) pointed to by
the expression that denotes the called function, the
behavior is undefined.
[#9] No other conversions are performed implicitly; in
particular, the number and types of arguments are not
compared with those of the parameters in a function
definition that does not include a function prototype
declarator.
[#10] The order of evaluation of the function designator,
the arguments, and subexpressions within the arguments is
unspecified, but there is a sequence point before the actual
call.
[#11] Recursive function calls shall be permitted, both
directly and indirectly through any chain of other
functions.
Examples
[#12] In the function call
(*pf[f1()]) (f2(), f3() + f4())
the functions f1, f2, f3, and f4 may be called in any order.
All side effects shall be completed before the function
pointed to by pf[f1()] is entered.
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Forward references: function declarators (including
prototypes) (6.5.5.3), function definitions (6.7.1), the
return statement (6.6.6.4), simple assignment (6.3.16.1).
6.3.2.3 Structure and union members
Constraints
[#1] The first operand of the . operator shall have a
qualified or unqualified structure or union type, and the
second operand shall name a member of that type.
[#2] The first operand of the -> operator shall have type
``pointer to qualified or unqualified structure'' or
``pointer to qualified or unqualified union,'' and the
second operand shall name a member of the type pointed to.
Semantics
[#3] A postfix expression followed by the . operator and an
identifier designates a member of a structure or union
object. The value is that of the named member, and is an
lvalue if the first expression is an lvalue. If the first
expression has qualified type, the result has the so-
qualified version of the type of the designated member.
[#4] A postfix expression followed by the -> operator and an
identifier designates a member of a structure or union
object. The value is that of the named member of the object
to which the first expression points, and is an lvalue.66
If the first expression is a pointer to a qualified type,
the result has the so-qualified version of the type of the
designated member.
[#5] With one exception, if the value of a member of a union
object is used when the most recent store to the object was
to a different member, the behavior is implementation-
defined.67 One special guarantee is made in order to
__________
66. If &E is a valid pointer expression (where & is the
``address-of'' operator, which generates a pointer to
its operand), the expression (&E)->MOS is the same as
E.MOS.
67. The ``byte orders'' for scalar types are invisible to
isolated programs that do not indulge in type punning
(for example, by assigning to one member of a union and
inspecting the storage by accessing another member that
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simplify the use of unions: If a union contains several
structures that share a common initial sequence (see below),
and if the union object currently contains one of these
structures, it is permitted to inspect the common initial
part of any of them anywhere that a declaration of the
completed type of the union is visible. Two structures
share a common initial sequence if corresponding members
have compatible types (and, for bit-fields, the same widths)
for a sequence of one or more initial members.
Examples
[#6]
1. If f is a function returning a structure or union, and
x is a member of that structure or union, f().x is a
valid postfix expression but is not an lvalue.
2. The following is a valid fragment:
union {
struct {
int alltypes;
} n;
struct {
int type;
int intnode;
} ni;
struct {
int type;
double doublenode;
} nf;
} u;
u.nf.type = 1;
u.nf.doublenode = 3.14;
/* ... */
if (u.n.alltypes == 1)
if (sin(u.nf.doublenode) == 0.0)
/* ... */
____________________________________________________________
is an appropriately sized array of character type), but
must be accounted for when conforming to externally
imposed storage layouts.
page 86 Language
page 87
3. The following is not a valid fragment (because the
union type is not visible within function f):
struct t1 { int m; };
struct t2 { int m; };
int f(struct t1 * p1, struct t2 * p2)
{
if (p1->m < 0)
p2->m = -p2->m;
return p1->m;
}
int g()
{
union {
struct t1 s1;
struct t2 s2;
} u;
/* ... */
return f(&u.s1, &u.s2);
}
Forward references: address and indirection operators
(6.3.3.2), structure and union specifiers (6.5.2.1).
6.3.2.4 Postfix increment and decrement operators
Constraints
[#1] The operand of the postfix increment or decrement
operator shall have qualified or unqualified real or pointer
type and shall be a modifiable lvalue.
Semantics
[#2] The result of the postfix ++ operator is the value of
the operand. After the result is obtained, the value of the
operand is incremented. (That is, the value 1 of the
appropriate type is added to it.) See the discussions of
additive operators and compound assignment for information
on constraints, types, and conversions and the effects of
operations on pointers. The side effect of updating the
stored value of the operand shall occur between the previous
and the next sequence point.
[#3] The postfix -- operator is analogous to the postfix ++
operator, except that the value of the operand is
decremented (that is, the value 1 of the appropriate type is
subtracted from it).
page 87 Language
page 88
Forward references: additive operators (6.3.6), compound
assignment (6.3.16.2).
6.3.2.5 Compound literals
Constraints
[#1] The type name shall specify an object type or an array
of unknown size.
[#2] No initializer shall attempt to provide a value for an
object not contained within the entire unnamed object
specified by the compound literal.
[#3] If the compound literal occurs outside the body of a
function, the initializer list shall consist of constant
expressions.
Semantics
[#4] A postfix expression that consists of a parenthesized
type name followed by a brace-enclosed list of initializers
is a compound literal. It provides an unnamed object whose
value is given by the initializer list.68
[#5] If the type name specifies an array of unknown size,
the size is determined by the initializer list as specified
in 6.5.7, and the type of the compound literal is that of
the completed array type. Otherwise (when the type name
specifies an object type), the type of the compound literal
is that specified by the type name. In either case, the
result is an lvalue.
[#6] The value of the compound literal is that of an unnamed
object initialized by the initializer list. The object has
static storage duration if and only if the compound literal
occurs outside the body of a function; otherwise, it has
automatic storage duration associated with the enclosing
block.
[#7] All the semantic rules and constraints for initializer
lists in 6.5.8 are applicable to compound literals.69
__________
68. Note that this differs from a cast expression. For
example, a cast specifies a conversion to scalar types
or void only, and the result of a cast expression is not
an lvalue.
page 88 Language
page 89
[#8] String literals, and compound literals with const-
qualified types, need not designate distinct objects.70
Examples
[#9]
1. The file scope definition
int *p = (int []){2, 4};
initializes p to point to the first element of an
array of two ints, the first having the value two and
the second, four. The expressions in this compound
literal must be constant. The unnamed object has
static storage duration.
2. In contrast, in
void f(void)
{
int *p;
/*...*/
p = (int [2]){*p};
/*...*/
}
p is assigned the address of the first element of an
array of two ints, the first having the value
previously pointed to by p and the second, zero. The
expressions in this compound literal need not be
constant. The unnamed object has automatic storage
duration.
3. Initializers with designations can be combined with
compound literals. Structure objects created using
compound literals can be passed to functions without
depending on member order:
____________________________________________________________
69. For example, subobjects without explicit initializers
are initialized to zero.
70. This allows implementations to share storage for string
literals and constant compound literals with the same or
overlapping representations.
page 89 Language
page 90
drawline((struct point){.x=1, .y=1},
(struct point){.x=3, .y=4});
Or, if drawline instead expected pointers to struct
point:
drawline(&(struct point){.x=1, .y=1},
&(struct point){.x=3, .y=4});
4. A read-only compound literal can be specified through
constructions like:
(const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
5. The following three expressions have different
meanings:
"/tmp/fileXXXXXX"
(char []){"/tmp/fileXXXXXX"}
(const char[]){"/tmp/fileXXXXXX"}
The first always has static storage duration and has
type array of char, but need not be modifiable; the
last two have automatic storage duration when they
occur within the body of a function, and the first of
these two is modifiable.
6. Like string literals, const-qualified compound
literals can be placed into read-only memory and can
even be shared. For example,
(const char[]){"abc"} == "abc"
might yield 1 if the literals' storage is shared.
7. Since compound literals are unnamed, a single compound
literal cannot specify a circularly linked object.
For example, there is no way to write a self-
referential compound literal that could be used as the
function argument in place of the named object
endless_zeros below:
struct int_list { int car; struct int_list *cdr; };
struct int_list endless_zeros = {0, &endless_zeros};
eval(endless_zeros);
8. Outside the body of a function, a compound literal is
an initialization of a static object; however, inside
a function body, it is an assignment to an automatic
object. Therefore, the following two loops produce
the same sequence of values for the objects associated
page 90 Language
page 91
with their respective compound literals.
for (int i = 0; i < 10; i++) {
f((struct s){.a = i, .b = 42});
}
for (int i = 0; i < 10; i++)
f((struct s){.a = i, .b = 42});
9. Each compound literal creates only a single object in
a given scope:
struct s { int i; };
int f (void)
{
struct s *p = 0, *q;
int j;
for (j = 0; j < 2; j++)
q = p, p = &((struct s){ j });
return p == q && q.i == 1;
}
The function f() always returns the value 1.
Note that if the body of the for loop were enclosed in
braces, the lifetime of the unnamed object would be
the body of the loop only, and on entry next time
around p would be pointing to an object which is no
longer guaranteed to exist, which is undefined
behavior.
6.3.3 Unary operators
Syntax
[#1]
unary-expr:
postfix-expr
++ unary-expr
-- unary-expr
unary-operator cast-expr
sizeof unary-expr
sizeof ( type-name )
unary-operator: one of
& * + - ~ !
page 91 Language
page 92
6.3.3.1 Prefix increment and decrement operators
Constraints
[#1] The operand of the prefix increment or decrement
operator shall have qualified or unqualified real or pointer
type and shall be a modifiable lvalue.
Semantics
[#2] The value of the operand of the prefix ++ operator is
incremented. The result is the new value of the operand
after incrementation. The expression ++E is equivalent to
(E+=1). See the discussions of additive operators and
compound assignment for information on constraints, types,
side effects, and conversions and the effects of operations
on pointers.
[#3] The prefix -- operator is analogous to the prefix ++
operator, except that the value of the operand is
decremented.
Forward references: additive operators (6.3.6), compound
assignment (6.3.16.2).
6.3.3.2 Address and indirection operators
Constraints
[#1] The operand of the unary & operator shall be either a
function designator, the result of a [] or unary * operator,
or an lvalue that designates an object that is not a bit-
field and is not declared with the register storage-class
specifier.
[#2] The operand of the unary * operator shall have pointer
type.
Semantics
[#3] The result of the unary & (address-of) operator is a
pointer to the object or function designated by its operand.
If the operand has type ``type'', the result has type
``pointer to type''. If the operand is the result of a
unary * operator, neither that operator nor the & operator
are evaluated, and the result shall be as if both were
omitted, even if the intermediate object does not exist,
except that the constraints on the operators still apply and
the result is not an lvalue. Similarly, if the operand is
the result of a [] operator, neither the & operator nor the
unary * that is implied by the [] are evaluated, and the
page 92 Language
page 93
result shall be as if the & operator was removed and the []
operator was changed to a + operator.
[#4] The unary * operator denotes indirection. If the
operand points to a function, the result is a function
designator; if it points to an object, the result is an
lvalue designating the object. If the operand has type
``pointer to type'', the result has type ``type''. If an
invalid value has been assigned to the pointer, the behavior
of the unary * operator is undefined.71
Forward references: storage-class specifiers (6.5.1),
structure and union specifiers (6.5.2.1).
6.3.3.3 Unary arithmetic operators
Constraints
[#1] The operand of the unary + or - operator shall have
arithmetic type; of the ~ operator, integer type; of the !
operator, scalar type.
Semantics
[#2] The result of the unary + operator is the value of its
operand. The integer promotion is performed on the operand,
and the result has the promoted type.
[#3] The result of the unary - operator is the negative of
its operand. The integer promotion is performed on the
operand, and the result has the promoted type.
__________
71. Thus &*E is equivalent to E (even if E is a null
pointer), and &(E1[E2]]) to (E1+(E2)). It is always
true that if E is a function designator or an lvalue
that is a valid operand of the unary & operator, *&E is
a function designator or an lvalue equal to E. If *P is
an lvalue and T is the name of an object pointer type,
*(T)P is an lvalue that has a type compatible with that
to which T points.
Among the invalid values for dereferencing a pointer by
the unary * operator are a null pointer, an address
inappropriately aligned for the type of object pointed
to, and the address of an automatic storage duration
object when execution of the block with which the object
is associated has terminated.
page 93 Language
page 94
[#4] The result of the ~ operator is the bitwise complement
of its operand (that is, each bit in the result is set if
and only if the corresponding bit in the converted operand
is not set). The integer promotion is performed on the
operand, and the result has the promoted type. The
expression ~E is equivalent to (ULLONG_MAX-E) if E is
promoted to type unsigned long long, to (ULONG_MAX-E) if E
is promoted to type unsigned long, to (UINT_MAX-E) if E is
promoted to type unsigned int. (The constants ULLONG_MAX,
ULONG_MAX, and UINT_MAX are defined in the header
<limits.h>.)
[#5] The result of the logical negation operator ! is 0 if
the value of its operand compares unequal to 0, 1 if the
value of its operand compares equal to 0. The result has
type int. The expression !E is equivalent to (0==E).
Forward references: limits <float.h> and <limits.h>
(7.1.5).
6.3.3.4 The sizeof operator
Constraints
[#1] The sizeof operator shall not be applied to an
expression that has function type or an incomplete type, to
the parenthesized name of such a type, or to an lvalue that
designates a bit-field object.
Semantics
[#2] The sizeof operator yields the size (in bytes) of its
operand, which may be an expression or the parenthesized
name of a type. The size is determined from the type of the
operand. The result is an integer. If the type of the
operand is a variable length array type, the operand is
evaluated; otherwise, the operand is not evaluated and the
result is an integer constant.
[#3] When applied to an operand that has type char, unsigned
char, or signed char, (or a qualified version thereof) the
result is 1. When applied to an operand that has array
type, the result is the total number of bytes in the
array.72 When applied to an operand that has structure or
__________
72. When applied to a parameter declared to have array or
function type, the sizeof operator yields the size of
the pointer obtained by converting as in 6.2.2.1; see
page 94 Language
page 95
union type, the result is the total number of bytes in such
an object, including internal and trailing padding.
[#4] The value of the result is implementation-defined, and
its type (an unsigned integer type) is size_t defined in the
<stddef.h> header.
Examples
[#5]
1. A principal use of the sizeof operator is in
communication with routines such as storage allocators
and I/O systems. A storage-allocation function might
accept a size (in bytes) of an object to allocate and
return a pointer to void. For example:
extern void *alloc(size_t);
double *dp = alloc(sizeof *dp);
The implementation of the alloc function should ensure
that its return value is aligned suitably for
conversion to a pointer to double.
2. Another use of the sizeof operator is to compute the
number of elements in an array:
sizeof array / sizeof array[0]
3. In this example, the size of a variable-length array
is computed and returned from a function:
size_t fsize3 (int n)
{
char b[n+3]; // Variable length array.
return sizeof b; // Execution time sizeof.
}
int main()
{
size_t size;
size = fsize3(10); // fsize3 returns 13.
return 0;
}
____________________________________________________________
6.7.1.
page 95 Language
page 96
Forward references: common definitions <stddef.h> (7.1.6),
declarations (6.5), structure and union specifiers
(6.5.2.1), type names (6.5.6), array declarators (6.5.5.2).
6.3.4 Cast operators
Syntax
[#1]
cast-expr:
unary-expr
( type-name ) cast-expr
Constraints
[#2] Unless the type name specifies a void type, the type
name shall specify qualified or unqualified scalar type and
the operand shall have scalar type.
[#3] Conversions that involve pointers, other than where
permitted by the constraints of 6.3.16.1, shall be specified
by means of an explicit cast.
Semantics
[#4] Preceding an expression by a parenthesized type name
converts the value of the expression to the named type.
This construction is called a cast.73 A cast that specifies
no conversion has no effect on the type or value of an
expression.74
__________
73. A cast does not yield an lvalue. Thus, a cast to a
qualified type has the same effect as a cast to the
unqualified version of the type.
74. If the value of the expression is represented with
greater precision or range than required by the type
named by the cast (6.2.1.7), then the cast specifies a
conversion even if the type of the expression is the
same as the named type.
page 96 Language
page 97
Forward references: equality operators (6.3.9), function
declarators (including prototypes) (6.5.5.3), simple
assignment (6.3.16.1), type names (6.5.6).
6.3.5 Multiplicative operators
Syntax
[#1]
multiplicative-expr:
cast-expr
multiplicative-expr * cast-expr
multiplicative-expr / cast-expr
multiplicative-expr % cast-expr
Constraints
[#2] Each of the operands shall have arithmetic type. The
operands of the % operator shall have integer type.
Semantics
[#3] The usual arithmetic conversions are performed on the
operands.
[#4] The result of the binary * operator is the product of
the operands.
[#5] The result of the / operator is the quotient from the
division of the first operand by the second; the result of
the % operator is the remainder. In both operations, if the
value of the second operand is zero, the behavior is
undefined.
[#6] When integers are divided, the result of the / operator
is the algebraic quotient with any fractional part
discarded.75 If the quotient a/b is representable, the
expression (a/b)*b + a%b shall equal a.
[#7] If either operand has complex type, the result has
complex type.
__________
75. This is often called ``truncation toward zero''.
page 97 Language
page 98
6.3.6 Additive operators
Syntax
[#1]
additive-expr:
multiplicative-expr
additive-expr + multiplicative-expr
additive-expr - multiplicative-expr
Constraints
[#2] For addition, either both operands shall have
arithmetic type, or one operand shall be a pointer to an
object type and the other shall have integer type.
(Incrementing is equivalent to adding 1.)
[#3] For subtraction, one of the following shall hold:
- both operands have arithmetic type;
- both operands are pointers to qualified or unqualified
versions of compatible object types; or
- the left operand is a pointer to an object type and the
right operand has integer type. (Decrementing is
equivalent to subtracting 1.)
Semantics
[#4] If both operands have arithmetic type, the usual
arithmetic conversions are performed on them.
[#5] The result of the binary + operator is the sum of the
operands.
[#6] The result of the binary - operator is the difference
resulting from the subtraction of the second operand from
the first.
[#7] For the purposes of these operators, a pointer to a
nonarray object behaves the same as a pointer to the first
element of an array of length one with the type of the
object as its element type.
[#8] When an expression that has integer type is added to or
subtracted from a pointer, the result has the type of the
pointer operand. If the pointer operand points to an
element of an array object, and the array is large enough,
the result points to an element offset from the original
page 98 Language
page 99
element such that the difference of the subscripts of the
resulting and original array elements equals the integer
expression. In other words, if the expression P points to
the i-th element of an array object, the expressions (P)+N
(equivalently, N+(P)) and (P)-N (where N has the value n)
point to, respectively, the i+n-th and i- n-th elements of
the array object, provided they exist. Moreover, if the
expression P points to the last element of an array object,
the expression (P)+1 points one past the last element of the
array object, and if the expression Q points one past the
last element of an array object, the expression (Q)-1 points
to the last element of the array object. If both the
pointer operand and the result point to elements of the same
array object, or one past the last element of the array
object, the evaluation shall not produce an overflow;
otherwise, the behavior is undefined. Unless both the
pointer operand and the result point to elements of the same
array object, or the pointer operand points one past the
last element of an array object and the result points to an
element of the same array object, the behavior is undefined
if the result is used as an operand of a unary * operator
that is actually evaluated.
[#9] When two pointers to elements of the same array object
are subtracted, the result is the difference of the
subscripts of the two array elements. The size of the
result is implementation-defined, and its type (a signed
integer type) is ptrdiff_t defined in the <stddef.h> header.
If the result is not representable in an object of that
type, the behavior is undefined. In other words, if the
expressions P and Q point to, respectively, the i-th and j-
th elements of an array object, the expression (P)-(Q) has
the value i-j provided the value fits in an object of type
ptrdiff_t. Moreover, if the expression P points either to
an element of an array object or one past the last element
of an array object, and the expression Q points to the last
element of the same array object, the expression ((Q)+1)-(P)
has the same value as ((Q)-(P))+1 and as -((P)-((Q)+1)), and
has the value zero if the expression P points one past the
last element of the array object, even though the expression
(Q)+1 does not point to an element of the array object.
Unless both pointers point to elements of the same array
object, or one past the last element of the array object,
the behavior is undefined.76
__________
76. Another way to approach pointer arithmetic is first to
convert the pointer(s) to character pointer(s): In this
scheme the integer expression added to or subtracted
page 99 Language
page 100
[#10] If either operand has complex type, the result has
complex type.
Examples
[#11] Pointer arithmetic is well defined with pointers to
variable length array types.
{
int n = 4, m = 3;
int a[n][m];
int (*p)[m] = a; // p == &a[0]
p += 1; // p == &a[1]
(*p)[2] = 99; // a[1][2] == 99
n = p - a; // n == 1
}
[#12] If array a in the above example is declared to be an
array of known constant size, and pointer p is declared to
be a pointer to an array of the same know constant size that
points to a, the results are the same.
Forward references: array declarators (6.5.5.2), common
definitions <stddef.h> (7.1.6).
____________________________________________________________
from the converted pointer is first multiplied by the
size of the object originally pointed to, and the
resulting pointer is converted back to the original
type. For pointer subtraction, the result of the
difference between the character pointers is similarly
divided by the size of the object originally pointed to.
When viewed in this way, an implementation need only
provide one extra byte (which may overlap another object
in the program) just after the end of the object in
order to satisfy the ``one past the last element''
requirements.
page 100 Language
page 101
6.3.7 Bitwise shift operators
Syntax
[#1]
shift-expr:
additive-expr
shift-expr << additive-expr
shift-expr >> additive-expr
Constraints
[#2] Each of the operands shall have integer type.
Semantics
[#3] The integer promotions are performed on each of the
operands. The type of the result is that of the promoted
left operand. If the value of the right operand is negative
or is greater than or equal to the number of value and sign
bits in the object representation of the promoted left
operand, the behavior is undefined.
[#4] The result of E1 << E2 is E1 left-shifted E2 bit
positions; vacated bits are filled with zeros. If E1 has an
unsigned type, the value of the result is E1 x 2E2, reduced
modulo ULLONG_MAX+1 if E1 has type unsigned long long,
ULONG_MAX+1 if E1 has type unsigned long, UINT_MAX+1
otherwise. (The constants ULLONG_MAX, ULONG_MAX, and
UINT_MAX are defined in the header <limits.h>.) If E1 has a
signed type and nonnegative value, and E1x2E2 is less than
or equal to INT_MAX (if E1 has type int), LONG_MAX (if E1
has type long int), or LLONG_MAX (if E1 has type long long
int), then that is the resulting value. Otherwise, the
behavior is undefined.
[#5] The result of E1 >> E2 is E1 right-shifted E2 bit
positions. If E1 has an unsigned type or if E1 has a signed
type and a nonnegative value, the value of the result is the
integral part of the quotient of E1 divided by the quantity,
2 raised to the power E2. If E1 has a signed type and a
negative value, the resulting value is implementation-
defined.
page 101 Language
page 102
6.3.8 Relational operators
Syntax
[#1]
relational-expr:
shift-expr
relational-expr < shift-expr
relational-expr > shift-expr
relational-expr <= shift-expr
relational-expr >= shift-expr
Constraints
[#2] One of the following shall hold:
- both operands have real type;
- both operands are pointers to qualified or unqualified
versions of compatible object types; or
- both operands are pointers to qualified or unqualified
versions of compatible incomplete types.
Semantics
[#3] If both of the operands have arithmetic type, the usual
arithmetic conversions are performed.
[#4] For the purposes of these operators, a pointer to a
nonarray object behaves the same as a pointer to the first
element of an array of length one with the type of the
object as its element type.
[#5] When two pointers are compared, the result depends on
the relative locations in the address space of the objects
pointed to. If two pointers to object or incomplete types
both point to the same object, or both point one past the
last element of the same array object, they compare equal.
If the objects pointed to are members of the same aggregate
object, pointers to structure members declared later compare
greater than pointers to members declared earlier in the
structure, and pointers to array elements with larger
subscript values compare greater than pointers to elements
of the same array with lower subscript values. All pointers
to members of the same union object compare equal. If the
expression P points to an element of an array object and the
expression Q points to the last element of the same array
object, the pointer expression Q+1 compares greater than P.
In all other cases, the behavior is undefined.
page 102 Language
page 103
[#6] Each of the operators < (less than), > (greater than),
<= (less than or equal to), and >= (greater than or equal
to) shall yield 1 if the specified relation is true and 0 if
it is false.77 The result has type int.
6.3.9 Equality operators
Syntax
[#1]
equality-expr:
relational-expr
equality-expr == relational-expr
equality-expr != relational-expr
Constraints
[#2] One of the following shall hold:
- both operands have arithmetic type;
- both operands are pointers to qualified or unqualified
versions of compatible types;
- one operand is a pointer to an object or incomplete
type and the other is a pointer to a qualified or
unqualified version of void; or
- one operand is a pointer and the other is a null
pointer constant.
Semantics
[#3] The == (equal to) and the != (not equal to) operators
are analogous to the relational operators except for their
lower precedence.78 Where the operands have types and
values suitable for the relational operators, the semantics
detailed in 6.3.8 apply.
__________
77. The expression a<b<c is not interpreted as in ordinary
mathematics. As the syntax indicates, it means (a<b)<c;
in other words, ``if a is less than b compare 1 to c;
otherwise, compare 0 to c.''
78. Because of the precedences, a<b == c<d is 1 whenever a<b
and c<d have the same truth-value.
page 103 Language
page 104
[#4] If two pointers to object or incomplete types are both
null pointers, they compare equal. If two pointers to
object or incomplete types compare equal, they both are null
pointers, or both point to the same object, or both point
one past the last element of the same array object.79 If
two pointers to function types are both null pointers or
both point to the same function, they compare equal. If two
pointers to function types compare equal, either both are
null pointers, or both point to the same function. If one
of the operands is a pointer to an object or incomplete type
and the other has type pointer to a qualified or unqualified
version of void, the pointer to an object or incomplete type
is converted to the type of the other operand.
[#5] Values of complex types are equal if and only if both
their real parts are equal and also their imaginary parts
are equal. Any two values of arithmetic types from
different type-domains are equal if and only if the results
of their conversion to the complex type corresponding to the
common real type determined by the usual arithmetic
conversions are equal.
6.3.10 Bitwise AND operator
Syntax
[#1]
AND-expr:
equality-expr
AND-expr & equality-expr
Constraints
[#2] Each of the operands shall have integer type.
Semantics
[#3] The usual arithmetic conversions are performed on the
operands.
[#4] The result of the binary & operator is the bitwise AND
of the operands (that is, each bit in the result is set if
__________
79. If invalid prior pointer operations, such as accesses
outside array bounds, produced undefined behavior, the
effect of subsequent comparisons is undefined.
page 104 Language
page 105
and only if each of the corresponding bits in the converted
operands is set).
6.3.11 Bitwise exclusive OR operator
Syntax
[#1]
exclusive-OR-expr:
AND-expr
exclusive-OR-expr ^ AND-expr
Constraints
[#2] Each of the operands shall have integer type.
Semantics
[#3] The usual arithmetic conversions are performed on the
operands.
[#4] The result of the ^ operator is the bitwise exclusive
OR of the operands (that is, each bit in the result is set
if and only if exactly one of the corresponding bits in the
converted operands is set).
6.3.12 Bitwise inclusive OR operator
Syntax
[#1]
inclusive-OR-expr:
exclusive-OR-expr
inclusive-OR-expr | exclusive-OR-expr
Constraints
[#2] Each of the operands shall have integer type.
Semantics
[#3] The usual arithmetic conversions are performed on the
operands.
[#4] The result of the | operator is the bitwise inclusive
OR of the operands (that is, each bit in the result is set
if and only if at least one of the corresponding bits in the
converted operands is set).
page 105 Language
page 106
6.3.13 Logical AND operator
Syntax
[#1]
logical-AND-expr:
inclusive-OR-expr
logical-AND-expr && inclusive-OR-expr
Constraints
[#2] Each of the operands shall have scalar type.
Semantics
[#3] The && operator shall yield 1 if both of its operands
compare unequal to 0; otherwise, it yields 0. The result
has type int.
[#4] Unlike the bitwise binary & operator, the && operator
guarantees left-to-right evaluation; there is a sequence
point after the evaluation of the first operand. If the
first operand compares equal to 0, the second operand is not
evaluated.
6.3.14 Logical OR operator
Syntax
[#1]
logical-OR-expr:
logical-AND-expr
logical-OR-expr || logical-AND-expr
Constraints
[#2] Each of the operands shall have scalar type.
Semantics
[#3] The || operator shall yield 1 if either of its operands
compare unequal to 0; otherwise, it yields 0. The result
has type int.
[#4] Unlike the bitwise | operator, the || operator
guarantees left-to-right evaluation; there is a sequence
point after the evaluation of the first operand. If the
first operand compares unequal to 0, the second operand is
not evaluated.
page 106 Language
page 107
6.3.15 Conditional operator
Syntax
[#1]
conditional-expr:
logical-OR-expr
logical-OR-expr ? expr : conditional-expr
Constraints
[#2] The first operand shall have scalar type.
[#3] One of the following shall hold for the second and
third operands:
- both operands have arithmetic type;
- both operands have compatible structure or union types;
- both operands have void type;
- both operands are pointers to qualified or unqualified
versions of compatible types;
- one operand is a pointer and the other is a null
pointer constant; or
- one operand is a pointer to an object or incomplete
type and the other is a pointer to a qualified or
unqualified version of void.
Semantics
[#4] The first operand is evaluated; there is a sequence
point after its evaluation. The second operand is evaluated
only if the first compares unequal to 0; the third operand
is evaluated only if the first compares equal to 0; the
value of the second or third operand (whichever is
evaluated) is the result.80
[#5] If both the second and third operands have arithmetic
type, the usual arithmetic conversions are performed to
bring them to a common type and the result has that type.
__________
80. A conditional expression does not yield an lvalue.
page 107 Language
page 108
If both the operands have structure or union type, the
result has that type. If both operands have void type, the
result has void type.
[#6] If both the second and third operands are pointers or
one is a null pointer constant and the other is a pointer,
the result type is a pointer to a type qualified with all
the type qualifiers of the types pointed-to by both
operands. Furthermore, if both operands are pointers to
compatible types or differently qualified versions of a
compatible type, the result has the composite type; if one
operand is a null pointer constant, the result has the type
of the other operand; otherwise, one operand is a pointer to
void or a qualified version of void, in which case the other
operand is converted to type pointer to void, and the result
has that type.
Examples
[#7] The common type that results when the second and third
operands are pointers is determined in two independent
stages. The appropriate qualifiers, for example, do not
depend on whether the two pointers have compatible types.
[#8] Given the declarations
const void *c_vp;
void *vp;
const int *c_ip;
volatile int *v_ip;
int *ip;
const char *c_cp;
the third column in the following table is the common type
that is the result of a conditional expression in which the
first two columns are the second and third operands (in
either order):
c_vp c_ip const void *
v_ip 0 volatile int *
c_ip v_ip const volatile int *
vp c_cp const void *
ip c_ip const int *
vp ip void *
page 108 Language
page 109
6.3.16 Assignment operators
Syntax
[#1]
assignment-expr:
conditional-expr
unary-expr assignment-operator assignment-expr
assignment-operator: one of
= *= /= %= += -= <<= >>= &= ^= |=
Constraints
[#2] An assignment operator shall have a modifiable lvalue
as its left operand.
Semantics
[#3] An assignment operator stores a value in the object
designated by the left operand. An assignment expression
has the value of the left operand after the assignment, but
is not an lvalue. The type of an assignment expression is
the type of the left operand unless the left operand has
qualified type, in which case it is the unqualified version
of the type of the left operand. The side effect of
updating the stored value of the left operand shall occur
between the previous and the next sequence point.
[#4] The order of evaluation of the operands is unspecified.
6.3.16.1 Simple assignment
Constraints
[#1] One of the following shall hold:81
- the left operand has qualified or unqualified
arithmetic type and the right has arithmetic type;
__________
81. The asymmetric appearance of these constraints with
respect to type qualifiers is due to the conversion
(specified in 6.2.2.1) that changes lvalues to ``the
value of the expression'' which removes any type
qualifiers from the type category of the expression.
page 109 Language
page 110
- the left operand has a qualified or unqualified version
of a structure or union type compatible with the type
of the right;
- both operands are pointers to qualified or unqualified
versions of compatible types, and the type pointed to
by the left has all the qualifiers of the type pointed
to by the right;
- one operand is a pointer to an object or incomplete
type and the other is a pointer to a qualified or
unqualified version of void, and the type pointed to by
the left has all the qualifiers of the type pointed to
by the right; or
- the left operand is a pointer and the right is a null
pointer constant.
Semantics
[#2] In simple assignment (=), the value of the right
operand is converted to the type of the assignment
expression and replaces the value stored in the object
designated by the left operand.
[#3] If the value being stored in an object is accessed from
another object that overlaps in any way the storage of the
first object, then the overlap shall be exact and the two
objects shall have qualified or unqualified versions of a
compatible type; otherwise, the behavior is undefined.
Examples
[#4]
1. In the program fragment
int f(void);
char c;
/* ... */
if ((c = f()) == -1)
/* ... */
the int value returned by the function may be
truncated when stored in the char, and then converted
back to int width prior to the comparison. In an
implementation in which ``plain'' char has the same
range of values as unsigned char (and char is narrower
than int), the result of the conversion cannot be
negative, so the operands of the comparison can never
compare equal. Therefore, for full portability, the
page 110 Language
page 111
variable c should be declared as int.
2. In the fragment:
char c;
int i;
long l;
l = (c = i);
the value of i is converted to the type of the
assignment-expression c = i, that is, char type. The
value of the expression enclosed in parentheses is
then converted to the type of the outer assignment-
expression, that is, long type.
6.3.16.2 Compound assignment
Constraints
[#1] For the operators += and -= only, either the left
operand shall be a pointer to an object type and the right
shall have integer type, or the left operand shall have
qualified or unqualified arithmetic type and the right shall
have arithmetic type.
[#2] For the other operators, each operand shall have
arithmetic type consistent with those allowed by the
corresponding binary operator.
Semantics
[#3] A compound assignment of the form E1 op= E2 differs
from the simple assignment expression E1 = E1 op (E2) only
in that the lvalue E1 is evaluated only once.
6.3.17 Comma operator
Syntax
[#1]
expression:
assignment-expr
expression , assignment-expr
page 111 Language
page 112
Semantics
[#2] The left operand of a comma operator is evaluated as a
void expression; there is a sequence point after its
evaluation. Then the right operand is evaluated; the result
has its type and value.82
Examples
[#3] As indicated by the syntax, in contexts where a comma
is a punctuator (in lists of arguments to functions and
lists of initializers) the comma operator as described in
this subclause cannot appear. On the other hand, it can be
used within a parenthesized expression or within the second
expression of a conditional operator in such contexts. In
the function call
f(a, (t=3, t+2), c)
the function has three arguments, the second of which has
the value 5.
Forward references: initialization (6.5.8).
__________
82. A comma operator does not yield an lvalue.
page 112 Language
page 113
6.4 Constant expressions
Syntax
[#1]
constant-expression:
conditional-expression
Description
[#2] A constant expression can be evaluated during
translation rather than runtime, and accordingly may be used
in any place that a constant may be.
Constraints
[#3] Constant expressions shall not contain assignment,
increment, decrement, function-call, or comma operators,
except when they are contained within the operand of a
sizeof operator.83
[#4] Each constant expression shall evaluate to a constant
that is in the range of representable values for its type.
Semantics
[#5] An expression that evaluates to a constant is required
in several contexts.84 If a floating expression is
evaluated in the translation environment, the arithmetic
precision and range shall be at least as great as if the
expression were being evaluated in the execution
environment.
[#6] An integer constant expression shall have integer type
and shall only have operands that are integer constants,
__________
83. The operand of a sizeof operator is not evaluated
(6.3.3.4), and thus any operator in 6.3 may be used.
84. An integer constant expression must be used to specify
the size of a bit-field member of a structure, the value
of an enumeration constant, the size of an array, or the
value of a case constant. Further constraints that
apply to the integer constant expressions used in
conditional-inclusion preprocessing directives are
discussed in 6.8.1.
page 113 Language
page 114
enumeration constants, character constants, sizeof
expressions whose operand does not have variable length
array type or a parenthesized name of such a type, and
floating constants that are the immediate operands of casts.
Cast operators in an integer constant expression shall only
convert arithmetic types to integer types, except as part of
an operand to the sizeof operator.
[#7] More latitude is permitted for constant expressions in
initializers. Such a constant expression shall be, or
evaluate to, one of the following:
- an arithmetic constant expression,
- a null pointer constant,
- an address constant, or
- an address constant for an object type plus or minus an
integer constant expression.
[#8] An arithmetic constant expression shall have arithmetic
type and shall only have operands that are integer
constants, floating constants, enumeration constants,
character constants, and sizeof expressions. Cast operators
in an arithmetic constant expression shall only convert
arithmetic types to arithmetic types, except as part of an
operand to the sizeof operator.
[#9] An address constant is a null pointer, a pointer to an
lvalue designating an object of static storage duration, or
to a function designator; it shall be created explicitly
using the unary & operator or an integer constant cast to
pointer type, or implicitly by the use of an expression of
array or function type. The array-subscript [] and member-
access . and -> operators, the address & and indirection *
unary operators, and pointer casts may be used in the
creation of an address constant, but the value of an object
shall not be accessed by use of these operators.
[#10] An implementation may accept other forms of constant
expressions.
[#11] The semantic rules for the evaluation of a constant
expression are the same as for nonconstant expressions.85
__________
85. Thus, in the following initialization,
page 114 Language
page 115
Forward references: array declarators (6.5.5.2),
initialization (6.5.8).
____________________________________________________________
static int i = 2 || 1 / 0;
the expression is a valid integer constant expression
with value one.
page 115 Language
page 116
6.5 Declarations
Syntax
[#1]
declaration:
declaration-specifiers init-declarator-list-opt ;
declaration-specifiers:
storage-class-specifier declaration-specifiers-opt
type-specifier declaration-specifiers-opt
type-qualifier declaration-specifiers-opt
function-specifiers
init-declarator-list:
init-declarator
init-declarator-list , init-declarator
init-declarator:
declarator
declarator = initializer
Constraints
[#2] A declaration shall declare at least a declarator
(excluding the parameters of a function or the members of a
structure or union), a tag, or the members of an
enumeration.
[#3] If an identifier has no linkage, there shall be no more
than one declaration of the identifier (in a declarator or
type specifier) with the same scope and in the same name
space, except for tags as specified in 6.5.2.3.
[#4] All declarations in the same scope that refer to the
same object or function shall specify compatible types.
Semantics
[#5] A declaration specifies the interpretation and
attributes of a set of identifiers. A definition of an
identifier is a declaration for that identifier that:
- for an object, causes storage to be reserved for that
object;
- for a function, includes the function body;86
page 116 Language
page 117
- for an enumeration constant or typedef name, is the
(only) declaration of the identifier.
[#6] The declaration specifiers consist of a sequence of
specifiers that indicate the linkage, storage duration, and
part of the type of the entities that the declarators
denote. The init-declarator-list is a comma-separated
sequence of declarators, each of which may have additional
type information, or an initializer, or both. The
declarators contain the identifiers (if any) being declared.
[#7] If an identifier for an object is declared with no
linkage, the type for the object shall be complete by the
end of its declarator, or by the end of its init-declarator
if it has an initializer.
Forward references: declarators (6.5.5), enumeration
specifiers (6.5.2.2), initialization (6.5.8), tags
(6.5.2.3).
6.5.1 Storage-class specifiers
Syntax
[#1]
storage-class-specifier:
typedef
extern
static
auto
register
Constraints
[#2] At most, one storage-class specifier may be given in
the declaration specifiers in a declaration.87
__________
86. Function definitions have a different syntax, described
in 6.7.1.
87. See ``future language directions'' (6.9.2).
page 117 Language
page 118
Semantics
[#3] The typedef specifier is called a ``storage-class
specifier'' for syntactic convenience only; it is discussed
in 6.5.7. The meanings of the various linkages and storage
durations were discussed in 6.1.2.2 and 6.1.2.4.
[#4] A declaration of an identifier for an object with
storage-class specifier register suggests that access to the
object be as fast as possible. The extent to which such
suggestions are effective is implementation-defined.88
[#5] The declaration of an identifier for a function that
has block scope shall have no explicit storage-class
specifier other than extern.
[#6] If an aggregate or union object is declared with a
storage-class specifier other than typedef, the properties
resulting from the storage-class specifier, except with
respect to linkage, also apply to the members of the object,
and so on recursively for any aggregate or union member
objects.
Forward references: type definitions (6.5.7).
__________
88. The implementation may treat any register declaration
simply as an auto declaration. However, whether or not
addressable storage is actually used, the address of any
part of an object declared with storage-class specifier
register may not be computed, either explicitly (by use
of the unary & operator as discussed in 6.3.3.2) or
implicitly (by converting an array name to a pointer as
discussed in 6.2.2.1). Thus the only operator that can
be applied to an array declared with storage-class
specifier register is sizeof.
page 118 Language
page 119
6.5.2 Type specifiers
Syntax
[#1]
type-specifier:
void
char
short
int
long
float
double
complex
signed
unsigned
struct-or-union-specifier
enum-specifier
typedef-name
Constraints
[#2] At least one type specifier shall be given in the
declaration specifiers in a declaration. Each list of type
specifiers shall be one of the following sets (delimited by
commas, when there is more than one set on a line); the type
specifiers may occur in any order, possibly intermixed with
the other declaration specifiers.
- void
- char
- signed char
- unsigned char
- short, signed short, short int, or signed short int
- unsigned short, or unsigned short int
- int, signed, or signed int
- unsigned, or unsigned int
- long, signed long, long int, or signed long int
- unsigned long, or unsigned long int
page 119 Language
page 120
- long long, signed long long, long long int, or signed
long long int
- unsigned long long, or unsigned long long int
- float
- double
- long double
- float complex
- double complex
- long double complex
- struct-or-union specifier
- enum-specifier
- typedef-name
Semantics
[#3] Specifiers for structures, unions, and enumerations are
discussed in 6.5.2.1 through 6.5.2.3. Declarations of
typedef names are discussed in 6.5.7. The characteristics
of the other types are discussed in 6.1.2.5.
[#4] Each of the comma-separated sets designates the same
type, except that for bit-fields, it is implementation-
defined whether the specified int is the same type as signed
int or is the same type as unsigned int.
Forward references: enumeration specifiers (6.5.2.2),
structure and union specifiers (6.5.2.1), tags (6.5.2.3),
type definitions (6.5.7).
6.5.2.1 Structure and union specifiers
Syntax
[#1]
struct-or-union-specifier:
struct-or-union identifier-opt { struct-declaration-list }
struct-or-union identifier
page 120 Language
page 121
struct-or-union:
struct
union
struct-declaration-list:
struct-declaration
struct-declaration-list struct-declaration
struct-declaration:
specifier-qualifier-list struct-declarator-list ;
specifier-qualifier-list:
type-specifier specifier-qualifier-list-opt
type-qualifier specifier-qualifier-list-opt
struct-declarator-list:
struct-declarator
struct-declarator-list , struct-declarator
struct-declarator:
declarator
declarator-opt : constant-expression
Constraints
[#2] A structure or union shall not contain a member with
incomplete or function type, except that the last element of
a structure may have incomplete array type. Hence it shall
not contain an instance of itself (but may contain a pointer
to an instance of itself).
[#3] The expression that specifies the width of a bit-field
shall be an integer constant expression that has nonnegative
value that shall not exceed the number of bits in an object
of the type that is specified if the colon and expression
are omitted. If the value is zero, the declaration shall
have no declarator.
Semantics
[#4] As discussed in 6.1.2.5, a structure is a type
consisting of a sequence of members, whose storage is
allocated in an ordered sequence, and a union is a type
consisting of a sequence of members whose storage overlap.
[#5] Structure and union specifiers have the same form.
[#6] The presence of a struct-declaration-list in a struct-
or-union-specifier declares a new type, within a translation
unit. The struct-declaration-list is a sequence of
declarations for the members of the structure or union. If
page 121 Language
page 122
the struct-declaration-list contains no named members, the
behavior is undefined. The type is incomplete until after
the } that terminates the list.
[#7] A member of a structure or union may have any object
type other than a variably modified type.89 In addition, a
member may be declared to consist of a specified number of
bits (including a sign bit, if any). Such a member is
called a bit-field;90 its width is preceded by a colon.
[#8] A bit-field shall have a type that is a qualified or
unqualified version of signed int or unsigned int. A bit-
field is interpreted as a signed or unsigned integer type
consisting of the specified number of bits.91
[#9] An implementation may allocate any addressable storage
unit large enough to hold a bit-field. If enough space
remains, a bit-field that immediately follows another bit-
field in a structure shall be packed into adjacent bits of
the same unit. If insufficient space remains, whether a
bit-field that does not fit is put into the next unit or
overlaps adjacent units is implementation-defined. The
order of allocation of bit-fields within a unit (high-order
to low-order or low-order to high-order) is implementation-
defined. The alignment of the addressable storage unit is
unspecified.
[#10] A bit-field declaration with no declarator, but only a
colon and a width, indicates an unnamed bit-field.92 As a
special case of this, a bit-field structure member with a
__________
89. A structure or union can not contain a member with a
variably modified type because member names are not
ordinary identifiers as defined in 6.1.2.3.
90. The unary & (address-of) operator may not be applied to
a bit-field object; thus, there are no pointers to or
arrays of bit-field objects.
91. As specified in 6.5.2 above, if the actual type
specifier used is int or there is no type specifier, or
is a typedef-name defined using either of these, then it
is implementation-defined whether the bit-field is
signed or unsigned.
92. An unnamed bit-field structure member is useful for
padding to conform to externally imposed layouts.
page 122 Language
page 123
width of 0 indicates that no further bit-field is to be
packed into the unit in which the previous bit-field, if
any, was placed.
[#11] Each non-bit-field member of a structure or union
object is aligned in an implementation-defined manner
appropriate to its type.
[#12] Within a structure object, the non-bit-field members
and the units in which bit-fields reside have addresses that
increase in the order in which they are declared. A pointer
to a structure object, suitably converted, points to its
initial member (or if that member is a bit-field, then to
the unit in which it resides), and vice versa. There may be
unnamed padding within a structure object, but not at its
beginning.
[#13] The size of a union is sufficient to contain the
largest of its members. The value of at most one of the
members can be stored in a union object at any time. A
pointer to a union object, suitably converted, points to
each of its members (or if a member is a bit-field, then to
the unit in which it resides), and vice versa.
[#14] There may be unnamed padding at the end of a structure
or union, were the structure or union to be an element of an
array.
[#15] As a special case, the last element of a structure may
be an incomplete array type. This is called a flexible
array member, and the size of the structure shall be equal
to the offset of the last element of an otherwise identical
structure that replaces the flexible array member with an
array of one element. When an lvalue whose type is a
structure with a flexible array member is used to access an
object, it behaves as if that member were replaced by the
longest array that would not make the structure larger than
the object being accessed. If this array would have no
elements, then it behaves as if it has one element, but the
behavior is undefined if any attempt is made to access that
element.
Examples
[#16] After the declarations:
struct s { int n; double d[]; };
struct ss { int n; double d[1]; };
the three expressions:
page 123 Language
page 124
sizeof (struct s)
offsetof(struct s, d)
offsetof(struct ss, d)
have the same value. The structure struct s has a flexible
array member d.
[#17] If sizeof (double) is 8, then after the following code
is executed:
struct s *s1;
struct s *s2;
s1 = malloc(sizeof (struct s) + 64);
s2 = malloc(sizeof (struct s) + 46);
and assuming that the calls to malloc succeed, s1 and s2
behave as if they had been declared as:
struct { int n; double d[8]; } *s1;
struct { int n; double d[5]; } *s2;
[#18] Following the further successful assignments:
s1 = malloc(sizeof (struct s) + 10);
s2 = malloc(sizeof (struct s) + 6);
they then behave as if they had been declared as:
struct { int n; double d[1]; } *s1, *s2;
and:
double *dp;
dp = &(s1->d[0]); // Permitted
*dp = 42; // Permitted
dp = &(s2->d[0]); // Permitted
*dp = 42; // Undefined behavior
Forward references: tags (6.5.2.3).
6.5.2.2 Enumeration specifiers
Syntax
[#1]
enum-specifier:
enum identifier-opt { enumerator-list }
enum identifier-opt { enumerator-list , }
enum identifier
page 124 Language
page 125
enumerator-list:
enumerator
enumerator-list , enumerator
enumerator:
enumeration-constant
enumeration-constant = constant-expression
Constraints
[#2] The expression that defines the value of an enumeration
constant shall be an integer constant expression that has a
value representable as an int.
Semantics
[#3] The identifiers in an enumerator list are declared as
constants that have type int and may appear wherever such
are permitted.93 An enumerator with = defines its
enumeration constant as the value of the constant
expression. If the first enumerator has no =, the value of
its enumeration constant is 0. Each subsequent enumerator
with no = defines its enumeration constant as the value of
the constant expression obtained by adding 1 to the value of
the previous enumeration constant. (The use of enumerators
with = may produce enumeration constants with values that
duplicate other values in the same enumeration.) The
enumerators of an enumeration are also known as its members.
[#4] Each enumerated type shall be compatible with an
integer type. The choice of type is implementation-defined,
but shall be capable of representing the values of all the
members of the enumeration.
[#5] The enumerated type is complete at the } that
terminates the list of enumerator declarations.
__________
93. Thus, the identifiers of enumeration constants declared
in the same scope shall all be distinct from each other
and from other identifiers declared in ordinary
declarators.
page 125 Language
page 126
Examples
[#6]
enum hue { chartreuse, burgundy, claret=20, winedark };
enum hue col, *cp;
col = claret;
cp = &col;
if (*cp != burgundy)
/* ... */
makes hue the tag of an enumeration, and then declares col
as an object that has that type and cp as a pointer to an
object that has that type. The enumerated values are in the
set {0, 1, 20, 21}.
Forward references: tags (6.5.2.3).
6.5.2.3 Tags
Constraints
[#1] A specific type shall have its content defined at most
once.
[#2] A type specifier of the form
enum identifier
without an enumerator list shall only appear after the type
it specifies is completed.
Semantics
[#3] All declarations of structure, union, or enumerated
types that have the same scope and use the same tag declare
the same type. The type is complete94 until the closing
brace of the list defining the content, and complete
__________
94. An incomplete type may only by used when the size of an
object of that type is not needed. It is not needed,
for example, when a typedef name is declared to be a
specifier for a structure or union, or when a pointer to
or a function returning a structure or union is being
declared. (See incomplete types in 6.1.2.5.) The
specification shall be complete before such a function
is called or defined.
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thereafter.
[#4] Two declarations of structure, union, or enumerated
types which are in different scopes or use different tags
declare distinct types. Each declaration of a structure,
union, or enumerated type which does not include a tag
declares a distinct type.
[#5] A type specifier of the form
struct-or-union identifier-opt { struct-declaration-list }
or
enum identifier { enumerator-list }
or
enum identifier { enumerator-list , }
declares a structure, union, or enumerated type. The list
defines the structure content, union content, or enumeration
content. If an identifier is provided,95 the type
specifier also declares the identifier to be the tag of that
type.
[#6] A declaration of the form
struct-or-union identifier
specifies a structure of union type and declares the
identifier as a tag of that type.96
[#7] If a type specifier of the form
struct-or-union identifier
occurs other than as part of one of the above forms, and no
other declaration of the identifier as a tag is visible,
then it declares an incomplete structure or union type, and
declares the identifier as the tag of that type.96
__________
95. If there is no identifier, the type can, within the
translation unit, only be referred to by the declaration
of which it is a part. Of course, when the declaration
is of a typedef name, subsequent declarations can make
use of that typedef name to declare objects having the
specified structure, union, or enumerated type.
96. A similar construction with enum does not exist.
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[#8] If a type specifier of the form
struct-or-union identifier
or
enum identifier
occurs other than as part of one of the above forms, and a
declaration of the identifier as a tag is visible, then it
specifies the same type as that other declaration, and does
not redeclare the tag.
Examples
[#9]
1. This mechanism allows declaration of a self-
referential structure.
struct tnode {
int count;
struct tnode *left, *right;
};
specifies a structure that contains an integer and two
pointers to objects of the same type. Once this
declaration has been given, the declaration
struct tnode s, *sp;
declares s to be an object of the given type and sp to
be a pointer to an object of the given type. With
these declarations, the expression sp->left refers to
the left struct tnode pointer of the object to which
sp points; the expression s.right->count designates
the count member of the right struct tnode pointed to
from s.
The following alternative formulation uses the typedef
mechanism:
typedef struct tnode TNODE;
struct tnode {
int count;
TNODE *left, *right;
};
TNODE s, *sp;
2. To illustrate the use of prior declaration of a tag to
specify a pair of mutually referential structures, the
declarations
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struct s1 { struct s2 *s2p; /* ... */ }; // D1
struct s2 { struct s1 *s1p; /* ... */ }; // D2
specify a pair of structures that contain pointers to
each other. Note, however, that if s2 were already
declared as a tag in an enclosing scope, the
declaration D1 would refer to it, not to the tag s2
declared in D2. To eliminate this context
sensitivity, the declaration
struct s2;
may be inserted ahead of D1. This declares a new tag
s2 in the inner scope; the declaration D2 then
completes the specification of the new type.
3. An enumeration type is compatible with some integer
type. An implementation may delay the choice of which
integer type until all enumeration constants have been
seen. Thus in:
enum f { c = sizeof (enum f) };
the behavior is undefined since the size of the
respective enumeration type is not necessarily known
when sizeof is encountered.
Forward references: declarators (6.5.5), array declarators
(6.5.5.2), type definitions (6.5.7).
6.5.3 Type qualifiers
Syntax
[#1]
type-qualifier:
const
restrict
volatile
Constraints
[#2] Types other than pointer types derived from object or
incomplete types shall not be restrict-qualified.
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Semantics
[#3] The properties associated with qualified types are
meaningful only for expressions that are lvalues.97
[#4] If the same qualifier appears more than once in the
same specifier-qualifier-list, either directly or via one or
more typedefs, the behavior is the same as if it appeared
only once.
[#5] If an attempt is made to modify an object defined with
a const-qualified type through use of an lvalue with non-
const-qualified type, the behavior is undefined. If an
attempt is made to refer to an object defined with a
volatile-qualified type through use of an lvalue with non-
volatile-qualified type, the behavior is undefined.98
[#6] An object that has volatile-qualified type may be
modified in ways unknown to the implementation or have other
unknown side effects. Therefore any expression referring to
such an object shall be evaluated strictly according to the
rules of the abstract machine, as described in 5.1.2.3.
Furthermore, at every sequence point the value last stored
in the object shall agree with that prescribed by the
abstract machine, except as modified by the unknown factors
mentioned previously.99 What constitutes an access to an
object that has volatile-qualified type is implementation-
defined.
__________
97. The implementation may place a const object that is not
volatile in a read-only region of storage. Moreover,
the implementation need not allocate storage for such an
object if its address is never used.
98. This applies to those objects that behave as if they
were defined with qualified types, even if they are
never actually defined as objects in the program (such
as an object at a memory-mapped input/output address).
99. A volatile declaration may be used to describe an object
corresponding to a memory-mapped input/output port or an
object accessed by an asynchronously interrupting
function. Actions on objects so declared shall not be
``optimized out'' by an implementation or reordered
except as permitted by the rules for evaluating
expressions.
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[#7] An object that is referenced through a restrict-
qualified pointer has a special association with that
pointer. This association, defined in 6.5.3.1 below,
requires that all references to that object shall use,
directly or indirectly, the value of that pointer. For
example, a statement that assigns a value returned by malloc
to a single pointer establishes this association between the
allocated object and the pointer. The intended use of the
restrict qualifier (like the register storage class) is to
promote optimization, and deleting all instances of the
qualifier from a conforming program does not change its
meaning (i.e., observable behavior).
[#8] If the specification of an array type includes any type
qualifiers, the element type is so-qualified, not the array
type. If the specification of a function type includes any
type qualifiers, the behavior is undefined.100
[#9] For two qualified types to be compatible, both shall
have the identically qualified version of a compatible type;
the order of type qualifiers within a list of specifiers or
qualifiers does not affect the specified type.
Examples
[#10]
1. An object declared
extern const volatile int real_time_clock;
may be modifiable by hardware, but cannot be assigned
to, incremented, or decremented.
2. The following declarations and expressions illustrate
the behavior when type qualifiers modify an aggregate
type:
__________
100. Both of these can occur through the use of typedefs.
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const struct s { int mem; } cs = { 1 };
struct s ncs; // the object ncs is modifiable
typedef int A[2][3];
const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of
// const int
int *pi;
const int *pci;
ncs = cs; // valid
cs = ncs; // violates modifiable lvalue constraint for =
pi = &ncs.mem; // valid
pi = &cs.mem; // violates type constraints for =
pci = &cs.mem; // valid
pi = a[0]; // invalid: a[0] has type ``const int *''
6.5.3.1 Formal definition of restrict
[#1] Let D be a declaration of an ordinary identifier that
provides a means of designating an object P as a restrict-
qualified pointer.
[#2] If D appears inside a block and does not have storage
class extern, let B denote the block. If D appears in the
list of parameter declarations of a function definition, let
B denote the associated block. Otherwise, let B denote the
block of main (or the block of whatever function is called
at program startup in a freestanding environment).
[#3] In what follows, a pointer expression E is said to be
based on object P if (at some sequence point in the
execution of B prior to the evaluation of E) modifying P to
point to a copy of the array object into which it formerly
pointed would change the value of E. (In other words, E
depends on the value of P itself rather than on the value of
an object referenced indirectly through P. For example, if
identifier p has type (int **restrict), then the pointer
expressions p and p+1 are based on the restricted pointer
object designated by p, but the pointer expressions *p and
p[1] are not.) Note that ``based'' is defined only for
expressions with pointer types.
[#4] During each execution of B, let A be the array object
that is determined dynamically by all references through
pointer expressions based on P. Then all references to
values of A shall be through pointer expressions based on P.
Furthermore, if P is assigned the value of a pointer
expression E that is based on another restricted pointer
object P2, associated with block B2, then either the
execution of B2 shall begin before the execution of B, or
the execution of B2 shall end prior to the assignment. If
these requirements are not met, then the behavior is
undefined.
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[#5] Here an execution of B means that portion of the
execution of the program during which storage is guaranteed
to be reserved for an instance of an object that is
associated with B and that has automatic storage duration.
A reference to a value means either an access to or a
modification of the value. During an execution of B,
attention is confined to those references that are actually
evaluated. (This excludes references that appear in
unevaluated expressions, and also excludes references that
are ``available'', in the sense of employing visible
identifiers, but do not actually appear in the text of B.)
[#6] A translator is free to ignore any or all aliasing
implications of uses of restrict.
Examples
[#7]
1. The file scope declarations
int * restrict a;
int * restrict b;
extern int c[];
assert that if an object is referenced using the value
of one of a, b, or c, then it is never referenced
using the value of either of the other two.
2. The function parameter declarations in the following
example
void f(int n, int * restrict p, int * restrict q)
{
while (n-- > 0)
*p++ = *q++;
}
assert that, during each execution of the function, if
an object is referenced through one of the pointer
parameters, then it is not also referenced through the
other.
The benefit of the restrict qualifiers is that they
enable a translator to make an effective dependence
analysis of function f without examining any of the
calls of f in the program. The cost is that the
programmer must examine all of those calls to ensure
that none give undefined behavior. For example, the
second call of f in g has undefined behavior because
each of d[1] through d[49] is referenced through both
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p and q.
void g(void)
{
extern float d[100];
f(50, d + 50, d); // ok
f(50, d + 1, d); // undefined behavior
}
3. The function parameter declarations
void h(int n, int * const restrict p,
int * const q, int * const r)
{
int i;
for (i = 0; i < n; i++)
p[i] = q[i] + r[i];
}
show how const can be used in conjunction with
restrict. The const qualifiers imply, without the
need to examine the body of h, that q and r cannot
become based on p. The fact that p is restrict-
qualified therefore implies that an object referenced
through p is never referenced through either of q or
r. This is the precise assertion required to optimize
the loop. Note that a call of the form h(100, a, b,
b) has defined behavior, which would not be true if
all three of p, q, and r were restrict-qualified.
4. The rule limiting assignments between restricted
pointers does not distinguish between a function call
and an equivalent nested block. With one exception,
only ``outer-to-inner'' assignments between restricted
pointers declared in nested blocks have defined
behavior.
{
int * restrict p1;
int * restrict q1;
p1 = q1; // undefined behavior
{
int * restrict p2 = p1; // ok
int * restrict q2 = q1; // ok
p1 = q2; // undefined behavior
p2 = q2; // undefined behavior
}
}
The exception allows the value of a restricted pointer
to be carried out of the block in which it (or, more
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precisely, the ordinary identifier used to designate
it) is declared when that block finishes execution.
For example, this permits new_vector to return a
vector.
typedef struct { int n; float * restrict v; } vector;
vector new_vector(int n)
{
vector t;
t.n = n;
t.v = malloc(n * sizeof (float));
return t;
}
6.5.4 Function specifiers
Syntax
[#1]
function-specifier:
inline
Constraints
[#2] Function specifiers shall be used only in the
declaration of an identifier for a function.
[#3] An inline definition (see below) of a function with
external linkage shall not contain a definition of an object
with static storage duration that can be modified, and shall
not contain a reference to an identifier with internal
linkage.
[#4] The inline function specifier shall not appear in a
declaration of main.
Semantics
[#5] A function declared with an inline function specifier
is an inline function. Making a function an inline function
suggests that calls to the function be as fast as possible
by using, for example, an alternative to the usual function
call mechanism known as ``inline substitution''.101 The
__________
101. Inline substitution is not textual substitution, nor
does it create a new function. Therefore, for example,
the expansion of a macro used within the body of the
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extent to which such suggestions are effective is
implementation-defined.102
[#6] Any function with internal linkage can be an inline
function. For a function with external linkage, the
following restrictions apply. If a function is declared
with an inline function specifier, then it shall also be
defined in the same translation unit. If all of the file
scope declarations for a function in a translation unit
include the inline function specifier without extern, then
the definition in that translation unit is an inline
definition. An inline definition does not provide an
external definition for the function, and does not forbid an
external definition in another translation unit. An inline
definition provides an alternative to an external
definition, which a translator may use to implement any call
to the function in the same translation unit. It is
unspecified whether a call to the function uses the inline
definition or the external definition.
Examples
[#7] The declaration of an inline function with external
linkage can result in either an external definition, or a
definition available for use only within the translation
unit. A file scope declaration with extern creates an
external definition. The following example shows an entire
translation unit.
____________________________________________________________
function uses the definition it had at the point the
function body appears, and not where the function is
called; and identifiers refer to the declarations in
scope where the body occurs. Similarly, the address of
the function is not affected by the function's being
inlined.
102. For example, an implementation might never perform
inline substitution, or might only perform inline
substitutions to calls in the scope of an inline
declaration.
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page 137
inline double fahr(double t)
{
return (9.0 * t) / 5.0 + 32.0;
}
inline double cels(double t)
{
return (5.0 * (t - 32.0)) / 9.0;
}
/* Creates an external definition. */
extern double fahr(double);
double convert(int is_fahr, double temp)
{
/* A translator may perform inline substitutions. */
return is_fahr ? cels(temp) : fahr(temp);
}
[#8] Note that the definition of fahr is an external
definition because fahr is also declared with extern, but
the definition of cels is an inline definition. Because
there is a call to cels, an external definition of cels in
another translation unit is still required by 6.7.
6.5.5 Declarators
Syntax
[#1]
declarator:
pointer-opt direct-declarator
direct-declarator:
identifier
( declarator )
direct-declarator [ assignment-expression-opt ]
direct-declarator [ * ]
direct-declarator ( parameter-type-list )
direct-declarator ( identifier-list-opt )
pointer:
* type-qualifier-list-opt
* type-qualifier-list-opt pointer
type-qualifier-list:
type-qualifier
type-qualifier-list type-qualifier
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parameter-type-list:
parameter-list
parameter-list , ...
parameter-list:
parameter-declaration
parameter-list , parameter-declaration
parameter-declaration:
declaration-specifiers declarator
declaration-specifiers abstract-declarator-opt
identifier-list:
identifier
identifier-list , identifier
Semantics
[#2] Each declarator declares one identifier, and asserts
that when an operand of the same form as the declarator
appears in an expression, it designates a function or object
with the scope, storage duration, and type indicated by the
declaration specifiers.
[#3] A full declarator is a declarator that is not part of
another declarator. The end of a full declarator is a
sequence point. If the nested sequence of declarators in a
full declarator contains a variable length array type, the
type specified by the full declarator is said to be variably
modified.
[#4] In the following subclauses, consider a declaration
T D1
where T contains the declaration specifiers that specify a
type T (such as int) and D1 is a declarator that contains an
identifier ident. The type specified for the identifier
ident in the various forms of declarator is described
inductively using this notation.
[#5] If, in the declaration ``T D1'', D1 has the form
identifier
then the type specified for ident is T.
[#6] If, in the declaration ``T D1'', D1 has the form
( D )
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then ident has the type specified by the declaration ``T
D''. Thus, a declarator in parentheses is identical to the
unparenthesized declarator, but the binding of complicated
declarators may be altered by parentheses.
Implementation limits
[#7] The implementation shall allow the specification of
types that have at least 12 pointer, array, and function
declarators (in any valid combinations) modifying an
arithmetic, structure, union, or incomplete type, either
directly or via one or more typedefs.
Forward references: array declarators (6.5.5.2), type
definitions (6.5.7).
6.5.5.1 Pointer declarators
Semantics
[#1] If, in the declaration ``T D1'', D1 has the form
* type-qualifier-list-opt D
and the type specified for ident in the declaration ``T D''
is ``derived-declarator-type-list T'', then the type
specified for ident is ``derived-declarator-type-list type-
qualifier-list pointer to T''. For each type qualifier in
the list, ident is a so-qualified pointer.
[#2] For two pointer types to be compatible, both shall be
identically qualified and both shall be pointers to
compatible types.
Examples
[#3] The following pair of declarations demonstrates the
difference between a ``variable pointer to a constant
value'' and a ``constant pointer to a variable value''.
const int *ptr_to_constant;
int *const constant_ptr;
The contents of an object pointed to by ptr_to_constant
shall not be modified through that pointer, but
ptr_to_constant itself may be changed to point to another
object. Similarly, the contents of the int pointed to by
constant_ptr may be modified, but constant_ptr itself shall
always point to the same location.
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[#4] The declaration of the constant pointer constant_ptr
may be clarified by including a definition for the type
``pointer to int''.
typedef int *int_ptr;
const int_ptr constant_ptr;
declares constant_ptr as an object that has type ``const-
qualified pointer to int''.
6.5.5.2 Array declarators
Constraints
[#1] The [ and ] may delimit an expression or *. If [ and ]
delimit an expression (which specifies the size of an
array), it shall have an integer type. If the expression is
a constant expression then it shall have a value greater
than zero. The element type shall not be an incomplete or
function type.
[#2] Only ordinary identifiers (as defined in 6.1.2.3) with
block scope or function prototype scope and without linkage
can have a variably modified type. If an identifier is
declared to be an object with static storage duration, it
shall not have a variable length array type.
Semantics
[#3] If, in the declaration ``T D1'', D1 has the form
D[assignment-expr-opt]
or
D[*]
and the type specified for ident in the declaration ``T D''
is ``derived-declarator-type-list T'', then the type
specified for ident is ``derived-declarator-type-list array
of T''.103 If the size is not present, the array type is an
incomplete type. If * is used instead of a size expression,
the array type is a variable length array type of
unspecified size, which can only be used in declarations
__________
103. When several ``array of'' specifications are adjacent,
a multidimensional array is declared.
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with function prototype scope. If the size expression is an
integer constant expression and the element type has a known
constant size, the array type is not a variable length array
type. Otherwise, the array type is a variable length array
type. If the size expression is not a constant expression,
and it is evaluated at program execution time, it shall
evaluate to a value greater than zero. It is unspecified
whether side effects are produced when the size expression
is evaluated. The size of each instance of a variable
length array type does not change during its lifetime.
[#4] For two array types to be compatible, both shall have
compatible element types, and if both size specifiers are
present, and are integer constant expressions, then both
size specifiers shall have the same constant value. If the
two array types are used in a context which requires them to
be compatible, it is undefined behavior if the two size
specifiers evaluate to unequal values.
Examples
[#5]
1. float fa[11], *afp[17];
declares an array of float numbers and an array of
pointers to float numbers.
2. Note the distinction between the declarations
extern int *x;
extern int y[];
The first declares x to be a pointer to int; the
second declares y to be an array of int of unspecified
size (an incomplete type), the storage for which is
defined elsewhere.
3. The following declarations demonstrate the
compatibility rules for variably modified types.
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extern int n;
extern int m;
void fcompat(void)
{
int a[n][6][m];
int (*p)[4][n+1];
int c[n][n][6][m];
int (*r)[n][n][n+1];
p = a; // Error - not compatible because 4 != 6.
r = c; // Compatible, but defined behavior
// only if n == 6 and m == n+1.
}
4. All declarations of variably modified (VM) types must
be declared at either block scope or function
prototype scope. Array objects declared with the
static or extern storage class specifier cannot have a
variable length array (VLA) type. However, an object
declared with the static storage class specifier can
have a VM type (that is, a pointer to a VLA type).
Finally, all identifiers declared with a VM type must
be ordinary identifiers, and can not, therefore, be
members of structures or unions.
extern int n;
int A[n]; // Error - file scope VLA.
extern int (*p2)[n]; // Error - file scope VM.
int B[100]; // OK - file scope but not VM.
void fvla(int n, int C[m][m]) // OK - VLA with prototype scope.
{
typedef int VLA[m][m] // OK - block scope typedef VLA.
struct tag {
int (*y)[n]; // Error - y not ordinary identifier.
int z[n]; // Error - z not ordinary identifier.
};
int D[m]; // OK - auto VLA.
static int E[m]; // Error - static block scope VLA.
extern int F[m]; // Error - F has linkage and is VLA.
int (*s)[m]; // OK - auto pointer to VLA.
extern int (*r)[m]; // Error - r had linkage and is
// a pointer to VLA.
static int (*q)[m] = &B; // OK - q is a static block
// pointer to VLA.
}
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Forward references: function definitions (6.7.1),
initialization (6.5.8).
6.5.5.3 Function declarators (including prototypes)
Constraints
[#1] A function declarator shall not specify a return type
that is a function type or an array type.
[#2] The only storage-class specifier that shall occur in a
parameter declaration is register.
[#3] An identifier list in a function declarator that is not
part of a function definition shall be empty.
[#4] After all rewrites, the parameters in a parameter-
type-list that is part of a function definition shall not
have incomplete type.104
Semantics
[#5] If, in the declaration ``T D1'', D1 has the form
D(parameter-type-list)
or
D(identifier-list-opt)
and the type specified for ident in the declaration ``T D''
is ``derived-declarator-type-list T'', then the type
specified for ident is ``derived-declarator-type-list
function returning T''.
[#6] A parameter type list specifies the types of, and may
declare identifiers for, the parameters of the function. A
declared parameter that is a member of a parameter type list
that is not part of a function definition, may use the [*]
notation in its sequence of declarator specifiers to specify
a variable length array type. If the list terminates with
an ellipsis (, ...), no information about the number or
types of the parameters after the comma is supplied.105 The
__________
104. Arrays and functions are rewritten as pointers.
105. The macros defined in the <stdarg.h> header (7.12) may
be used to access arguments that correspond to the
ellipsis.
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special case of an unnamed parameter of type void as the
only item in the list specifies that the function has no
parameters.
[#7] If, in a parameter declaration, an identifier can be
treated as a typedef name or as a parameter name, it shall
be taken as a typedef name.
[#8] If the function declarator is not part of a function
definition, the parameters may have incomplete type.
[#9] The storage-class specifier in the declaration
specifiers for a parameter declaration, if present, is
ignored unless the declared parameter is one of the members
of the parameter type list for a function definition.
[#10] An identifier list declares only the identifiers of
the parameters of the function. An empty list in a function
declarator that is part of a function definition specifies
that the function has no parameters. The empty list in a
function declarator that is not part of a function
definition specifies that no information about the number or
types of the parameters is supplied.106
[#11] For two function types to be compatible, both shall
specify compatible return types.107 Moreover, the parameter
type lists, if both are present, shall agree in the number
of parameters and in use of the ellipsis terminator;
corresponding parameters shall have compatible types. If
one type has a parameter type list and the other type is
specified by a function declarator that is not part of a
function definition and that contains an empty identifier
list, the parameter list shall not have an ellipsis
terminator and the type of each parameter shall be
compatible with the type that results from the application
of the default argument promotions. If one type has a
parameter type list and the other type is specified by a
function definition that contains a (possibly empty)
identifier list, both shall agree in the number of
parameters, and the type of each prototype parameter shall
be compatible with the type that results from the
application of the default argument promotions to the type
__________
106. See ``future language directions'' (6.9.3).
107. If both function types are ``old style'', parameter
types are not compared.
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of the corresponding identifier. (In the determination of
type compatibility and of a composite type, each parameter
declared with function or array type is taken as having the
type that results from conversion to a pointer type, as in
6.7.1, and each parameter declared with qualified type is
taken as having the unqualified version of its declared
type.)
Examples
[#12]
1. The declaration
int f(void), *fip(), (*pfi)();
declares a function f with no parameters returning an
int, a function fip with no parameter specification
returning a pointer to an int, and a pointer pfi to a
function with no parameter specification returning an
int. It is especially useful to compare the last two.
The binding of *fip() is *(fip()), so that the
declaration suggests, and the same construction in an
expression requires, the calling of a function fip,
and then using indirection through the pointer result
to yield an int. In the declarator (*pfi)(), the
extra parentheses are necessary to indicate that
indirection through a pointer to a function yields a
function designator, which is then used to call the
function; it returns an int.
If the declaration occurs outside of any function, the
identifiers have file scope and external linkage. If
the declaration occurs inside a function, the
identifiers of the functions f and fip have block
scope and either internal or external linkage
(depending on what file scope declarations for these
identifiers are visible), and the identifier of the
pointer pfi has block scope and no linkage.
2. The declaration
int (*apfi[3])(int *x, int *y);
declares an array apfi of three pointers to functions
returning int. Each of these functions has two
parameters that are pointers to int. The identifiers
x and y are declared for descriptive purposes only and
go out of scope at the end of the declaration of apfi.
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3. The declaration
int (*fpfi(int (*)(long), int))(int, ...);
declares a function fpfi that returns a pointer to a
function returning an int. The function fpfi has two
parameters: a pointer to a function returning an int
(with one parameter of type long), and an int. The
pointer returned by fpfi points to a function that has
one int parameter and accepts zero or more additional
arguments of any type.
4. The following prototype has a variably modified
parameter.
void addscalar(int n, int m,
double a[n][n*m+300], double x);
int main()
{
double b[4][308];
addscalar(4, 2, b, 2.17);
return 0;
}
void addscalar(int n, int m,
double a[n][n*m+300], double x)
{
for (int i = 0; i < n; i++)
for (int j = 0, k = n*m+300; j < k; j++)
// a is a pointer to a VLA
// with n*m+300 elements
a[i][j] += x;
}
5. The following are all compatible function prototype
declarators.
double maximum(int n, int m, double a[n][m]);
double maximum(int n, int m, double a[*][*]);
double maximum(int n, int m, double a[ ][*]);
double maximum(int n, int m, double a[ ][m]);
Forward references: function definitions (6.7.1), type
names (6.5.6).
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6.5.6 Type names
Syntax
[#1]
type-name:
specifier-qualifier-list abstract-declarator-opt
abstract-declarator:
pointer
pointer-opt direct-abstract-declarator
direct-abstract-declarator:
( abstract-declarator )
direct-abstract-declarator-opt [ assignment-expression-opt ]
direct-abstract-declarator-opt [ * ]
direct-abstract-declarator-opt ( parameter-type-list-opt )
Semantics
[#2] In several contexts, it is desired to specify a type.
This is accomplished using a type name, which is
syntactically a declaration for a function or an object of
that type that omits the identifier.108
Examples
[#3] The constructions
(a) int
(b) int *
(c) int *[3]
(d) int (*)[3]
(e) int *()
(f) int (*)(void)
(g) int (*const [])(unsigned int, ...)
name respectively the types (a) int, (b) pointer to int, (c)
array of three pointers to int, (d) pointer to an array of
three ints, (e) function with no parameter specification
returning a pointer to int, (f) pointer to function with no
__________
108. As indicated by the syntax, empty parentheses in a type
name are interpreted as ``function with no parameter
specification'', rather than redundant parentheses
around the omitted identifier.
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parameters returning an int, and (g) array of an unspecified
number of constant pointers to functions, each with one
parameter that has type unsigned int and an unspecified
number of other parameters, returning an int.
6.5.7 Type definitions
Syntax
[#1]
typedef-name:
identifier
Constraints
[#2] If a typedef name specifies a variably modified type
then it shall have block scope.
Semantics
[#3] In a declaration whose storage-class specifier is
typedef, each declarator defines an identifier to be a
typedef name that specifies the type specified for the
identifier in the way described in 6.5.5. Any array size
expressions associated with variable length array
declarators shall be evaluated with the typedef name at the
beginning of its scope upon each normal entry to the block.
A typedef declaration does not introduce a new type, only a
synonym for the type so specified. That is, in the
following declarations:
typedef T type_ident;
type_ident D;
type_ident is defined as a typedef name with the type
specified by the declaration specifiers in T (known as T),
and the identifier in D has the type ``derived-declarator-
type-list T'' where the derived-declarator-type-list is
specified by the declarators of D. A typedef name shares
the same name space as other identifiers declared in
ordinary declarators. If the identifier is redeclared in an
inner scope or is declared as a member of a structure or
union in the same or an inner scope, the type specifiers
shall not be omitted in the inner declaration.
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Examples
[#4]
1. After
typedef int MILES, KLICKSP();
typedef struct { double hi, lo; } range;
the constructions
MILES distance;
extern KLICKSP *metricp;
range x;
range z, *zp;
are all valid declarations. The type of distance is
int, that of metricp is ``pointer to function with no
parameter specification returning int'', and that of x
and z is the specified structure; zp is a pointer to
such a structure. The object distance has a type
compatible with any other int object.
2. After the declarations
typedef struct s1 { int x; } t1, *tp1;
typedef struct s2 { int x; } t2, *tp2;
type t1 and the type pointed to by tp1 are compatible.
Type t1 is also compatible with type struct s1, but
not compatible with the types struct s2, t2, the type
pointed to by tp2, and int.
3. The following obscure constructions
typedef signed int t;
typedef int plain;
struct tag {
unsigned t:4;
const t:5;
plain r:5;
};
declare a typedef name t with type signed int, a
typedef name plain with type int, and a structure with
three bit-field members, one named t that contains
values in the range [0, 15], an unnamed const-
qualified bit-field which (if it could be accessed)
would contain values in at least the range [-15, +15],
and one named r that contains values in the range [0,
31] or values in at least the range [-15, +15]. (The
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choice of range is implementation-defined.) The first
two bit-field declarations differ in that unsigned is
a type specifier (which forces t to be the name of a
structure member), while const is a type qualifier
(which modifies t which is still visible as a typedef
name). If these declarations are followed in an inner
scope by
t f(t (t));
long t;
then a function f is declared with type ``function
returning signed int with one unnamed parameter with
type pointer to function returning signed int with one
unnamed parameter with type signed int'', and an
identifier t with type long.
4. On the other hand, typedef names can be used to
improve code readability. All three of the following
declarations of the signal function specify exactly
the same type, the first without making use of any
typedef names.
typedef void fv(int), (*pfv)(int);
void (*signal(int, void (*)(int)))(int);
fv *signal(int, fv *);
pfv signal(int, pfv);
5. The following is a block scope declaration of a
typedef name A with a variable length array type.
void tdef(int n)
{
typedef int A[n];
A a;
A *p;
p = &a;
}
6. The size expression that is part of the variable
length array type named by typedef name B is evaluated
each time function copyt is entered. However, the
size of the variable length array type does not change
if the value of n is subsequently changed.
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void copyt(int n)
{
typedef int B[n]; // B is n ints, n evaluated now.
n += 1;
{
B a; // a is n ints, n without += 1.
int b[n]; // a and b are different sizes
for (i = 1; i < n; i++)
a[i-1] = b[i];
}
}
Forward references: the signal function (7.11.1.1).
6.5.8 Initialization
Syntax
[#1]
initializer:
assignment-expression
{ initializer-list }
{ initializer-list , }
initializer-list:
designation-opt initializer
initializer-list , designation-opt initializer
designation:
designator-list =
designator-list:
designator
designator-list designator
designator:
[ constant-expression ]
. identifier
Constraints
[#2] No initializer shall attempt to provide a value for an
object not contained within the entity being initialized.
[#3] The type of the entity to be initialized shall be an
array of unknown size or an object type that is not a
variable length array type.
[#4] All the expressions in an initializer for an object
that has static storage duration shall be constant
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expressions or string literals.
[#5] If the declaration of an identifier has block scope,
and the identifier has external or internal linkage, the
declaration shall have no initializer for the identifier.
[#6] If a designator has the form
[ constant-expression ]
then the current object (defined below) shall have array
type and the expression shall be an integer constant
expression. If the array is of unknown size, any
nonnegative value is valid.
[#7] If a designator has the form
. identifier
then the current object (defined below) shall have structure
or union type and the identifier shall be a member of that
type.
Semantics
[#8] An initializer specifies the initial value stored in an
object.
[#9] Except where explicitly stated otherwise, for the
purposes of this subclause unnamed members of objects of
structure and union type do not participate in
initialization. Unnamed members of structure objects have
indeterminate value even after initialization. A union
object containing only unnamed members has indeterminate
value even after initialization.
[#10] If an object that has automatic storage duration is
not initialized explicitly, its value is indeterminate. If
an object that has static storage duration is not
initialized explicitly, then:
- if it has pointer type, it is initialized to a null
pointer;
- if it has arithmetic type, it is initialized to zero;
- if it is an aggregate, every member is initialized
(recursively) according to these rules;
- if it is a union, the first named member is initialized
(recursively) according to these rules.
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[#11] The initializer for a scalar shall be a single
expression, optionally enclosed in braces. The initial
value of the object, including unnamed members, is that of
the expression; the same type constraints and conversions as
for simple assignment apply, taking the type of the scalar
to be the unqualified version of its declared type.
[#12] Each brace-enclosed initializer list has an associated
current object. When no designations are present,
subobjects of the current object are initialized in order
according to the type of the current object: array elements
in increasing subscript order, structure members in
declaration order, and the first named member of a
union.109 In contrast, a designation causes the following
initializer to begin initialization of the subobject
described by the designator. Initialization then continues
forward in order, beginning with the next subobject after
that described by the designator.110
[#13] Each designator list begins its description with the
current object associated with the closest surrounding brace
pair. Each item in the designator list (in order) specifies
a particular member of its current object and changes the
current object for the next designator (if any) to be that
member.111 The current object that results at the end of
the designator list is the subobject to be initialized by
the following initializer.
[#14] The initialization shall occur in initializer list
order, each initializer provided for a particular subobject
__________
109. If the initializer list for a subaggregate or
contained union does not begin with a left brace, its
subobjects are initialized as usual, but the
subaggregate or contained union does not become the
current object: current objects are associated only
with brace-enclosed initializer lists.
110. After a union member is initialized, the next object
is not the next member of the union; instead, it
is the next subobject of an object containing the
union.
111. Thus, a designator can only specify a strict subobject
of the aggregate or union that is associated with the
surrounding brace pair. Note, too, that each separate
designator list is independent.
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page 154
overriding any previously listed initializer for the same
subobject; all subobjects that are not initialized
explicitly shall be initialized implicitly the same as
objects that have static storage duration.
[#15] The initializer for a structure or union object that
has automatic storage duration either shall be an
initializer list as described below, or shall be a single
expression that has compatible structure or union type. In
the latter case, the initial value of the object is that of
the expression.
[#16] The rest of this subclause deals with initializers for
objects that have aggregate or union type.
[#17] An array of character type may be initialized by a
character string literal, optionally enclosed in braces.
Successive characters of the character string literal
(including the terminating null character if there is room
or if the array is of unknown size) initialize the elements
of the array.
[#18] An array with element type compatible with wchar_t may
be initialized by a wide string literal, optionally enclosed
in braces. Successive codes of the wide string literal
(including the terminating zero-valued code if there is room
or if the array is of unknown size) initialize the elements
of the array.
[#19] Otherwise, the initializer for an object that has
aggregate type shall be a brace-enclosed list of
initializers for the named members of the aggregate, written
in increasing subscript or member order; and the initializer
for an object that has union type shall be a brace-enclosed
initializer for the first named member of the union.
[#20] If the aggregate contains members that are aggregates
or unions, or if the first member of a union is an aggregate
or union, the rules apply recursively to the subaggregates
or contained unions. If the initializer of a subaggregate
or contained union begins with a left brace, the
initializers enclosed by that brace and its matching right
brace initialize the members of the subaggregate or the
first member of the contained union. Otherwise, only enough
initializers from the list are taken to account for the
members of the subaggregate or the first member of the
contained union; any remaining initializers are left to
initialize the next member of the aggregate of which the
current subaggregate or contained union is a part.
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[#21] If there are fewer initializers in a brace-enclosed
list than there are members of an aggregate, or fewer
characters in a string literal used to initialize an array
of known size than there are elements in the array, the
remainder of the aggregate shall be initialized implicitly
the same as objects that have static storage duration.
[#22] If an array of unknown size is initialized, its size
is determined by the largest indexed element with an
explicit initializer. At the end of its initializer list,
the array no longer has incomplete type.
[#23] The order in which any side effects occur among the
initialization list expressions is unspecified.112
Examples
[#24]
1. The declaration
int x[] = { 1, 3, 5 };
defines and initializes x as a one-dimensional array
object that has three elements, as no size was
specified and there are three initializers.
2. The declaration
int y[4][3] = {
{ 1, 3, 5 },
{ 2, 4, 6 },
{ 3, 5, 7 },
};
is a definition with a fully bracketed initialization:
1, 3, and 5 initialize the first row of y (the array
object y[0]), namely y[0][0], y[0][1], and y[0][2].
Likewise the next two lines initialize y[1] and y[2].
The initializer ends early, so y[3] is initialized
with zeros. Precisely the same effect could have been
achieved by
__________
112. In particular, the evaluation order need not be the
same as the order of subobject initialization.
page 155 Language
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int y[4][3] = {
1, 3, 5, 2, 4, 6, 3, 5, 7
};
The initializer for y[0] does not begin with a left
brace, so three items from the list are used.
Likewise the next three are taken successively for
y[1] and y[2].
3. The declaration
int z[4][3] = {
{ 1 }, { 2 }, { 3 }, { 4 }
};
initializes the first column of z as specified and
initializes the rest with zeros.
4. The declaration
struct { int a[3], b; } w[] = { { 1 }, 2 };
is a definition with an inconsistently bracketed
initialization. It defines an array with two element
structures: w[0].a[0] is 1 and w[1].a[0] is 2; all
the other elements are zero.
5. The declaration
short q[4][3][2] = {
{ 1 },
{ 2, 3 },
{ 4, 5, 6 }
};
contains an incompletely but consistently bracketed
initialization. It defines a three-dimensional array
object: q[0][0][0] is 1, q[1][0][0] is 2, q[1][0][1]
is 3, and 4, 5, and 6 initialize q[2][0][0],
q[2][0][1], and q[2][1][0], respectively; all the rest
are zero. The initializer for q[0][0] does not begin
with a left brace, so up to six items from the current
list may be used. There is only one, so the values
for the remaining five elements are initialized with
zero. Likewise, the initializers for q[1][0] and
q[2][0] do not begin with a left brace, so each uses
up to six items, initializing their respective two-
dimensional subaggregates. If there had been more
than six items in any of the lists, a diagnostic
message would have been issued. The same
initialization result could have been achieved by:
page 156 Language
page 157
short q[4][3][2] = {
1, 0, 0, 0, 0, 0,
2, 3, 0, 0, 0, 0,
4, 5, 6
};
or by:
short q[4][3][2] = {
{
{ 1 },
},
{
{ 2, 3 },
},
{
{ 4, 5 },
{ 6 },
}
};
in a fully bracketed form.
Note that the fully bracketed and minimally bracketed
forms of initialization are, in general, less likely
to cause confusion.
6. One form of initialization that completes array types
involves typedef names. Given the declaration
typedef int A[]; // OK - declared with block scope
the declaration
A a = { 1, 2 }, b = { 3, 4, 5 };
is identical to
int a[] = { 1, 2 }, b[] = { 3, 4, 5 };
due to the rules for incomplete types.
7. The declaration
char s[] = "abc", t[3] = "abc";
defines ``plain'' char array objects s and t whose
elements are initialized with character string
literals. This declaration is identical to
page 157 Language
page 158
char s[] = { 'a', 'b', 'c', '\0' },
t[] = { 'a', 'b', 'c' };
The contents of t>
<HR><H3>Transfer interrupted!</H3>
other hand, the declaration
char *p = "abc";
defines p with type ``pointer to char'' that is
initialized to point to an object with type ``array of
char'' with length 4 whose elements are initialized
with a character string literal. If an attempt is
made to use p to modify the contents of the array, the
behavior is undefined.
8. Arrays can be initialized to correspond to the
elements of an enumeration by using designators:
enum { member_one, member_two };
const char *nm[] = {
[member_two] = "member two",
[member_one] = "member one",
};
9. Structure members can be initialized to nonzero values
without depending on their order:
div_t answer = { .quot = 2, .rem = -1 };
10. Designators can be used to provide explicit
initialization when unadorned initializer lists might
be misunderstood:
struct { int a[3], b; } w[] =
{ [0].a = {1}, [1].a[0] = 2 };
11. Space can be ``allocated'' from both ends of an array
by using a single designator:
int a[MAX] = {
1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
};
In the above, if MAX is greater than ten, there will
be some zero-valued elements in the middle; if it is
less than ten, some of the values provided by the
first five initializers will be overridden by the
second five.
12. Any member of a union can be initialized:
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union { /* ... */ } u = { .any_member = 42 };
Forward references: common definitions <stddef.h> (7.1.6).
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6.6 Statements
Syntax
[#1]
statement:
labeled-statement
compound-statement
expression-statement
selection-statement
iteration-statement
jump-statement
Semantics
[#2] A statement specifies an action to be performed.
Except as indicated, statements are executed in sequence.
[#3] A full expression is an expression that is not part of
another expression. Each of the following is a full
expression: an initializer; the expression in an expression
statement; the controlling expression of a selection
statement (if or switch); the controlling expression of a
while or do statement; each of the (optional) expressions of
a for statement; the (optional) expression in a return
statement. The end of a full expression is a sequence
point.
Forward references: expression and null statements (6.6.3),
selection statements (6.6.4), iteration statements (6.6.5),
the return statement (6.6.6.4).
6.6.1 Labeled statements
Syntax
[#1]
labeled-statement:
identifier : statement
case constant-expr : statement
default : statement
Constraints
[#2] A case or default label shall appear only in a switch
statement. Further constraints on such labels are discussed
under the switch statement.
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page 161
Semantics
[#3] Any statement may be preceded by a prefix that declares
an identifier as a label name. Labels in themselves do not
alter the flow of control, which continues unimpeded across
them.
Forward references: the goto statement (6.6.6.1), the
switch statement (6.6.4.2).
6.6.2 Compound statement, or block
Syntax
[#1]
compound-statement:
{ block-item-list-opt }
block-item-list:
block-item
block-item-list block-item
block-item:
declaration
statement
Semantics
[#2] A compound statement (also called a block) allows a set
of statements to be grouped into one syntactic unit, which
may have its own set of declarations and initializations (as
discussed in 6.1.2.4). The initializers of objects that
have automatic storage duration, and the variable length
array declarators of ordinary identifiers with block scope
are evaluated and the values are stored in the objects
(including storing an indeterminate value in objects without
an initializer) each time that the declaration is reached in
the order of execution, as if it were a statement, and
within each declaration in the order that declarators
appear.
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6.6.3 Expression and null statements
Syntax
[#1]
expression-statement:
expression-opt ;
Semantics
[#2] The expression in an expression statement is evaluated
as a void expression for its side effects.113
[#3] A null statement (consisting of just a semicolon)
performs no operations.
Examples
[#4]
1. If a function call is evaluated as an expression
statement for its side effects only, the discarding of
its value may be made explicit by converting the
expression to a void expression by means of a cast:
int p(int);
/* ... */
(void)p(0);
2. In the program fragment
char *s;
/* ... */
while (*s++ != '\0')
;
a null statement is used to supply an empty loop body
to the iteration statement.
3. A null statement may also be used to carry a label
just before the closing } of a compound statement.
__________
113. Such as assignments, and function calls which have side
effects.
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page 163
while (loop1) {
/* ... */
while (loop2) {
/* ... */
if (want_out)
goto end_loop1;
/* ... */
}
/* ... */
end_loop1: ;
}
Forward references: iteration statements (6.6.5).
6.6.4 Selection statements
Syntax
[#1]
selection-statement:
if ( expression ) statement
if ( expression ) statement else statement
switch ( expression ) statement
Semantics
[#2] A selection statement selects among a set of statements
depending on the value of a controlling expression.
6.6.4.1 The if statement
Constraints
[#1] The controlling expression of an if statement shall
have scalar type.
Semantics
[#2] In both forms, the first substatement is executed if
the expression compares unequal to 0. In the else form, the
second substatement is executed if the expression compares
equal to 0. If the first substatement is reached via a
label, the second substatement is not executed.
[#3] An else is associated with the lexically nearest
preceeding if that is allowed by the grammar.
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6.6.4.2 The switch statement
Constraints
[#1] The controlling expression of a switch statement shall
have integer type, and shall not cause a block to be entered
by a jump from outside the block to a statement that follows
a case or default label in the block (or an enclosed block)
if that block contains the declaration of a variably
modified object or variably modified typedef name. The
expression of each case label shall be an integer constant
expression. No two of the case constant expressions in the
same switch statement shall have the same value after
conversion. There may be at most one default label in a
switch statement. (Any enclosed switch statement may have a
default label or case constant expressions with values that
duplicate case constant expressions in the enclosing switch
statement.)
Semantics
[#2] A switch statement causes control to jump to, into, or
past the statement that is the switch body, depending on the
value of a controlling expression, and on the presence of a
default label and the values of any case labels on or in the
switch body. A case or default label is accessible only
within the closest enclosing switch statement.
[#3] The integer promotions are performed on the controlling
expression. The constant expression in each case label is
converted to the promoted type of the controlling
expression. If a converted value matches that of the
promoted controlling expression, control jumps to the
statement following the matched case label. Otherwise, if
there is a default label, control jumps to the labeled
statement. If no converted case constant expression matches
and there is no default label, no part of the switch body is
executed.
Implementation limits
[#4] As discussed previously (5.2.4.1), the implementation
may limit the number of case values in a switch statement.
Examples
[#5] In the artificial program fragment
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page 165
switch (expr)
{
int i = 4;
f(i);
case 0:
i = 17;
/* falls through into default code */
default:
printf("%d\n", i);
}
the object whose identifier is i exists with automatic
storage duration (within the block) but is never
initialized, and thus if the controlling expression has a
nonzero value, the call to the printf function will access
an indeterminate value. Similarly, the call to the function
f cannot be reached.
6.6.5 Iteration statements
Syntax
[#1]
iteration-statement:
while ( expression ) statement
do statement while ( expression ) ;
for ( expr-opt ; expr-opt ; expr-opt ) statement
for ( declaration ; expr-opt ; expr-opt ) statement
Constraints
[#2] The controlling expression of an iteration statement
shall have scalar type.
[#3] The declaration part of a for statement shall only
declare identifiers for objects having storage class auto or
register.
Semantics
[#4] An iteration statement causes a statement called the
loop body to be executed repeatedly until the controlling
expression compares equal to 0.
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6.6.5.1 The while statement
[#1] The evaluation of the controlling expression takes
place before each execution of the loop body.
6.6.5.2 The do statement
[#1] The evaluation of the controlling expression takes
place after each execution of the loop body.
6.6.5.3 The for statement
[#1] Except for the behavior of a continue statement in the
loop body, the statement
for ( clause-1 ; expr-2 ; expr-3 ) statement
and the sequence of statements
{
clause-1 ;
while ( expr-2 ) {
statement
expr-3 ;
}
}
are equivalent (where clause-1 can be an expression or a
declaration).114
[#2] Both clause-1 and expr-3 can be omitted. If either or
both are an expression, they are evaluated as a void
expression. An omitted expr-2 is replaced by a nonzero
constant.
__________
114. Thus, clause-1 specifies initialization for the loop,
possibly declaring one or more variables for use in the
loop; expr-2, the controlling expression, specifies an
evaluation made before each iteration, such that
execution of the loop continues until the expression
compares equal to 0; expr-3 specifies an operation (such
as incrementing) that is performed after each iteration.
If clause-1 is a declaration, then the scope of any
variable it declares is the remainder of the declaration
and the entire loop, including the other two
expressions.
page 166 Language
page 167
Forward references: the continue statement (6.6.6.2).
6.6.6 Jump statements
Syntax
[#1]
jump-statement:
goto identifier ;
continue ;
break ;
return expression-opt ;
Semantics
[#2] A jump statement causes an unconditional jump to
another place.
6.6.6.1 The goto statement
Constraints
[#1] The identifier in a goto statement shall name a label
located somewhere in the enclosing function. A goto
statement shall not cause a block to be entered by a jump
from outside the block to a labeled statement in the block
(or an enclosed block) if that block contains the
declaration of a variably modified object or variably
modified typedef name.
Semantics
[#2] A goto statement causes an unconditional jump to the
statement prefixed by the named label in the enclosing
function.
Examples
[#3]
1. It is sometimes convenient to jump into the middle of
a complicated set of statements. The following
outline presents one possible approach to a problem
based on these three assumptions:
1. The general initialization code accesses objects
only visible to the current function.
2. The general initialization code is too large to
warrant duplication.
page 167 Language
page 168
3. The code to determine the next operation must be
at the head of the loop. (To allow it to be
reached by continue statements, for example.)
/* ... */
goto first_time;
for (;;) {
// determine next operation
/* ... */
if (need to reinitialize) {
// reinitialize-only code
/* ... */
first_time:
// general initialization code
/* ... */
continue;
}
// handle other operations
/* ... */
}
2. A goto statement is not allowed to jump past any
declarations of objects with variably modified types.
A jump within the block, however, is permitted.
goto lab3; // Error: going INTO scope of VLA.
{
double a[n];
a[j] = 4.4;
lab3:
a[j] = 3.3;
goto lab 4; // OK, going WITHIN scope of VLA.
a[j] = 5.5;
lab4:
a[j] = 6.6;
}
goto lab4; // Error: going INTO scope of VLA.
6.6.6.2 The continue statement
Constraints
[#1] A continue statement shall appear only in or as a loop
body.
Semantics
[#2] A continue statement causes a jump to the loop-
continuation portion of the smallest enclosing iteration
statement; that is, to the end of the loop body. More
precisely, in each of the statements
page 168 Language
page 169
while (/* ... */) { do { for (/* ... */) {
/* ... */ /* ... */ /* ... */
continue; continue; continue;
/* ... */ /* ... */ /* ... */
contin: ; contin: ; contin: ;
} } while (/* ... */); }
unless the continue statement shown is in an enclosed
iteration statement (in which case it is interpreted within
that statement), it is equivalent to goto contin;.115
6.6.6.3 The break statement
Constraints
[#1] A break statement shall appear only in or as a switch
body or loop body.
Semantics
[#2] A break statement terminates execution of the smallest
enclosing switch or iteration statement.
6.6.6.4 The return statement
Constraints
[#1] A return statement with an expression shall not appear
in a function whose return type is void. A return statement
without an expression shall only appear in a function whose
return type is void.
Semantics
[#2] A return statement terminates execution of the current
function and returns control to its caller. A function may
have any number of return statements.
[#3] If a return statement with an expression is executed,
the value of the expression is returned to the caller as the
value of the function call expression. If the expression
has a type different from the return type of the function in
which it appears, the value is converted as if by assignment
to an object having the return type of the function.116
__________
115. Following the contin: label is a null statement.
116. The return statement is not an assignment. The overlap
page 169 Language
page 170
[#4] If a return statement without an expression is
executed, and the value of the function call is used by the
caller, the behavior is undefined.
Examples
[#5] In:
struct s { double i; } f(void);
union {
struct {
int f1;
struct s f2;
} u1;
struct {
struct s f3;
int f4;
} u2;
} g;
struct s f(void)
{
return g.u1.f2;
}
/* ... */
g.u2.f3 = f();
there is no undefined behavior.
____________________________________________________________
restriction of subclause 6.3.16.1 does not apply to the
case of function return.
page 170 Language
page 171
6.7 External definitions
Syntax
[#1]
translation-unit:
external-declaration
translation-unit external-declaration
external-declaration:
function-definition
declaration
Constraints
[#2] The storage-class specifiers auto and register shall
not appear in the declaration specifiers in an external
declaration.
[#3] There shall be no more than one external definition for
each identifier declared with internal linkage in a
translation unit. Moreover, if an identifier declared with
internal linkage is used in an expression (other than as a
part of the operand of a sizeof operator), there shall be
exactly one external definition for the identifier in the
translation unit.
Semantics
[#4] As discussed in 5.1.1.1, the unit of program text after
preprocessing is a translation unit, which consists of a
sequence of external declarations. These are described as
``external'' because they appear outside any function (and
hence have file scope). As discussed in 6.5, a declaration
that also causes storage to be reserved for an object or a
function named by the identifier is a definition.
[#5] An external definition is an external declaration that
is also a definition of a function or an object. If an
identifier declared with external linkage is used in an
expression (other than as part of the operand of a sizeof
operator), somewhere in the entire program there shall be
exactly one external definition for the identifier;
otherwise, there shall be no more than one.117
__________
117. Thus, if an identifier declared with external linkage
is not used in an expression, there need be no external
page 171 Language
page 172
6.7.1 Function definitions
Syntax
[#1]
function-definition:
declaration-specifiers declarator declaration-list-opt compound-statement
Constraints
[#2] The identifier declared in a function definition (which
is the name of the function) shall have a function type, as
specified by the declarator portion of the function
definition.118
[#3] The return type of a function shall be void or an
object type other than array type.
[#4] The storage-class specifier, if any, in the declaration
specifiers shall be either extern or static.
[#5] If the declarator includes a parameter type list, the
declaration of each parameter shall include an identifier
(except for the special case of a parameter list consisting
of a single parameter of type void, in which there shall not
be an identifier). No declaration list shall follow.
[#6] If the declarator includes an identifier list, each
declaration in the declaration list shall have at least one
declarator, those declarators shall declare only identifiers
____________________________________________________________
definition for it.
118. The intent is that the type category in a function
definition cannot be inherited from a typedef:
typedef int F(void); /* type F is ``function of no arguments returning int'' */
F f, g; /* f and g both have type compatible with F */
F f { /* ... */ } /* WRONG: syntax/constraint error */
F g() { /* ... */ } /* WRONG: declares that g returns a function */
int f(void) { /* ... */ } /* RIGHT: f has type compatible with F */
int g() { /* ... */ } /* RIGHT: g has type compatible with F */
F *e(void) { /* ... */ } /* e returns a pointer to a function */
F *((e))(void) { /* ... */ } /* same: parentheses irrelevant */
int (*fp)(void); /* fp points to a function that has type F */
F *Fp; /* Fp points to a function that has type F */
page 172 Language
page 173
from the identifier list, and every identifier in the
identifier list shall be declared. An identifier declared
as a typedef name shall not be redeclared as a parameter.
The declarations in the declaration list shall contain no
storage-class specifier other than register and no
initializations.
Semantics
[#7] The declarator in a function definition specifies the
name of the function being defined and the identifiers of
its parameters. If the declarator includes a parameter type
list, the list also specifies the types of all the
parameters; such a declarator also serves as a function
prototype for later calls to the same function in the same
translation unit. If the declarator includes an identifier
list,119 the types of the parameters shall be declared in a
following declaration list.
[#8] If a function that accepts a variable number of
arguments is defined without a parameter type list that ends
with the ellipsis notation, the behavior is undefined.
[#9] Each parameter has automatic storage duration. Its
identifier is an lvalue.120 The layout of the storage for
parameters is unspecified.
[#10] On entry to the function all size expressions of its
variably modified parameters are evaluated, and the value of
each argument expression shall be converted to the type of
its corresponding parameter, as if by assignment to the
parameter. Array expressions and function designators as
arguments are converted to pointers before the call. A
declaration of a parameter as ``array of type'' shall be
adjusted to ``pointer to type,'' and a declaration of a
parameter as ``function returning type'' shall be adjusted
to ``pointer to function returning type,'' as in 6.2.2.1.
The resulting parameter type shall be an object type.
__________
119. See ``future language directions'' (6.9.4).
120. A parameter is in effect declared at the head of the
compound statement that constitutes the function body,
and therefore may not be redeclared in the function body
(except in an enclosed block).
page 173 Language
page 174
[#11] After all parameters have been assigned, the compound
statement that constitutes the body of the function
definition is executed.
[#12] If the } that terminates a function is reached, and
the value of the function call is used by the caller, the
behavior is undefined.
Examples
[#13]
1. In the following:
extern int max(int a, int b)
{
return a > b ? a : b;
}
extern is the storage-class specifier and int is the
type specifier; max(int a, int b) is the function
declarator; and
{ return a > b ? a : b; }
is the function body. The following similar
definition uses the identifier-list form for the
parameter declarations:
extern int max(a, b)
int a, b;
{
return a > b ? a : b;
}
Here int a, b; is the declaration list for the
parameters. The difference between these two
definitions is that the first form acts as a prototype
declaration that forces conversion of the arguments of
subsequent calls to the function, whereas the second
form may not.
2. To pass one function to another, one might say
int f(void);
/* ... */
g(f);
Then the definition of g might read
page 174 Language
page 175
void g(int (*funcp)(void))
{
/* ... */ (*funcp)() /* or funcp() ... */
}
or, equivalently,
void g(int func(void))
{
/* ... */ func() /* or (*func)() ... */
}
6.7.2 External object definitions
Semantics
[#1] If the declaration of an identifier for an object has
file scope and an initializer, the declaration is an
external definition for the identifier.
[#2] A declaration of an identifier for an object that has
file scope without an initializer, and without a storage-
class specifier or with the storage-class specifier static,
constitutes a tentative definition. If a translation unit
contains one or more tentative definitions for an
identifier, and the translation unit contains no external
definition for that identifier, then the behavior is exactly
as if the translation unit contains a file scope declaration
of that identifier, with the composite type as of the end of
the translation unit, with an initializer equal to 0.
[#3] If the declaration of an identifier for an object is a
tentative definition and has internal linkage, the declared
type shall not be an incomplete type.
Examples
[#4]
1.
int i1 = 1; // definition, external linkage
static int i2 = 2; // definition, internal linkage
extern int i3 = 3; // definition, external linkage
int i4; // tentative definition, external linkage
static int i5; // tentative definition, internal linkage
page 175 Language
page 176
int i1; // valid tentative definition, refers to previous
int i2; // 6.1.2.2 renders undefined, linkage disagreement
int i3; // valid tentative definition, refers to previous
int i4; // valid tentative definition, refers to previous
int i5; // 6.1.2.2 renders undefined, linkage disagreement
extern int i1; // refers to previous, whose linkage is external
extern int i2; // refers to previous, whose linkage is internal
extern int i3; // refers to previous, whose linkage is external
extern int i4; // refers to previous, whose linkage is external
extern int i5; // refers to previous, whose linkage is internal
2. If at the end of the translation unit containing
int i[];
the array i still has incomplete type, the array is
assumed to have one element. This element is
initialized to zero on program startup.
page 176 Language
page 177
6.8 Preprocessing directives
Syntax
[#1]
preprocessing-file:
group-opt
group:
group-part
group group-part
group-part:
pp-tokens-opt new-line
if-section
control-line
if-section:
if-group elif-groups-opt else-group-opt endif-line
if-group:
# if constant-expr new-line group-opt
# ifdef identifier new-line group-opt
# ifndef identifier new-line group-opt
elif-groups:
elif-group
elif-groups elif-group
elif-group:
# elif constant-expr new-line group-opt
else-group:
# else new-line group-opt
endif-line:
# endif new-line
page 177 Language
page 178
control-line:
# include pp-tokens new-line
# define identifier replacement-list new-line
# define identifier lparen identifier-list-opt )
replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... )
replacement-list new-line
# undef identifier new-line
# line pp-tokens new-line
# error pp-tokens-opt new-line
# pragma pp-tokens-opt new-line
# new-line
lparen:
the left-parenthesis character without preceding white-space
replacement-list:
pp-tokens-opt
pp-tokens:
preprocessing-token
pp-tokens preprocessing-token
new-line:
the new-line character
Description
[#2] A preprocessing directive consists of a sequence of
preprocessing tokens that begins with a # preprocessing
token that (at the start of translation phase 4) is either
the first character in the source file (optionally after
white space containing no new-line characters) or that
follows white space containing at least one new-line
character, and is ended by the next new-line character.121 A
new-line character ends the preprocessing directive even if
it occurs within what would otherwise be an invocation of a
function-like macro.
__________
121. Thus, preprocessing directives are commonly called
``lines.'' These ``lines'' have no other syntactic
significance, as all white space is equivalent except in
certain situations during preprocessing (see the #
character string literal creation operator in 6.8.3.2,
for example).
page 178 Language
page 179
Constraints
[#3] The only white-space characters that shall appear
between preprocessing tokens within a preprocessing
directive (from just after the introducing # preprocessing
token through just before the terminating new-line
character) are space and horizontal-tab (including spaces
that have replaced comments or possibly other white-space
characters in translation phase 3).
[#4] In the definition of an object-like macro, if the first
character of a replacement list is not a character required
by subclause 5.2.1, then there shall be white-space
separation between the identifier and the replacement
list.122
Semantics
[#5] The implementation can process and skip sections of
source files conditionally, include other source files, and
replace macros. These capabilities are called
preprocessing, because conceptually they occur before
translation of the resulting translation unit.
[#6] The preprocessing tokens within a preprocessing
directive are not subject to macro expansion unless
otherwise stated.
Examples
[#7] In:
#define EMPTY
EMPTY # include <file.h>
the sequence of preprocessing tokens on the second line is
not a preprocessing directive, because it does not begin
with a # at the start of translation phase 4, even though it
will do so after the macro EMPTY has been replaced.
__________
122. This allows an implementation to choose to interpret
the directive:
#define THIS$AND$THAT(a, b) ((a) + (b))
as defining a function-like macro THIS$AND$THAT, rather
than an object-like macro THIS. Whichever choice it
makes, it must also issue a diagnostic.
page 179 Language
page 180
6.8.1 Conditional inclusion
Constraints
[#1] The expression that controls conditional inclusion
shall be an integer constant expression except that: it
shall not contain a cast; identifiers (including those
lexically identical to keywords) are interpreted as
described below;123 and it may contain unary operator
expressions of the form
defined identifier
or
defined ( identifier )
which evaluate to 1 if the identifier is currently defined
as a macro name (that is, if it is predefined or if it has
been the subject of a #define preprocessing directive
without an intervening #undef directive with the same
subject identifier), 0 if it is not.
Semantics
[#2] Preprocessing directives of the forms
# if constant-expr new-line group-opt
# elif constant-expr new-line group-opt
check whether the controlling constant expression evaluates
to nonzero.
[#3] Prior to evaluation, macro invocations in the list of
preprocessing tokens that will become the controlling
constant expression are replaced (except for those macro
names modified by the defined unary operator), just as in
normal text. If the token defined is generated as a result
of this replacement process or use of the defined unary
operator does not match one of the two specified forms prior
to macro replacement, the behavior is undefined. After all
replacements due to macro expansion and the defined unary
operator have been performed, all remaining identifiers are
replaced with the pp-number 0, and then each preprocessing
__________
123. Because the controlling constant expression is
evaluated during translation phase 4, all identifiers
either are or are not macro names - there simply are no
keywords, enumeration constants, etc.
page 180 Language
page 181
token is converted into a token. The resulting tokens
compose the controlling constant expression which is
evaluated according to the rules of 6.4, except that all
signed integer types and all unsigned integer types act as
if they have the same representation as, respectively, the
types intmax_t and uintmax_t defined in the header
<inttypes.h>. This includes interpreting character
constants, which may involve converting escape sequences
into execution character set members. Whether the numeric
value for these character constants matches the value
obtained when an identical character constant occurs in an
expression (other than within a #if or #elif directive) is
implementation-defined.124 Also, whether a single-character
character constant may have a negative value is
implementation-defined.
[#4] Preprocessing directives of the forms
# ifdef identifier new-line group-opt
# ifndef identifier new-line group-opt
check whether the identifier is or is not currently defined
as a macro name. Their conditions are equivalent to #if
defined identifier and #if !defined identifier respectively.
[#5] Each directive's condition is checked in order. If it
evaluates to false (zero), the group that it controls is
skipped: directives are processed only through the name
that determines the directive in order to keep track of the
level of nested conditionals; the rest of the directives'
preprocessing tokens are ignored, as are the other
preprocessing tokens in the group. Only the first group
whose control condition evaluates to true (nonzero) is
processed. If none of the conditions evaluates to true, and
there is a #else directive, the group controlled by the
#else is processed; lacking a #else directive, all the
groups until the #endif are skipped.125
__________
124. Thus, the constant expression in the following #if
directive and if statement is not guaranteed to evaluate
to the same value in these two contexts.
#if 'z' - 'a' == 25
if ('z' - 'a' == 25)
125. As indicated by the syntax, a preprocessing token shall
not follow a #else or #endif directive before the
page 181 Language
page 182
Forward references: macro replacement (6.8.3), source file
inclusion (6.8.2), largest integer types (7.4.1.5).
6.8.2 Source file inclusion
Constraints
[#1] A #include directive shall identify a header or source
file that can be processed by the implementation.
Semantics
[#2] A preprocessing directive of the form
# include <h-char-sequence> new-line
searches a sequence of implementation-defined places for a
header identified uniquely by the specified sequence between
the < and > delimiters, and causes the replacement of that
directive by the entire contents of the header. How the
places are specified or the header identified is
implementation-defined.
[#3] A preprocessing directive of the form
# include "q-char-sequence" new-line
causes the replacement of that directive by the entire
contents of the source file identified by the specified
sequence between the " delimiters. The named source file is
searched for in an implementation-defined manner. If this
search is not supported, or if the search fails, the
directive is reprocessed as if it read
# include <h-char-sequence> new-line
with the identical contained sequence (including >
characters, if any) from the original directive.
[#4] A preprocessing directive of the form
# include pp-tokens new-line
____________________________________________________________
terminating new-line character. However, comments may
appear anywhere in a source file, including within a
preprocessing directive.
page 182 Language
page 183
(that does not match one of the two previous forms) is
permitted. The preprocessing tokens after include in the
directive are processed just as in normal text. (Each
identifier currently defined as a macro name is replaced by
its replacement list of preprocessing tokens.) The
directive resulting after all replacements shall match one
of the two previous forms.126 The method by which a
sequence of preprocessing tokens between a < and a >
preprocessing token pair or a pair of " characters is
combined into a single header name preprocessing token is
implementation-defined.
[#5] The implementation shall provide unique mappings for
sequences consisting of one or more letters or digits (as
defined in 5.2.1) followed by a period (.) and a single
letter. The first character shall be a letter. The
implementation may ignore the distinctions of alphabetical
case and restrict the mapping to eight significant
characters before the period.
[#6] A #include preprocessing directive may appear in a
source file that has been read because of a #include
directive in another file, up to an implementation-defined
nesting limit (see 5.2.4.1).
Examples
[#7]
1. The most common uses of #include preprocessing
directives are as in the following:
#include <stdio.h>
#include "myprog.h"
2. This illustrates macro-replaced #include directives:
__________
126. Note that adjacent string literals are not concatenated
into a single string literal (see the translation phases
in 5.1.1.2); thus, an expansion that results in two
string literals is an invalid directive.
page 183 Language
page 184
#if VERSION == 1
#define INCFILE "vers1.h"
#elif VERSION == 2
#define INCFILE "vers2.h" // and so on
#else
#define INCFILE "versN.h"
#endif
#include INCFILE
Forward references: macro replacement (6.8.3).
6.8.3 Macro replacement
Constraints
[#1] Two replacement lists are identical if and only if the
preprocessing tokens in both have the same number, ordering,
spelling, and white-space separation, where all white-space
separations are considered identical.
[#2] An identifier currently defined as a macro without use
of lparen (an object-like macro) shall not be redefined by
another #define preprocessing directive unless the second
definition is an object-like macro definition and the two
replacement lists are identical.
[#3] An identifier currently defined as a macro using lparen
(a function-like macro) shall not be redefined by another
#define preprocessing directive unless the second definition
is a function-like macro definition that has the same number
and spelling of parameters, and the two replacement lists
are identical.
[#4] If the identifier-list in the macro definition does not
end with an ellipsis, the number of arguments, including
those arguments consisting of no preprocessing tokens, in an
invocation of a function-like macro shall agree with the
number of parameters in the macro definition. Otherwise,
there shall be more arguments in the invocation than there
are parameters in the macro definition (excluding the ...).
There shall exist a ) preprocessing token that terminates
the invocation.
[#5] The identifier __VA_ARGS__ shall only occur in the
replacement-list of a #define preprocessing directive using
the ellipsis notation in the arguments.
[#6] A parameter identifier in a function-like macro shall
be uniquely declared within its scope.
page 184 Language
page 185
Semantics
[#7] The identifier immediately following the define is
called the macro name. There is one name space for macro
names. Any white-space characters preceding or following
the replacement list of preprocessing tokens are not
considered part of the replacement list for either form of
macro.
[#8] If a # preprocessing token, followed by an identifier,
occurs lexically at the point at which a preprocessing
directive could begin, the identifier is not subject to
macro replacement.
[#9] A preprocessing directive of the form
# define identifier replacement-list new-line
defines an object-like macro that causes each subsequent
instance of the macro name127 to be replaced by the
replacement list of preprocessing tokens that constitute the
remainder of the directive. The replacement list is then
rescanned for more macro names as specified below.
[#10] A preprocessing directive of the form
# define identifier lparen identifier-list-opt ) replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... ) replacement-list new-line
defines a function-like macro with arguments, similar
syntactically to a function call. The parameters are
specified by the optional list of identifiers, whose scope
extends from their declaration in the identifier list until
the new-line character that terminates the #define
preprocessing directive. Each subsequent instance of the
function-like macro name followed by a ( as the next
preprocessing token introduces the sequence of preprocessing
tokens that is replaced by the replacement list in the
definition (an invocation of the macro). The replaced
sequence of preprocessing tokens is terminated by the
__________
127. Since, by macro-replacement time, all character
constants and string literals are preprocessing tokens,
not sequences possibly containing identifier-like
subsequences (see 5.1.1.2, translation phases), they are
never scanned for macro names or parameters.
page 185 Language
page 186
matching ) preprocessing token, skipping intervening matched
pairs of left and right parenthesis preprocessing tokens.
Within the sequence of preprocessing tokens making up an
invocation of a function-like macro, new-line is considered
a normal white-space character.
[#11] The sequence of preprocessing tokens bounded by the
outside-most matching parentheses forms the list of
arguments for the function-like macro. The individual
arguments within the list are separated by comma
preprocessing tokens, but comma preprocessing tokens between
matching inner parentheses do not separate arguments. If
there are sequences of preprocessing tokens within the list
of arguments that would otherwise act as preprocessing
directives, the behavior is undefined.
[#12] If there is a ... in the identifier-list in the macro
definition, then the trailing arguments, including any
separating comma preprocessing tokens, are merged to form a
single item: the variable arguments. The number of
arguments so combined is such that, following merger, the
number of arguments is one more than the number of
parameters in the macro definition (excluding the ...).
6.8.3.1 Argument substitution
[#1] After the arguments for the invocation of a function-
like macro have been identified, argument substitution takes
place. A parameter in the replacement list, unless preceded
by a # or ## preprocessing token or followed by a ##
preprocessing token (see below), is replaced by the
corresponding argument after all macros contained therein
have been expanded. Before being substituted, each
argument's preprocessing tokens are completely macro
replaced as if they formed the rest of the preprocessing
file; no other preprocessing tokens are available.
[#2] An identifier __VA_ARGS__ that occurs in the
replacement list shall be treated as if it were a parameter,
and the variable arguments shall form the preprocessing
tokens used to replace it.
page 186 Language
page 187
6.8.3.2 The # operator
Constraints
[#1] Each # preprocessing token in the replacement list for
a function-like macro shall be followed by a parameter as
the next preprocessing token in the replacement list.
Semantics
[#2] If, in the replacement list, a parameter is immediately
preceded by a # preprocessing token, both are replaced by a
single character string literal preprocessing token that
contains the spelling of the preprocessing token sequence
for the corresponding argument. Each occurrence of white
space between the argument's preprocessing tokens becomes a
single space character in the character string literal.
White space before the first preprocessing token and after
the last preprocessing token comprising the argument is
deleted. Otherwise, the original spelling of each
preprocessing token in the argument is retained in the
character string literal, except for special handling for
producing the spelling of string literals and character
constants: a \ character is inserted before each " and \
character of a character constant or string literal
(including the delimiting " characters). If the replacement
that results is not a valid character string literal, the
behavior is undefined. The character string literal
corresponding to an empty argument is "". The order of
evaluation of # and ## operators is unspecified.
6.8.3.3 The ## operator
Constraints
[#1] A ## preprocessing token shall not occur at the
beginning or at the end of a replacement list for either
form of macro definition.
Semantics
[#2] If, in the replacement list, a parameter is immediately
preceded or followed by a ## preprocessing token, the
parameter is replaced by the corresponding argument's
preprocessing token sequence; however, if an argument
consists of no preprocessing tokens, the parameter is
replaced by a placemarker preprocessing token instead.
[#3] For both object-like and function-like macro
invocations, before the replacement list is reexamined for
more macro names to replace, each instance of a ##
page 187 Language
page 188
preprocessing token in the replacement list (not from an
argument) is deleted and the preceding preprocessing token
is concatenated with the following preprocessing token
(placemarker preprocessing tokens are handled specially:
concatenation of two placemarkers results in a single
placemarker preprocessing token; concatenation of a
placemarker with a non - placemarker preprocessing token
results in the non-placemarker preprocessing token). If the
result is not a valid preprocessing token, the behavior is
undefined. The resulting token is available for further
macro replacement. The order of evaluation of ## operators
is unspecified.
Examples
[#4]
#define hash_hash # ## #
#define mkstr(a) # a
#define in_between(a) mkstr(a)
#define join(c, d) in_between(c hash_hash d)
char p[] = join(x, y); // equivalent to
// char p[] = "x ## y";
The expansion produces, at various stages:
join(x, y)
in_between(x hash_hash y)
in_between(x ## y)
mkstr(x ## y)
"x ## y"
In other words, expanding hash_hash produces a new token,
consisting of two adjacent sharp signs, but this new token
is not the catenation operator.
6.8.3.4 Rescanning and further replacement
[#1] After all parameters in the replacement list have been
substituted and # and ## processing has taken place, all
placemarker preprocessing tokens are removed, then the
resulting preprocessing token sequence is rescanned with all
subsequent preprocessing tokens of the source file for more
macro names to replace.
page 188 Language
page 189
[#2] If the name of the macro being replaced is found during
this scan of the replacement list (not including the rest of
the source file's preprocessing tokens), it is not replaced.
Further, if any nested replacements encounter the name of
the macro being replaced, it is not replaced. These
nonreplaced macro name preprocessing tokens are no longer
available for further replacement even if they are later
(re)examined in contexts in which that macro name
preprocessing token would otherwise have been replaced.
[#3] The resulting completely macro-replaced preprocessing
token sequence is not processed as a preprocessing directive
even if it resembles one, but all pragma unary operator
expressions within it are then processed as specified in
6.8.9 below.
6.8.3.5 Scope of macro definitions
[#1] A macro definition lasts (independent of block
structure) until a corresponding #undef directive is
encountered or (if none is encountered) until the end of
translation phase 4.
[#2] A preprocessing directive of the form
# undef identifier new-line
causes the specified identifier no longer to be defined as a
macro name. It is ignored if the specified identifier is
not currently defined as a macro name.
Examples
[#3]
1. The simplest use of this facility is to define a
``manifest constant,'' as in
#define TABSIZE 100
int table[TABSIZE];
2. The following defines a function-like macro whose
value is the maximum of its arguments. It has the
advantages of working for any compatible types of the
arguments and of generating in-line code without the
overhead of function calling. It has the
disadvantages of evaluating one or the other of its
arguments a second time (including side effects) and
generating more code than a function if invoked
several times. It also cannot have its address taken,
as it has none.
page 189 Language
page 190
#define max(a, b) ((a) > (b) ? (a) : (b))
The parentheses ensure that the arguments and the
resulting expression are bound properly.
3. To illustrate the rules for redefinition and
reexamination, the sequence
#define x 3
#define f(a) f(x * (a))
#undef x
#define x 2
#define g f
#define z z[0]
#define h g(~
#define m(a) a(w)
#define w 0,1
#define t(a) a
#define p() int
#define q(x) x
#define r(x,y) x ## y
#define str(x) # x
f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
g(x+(3,4)-w) | h 5) & m
(f)^m(m);
p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) };
char c[2][6] = { str(hello), str() };
results in
f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
f(2 * (2+(3,4)-0,1)) | f(2 * (~ 5)) & f(2 * (0,1))^m(0,1);
int i[] = { 1, 23, 4, 5, };
char c[2][6] = { "hello", "" };
4. To illustrate the rules for creating character string
literals and concatenating tokens, the sequence
page 190 Language
page 191
#define str(s) # s
#define xstr(s) str(s)
#define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \
x ## s, x ## t)
#define INCFILE(n) vers ## n // from previous #include example
#define glue(a, b) a ## b
#define xglue(a, b) glue(a, b)
#define HIGHLOW "hello"
#define LOW LOW ", world"
debug(1, 2);
fputs(str(strncmp("abc\0d", "abc", '\4') // this goes away
== 0) str(: @\n), s);
#include xstr(INCFILE(2).h)
glue(HIGH, LOW);
xglue(HIGH, LOW)
results in
printf("x" "1" "= %d, x" "2" "= %s", x1, x2);
fputs(
"strncmp(\"abc\\0d\", \"abc\", '\\4') == 0" ": @\n",
s);
#include "vers2.h" (after macro replacement, before file access)
"hello";
"hello" ", world"
or, after concatenation of the character string
literals,
printf("x1= %d, x2= %s", x1, x2);
fputs(
"strncmp(\"abc\\0d\", \"abc\", '\\4') == 0: @\n",
s);
#include "vers2.h" (after macro replacement, before file access)
"hello";
"hello, world"
Space around the # and ## tokens in the macro
definition is optional.
5. To illustrate the rules for
placemarker ## placemarker
the sequence
#define t(x,y,z) x ## y ## z
int j[] = { t(1,2,3), t(,4,5), t(6,,7), t(8,9,),
t(10,,), t(,11,), t(,,12), t(,,) };
results in
page 191 Language
page 192
int j[] = { 123, 45, 67, 89,
10, 11, 12, };
6. To demonstrate the redefinition rules, the following
sequence is valid.
#define OBJ_LIKE (1-1)
#define OBJ_LIKE /* white space */ (1-1) /* other */
#define FUNC_LIKE(a) ( a )
#define FUNC_LIKE( a )( /* note the white space */ \
a /* other stuff on this line
*/ )
But the following redefinitions are invalid:
#define OBJ_LIKE (0) /* different token sequence */
#define OBJ_LIKE (1 - 1) /* different white space */
#define FUNC_LIKE(b) ( a ) /* different parameter usage */
#define FUNC_LIKE(b) ( b ) /* different parameter spelling */
7. Finally, to show the variable argument list macro
facilities:
#define debug(...) fprintf(stderr, __VA_ARGS__)
#define showlist(...) puts(#__VA_ARGS__)
#define report(test, ...) ((test)?puts(#test):\
printf(__VA_ARGS__))
debug("Flag");
debug("X = %d\n", x);
showlist(The first, second, and third items.);
report(x>y, "x is %d but y is %d", x, y);
results in
fprintf(stderr, "Flag" );
fprintf(stderr, "X = %d\n", x );
puts( "The first, second, and third items." );
((x>y)?puts("x>y"):
printf("x is %d but y is %d", x, y));
page 192 Language
page 193
6.8.4 Line control
Constraints
[#1] The string literal of a #line directive, if present,
shall be a character string literal.
Semantics
[#2] The line number of the current source line is one
greater than the number of new-line characters read or
introduced in translation phase 1 (5.1.1.2) while processing
the source file to the current token.
[#3] A preprocessing directive of the form
# line digit-sequence new-line
causes the implementation to behave as if the following
sequence of source lines begins with a source line that has
a line number as specified by the digit sequence
(interpreted as a decimal integer). The digit sequence
shall not specify zero, nor a number greater than
2147483647.
[#4] A preprocessing directive of the form
# line digit-sequence "s-char-sequence-opt" new-line
sets the line number similarly and changes the presumed name
of the source file to be the contents of the character
string literal.
[#5] A preprocessing directive of the form
# line pp-tokens new-line
(that does not match one of the two previous forms) is
permitted. The preprocessing tokens after line on the
directive are processed just as in normal text (each
identifier currently defined as a macro name is replaced by
its replacement list of preprocessing tokens). The
directive resulting after all replacements shall match one
of the two previous forms and is then processed as
appropriate.
page 193 Language
page 194
6.8.5 Error directive
Semantics
[#1] A preprocessing directive of the form
# error pp-tokens-opt new-line
causes the implementation to produce a diagnostic message
that includes the specified sequence of preprocessing
tokens.
6.8.6 Pragma directive
Semantics
[#1] A preprocessing directive of the form
# pragma pp-tokens-opt new-line
where the preprocessing token STDC does not immediately
follow the pragma on the directive causes the implementation
to behave in a manner which it shall document. The behavior
might cause translation to fail or the resulting program to
behave in a non-conforming manner. Any such pragma that is
not recognized by the implementation is ignored.
[#2] If the preprocessing token STDC does immediately follow
the pragma on the directive, then no macro replacements are
performed on the directive, and the directive shall have one
of the following forms whose meaning is described elsewhere:
#pragma STDC FP_CONTRACT on-off-switch
#pragma STDC FENV_ACCESS on-off-switch
#pragma STDC CX_LIMITED_RANGE on-off-switch
on-off-switch: one of
ON OFF DEFAULT
Forward references: the FP_CONTRACT pragma (7.7.2), the
FENV_ACCESS pragma (7.6.1), the CX_LIMITED_RANGE pragma
(7.8.1).
page 194 Language
page 195
6.8.7 Null directive
Semantics
[#1] A preprocessing directive of the form
# new-line
has no effect.
6.8.8 Predefined macro names
[#1] The following macro names shall be defined by the
implementation:
__LINE__ The line number of the current source line (a
decimal constant).
__FILE__ The presumed name of the source file (a character
string literal).
__DATE__ The date of translation of the source file (a
character string literal of the form "Mmm dd yyyy",
where the names of the months are the same as those
generated by the asctime function, and the first
character of dd is a space character if the value
is less than 10). If the date of translation is
not available, an implementation-defined valid date
shall be supplied.
__TIME__ The time of translation of the source file (a
character string literal of the form "hh:mm:ss" as
in the time generated by the asctime function). If
the time of translation is not available, an
implementation-defined valid time shall be
supplied.
__STDC__ The decimal constant 1, intended to indicate a
conforming implementation.
__STDC_VERSION__ The decimal constant 199901L.128
[#2] The following macro names are conditionally defined by
the implementation: 624.nr:c 0u+000m'unu
__________
128. The value in ISO/IEC 9899:1994 was 199409L.
page 195 Language
page 196
__STDC_IEC_559__ The decimal constant 1, intended to
indicate conformance to the specifications in Annex F (IEC
559 floating-point arithmetic).
__STDC_IEC_559_COMPLEX__ The decimal constant 1, intended to
indicate adherence to the specifications in informative
Annex G (IEC 559 compatible complex arithmetic).
[#3] The values of the predefined macros (except for
__LINE__ and __FILE__) remain constant throughout the
translation unit.
[#4] None of these macro names, nor the identifier defined,
shall be the subject of a #define or a #undef preprocessing
directive. All predefined macro names shall begin with a
leading underscore followed by an uppercase letter or a
second underscore.
Forward references: the asctime function (7.16.3.1).
6.8.9 Pragma operator
Semantics
[#1] A unary operator expression of the form:
_Pragma ( string-literal )
is processed as follows. The string-literal is destringized
by deleting the L prefix, if present, deleting the leading
and trailing double-quotes, replacing each escape sequence
\" by a double-quote, and replacing each escape sequence \\
by a single backslash. The resulting sequence of characters
is processed through translation phase 3 to produce
preprocessing tokens that are executed as if they were the
pp-tokens in a pragma directive. The original four
preprocessing tokens in the unary operator expression are
removed.
Examples
[#2] A directive of the form:
#pragma list on "..\listing.dir"
can also be expressed as:
_Pragma ( "listing on \"..\\listing.dir\"" )
The latter form is processed in the same way whether it
appears literally as shown, or results from macro
page 196 Language
page 197
replacement, as in:
#define LISTING(x) PRAGMA(listing on #x)
#define PRAGMA(x) _Pragma(#x)
LISTING ( ..\listing.dir )
page 197 Language
page 198
6.9 Future language directions
6.9.1 Character escape sequences
[#1] Lowercase letters as escape sequences are reserved for
future standardization. Other characters may be used in
extensions.
6.9.2 Storage-class specifiers
[#1] The placement of a storage-class specifier other than
at the beginning of the declaration specifiers in a
declaration is an obsolescent feature.
6.9.3 Function declarators
[#1] The use of function declarators with empty parentheses
(not prototype-format parameter type declarators) is an
obsolescent feature.
6.9.4 Function definitions
[#1] The use of function definitions with separate parameter
identifier and declaration lists (not prototype-format
parameter type and identifier declarators) is an obsolescent
feature.
6.9.5 Pragma directives
[#1] Pragmas whose first pp-token is STDC are reserved for
future standardization.
page 198 Language
page 199
7. Library
7.1 Introduction
7.1.1 Definitions of terms
[#1] A string is a contiguous sequence of characters
terminated by and including the first null character. A
``pointer to'' a string is a pointer to its initial (lowest
addressed) character. The ``length'' of a string is the
number of characters preceding the null character and its
``value'' is the sequence of the values of the contained
characters, in order.
[#2] A letter is a printing character in the execution
character set corresponding to any of the 52 required
lowercase and uppercase letters in the source character set,
listed in 5.2.1.
[#3] The decimal-point character is the character used by
functions that convert floating-point numbers to or from
character sequences to denote the beginning of the
fractional part of such character sequences.129 It is
represented in the text and examples by a period, but may be
changed by the setlocale function.
[#4] A wide character is a code value (a binary encoded
integer) of an object of type wchar_t that corresponds to a
member of the extended character set.130
[#5] A null wide character is a wide character with code
value zero.
[#6] A wide string is a contiguous sequence of wide
characters terminated by and including the first null wide
__________
129. The functions that make use of the decimal-point
character are atof, fprintf, fscanf, fwprintf, fwscanf,
localeconv, printf, scanf, sprintf, sscanf, strtod,
swprintf, swscanf, vfprintf, vfscanf, vfwprintf,
vfwscanf, vprintf, vscanf, vsprintf, vsscanf, vswprintf,
vswscanf, vwprintf, vwscanf, wprintf, and wscanf.
130. An equivalent definition can be found in subclause
6.1.3.4.
page 199 Library
page 200
character. A pointer to a wide string is a pointer to its
initial (lowest addressed) wide character. The length of a
wide string is the number of wide characters preceding the
null wide character and the value of a wide string is the
sequence of code values of the contained wide characters, in
order.
[#7] A shift sequence is a contiguous sequence of bytes
within a multibyte string that (potentially) causes a change
in shift state. (See subclause 5.2.1.2.) A shift sequence
shall not have a corresponding wide character; it is instead
taken to be an adjunct to an adjacent multibyte
character.131
Forward references: character handling (7.3), the setlocale
function (7.5.1.1).
7.1.2 Standard headers
[#1] Each library function is declared, with a type that
includes a prototype, in a header,132 whose contents are
made available by the #include preprocessing directive. The
header declares a set of related functions, plus any
necessary types and additional macros needed to facilitate
their use. Declarations of types described in this clause
shall not include type qualifiers, unless explicity stated
otherwise.
[#2] The standard headers are
__________
131. For state-dependent encodings, the values for
MB_CUR_MAX and MB_LEN_MAX must thus be large enough to
count all the bytes in any complete multibyte character
plus at least one adjacent shift sequence of maximum
length. Whether these counts provide for more than one
shift sequence is the implementation's choice.
132. A header is not necessarily a source file, nor are the
< and > delimited sequences in header names necessarily
valid source file names.
page 200 Library
page 201
<assert.h> <complex.h> <ctype.h>
<errno.h> <fenv.h> <float.h>
<inttypes.h> <iso646.h> <limits.h>
<locale.h> <math.h> <setjmp.h>
<signal.h> <stdarg.h> <stdbool.h>
<stddef.h> <stdio.h> <stdlib.h>
<string.h> <tgmath.h> <time.h>
<wchar.h> <wctype.h>
[#3] If a file with the same name as one of the above < and
> delimited sequences, not provided as part of the
implementation, is placed in any of the standard places for
a source file to be included, the behavior is undefined.
[#4] Standard headers may be included in any order; each may
be included more than once in a given scope, with no effect
different from being included only once, except that the
effect of including <assert.h> depends on the definition of
NDEBUG. If used, a header shall be included outside of any
external declaration or definition, and it shall first be
included before the first reference to any of the functions
or objects it declares, or to any of the types or macros it
defines. However, if an identifier is declared or defined
in more than one header, the second and subsequent
associated headers may be included after the initial
reference to the identifier. The program shall not have any
macros with names lexically identical to keywords currently
defined prior to the inclusion.
[#5] Any definition of an object-like macro described in
this clause shall expand to code that is fully protected by
parentheses where necessary, so that it groups in an
arbitrary expression as if it were a single identifier.
[#6] Any declaration of a library function shall have
external linkage.
Forward references: diagnostics (7.2).
page 201 Library
page 202
7.1.3 Reserved identifiers
[#1] Each header declares or defines all identifiers listed
in its associated subclause, and optionally declares or
defines identifiers listed in its associated future library
directions subclause and identifiers which are always
reserved either for any use or for use as file scope
identifiers.
- All identifiers that begin with an underscore and
either an uppercase letter or another underscore are
always reserved for any use.
- All identifiers that begin with an underscore are
always reserved for use as macros and as identifiers
with file scope in both the ordinary and tag name
spaces.
- Each macro name in any of the following subclauses
(including the future library directions) is reserved
for use as specified if any of its associated headers
is included; unless explicitly stated otherwise (see
7.1.8).
- All identifiers with external linkage in any of the
following subclauses (including the future library
directions) are always reserved for use as identifiers
with external linkage.133
- Each identifier with file scope listed in any of the
following subclauses (including the future library
directions) is reserved for use as macro and as an
identifier with file scope in the same name space if
any of its associated headers is included.
[#2] No other identifiers are reserved. If the program
declares or defines an identifier that is reserved in that
context (other than as allowed by 7.1.8), the behavior is
undefined.134
__________
133. The list of reserved identifiers with external linkage
includes errno, setjmp, and va_end.
134. Since macro names are replaced whenever found,
independent of scope and name space, macro names
matching any of the reserved identifier names must not
be defined if an associated header, if any, is included.
page 202 Library
page 203
[#3] If the program removes (with #undef) any macro
definition of an identifier in the first group listed above,
the behavior is undefined.
7.1.4 Errors <errno.h>
[#1] The header <errno.h> defines several macros, all
relating to the reporting of error conditions.
[#2] The macros are
EDOM
EILSEQ
ERANGE
which expand to integer constant expressions with type int,
distinct positive values, and which are suitable for use in
#if preprocessing directives; and
errno
which expands to a modifiable lvalue135 that has type int,
the value of which is set to a positive error number by
several library functions. It is unspecified whether errno
is a macro or an identifier declared with external linkage.
If a macro definition is suppressed in order to access an
actual object, or a program defines an identifier with the
name errno, the behavior is undefined.
[#3] The value of errno is zero at program startup, but is
never set to zero by any library function.136 The value of
errno may be set to nonzero by a library function call
whether or not there is an error, provided the use of errno
is not documented in the description of the function in this
International Standard.
__________
135. The macro errno need not be the identifier of an
object. It might expand to a modifiable lvalue
resulting from a function call (for example, *errno()).
136. Thus, a program that uses errno for error checking
should set it to zero before a library function call,
then inspect it before a subsequent library function
call. Of course, a library function can save the value
of errno on entry and then set it to zero, as long as
the original value is restored if errno's value is still
zero just before the return.
page 203 Library
page 204
[#4] Additional macro definitions, beginning with E and a
digit or E and an uppercase letter,137 may also be specified
by the implementation.
7.1.5 Limits <float.h> and <limits.h>
[#1] The headers <float.h> and <limits.h> define several
macros that expand to various limits and parameters.
[#2] The macros, their meanings, and the constraints (or
restrictions) on their values are listed in 5.2.4.2.
7.1.6 Common definitions <stddef.h>
[#1] The following types and macros are defined in the
standard header <stddef.h>. Some are also defined in other
headers, as noted in their respective subclauses.
[#2] The types are
ptrdiff_t
which is the signed integer type of the result of
subtracting two pointers;
size_t
which is the unsigned integer type of the result of the
sizeof operator; and
wchar_t
which is an integer type whose range of values can represent
distinct codes for all members of the largest extended
character set specified among the supported locales; the
null character shall have the code value zero and each
member of the basic character set defined in 5.2.1 shall
have a code value equal to its value when used as the lone
character in an integer character constant.
[#3] The macros are
NULL
which expands to an implementation-defined null pointer
__________
137. See ``future library directions'' (7.20.1).
page 204 Library
page 205
constant; and
offsetof(type, member-designator)
which expands to an integer constant expression that has
type size_t, the value of which is the offset in bytes, to
the structure member (designated by member-designator), from
the beginning of its structure (designated by type). The
member-designator shall be such that given
static type t;
then the expression &(t.member-designator) evaluates to an
address constant. (If the specified member is a bit-field,
the behavior is undefined.)
Forward references: localization (7.5).
7.1.7 Boolean type and values <stdbool.h>
[#1] The header <stdbool.h> defines one type and three
macros.
[#2] The type is
bool
which is an integer type that promotes to int or unsigned
int, and that is suitable to be used as the type of a bit-
field. A bit-field of any width and type bool shall be
capable for representing the value 1.138
__________
138. The traditional choice for type bool has been int, but
this is not a requirement of this International
Standard. Other available choices include, but are not
limited to, char, unsigned int, and an enumeration type.
If an enumeration type is chosen, the names of its true
and false members are "masked" by the macros true and
false, but the member names might be available to the
debugger:
typedef enum { false=0, true=1 } bool;
#define false 0
#define true 1
The type is suitable for bit-fields if it is int,
unsigned int, signed int, or some type allowed by an
page 205 Library
page 206
[#3] The macros are
true
which expands to the decimal constant 1,
false
which expands to the decimal constant 0, and
__bool_true_false_are_defined
which expands to the decimal constant 1. The macros are
suitable for use in #if preprocessing directives.
7.1.8 Use of library functions
[#1] Each of the following statements applies unless
explicitly stated otherwise in the detailed descriptions
that follow. If an argument to a function has an invalid
value (such as a value outside the domain of the function,
or a pointer outside the address space of the program, or a
null pointer) or a type (after promotion) not expected by a
function with variable number of arguments, the behavior is
undefined. If a function argument is described as being an
array, the pointer actually passed to the function shall
have a value such that all address computations and accesses
to objects (that would be valid if the pointer did point to
the first element of such an array) are in fact valid. Any
function declared in a header may be additionally
implemented as a function-like macro defined in the header,
so if a library function is declared explicitly when its
header is included, one of the techniques shown below can be
used to ensure the declaration is not affected by such a
macro. Any macro definition of a function can be suppressed
locally by enclosing the name of the function in
parentheses, because the name is then not followed by the
left parenthesis that indicates expansion of a macro
function name. For the same syntactic reason, it is
permitted to take the address of a library function even if
it is also defined as a macro.139 The use of #undef to
____________________________________________________________
implementation extension. It is required that a bool
bit-field of width 1 be unsigned. Thus, bool cannot be
signed int, nor can it be plain int if width 1 plain int
bit-fields are signed.
139. This means that an implementation must provide an
page 206 Library
page 207
remove any macro definition will also ensure that an actual
function is referred to. Any invocation of a library
function that is implemented as a macro shall expand to code
that evaluates each of its arguments exactly once, fully
protected by parentheses where necessary, so it is generally
safe to use arbitrary expressions as arguments.140
Likewise, those function-like macros described in the
following subclauses may be invoked in an expression
anywhere a function with a compatible return type could be
called.141 All object-like macros listed as expanding to
integer constant expressions shall additionally be suitable
for use in #if preprocessing directives.
[#2] Provided that a library function can be declared
without reference to any type defined in a header, it is
also permissible to declare the function and use it without
including its associated header.
____________________________________________________________
actual function for each library function, even if it
also provides a macro for that function.
140. Such macros might not contain the sequence points that
the corresponding function calls do.
141. Because external identifiers and some macro names
beginning with an underscore are reserved,
implementations may provide special semantics for such
names. For example, the identifier _BUILTIN_abs could
be used to indicate generation of in-line code for the
abs function. Thus, the appropriate header could
specify
#define abs(x) _BUILTIN_abs(x)
for a compiler whose code generator will accept it.
In this manner, a user desiring to guarantee that a
given library function such as abs will be a genuine
function may write
#undef abs
whether the implementation's header provides a macro
implementation of abs or a built-in implementation. The
prototype for the function, which precedes and is hidden
by any macro definition, is thereby revealed also.
page 207 Library
page 208
[#3] There is a sequence point immediately before a library
function return.
[#4] The functions in the standard library are not
guaranteed to be reentrant and may modify objects with
static storage duration.142
Examples
[#5] The function atoi may be used in any of several ways:
- by use of its associated header (possibly generating a
macro expansion)
#include <stdlib.h>
const char *str;
/* ... */
i = atoi(str);
- by use of its associated header (assuredly generating a
true function reference)
#include <stdlib.h>
#undef atoi
const char *str;
/* ... */
i = atoi(str);
or
#include <stdlib.h>
const char *str;
/* ... */
i = (atoi)(str);
- by explicit declaration
extern int atoi(const char *);
const char *str;
/* ... */
i = atoi(str);
__________
142. Thus, a signal handler cannot, in general, call
standard library functions.
page 208 Library
page 209
7.2 Diagnostics <assert.h>
[#1] The header <assert.h> defines the assert macro and
refers to another macro,
NDEBUG
which is not defined by <assert.h>. If NDEBUG is defined as
a macro name at the point in the source file where
<assert.h> is included, the assert macro is defined simply
as
#define assert(ignore) ((void)0)
[#2] The assert macro shall be implemented as a macro, not
as an actual function. If the macro definition is
suppressed in order to access an actual function, the
behavior is undefined.
7.2.1 Program diagnostics
7.2.1.1 The assert macro
Synopsis
[#1]
#include <assert.h>
void assert(int expression);
Description
[#2] The assert macro puts diagnostic tests into programs.
When it is executed, if expression is false (that is,
compares equal to 0), the assert macro writes information
about the particular call that failed (including the text of
the argument, the name of the source file, and the source
line number -- the latter are respectively the values of
the preprocessing macros __FILE__ and __LINE__ and the
identifier __func__) on the standard error file in an
implementation-defined format.143 It then calls the abort
function.
__________
143. The message written might be of the form
Assertion failed: expression, function abc, file xyz,
line nnn
page 209 Library
page 210
Returns
[#3] The assert macro returns no value.
Forward references: the abort function (7.14.4.1).
page 210 Library
page 211
7.3 Character handling <ctype.h>
[#1] The header <ctype.h> declares several functions useful
for testing and mapping characters.144 In all cases the
argument is an int, the value of which shall be
representable as an unsigned char or shall equal the value
of the macro EOF. If the argument has any other value, the
behavior is undefined.
[#2] The behavior of these functions is affected by the
current locale. Those functions that have locale-specific
aspects only when not in the "C" locale are noted below.
[#3] The term printing character refers to a member of a
locale-specific set of characters, each of which occupies
one printing position on a display device; the term control
character refers to a member of a locale-specific set of
characters that are not printing characters.145
Forward references: EOF (7.13.1), localization (7.5).
7.3.1 Character testing functions
[#1] The functions in this subclause return nonzero (true)
if and only if the value of the argument c conforms to that
in the description of the function.
7.3.1.1 The isalnum function
Synopsis
[#1]
#include <ctype.h>
int isalnum(int c);
__________
144. See ``future library directions'' (7.20.2).
145. In an implementation that uses the seven-bit ASCII
character set, the printing characters are those whose
values lie from 0x20 (space) through 0x7E (tilde); the
control characters are those whose values lie from 0
(NUL) through 0x1F (US), and the character 0x7F (DEL).
page 211 Library
page 212
Description
[#2] The isalnum function tests for any character for which
isalpha or isdigit is true.
7.3.1.2 The isalpha function
Synopsis
[#1]
#include <ctype.h>
int isalpha(int c);
Description
[#2] The isalpha function tests for any character for which
isupper or islower is true, or any character that is one of
a locale-specific set of characters for which none of
iscntrl, isdigit, ispunct, or isspace is true.146 In the
"C" locale, isalpha returns true only for the characters for
which isupper or islower is true.
7.3.1.3 The isblank function
Synopsis
[#1]
#include <ctype.h>
int isblank(int c);
Description
[#2] The isblank function tests for any character for that
is a standard blank character or is one of a locale-specific
set of characters, for which isalnum is false. The standard
blank characters are the following: space (' '), and
horizontal tab ('\t'). In the "C" locale, isblank returns
true only for the standard blank characters.
__________
146. The functions islower and isupper test true or false
separately for each of these additional characters; all
four combinations are possible.
page 212 Library
page 213
7.3.1.4 The iscntrl function
Synopsis
[#1]
#include <ctype.h>
int iscntrl(int c);
Description
[#2] The iscntrl function tests for any control character.
7.3.1.5 The isdigit function
Synopsis
[#1]
#include <ctype.h>
int isdigit(int c);
Description
[#2] The isdigit function tests for any decimal-digit
character (as defined in 5.2.1).
7.3.1.6 The isgraph function
Synopsis
[#1]
#include <ctype.h>
int isgraph(int c);
Description
[#2] The isgraph function tests for any printing character
except space (' ').
7.3.1.7 The islower function
Synopsis
[#1]
#include <ctype.h>
int islower(int c);
page 213 Library
page 214
Description
[#2] The islower function tests for any character that is a
lowercase letter or is one of a locale-specific set of
characters for which none of iscntrl, isdigit, ispunct, or
isspace is true. In the "C" locale, islower returns true
only for the characters defined as lowercase letters (as
defined in 5.2.1).
7.3.1.8 The isprint function
Synopsis
[#1]
#include <ctype.h>
int isprint(int c);
Description
[#2] The isprint function tests for any printing character
including space (' ').
7.3.1.9 The ispunct function
Synopsis
[#1]
#include <ctype.h>
int ispunct(int c);
Description
[#2] The ispunct function tests for any printing character
that is one of a locale-specific set of characters for which
neither isspace nor isalnum is true.
7.3.1.10 The isspace function
Synopsis
[#1]
#include <ctype.h>
int isspace(int c);
page 214 Library
page 215
Description
[#2] The isspace function tests for any character that is a
standard white-space character or is one of a locale-
specific set of characters for which isalnum is false. The
standard white-space characters are the following: space
(' '), form feed ('\f'), new-line ('\n'), carriage return
('\r'), horizontal tab ('\t'), and vertical tab ('\v'). In
the "C" locale, isspace returns true only for the standard
white-space characters.
7.3.1.11 The isupper function
Synopsis
[#1]
#include <ctype.h>
int isupper(int c);
Description
[#2] The isupper function tests for any character that is an
uppercase letter or is one of a locale-specific set of
characters for which none of iscntrl, isdigit, ispunct, or
isspace is true. In the "C" locale, isupper returns true
only for the characters defined as uppercase letters (as
defined in 5.2.1).
7.3.1.12 The isxdigit function
Synopsis
[#1]
#include <ctype.h>
int isxdigit(int c);
Description
[#2] The isxdigit function tests for any hexadecimal-digit
character (as defined in 6.1.3.1).
page 215 Library
page 216
7.3.2 Character case mapping functions
7.3.2.1 The tolower function
Synopsis
[#1]
#include <ctype.h>
int tolower(int c);
Description
[#2] The tolower function converts an uppercase letter to a
corresponding lowercase letter.
Returns
[#3] If the argument is a character for which isupper is
true and there are one or more corresponding characters, as
specified by the current locale, for which islower is true,
the tolower function returns one of the corresponding
characters (always the same one for any given locale);
otherwise, the argument is returned unchanged.
7.3.2.2 The toupper function
Synopsis
[#1]
#include <ctype.h>
int toupper(int c);
Description
[#2] The toupper function converts a lowercase letter to a
corresponding uppercase letter.
Returns
[#3] If the argument is a character for which islower is
true and there are one or more corresponding characters, as
specified by the current locale, for which isupper is true,
the toupper function returns one of the corresponding
characters (always the same one for any given locale);
otherwise, the argument is returned unchanged.
page 216 Library
page 217
7.4 Integer types <inttypes.h>
[#1] The header <inttypes.h> defines sets of typedef names
for integer types having specified widths, and defines
corresponding sets of macros. It also defines macros that
specify limits of integer types corresponding to typedef
names defined in other standard headers, and declares four
functions for converting numeric character strings to
greatest-width integers.
[#2] Typedef names are defined in the following categories:
- integer types having certain exact widths;
- integer types having at least certain specified widths;
- fastest integer types having at least certain specified
widths;
- integer types wide enough to hold pointers to objects;
- integer types having greatest width.
(Some of these typedef names may denote the same type.)
[#3] Corresponding macros specify limits of the defined
types, construct suitable character constants, and provide
conversion specifiers for use with the formatted
input/output functions.
[#4] For each typedef name described herein that can be
defined as a type existing in the implementation,147
<inttypes.h> shall define that typedef name, and it shall
define the associated macros. Conversely, for each typedef
name described herein that cannot be defined as a type
existing in the implementation, <inttypes.h> shall not
define that typedef name, nor shall it define the associated
macros.
__________
147. Some of these typedef names may denote implementation-
defined extended integer types.
page 217 Library
page 218
7.4.1 Typedef names for integer types
[#1] When typedef names differing only in the absence or
presence of the initial u are defined, they shall denote
corresponding signed and unsigned types as described in
subclause 6.1.2.5.
7.4.1.1 Exact-width integer types
[#1] Each of the following typedef names designates an
integer type that has exactly the specified width. These
typedef names have the general form of intn_t or uintn_t
where n is the required width. For example, uint8_t denotes
an unsigned integer type that has a width of exactly 8 bits.
[#2] The following designate exact-width signed integer
types:
int8_t int16_t int32_t int64_t
[#3] The following designate exact-width unsigned integer
types:
uint8_t uint16_t uint32_t uint64_t
(Any of these types might not exist.)
7.4.1.2 Minimum-width integer types
[#1] Each of the following typedef names designates an
integer type that has at least the specified width, such
that no integer type of lesser size has at least the
specified width. These typedef names have the general form
of int_leastn_t or uint_leastn_t where n is the minimum
required width. For example, int_least32_t denotes a signed
integer type that has a width of at least 32 bits.
[#2] The following designate minimum-width signed integer
types:
int_least8_t int_least16_t
int_least32_t int_least64_t
[#3] The following designate minimum-width unsigned integer
types:
uint_least8_t uint_least16_t
uint_least32_t uint_least64_t
(These types must exist.)
page 218 Library
page 219
7.4.1.3 Fastest minimum-width integer types
[#1] Each of the following typedef names designates an
integer type that is usually fastest148 to operate with
among all integer types that have at least the specified
width. These typedef names have the general form of
int_fastn_t or uint_fastn_t where n is the minimum required
width. For example, int_fast16_t denotes the fastest signed
integer type that has a width of at least 16 bits.
[#2] The following designate fastest minimum-width signed
integer types:
int_fast8_t int_fast16_t
int_fast32_t int_fast64_t
[#3] The following designate fastest minimum-width unsigned
integer types:
uint_fast8_t uint_fast16_t
uint_fast32_t uint_fast64_t
(These types must exist.)
7.4.1.4 Integer types capable of holding object pointers
[#1] The following typedef name designates a signed integer
type with the property that any valid pointer to void can be
converted to this type, then converted back to pointer to
void, and the result will compare equal to the original
pointer:
intptr_t
[#2] The following typedef name designates an unsigned
integer type with the property that any valid pointer to
void can be converted to this type, then converted back to
pointer to void, and the result will compare equal to the
original pointer:
__________
148. The designated type is not guaranteed to be fastest for
all purposes; if the implementation has no clear grounds
for choosing one type over another, it will simply pick
some integer type satisfying the signedness and width
requirements.
page 219 Library
page 220
uintptr_t
(Either or both of these types might not exist.)
7.4.1.5 Greatest-width integer types
[#1] The following typedef name designates a signed integer
type capable of representing any value of any signed integer
type:
intmax_t
[#2] The following typedef name designates an unsigned
integer type capable of representing any value of any
unsigned integer type:
uintmax_t
(These types must exist.)
7.4.2 Limits of specified-width integer types
[#1] The following object-like macros149 specify the minimum
and maximum limits of integer types corresponding to the
typedef names defined in <inttypes.h>. Each macro name
corresponds to a similar typedef name in subclause 7.4.1.
[#2] Each instance of any defined macro shall be replaced by
a constant expression suitable for use in #if preprocessing
directives, and this expression shall have the same type as
would an expression that is an object of the corresponding
type converted according to the integer promotions. Its
implementation-defined value shall be equal to or greater in
magnitude (absolute value) than the corresponding value
given below, with the same sign.
__________
149. C++ implementations should define these macros only
when __STDC_LIMIT_MACROS is defined before <inttypes.h>
is included.
page 220 Library
page 221
7.4.2.1 Limits of exact-width integer types
- minimum values of exact-width signed integer types
INT8_MIN -127
INT16_MIN -32767
INT32_MIN -2147483647
INT64_MIN -9223372036854775807
(The value must be either that given or exactly 1
less.)
- maximum values of exact-width signed integer types
INT8_MAX +127
INT16_MAX +32767
INT32_MAX +2147483647
INT64_MAX +9223372036854775807
(The value must be exactly that given.)
- maximum values of exact-width unsigned integer types
UINT8_MAX 255
UINT16_MAX 65535
UINT32_MAX 4294967295
UINT64_MAX 18446744073709551615
(The value must be exactly that given.)
7.4.2.2 Limits of minimum-width integer types
- minimum values of minimum-width signed integer types
INT_LEAST8_MIN -127
INT_LEAST16_MIN -32767
INT_LEAST32_MIN -2147483647
INT_LEAST64_MIN -9223372036854775807
- maximum values of minimum-width signed integer types
INT_LEAST8_MAX +127
INT_LEAST16_MAX +32767
INT_LEAST32_MAX +2147483647
INT_LEAST64_MAX +9223372036854775807
- maximum values of minimum-width unsigned integer types
UINT_LEAST8_MAX 255
UINT_LEAST16_MAX 65535
UINT_LEAST32_MAX 4294967295
UINT_LEAST64_MAX 18446744073709551615
page 221 Library
page 222
7.4.2.3 Limits of fastest minimum-width integer types
- minimum values of fastest minimum-width signed integer
types
INT_FAST8_MIN -127
INT_FAST16_MIN -32767
INT_FAST32_MIN -2147483647
INT_FAST64_MIN -9223372036854775807
- maximum values of fastest minimum-width signed integer
types
INT_FAST8_MAX +127
INT_FAST16_MAX +32767
INT_FAST32_MAX +2147483647
INT_FAST64_MAX +9223372036854775807
- maximum values of fastest minimum-width unsigned
integer types
UINT_FAST8_MAX 255
UINT_FAST16_MAX 65535
UINT_FAST32_MAX 4294967295
UINT_FAST64_MAX 18446744073709551615
7.4.2.4 Limits of integer types capable of holding object
pointers
- minimum value of pointer-holding signed integer type
INTPTR_MIN -32767
- maximum value of pointer-holding signed integer type
INTPTR_MAX +32767
- maximum value of pointer-holding unsigned integer type
UINTPTR_MAX 65535
7.4.2.5 Limits of greatest-width integer types
- minimum value of greatest-width signed integer type
INTMAX_MIN -9223372036854775807
- maximum value of greatest-width signed integer type
INTMAX_MAX +9223372036854775807
- maximum value of greatest-width unsigned integer type
UINTMAX_MAX 18446744073709551615
page 222 Library
page 223
7.4.3 Macros for integer constants
[#1] The following function-like macros150 expand to integer
constants suitable for initializing objects that have
integer types corresponding to typedef names defined in
<inttypes.h>. Each macro name corresponds to a similar
typedef name in subclause 7.4.1.2 or 7.4.1.5.
[#2] The argument in any instance of these macros shall be a
decimal, octal, or hexadecimal constant (as defined in
subclause 6.1.3.2) with a value that does not exceed the
limits for the corresponding type.
7.4.3.1 Macros for minimum-width integer constants
[#1] Each of the following macros expands to an integer
constant having the value specified by its argument and a
type with at least the specified width. These macro names
have the general form of INTn_C or UINTn_C where n is the
minimum required width. For example, UINT64_C(0x123) might
expand to the integer constant 0x123ULL.
[#2] The following expand to integer constants that have
signed integer types:
INT8_C(value) INT16_C(value)
INT32_C(value) INT64_C(value)
[#3] The following expand to integer constants that have
unsigned integer types:
UINT8_C(value) UINT16_C(value)
UINT32_C(value) UINT64_C(value)
__________
150. C++ implementations should define these macros only
when __STDC_CONSTANT_MACROS is defined before
<inttypes.h> is included.
page 223 Library
page 224
7.4.3.2 Macros for greatest-width integer constants
[#1] The following macro expands to an integer constant
having the value specified by its argument and the type
intmax_t:
INTMAX_C(value)
[#2] The following macro expands to an integer constant
having the value specified by its argument and the type
uintmax_t:
UINTMAX_C(value)
7.4.4 Macros for format specifiers
[#1] Each of the following object-like macros151 expands to
a string literal containing a conversion specifier, possibly
modified by a prefix such as hh, h, l, or ll, suitable for
use within the format argument of a formatted input/output
function when converting the corresponding integer type.
These macro names have the general form of PRI (character
string literals for the fprintf family) or SCN (character
string literals for the fscanf family),152 followed by the
conversion specifier, followed by a name corresponding to a
similar typedef name in subclause 7.4.1. For example,
PRIdFAST32 can be used in a format string to print the value
of an integer of type int_fast32_t.
[#2] The fprintf macros for signed integers are:
__________
151. C++ implementations should define these macros only
when __STDC_FORMAT_MACROS is defined before <inttypes.h>
is included.
152. Separate macros are given for use with fprintf and
fscanf functions because, typically, different format
specifiers are required for fprintf and fscanf even when
the type is the same.
page 224 Library
page 225
PRId8 PRId16 PRId32 PRId64
PRIdLEAST8 PRIdLEAST16 PRIdLEAST32 PRIdLEAST64
PRIdFAST8 PRIdFAST16 PRIdFAST32 PRIdFAST64
PRIdMAX PRIdPTR
PRIi8 PRIi16 PRIi32 PRIi64
PRIiLEAST8 PRIiLEAST16 PRIiLEAST32 PRIiLEAST64
PRIiFAST8 PRIiFAST16 PRIiFAST32 PRIiFAST64
PRIiMAX PRIiPTR
[#3] The fprintf macros for unsigned integers are:
PRIo8 PRIo16 PRIo32 PRIo64
PRIoLEAST8 PRIoLEAST16 PRIoLEAST32 PRIoLEAST64
PRIoFAST8 PRIoFAST16 PRIoFAST32 PRIoFAST64
PRIoMAX PRIoPTR
PRIu8 PRIu16 PRIu32 PRIu64
PRIuLEAST8 PRIuLEAST16 PRIuLEAST32 PRIuLEAST64
PRIuFAST8 PRIuFAST16 PRIuFAST32 PRIuFAST64
PRIuMAX PRIuPTR
PRIx8 PRIx16 PRIx32 PRIx64
PRIxLEAST8 PRIxLEAST16 PRIxLEAST32 PRIxLEAST64
PRIxFAST8 PRIxFAST16 PRIxFAST32 PRIxFAST64
PRIxMAX PRIxPTR
PRIX8 PRIX16 PRIX32 PRIX64
PRIXLEAST8 PRIXLEAST16 PRIXLEAST32 PRIXLEAST64
PRIXFAST8 PRIXFAST16 PRIXFAST32 PRIXFAST64
PRIXMAX PRIXPTR
[#4] The fscanf macros for signed integers are:
SCNd8 SCNd16 SCNd32 SCNd64
SCNdLEAST8 SCNdLEAST16 SCNdLEAST32 SCNdLEAST64
SCNdFAST8 SCNdFAST16 SCNdFAST32 SCNdFAST64
SCNdMAX SCNdPTR
SCNi8 SCNi16 SCNi32 SCNi64
SCNiLEAST8 SCNiLEAST16 SCNiLEAST32 SCNiLEAST64
SCNiFAST8 SCNiFAST16 SCNiFAST32 SCNiFAST64
SCNiMAX SCNiPTR
[#5] The fscanf macros for unsigned integers are:
page 225 Library
page 226
SCNo8 SCNo16 SCNo32 SCNo64
SCNoLEAST8 SCNoLEAST16 SCNoLEAST32 SCNoLEAST64
SCNoFAST8 SCNoFAST16 SCNoFAST32 SCNoFAST64
SCNoMAX SCNoPTR
SCNu8 SCNu16 SCNu32 SCNu64
SCNuLEAST8 SCNuLEAST16 SCNuLEAST32 SCNuLEAST64
SCNuFAST8 SCNuFAST16 SCNuFAST32 SCNuFAST64
SCNuMAX SCNuPTR
SCNx8 SCNx16 SCNx32 SCNx64
SCNxLEAST8 SCNxLEAST16 SCNxLEAST32 SCNxLEAST64
SCNxFAST8 SCNxFAST16 SCNxFAST32 SCNxFAST64
SCNxMAX SCNxPTR
[#6] Because the default argument promotions do not affect
pointer parameters, there might not exist suitable fscanf
format specifiers for some of the typedef names defined in
this header. Consequently, as a special exception to the
requirement that the implementation shall define all macros
associated with each typedef name defined in this header, in
such a case the problematic fscanf macros may be left
undefined.
Examples
#include <inttypes.h>
#include <wchar.h>
int main(void)
{
uintmax_t i = UINTMAX_MAX; // this type always exists
wprintf(L"The largest integer value is %020"
PRIxMAX "\n", i);
return 0;
}
7.4.5 Limits of other integer types
[#1] The following object-like macros151 specify the minimum
and maximum limits of integer types corresponding to typedef
names defined in other standard headers.
[#2] Each instance of these macros shall be replaced by a
constant expression suitable for use in #if preprocessing
directives, and this expression shall have the same type as
would an expression that is an object of the corresponding
type converted according to the integer promotions. Its
implementation-defined value shall be equal to or greater in
magnitude (absolute value) than the corresponding value
given below, with the same sign.
page 226 Library
page 227
- limits of ptrdiff_t
PTRDIFF_MIN -65535
PTRDIFF_MAX +65535
- limits of sig_atomic_t
SIG_ATOMIC_MIN see below
SIG_ATOMIC_MAX see below
- limit of size_t
SIZE_MAX 65535
- limits of wchar_t
WCHAR_MIN see below
WCHAR_MAX see below
- limits of wint_t
WINT_MIN see below
WINT_MAX see below
[#3] If sig_atomic_t is defined as a signed integer type,
the value of SIG_ATOMIC_MIN shall be no greater than -127
and the value of SIG_ATOMIC_MAX shall be no less than 127;
otherwise, sig_atomic_t is defined as an unsigned integer
type, and the value of SIG_ATOMIC_MIN shall be 0 and the
value of SIG_ATOMIC_MAX shall be no less than 255.
[#4] If wchar_t is defined as a signed integer type, the
value of WCHAR_MIN shall be no greater than -127 and the
value of WCHAR_MAX shall be no less than 127; otherwise,
wchar_t is defined as an unsigned integer type, and the
value of WCHAR_MIN shall be 0 and the value of WCHAR_MAX
shall be no less than 255.
[#5] If wint_t is defined as a signed integer type, the
value of WINT_MIN shall be no greater than -32767 and the
value of WINT_MAX shall be no less than 32767; otherwise,
wint_t is defined as an unsigned integer type, and the value
of WINT_MIN shall be 0 and the value of WINT_MAX shall be no
less than 65535.
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7.4.6 Conversion functions for greatest-width integer types
7.4.6.1 The strtoimax function
Synopsis
[#1]
#include <inttypes.h>
intmax_t strtoimax(const char * restrict nptr,
char ** restrict endptr, int base);
Description
[#2] The strtoimax function is equivalent to strtol, except
that the initial portion of the string is converted to
intmax_t representation.
Returns
[#3] The strtoimax function returns the converted value, if
any. If no conversion could be performed zero is returned.
If the correct value is outside the range of representable
values, INTMAX_MAX or INTMAX_MIN is returned (according to
the sign of the value), and the value of the macro ERANGE is
stored in errno.
7.4.6.2 The strtoumax function
Synopsis
[#1]
#include <inttypes.h>
uintmax_t strtoumax(const char * restrict nptr,
char ** restrict endptr, int base);
Description
[#2] The strtoumax function is equivalent to strtoul, except
that the initial portion of the string is converted to
uintmax_t representation.
Returns
[#3] The strtoumax function returns the converted value, if
any. If no conversion could be performed zero is returned.
If the correct value is outside the range of representable
values, UINTMAX_MAX is returned, and the value of the macro
ERANGE is stored in errno.
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7.4.6.3 The wcstoimax function
Synopsis
[#1]
#include <stddef.h> // for wchar_t
#include <inttypes.h>
intmax_t wcstoimax(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
Description
[#2] The wcstoimax function is equivalent to wcstol, except
that the initial portion of the wide string is converted to
intmax_t representation.
Returns
[#3] The wcstoimax function returns the converted value, if
any. If no conversion could be performed zero is returned.
If the correct value is outside the range of representable
values, INTMAX_MAX or INTMAX_MIN is returned (according to
the sign of the value), and the value of the macro ERANGE is
stored in errno.
7.4.6.4 The wcstoumax function
Synopsis
[#1]
#include <stddef.h> // for wchar_t
#include <inttypes.h>
uintmax_t wcstoumax(const wchar_t * restrict nptr,
wchar_t ** restrict endptr, int base);
Description
[#2] The wcstoumax function is equivalent to wcstoul, except
that the initial portion of the wide string is converted to
uintmax_t representation.
Returns
[#3] The wcstoumax function returns the converted value, if
any. If no conversion could be performed zero is returned.
If the correct value is outside the range of representable
values, UINTMAX_MAX is returned, and the value of the macro
ERANGE is stored in errno.
page 229 Library
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7.5 Localization <locale.h>
[#1] The header <locale.h> declares two functions, one type,
and defines several macros.
[#2] The type is
struct lconv
which contains members related to the formatting of numeric
values. The structure shall contain at least the following
members, in any order. The semantics of the members and
their normal ranges is explained in 7.5.2.1. In the "C"
locale, the members shall have the values specified in the
comments.
char *decimal_point; // "."
char *thousands_sep; // ""
char *grouping; // ""
char *int_curr_symbol; // ""
char *currency_symbol; // ""
char *mon_decimal_point; // ""
char *mon_thousands_sep; // ""
char *mon_grouping; // ""
char *positive_sign; // ""
char *negative_sign; // ""
char int_frac_digits; // CHAR_MAX
char frac_digits; // CHAR_MAX
char p_cs_precedes; // CHAR_MAX
char p_sep_by_space; // CHAR_MAX
char n_cs_precedes; // CHAR_MAX
char n_sep_by_space; // CHAR_MAX
char p_sign_posn; // CHAR_MAX
char n_sign_posn; // CHAR_MAX
[#3] The macros defined are NULL (described in 7.1.6);
and153
__________
153. ISO/IEC 9945-2, Information technology - Portable
operating system interface (POSIX) - Part 2: shell and
utilities specifies locale and charmap formats that may
be used to specify locales for C.
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LC_ALL
LC_COLLATE
LC_CTYPE
LC_MONETARY
LC_NUMERIC
LC_TIME
which expand to integer constant expressions with distinct
values, suitable for use as the first argument to the
setlocale function. Additional macro definitions, beginning
with the characters LC_ and an uppercase letter,154 may also
be specified by the implementation.
7.5.1 Locale control
7.5.1.1 The setlocale function
Synopsis
[#1]
#include <locale.h>
char *setlocale(int category, const char *locale);
Description
[#2] The setlocale function selects the appropriate portion
of the program's locale as specified by the category and
locale arguments. The setlocale function may be used to
change or query the program's entire current locale or
portions thereof. The value LC_ALL for category names the
program's entire locale; the other values for category name
only a portion of the program's locale. LC_COLLATE affects
the behavior of the strcoll and strxfrm functions. LC_CTYPE
affects the behavior of the character handling functions155
and the multibyte functions. LC_MONETARY affects the
monetary formatting information returned by the localeconv
function. LC_NUMERIC affects the decimal-point character
for the formatted input/output functions and the string
conversion functions, as well as the nonmonetary formatting
information returned by the localeconv function. LC_TIME
affects the behavior of the strftime function.
__________
154. See ``future library directions'' (7.20.4).
155. The only functions in 7.3 whose behavior is not
affected by the current locale are isdigit and isxdigit.
page 231 Library
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[#3] A value of "C" for locale specifies the minimal
environment for C translation; a value of "" for locale
specifies the locale-specific native environment. Other
implementation-defined strings may be passed as the second
argument to setlocale.
[#4] At program startup, the equivalent of
setlocale(LC_ALL, "C");
is executed.
[#5] The implementation shall behave as if no library
function calls the setlocale function.
Returns
[#6] If a pointer to a string is given for locale and the
selection can be honored, the setlocale function returns a
pointer to the string associated with the specified category
for the new locale. If the selection cannot be honored, the
setlocale function returns a null pointer and the program's
locale is not changed.
[#7] A null pointer for locale causes the setlocale function
to return a pointer to the string associated with the
category for the program's current locale; the program's
locale is not changed.156
[#8] The pointer to string returned by the setlocale
function is such that a subsequent call with that string
value and its associated category will restore that part of
the program's locale. The string pointed to shall not be
modified by the program, but may be overwritten by a
subsequent call to the setlocale function.
Forward references: formatted input/output functions
(7.13.6), the multibyte character functions (7.14.7), the
multibyte string functions (7.14.8), string conversion
functions (7.14.1), the strcoll function (7.15.4.3), the
strftime function (7.16.3.6), the strxfrm function
(7.15.4.5).
__________
156. The implementation must arrange to encode in a string
the various categories due to a heterogeneous locale
when category has the value LC_ALL.
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7.5.2 Numeric formatting convention inquiry
7.5.2.1 The localeconv function
Synopsis
[#1]
#include <locale.h>
struct lconv *localeconv(void);
Description
[#2] The localeconv function sets the components of an
object with type struct lconv with values appropriate for
the formatting of numeric quantities (monetary and
otherwise) according to the rules of the current locale.
[#3] The members of the structure with type char * are
pointers to strings, any of which (except decimal_point) can
point to "", to indicate that the value is not available in
the current locale or is of zero length. Apart from
grouping and mon_grouping, the strings shall start and end
in the initial shift state. The members with type char are
nonnegative numbers, any of which can be CHAR_MAX to
indicate that the value is not available in the current
locale. The members include the following:
char *decimal_point
The decimal-point character used to format
nonmonetary quantities.
char *thousands_sep
The character used to separate groups of digits
before the decimal-point character in formatted
nonmonetary quantities.
char *grouping
A string whose elements indicate the size of each
group of digits in formatted nonmonetary quantities.
char *int_curr_symbol
The international currency symbol applicable to the
current locale. The first three characters contain
the alphabetic international currency symbol in
accordance with those specified in ISO 4217:1987.
The fourth character (immediately preceding the null
character) is the character used to separate the
international currency symbol from the monetary
quantity.
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char *currency_symbol
The local currency symbol applicable to the current
locale.
char *mon_decimal_point
The decimal-point used to format monetary quantities.
char *mon_thousands_sep
The separator for groups of digits before the
decimal-point in formatted monetary quantities.
char *mon_grouping
A string whose elements indicate the size of each
group of digits in formatted monetary quantities.
char *positive_sign
The string used to indicate a nonnegative-valued
formatted monetary quantity.
char *negative_sign
The string used to indicate a negative-valued
formatted monetary quantity.
char int_frac_digits
The number of fractional digits (those after the
decimal-point) to be displayed in an internationally
formatted monetary quantity.
char frac_digits
The number of fractional digits (those after the
decimal-point) to be displayed in a formatted
monetary quantity.
char p_cs_precedes
Set to 1 or 0 if the currency_symbol respectively
precedes or succeeds the value for a nonnegative
formatted monetary quantity.
char p_sep_by_space
Set to 1 or 0 if the currency_symbol respectively is
or is not separated by a space from the value for a
nonnegative formatted monetary quantity.
char n_cs_precedes
Set to 1 or 0 if the currency_symbol respectively
precedes or succeeds the value for a negative
formatted monetary quantity.
char n_sep_by_space
Set to 1 or 0 if the currency_symbol respectively is
or is not separated by a space from the value for a
page 234 Library
page 235
negative formatted monetary quantity.
char p_sign_posn
Set to a value indicating the positioning of the
positive_sign for a nonnegative formatted monetary
quantity.
char n_sign_posn
Set to a value indicating the positioning of the
negative_sign for a negative formatted monetary
quantity.
[#4] The elements of grouping and mon_grouping are
interpreted according to the following:
CHAR_MAX No further grouping is to be performed.
0 The previous element is to be repeatedly used for
the remainder of the digits.
other The integer value is the number of digits that
comprise the current group. The next element is
examined to determine the size of the next group
of digits before the current group.
[#5] The value of p_sign_posn and n_sign_posn is interpreted
according to the following:
0 Parentheses surround the quantity and currency_symbol.
1 The sign string precedes the quantity and
currency_symbol.
2 The sign string succeeds the quantity and
currency_symbol.
3 The sign string immediately precedes the currency_symbol.
4 The sign string immediately succeeds the currency_symbol.
[#6] The implementation shall behave as if no library
function calls the localeconv function.
Returns
[#7] The localeconv function returns a pointer to the
filled-in object. The structure pointed to by the return
value shall not be modified by the program, but may be
overwritten by a subsequent call to the localeconv function.
In addition, calls to the setlocale function with categories
LC_ALL, LC_MONETARY, or LC_NUMERIC may overwrite the
page 235 Library
page 236
contents of the structure.
Examples
[#8] The following table illustrates the rules which may
well be used by four countries to format monetary
quantities.
Country Positive format Negative formatInternational format
Italy L.1.234 -L.1.234 ITL.1.234
Netherlands F 1.234,56 F -1.234,56 NLG 1.234,56
Norway kr1.234,56 kr1.234,56- NOK 1.234,56
Switzerland SFrs.1,234.56 SFrs.1,234.56C CHF 1,234.56
[#9] For these four countries, the respective values for the
monetary members of the structure returned by localeconv
are:
Italy Netherlands NorwaySwitzerland
int_curr_symbol "ITL." "NLG " "NOK " "CHF "
currency_symbol "L." "F" "kr" "SFrs."
mon_decimal_point "" "," ",""."
mon_thousands_sep "." "." "."","
mon_grouping "\3" "\3" "\3" "\3"
positive_sign "" "" "" ""
negative_sign "-" "-" "-" "C"
int_frac_digits 0 2 2 2
frac_digits 0 2 2 2
p_cs_precedes 1 1 1 1
p_sep_by_space 0 1 0 0
n_cs_precedes 1 1 1 1
n_sep_by_space 0 1 0 0
p_sign_posn 1 1 1 1
n_sign_posn 1 4 2 2
page 236 Library
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7.6 Floating-point environment <fenv.h>
[#1] The header <fenv.h> declares two types and several
macros and functions to provide access to the floating-point
environment. The floating-point environment refers
collectively to any floating-point status flags and control
modes supported by the implementation.157 A floating-point
status flag is a system variable whose value is set as a
side effect of the arithmetic to provide auxiliary
information. A floating-point control mode is a system
variable whose value may be set by the user to affect the
subsequent behavior of the arithmetic.
[#2] Certain programming conventions support the intended
model of use for the floating-point environment:158
- a function call must not alter its caller's modes,
clear its caller's flags, nor depend on the state of
its caller's flags unless the function is so
documented;
- a function call is assumed to require default modes,
unless its documentation promises otherwise or unless
the function is known not to use floating-point;
- a function call is assumed to have the potential for
raising floating-point exceptions, unless its
documentation promises otherwise, or unless the
function is known not to use floating-point.
[#3] The type
fenv_t
represents the entire floating-point environment.
__________
157. This header is designed to support the exception status
flags and directed-rounding control modes required by
IEC 559, and other similar floating-point state
information. Also it is designed to facilitate code
portability among all systems.
158. With these conventions, a programmer can safely assume
default modes (or be unaware of them). The
responsibilities associated with accessing the
floating-point environment fall on the programmer or
program that does so explicitly.
page 237 Library
page 238
[#4] The type
fexcept_t
represents the floating-point exception flags collectively,
including any status the implementation associates with the
flags.
[#5] Each of the macros
FE_DIVBYZERO
FE_INEXACT
FE_INVALID
FE_OVERFLOW
FE_UNDERFLOW
is defined if and only if the implementation supports the
exception by means of the functions in 7.6.2. The defined
macros expand to integer constant expressions with values
such that bitwise ORs of all combinations of the macros
result in distinct values.
[#6] The macro
FE_ALL_EXCEPT
is simply the bitwise OR of all exception macros defined by
the implementation.
[#7] Each of the macros
FE_DOWNWARD
FE_TONEAREST
FE_TOWARDZERO
FE_UPWARD
is defined if and only if the implementation supports
getting and setting the represented rounding direction by
means of the fegetround and fesetround functions. The
defined macros expand to integer constant expressions whose
values are distinct nonnegative values.159
__________
159. Even though the rounding direction macros may expand to
constants corresponding to the values of FLT_ROUNDS,
they are not required to do so.
page 238 Library
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[#8] The macro
FE_DFL_ENV
represents the default floating-point environment - the one
installed at program startup - and has type pointer to
const-qualified fenv_t. It can be used as an argument to
<fenv.h> functions that manage the floating-point
environment.
[#9] Additional macro definitions, beginning with FE_ and
having type pointer to const-qualified fenv_t, may also be
specified by the implementation.
7.6.1 The FENV_ACCESS pragma
Synopsis
[#1]
#include <fenv.h>
#pragma STDC FENV_ACCESS on-off-switch
Description
[#2] The FENV_ACCESS pragma provides a means to inform the
implementation when a program might access the floating-
point environment to test flags or run under non-default
modes.160 The pragma can occur either outside external
declarations or preceding all explicit declarations and
statements inside a compound statement. When outside
external declarations, the pragma takes effect from its
occurrence until another FENV_ACCESS pragma is encountered,
or until the end of the translation unit. When inside a
compound statement, the pragma takes effect from its
occurrence until another FENV_ACCESS pragma is encountered
(within a nested compound statement), or until the end of
the compound statement; at the end of a compound statement
the state for the pragma is restored to its condition just
__________
160. The purpose of the FENV_ACCESS pragma is to allow
certain optimizations, for example global common
subexpression elimination, code motion, and constant
folding, that could subvert flag tests and mode changes.
In general, if the state of FENV_ACCESS is off then the
translator can assume that default modes are in effect
and the flags are not tested.
page 239 Library
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before the compound statement. If this pragma is used in
any other context, the behavior is undefined. If part of a
program tests flags or runs under non-default mode settings,
but was translated with the state for the FENV_ACCESS pragma
off, then the behavior of that program is undefined. The
default state (on or off) for the pragma is implementation-
defined.
Examples
[#3]
#include <fenv.h>
void f(double x)
{
#pragma STDC FENV_ACCESS ON
void g(double);
void h(double);
/* ... */
g(x + 1);
h(x + 1);
/* ... */
}
If the function g might depend on status flags set as a side
effect of the first x + 1, or if the second x + 1 might
depend on control modes set as a side effect of the call to
function g, then the program must contain an appropriately
placed invocation of #pragma STDC FENV_ACCESS ON.161
__________
161. The side effects impose a temporal ordering that
requires two evaluations of x + 1. On the other hand,
without the #pragma STDC FENV_ACCESS ON pragma, and
assuming the default state is off, just one evaluation
of x + 1 would suffice.
page 240 Library
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7.6.2 Exceptions
[#1] The following functions provide access to the exception
flags.162 The int input argument for the functions
represents a subset of floating-point exceptions, and can be
constructed by bitwise ORs of the exception macros, for
example FE_OVERFLOW | FE_INEXACT. For other argument values
the behavior of these functions is undefined.
7.6.2.1 The feclearexcept function
Synopsis
[#1]
#include <fenv.h>
void feclearexcept(int excepts);
Description
[#2] The feclearexcept function clears the supported
exceptions represented by its argument.
7.6.2.2 The fegetexceptflag function
Synopsis
[#1]
#include <fenv.h>
void fegetexceptflag(fexcept_t *flagp,
int excepts);
Description
[#2] The fegetexceptflag function stores an implementation-
defined representation of the exception flags indicated by
the argument excepts in the object pointed to by the
argument flagp.
__________
162. The functions fetestexcept, feraiseexcept, and
feclearexcept support the basic abstraction of flags
that are either set or clear. An implementation may
endow exception flags with more information - for
example, the address of the code which first raised the
exception; the functions fegetexceptflag and
fesetexceptflag deal with the full content of flags.
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