ISO/IEC 9899:2024
(Main)Information technology — Programming languages — C
Information technology — Programming languages — C
This document specifies the form and establishes the interpretation of programs written in the C programming language. It is designed to promote the portability of C programs among a variety of data-processing systems. It is intended for use by implementers and programmers. 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. This document 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; — all minimal requirements of a data-processing system that is capable of supporting a conforming implementation. Annex J gives an overview of portability issues that a C program can encounter.
Technologies de l'information — Langages de programmation — C
General Information
Relations
Standards Content (Sample)
International
Standard
ISO/IEC 9899
Fifth edition
Information technology —
2024-10
Programming languages — C
Technologies de l'information — Langages de programmation — C
Reference number
© ISO/IEC 2024
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© ISO/IEC 2024 – All rights reserved
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Contents
Foreword xii
Introduction xiii
1 Scope 1
2 Normative references 2
3 Terms, definitions, and symbols 3
4 Conformance 9
5 Environment 11
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2 Conceptual models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.1 Translation environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.2 Execution environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3 Environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.3.1 Character sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.3.2 Multibyte characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.3.3 Character display semantics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.3.4 Signals and interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.3.5 Environmental limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6 Language 35
6.1 Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6.2 Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6.2.1 Scopes of identifiers, type names, and compound literals . . . . . . . . . . . . 35
6.2.2 Linkages of identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
6.2.3 Name spaces of identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6.2.4 Storage durations of objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6.2.5 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
6.2.6 Representations of types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.2.7 Compatible type and composite type . . . . . . . . . . . . . . . . . . . . . . . 43
6.2.8 Alignment of objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.2.9 Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.3 Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
6.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
6.3.2 Arithmetic operands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
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6.3.3 Other operands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
6.4 Lexical elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
6.4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
6.4.2 Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.4.3 Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
6.4.4 Universal character names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
6.4.5 Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
6.4.6 String literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6.4.7 Punctuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.4.8 Header names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.4.9 Preprocessing numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.4.10 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.5 Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.5.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.5.2 Primary expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.5.3 Postfix operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.5.4 Unary operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
6.5.5 Cast operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
6.5.6 Multiplicative operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.5.7 Additive operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6.5.8 Bitwise shift operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.5.9 Relational operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
6.5.10 Equality operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.5.11 Bitwise AND operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6.5.12 Bitwise exclusive OR operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6.5.13 Bitwise inclusive OR operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.5.14 Logical AND operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.5.15 Logical OR operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.5.16 Conditional operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.5.17 Assignment operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
6.5.18 Comma operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
6.6 Constant expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.7 Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.7.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.7.2 Storage-class specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.7.3 Type specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.7.4 Type qualifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
6.7.5 Function specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
6.7.6 Alignment specifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
6.7.7 Declarators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
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6.7.8 Type names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
6.7.9 Type definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.7.10 Type inference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
6.7.11 Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
6.7.12 Static assertions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
6.7.13 Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
6.8 Statements and blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
6.8.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
6.8.2 Labeled statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
6.8.3 Compound statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
6.8.4 Expression and null statements . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
6.8.5 Selection statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
6.8.6 Iteration statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
6.8.7 Jump statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
6.9 External definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
6.9.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
6.9.2 Function definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
6.9.3 External object definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
6.10 Preprocessing directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
6.10.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
6.10.2 Conditional inclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
6.10.3 Source file inclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
6.10.4 Binary resource inclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
6.10.5 Macro replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
6.10.6 Line control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
6.10.7 Diagnostic directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
6.10.8 Pragma directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
6.10.9 Null directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
6.10.10 Predefined macro names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
6.10.11 Pragma operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
6.11 Future language directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
6.11.1 Floating types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
6.11.2 Linkages of identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
6.11.3 External names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
6.11.4 Character escape sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
6.11.5 Storage-class specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
6.11.6 Pragma directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
6.11.7 Predefined macro names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
7 Library 191
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7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
7.1.1 Definitions of terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
7.1.2 Standard headers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
7.1.3 Reserved identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
7.1.4 Use of library functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
7.2 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
7.2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
7.2.2 Program diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
7.3 Complex arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
7.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
7.3.2 Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
7.3.3 Branch cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
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7.3.4 TheCX LIMITED RANGE pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 197
7.3.5 Trigonometric functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
7.3.6 Hyperbolic functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
7.3.7 Exponential and logarithmic functions . . . . . . . . . . . . . . . . . . . . . . 200
7.3.8 Power and absolute-value functions . . . . . . . . . . . . . . . . . . . . . . . . 201
7.3.9 Manipulation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
7.4 Character handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
7.4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
7.4.2 Character classification functions . . . . . . . . . . . . . . . . . . . . . . . . . . 205
7.4.3 Character case mapping functions . . . . . . . . . . . . . . . . . . . . . . . . . 207
7.5 Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
7.6 Floating-point environment . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
7.6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
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7.6.2 TheFENV ACCESS pragma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
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7.6.3 TheFENV ROUND pragma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
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7.6.4 TheFENV DEC ROUND pragma . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
7.6.5 Floating-point exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
7.6.6 Rounding and other control modes . . . . . . . . . . . . . . . . . . . . . . . . 218
7.6.7 Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
7.7 Characteristics of floating types . . . . . . . . . . . . . . . . . . . . . . . . 223
7.8 Format conversion of integer types . . . . . . . . . . . . . . . . . . . . 224
7.8.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
7.8.2 Macros for format specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
7.8.3 Functions for greatest-width integer types . . . . . . . . . . . . . . . . . . . . 225
7.9 Alternative spellings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
7.10 Characteristics of integer types . . . . . . . . . . . . . . . . . . . . . . . . 228
7.11 Localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
7.11.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
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7.11.2 Thesetlocale function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
7.11.3 Numeric formatting convention inquiry . . . . . . . . . . . . . . . . . . . . . . 230
7.12 Mathematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
7.12.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
7.12.2 Treatment of error conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
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7.12.3 TheFP CONTRACT pragma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
7.12.4 Classification macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
7.12.5 Trigonometric functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
7.12.6 Hyperbolic functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
7.12.7 Exponential and logarithmic functions . . . . . . . . . . . . . . . . . . . . . . 249
7.12.8 Power and absolute-value functions . . . . . . . . . . . . . . . . . . . . . . . . 257
7.12.9 Error and gamma functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
7.12.10 Nearest integer functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
7.12.11 Remainder functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
7.12.12 Manipulation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
7.12.13 Maximum, minimum, and positive difference functions . . . . . . . . . . . . 270
7.12.14 Fused multiply-add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
7.12.15 Functions that round result to narrower type . . . . . . . . . . . . . . . . . . . 275
7.12.16 Quantum and quantum exponent functions . . . . . . . . . . . . . . . . . . . 277
7.12.17 Decimal re-encoding functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
7.12.18 Comparison macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
7.13 Non-local jumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
7.13.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
7.13.2 Save calling environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
7.13.3 Restore calling environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
7.14 Signal handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
7.14.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
7.14.2 Specify signal handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
7.14.3 Send signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
7.15 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289
7.16 Variable arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
7.16.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
7.16.2 Variable argument list access macros . . . . . . . . . . . . . . . . . . . . . . . . 290
7.17 Atomics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
7.17.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
7.17.2 Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
7.17.3 Order and consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296
7.17.4 Fences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298
7.17.5 Lock-free property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
7.17.6 Atomic integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
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7.17.7 Operations on atomic types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301
7.17.8 Atomic flag type and operations . . . . . . . . . . . . . . . . . . . . . . . . . . 303
7.18 Bit and byte utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
7.18.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
7.18.2 Endian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
7.18.3 Count Leading Zeros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
7.18.4 Count Leading Ones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
7.18.5 Count Trailing Zeros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
7.18.6 Count Trailing Ones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
7.18.7 First Leading Zero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
7.18.8 First Leading One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
7.18.9 First Trailing Zero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
7.18.10 First Trailing One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309
7.18.11 Count Zeros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
7.18.12 Count Ones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
7.18.13 Single-bit Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
7.18.14 Bit Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
7.18.15 Bit Floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
7.18.16 Bit Ceiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
7.19 Boolean type and values . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
7.20 Checked Integer Arithmetic . . . . . . . . . . . . . . . . . . . . . . . 314
7.20.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314
7.20.2 Checked Integer Operation Type-generic Macros . . . . . . . . . . . . . . . . . 314
7.21 Common definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
7.21.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
7.21.2 Theunreachable macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
_
7.21.3 Thenullptr t type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317
7.22 Integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318
7.22.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318
7.22.2 Integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318
7.22.3 Widths of specified-width integer types . . . . . . . . . . . . . . . . . . . . . . 320
7.22.4 Width of other integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320
7.22.5 Macros for integer constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
7.22.6 Maximal and minimal values of integer types . . . . . . . . . . . . . . . . . . 321
7.23 Input/output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322
7.23.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322
7.23.2 Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324
7.23.3 Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
7.23.4 Operations on files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326
7.23.5 File access functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
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7.23.6 Formatted input/output functions . . . . . . . . . . . . . . . . . . . . . . . . . 331
7.23.7 Character input/output functions . . . . . . . . . . . . . . . . . . . . . . . . . 349
7.23.8 Direct input/output functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
7.23.9 File positioning functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353
7.23.10 Error-handling functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
7.24 General utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
7.24.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
7.24.2 Numeric conversion functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
7.24.3 Pseudo-random sequence generation functions . . . . . . . . . . . . . . . . . 364
7.24.4 Memory management functions . . . . . . . . . . . . . . . . . . . . . . . . . . 365
7.24.5 Communication with the environment . . . . . . . . . . . . . . . . . . . . . . 368
7.24.6 Searching and sorting utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
7.24.7 Integer arithmetic functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372
7.24.8 Multibyte/wide character conversion functions . . . . . . . . . . . . . . . . . 373
7.24.9 Multibyte/wide string conversion functions . . . . . . . . . . . . . . . . . . . 374
7.24.10 Alignment of memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
_
7.25 Noreturn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
7.26 String handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
7.26.1 String function conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
7.26.2 Copying functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
7.26.3 Concatenation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380
7.26.4 Comparison functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
7.26.5 Search functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382
7.26.6 Miscellaneous functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385
7.27 Type-generic math . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
7.28 Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392
7.28.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392
7.28.2 Initialization functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393
7.28.3 Condition variable functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393
7.28.4 Mutex functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395
7.28.5 Thread functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397
7.28.6 Thread-specific storage functions . . . . . . . . . . . . . . . . . . . . . . . . . . 399
7.29 Date and time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402
7.29.1 Components of time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402
7.29.2 Time manipulation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403
7.29.3 Time conversion functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406
7.30 Unicode utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
7.30.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
7.30.2 Restartable multibyte/wide character conversion functions . . . . . . . . . . 412
7.31 Extended multibyte and wide character utilities . . . . . . . . . . . . . . . 417
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7.31.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417
7.31.2 Formatted wide character input/output functions . . . . . . . . . . . . . . . . 418
7.31.3 Wide character input/output functions . . . . . . . . . . . . . . . . . . . . . . 431
7.31.4 General wide string utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
7.31.4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
7.31.4.2 Wide string numeric conversion functions . . . . . . . . . . . . . . . 435
7.31.4.3 Wide string copying functions . . . . . . . . . . . . . . . . . . . . . . 440
7.31.4.4 Wide string concatenation functions . . . . . . . . . . . . . . . . . . . 441
7.31.4.5 Wide string comparison functions . . . . . . . . . . . . . . . . . . . . 441
7.31.4.6 Wide string search functions . . . . . . . . . . . . . . . . . . . . . . . 443
7.31.4.7 Miscellaneous functions . . . . . . . . . . . . . . . . . . . . . . . . . 446
7.31.5 Wide character time conversion functions . . . . . . . . . . . . . . . . . . . . . 446
7.31.6 Extended multibyte/wide character conversion utilities . . . . . . . . . . . . 447
7.31.6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
7.31.6.2 Single-byte/wide character conversion functions . . . . . . . . . . . 447
7.31.6.3 Conversion state functions . . . . . . . . . . . . . . . . . . . . . . . . 448
7.31.6.4 Restartable multibyte/wide character conversion functions . . . . . 448
7.31.6.5 Restartable multibyte/wide string conversion functions . . . . . . . 450
7.32 Wide character classification and mapping utilities . . . . . . . . . . . . 452
7.32.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452
7.32.2 Wide character classification utilities . . . . . . . . . . . . . . . . . . . . . . . . 452
7.32.2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452
7.32.2.2 Wide character classification functions . . . . . . . . . . . . . . . . . 452
7.32.2.3 Extensible wide character classification functions . . . . . . . . . . . 455
7.32.3 Wide character case mapping utilities . . . . . . . . . . . . . . . . . . . . . . . 456
7.32.3.1 Wide character case mapping functions . . . . . . . . . . . . . . . . . 456
7.32.3.2 Extensible wide character case mapping functions . . . . . . . . . . 456
7.33 Future library directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458
7.33.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458
7.33.2 Complex arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . 458
7.33.3 Character handling . . . . . . . . . . . . . . . . . . . . . . . . . . . 458
7.33.4 Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458
7.33.5 Floating-point environment . . . . . . . . . . . . . . . . . . . . . . . 458
7.33.6 Characteristics of floating types . . . . . . . . . . . . . . . . . . . . 458
7.33.7 Format conversion of integer types . . . . . . . . . . . . . . . . 458
7.33.8 Localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458
7.33.9 Mathematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458
7.33.10 Signal handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
7.33.11 Atomics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
7.33.12 Boolean type and values . . . . . . . . . . . . . . . . . . . . . . 459
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7.33.13 Bit and byte utilities . . . . . . . . . . . . . . . . . . . . . . . . . . 459
7.33.14 Checked Arithmetic Functions . . . . . . . . . . . . . . . . . 459
7.33.15 Integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
7.33.16 Input/output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
7.33.17 General utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
7.33.18 String handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460
7.33.19 Date and time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460
7.33.20 Thr eads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460
7.33.21 Extended multibyte and wide character utilities . . . . . . . . . . 460
7.33.22 Wide character classification and mapping utilities . . . . . . . . 460
Annex A (informative) Language syntax summary 461
Annex B (informative) Library summary 476
Annex C (informative) Sequence points 516
Annex D (informative) Universal character names for identifiers 517
Annex E (informative) Implementation limits 519
Annex F (normative) ISO/IEC 60559 floating-point arithmetic 522
Annex G (normative) ISO/IEC 60559-compatible complex arithmetic 553
Annex H (normative) ISO/IEC 60559 interchange and extended types 564
Annex I (informative) Common warnings 597
Annex J (informative) Portability issues 598
Annex K (normative) Bounds-checking interfaces 636
Annex L (normative) Analyzability 684
Annex M (informative) Change History 685
Bibliography 691
Index 692
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Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that
are members of ISO or IEC participate in the development of International Standards through
technical committees established by the respective organization to deal with particular fields of
technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other
international organizations, governmental and non-governmental, in liaison with ISO and IEC, also
take part in the work.
The procedures used to develop this document and those intended for its further maintenance
are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria
needed for the different types of document should be noted. This document was drafted in ac-
cordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives or
www.iec.ch/members_experts/refdocs).
ISO and IEC draw attention to the possibility that the implementation of this document may involve
the use of (a) patent(s). ISO and IEC take no position concerning the evidence, validity or applicability
of any claimed patent rights in respect thereof. As of the date of publication of this document, ISO
and IEC had not received notice of (a) patent(s) which may be required to implement this document.
However, implementers are cautioned that this may not represent the latest information, which may
be obtained from the patent database available at www.iso.org/patents and patents.iec.ch. ISO and
IEC shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does
not constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see
www.iso.org/iso/foreword.html. In the IEC, see www.iec.ch/understanding-standards.
This document was prepared by Joint Technical Committee ISO/IEC JTC1, Information technology,
Subcommittee SC22, Programming languages, their environments and system software interfaces.
This fifth edition cancels and replaces the fourth edition (ISO/IEC 9899:2018), which has been
technically revised. The main changes are contained in Annex M.
Any feedback or questions on this document should be directed to the user’s national stan-
dards body. A complete listing of these bodies can be found at www.iso.org/members.html and
www.iec.ch/national-committees.
© ISO/IEC 2024 — All rights reserved
xii
Introduction
With the introduction of new devices and extended character sets, new features could be added to
future editions of this document. Subclauses in the language and library clauses warn implementers
and programmers of usages which, though valid in themselves, could conflict with future additions.
Certain features are obsolescent, which means that they could be considered for withdrawal in future
revisions of this document. They are retained because of their widespread use, but their use in
new implementations (for implementation features) or new programs (for language [6.11] or library
features [7.33]) is discouraged.
This document is divided into four major subdivisions:
— preliminary elements (Clauses 1–4);
— the characteristics of environments that translate and execute C programs (Clause 5);
— the language syntax, constraints, and semantics (Clause 6);
— the library facilities (Clause 7).
In any given subsequent clause or subclause, there are section delineations in bold to describe the
semantics, restrictions, and behaviors of programs for this language and potentially the use of its
library clauses in this document:
— Syntax
which pertains to the spelling and organization of the language and library;
— Constraints
which detail and enumerate various requirements for the correct interpretation of the language
and library, typically during translation;
— Semantics
which explain the behavior of language features and similar constructs;
— Description
which explain the behavior of library usage and similar constructs;
— Returns
which describes the effects of constructs provided back to a user of the library;
— Runtime-constraints
which detail and enumerate various requirements that are expected to be checked and which
shall not be violated, typically during execution;
— Environmental limits
which list limitations an implementation may impose on a library or language construct which
might otherwise be unlimited;
— Recommended practice
which provides guidance and important considerations for implementers of this document.
Examples are provided to illustrate possible forms of the constructions described. Footnotes are
provided to emphasize consequences of the rules described in that subclause or elsewhere in this
document. References are used to refer to other related subclauses. Recommendations are provided
to give advice or guidance to implementers. Annexes define optional features, provide additional
© ISO/IEC 2024 — All rights reserved
xiii
information and summarize the information contained in this document. A bibliography lists
documents that were referred to during the preparation of this document.
The language clause (Clause 6) is derived from "The C Reference Manual"[15].
The library clause (Clause 7) is based on the 1984 /usr/group Standard[16].
© ISO/IEC 2024 — All rights reserved
xiv
INTERNATIONAL STANDARD ©ISO/IEC ISO/IEC 9899:2024
Information technology — Programming languages — C
1. Scope
This document specifies the form and establishes the interpretation of programs written in the C
programming language. It is designed to promote the portability of C programs among a variety of
data-processing systems. It is intended for use by implementers and programmers. 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.
This document 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;
— all minimal requirements of a data-processing system that is capable of supporting a conform-
ing implementation.
Annex J gives an overview of portability issues that a C program can encounter.
© ISO/IEC 2024 — All rights reserved
2. Normative references
The following documents are referred to in the text in such a way that some or all their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any amendments)
applies.
ISO/IEC 2382:2015, Information technology — Vocabulary.
ISO 4217, Codes for the representation of currencies.
ISO 8601 series, Data elements and interchange formats — Information interchange — Representation of
dates and times.
ISO/IEC 10646, Information technology —Universal Coded Character Set (UCS).
ISO/IEC 60559:2020, Information technology — Microprocessor Systems — Floating-Point arithmetic.
ISO 80000–2, Quantities and units — Part 2: Mathematics.
The Unicode Consortium. Unicode Standard Annex, UAX #44, Unicode Character Database [online].
Edited by Ken Whistler. Available athttps://www.unicode.org/reports/tr44.
The Unicode Consortium. The Unicode Standard, Derived Core Properties. Available athttps://www.
unicode.org/Public/UCD/latest/ucd/DerivedCoreProperties.txt.
© ISO/IEC 2024 — All rights reserved
3. Terms, definitions, and symbols
For the purposes of this document, the terms and definitions given in ISO/IEC 2382, ISO 80000–2,
and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available athttps://www.iso.org/obp
— IEC Electropedia: available athttps://www.electropedia.org/
Additional terms are defined where they appear in italic type or on the left side of a syntax rule.
Terms explicitly defined in this document are not to be presumed to refer implicitly to similar terms
defined elsewhere.
3.1
access (verb)
⟨execution-time action⟩ read or modify the value of an object
Note 1 to entry: Where only one of these two actions is meant, "read" or "modify" is used.
Note 2 to entry: "Modify" includes the case where the new value being stored is the same as the previous
value.
Note 3 to entry: Expressions that are not evaluated do not access objects.
3.2
alignment
requirement that objects of a particular type be located on storage boundaries with addresses that
are particular multiples of a byte (3.7) address
3.3
argument
actual argument
DEPRECATED: actual parameter
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
3.4
arithmetically negate
produce the negative of a given number
Note 1 to entry: For a floating-point number (5.3.5.3.3), this changes the sign; for an integer, this is equivalent
to subtracting from zero.
3.5
behavior
external appearance or action
3.5.1
implementation-defined behavior
unspecified behavior where each implementation documents how the choice is made
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Note 1 to entry: J.3 gives an overview over properties of C programs that lead to implementation-defined
behavior.
EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a
signed integer is shifted right.
3.5.2
locale-specific behavior
behavior that depends on local conventions of nationality, culture, and language that each implemen-
tation documents
Note 1 to entry: J.4 gives an overview over properties of C programs that lead to locale-specific behavior.
EXAMPLE An e
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