ISO/IEC 9899:2018

INTERNATIONAL ISO/IEC
STANDARD 9899
Fourth edition
2018-07
Information technology —
Programming languages — C
Technologies de l'information — Langages de programmation — C
Reference number
©
ISO/IEC 2018
© ISO/IEC 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
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Published in Switzerland
ii © ISO/IEC 2018 – All rights reserved

© ISO/IEC 2018– All rights reserved ISO/IEC 9899:2018 (E)
Contents
Foreword xi
Introduction xii
1 Scope
2 Normative references 2
3 Terms, definitions and symbols 3
4 Conformance
5 Environment 9
5.1 Conceptual models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1.1 Translation environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1.2 Execution environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2 Environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2.1 Character sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2.2 Character display semantics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2.3 Signals and interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2.4 Environmental limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6 Language 28
6.1 Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
6.2 Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
6.2.1 Scopes of identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
6.2.2 Linkages of identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.2.3 Name spaces of identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.2.4 Storage durations of objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6.2.5 Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
6.2.6 Representations of types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.2.7 Compatible type and composite type . . . . . . . . . . . . . . . . . . . . . . . 35
6.2.8 Alignment of objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
6.3 Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6.3.1 Arithmetic operands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6.3.2 Other operands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
6.4 Lexical elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6.4.1 Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.4.2 Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Contents iii
6.4.3 Universal character names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.4.4 Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.4.5 String literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
6.4.6 Punctuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
6.4.7 Header names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.4.8 Preprocessing numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.4.9 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
6.5 Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6.5.1 Primary expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
6.5.2 Postfix operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
6.5.3 Unary operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
6.5.4 Cast operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
6.5.5 Multiplicative operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
6.5.6 Additive operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
6.5.7 Bitwise shift operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.5.8 Relational operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.5.9 Equality operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.5.10 Bitwise AND operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.5.11 Bitwise exclusive OR operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.5.12 Bitwise inclusive OR operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.5.13 Logical AND operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.5.14 Logical OR operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.5.15 Conditional operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.5.16 Assignment operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.5.17 Comma operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
6.6 Constant expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.7 Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
6.7.1 Storage-class specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.7.2 Type specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.7.3 Type qualifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.7.4 Function specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6.7.5 Alignment specifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.7.6 Declarators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.7.7 Type names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.7.8 Type definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.7.9 Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
6.7.10 Static assertions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.8 Statements and blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.8.1 Labeled statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.8.2 Compound statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
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© ISO/IEC 2018– All rights reserved ISO/IEC 9899:2018 (E)
6.8.3 Expression and null statements . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.8.4 Selection statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
6.8.5 Iteration statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
6.8.6 Jump statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
6.9 External definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
6.9.1 Function definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
6.9.2 External object definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
6.10 Preprocessing directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
6.10.1 Conditional inclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
6.10.2 Source file inclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
6.10.3 Macro replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
6.10.4 Line control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.10.5 Error directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
6.10.6 Pragma directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
6.10.7 Null directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
6.10.8 Predefined macro names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
6.10.9 Pragma operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
6.11 Future language directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.11.1 Floating types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.11.2 Linkages of identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.11.3 External names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.11.4 Character escape sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.11.5 Storage-class specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.11.6 Function declarators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.11.7 Function definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.11.8 Pragma directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.11.9 Predefined macro names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
7 Library 131
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
7.1.1 Definitions of terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
7.1.2 Standard headers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
7.1.3 Reserved identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
7.1.4 Use of library functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
7.2 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7.2.1 Program diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7.3 Complex arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
7.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
7.3.2 Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
7.3.3 Branch cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
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7.3.4 TheCX LIMITED RANGE pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 137
7.3.5 Trigonometric functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
7.3.6 Hyperbolic functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
7.3.7 Exponential and logarithmic functions . . . . . . . . . . . . . . . . . . . . . . 140
7.3.8 Power and absolute-value functions . . . . . . . . . . . . . . . . . . . . . . . . 141
7.3.9 Manipulation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
7.4 Character handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
7.4.1 Character classification functions . . . . . . . . . . . . . . . . . . . . . . . . . . 145
7.4.2 Character case mapping functions . . . . . . . . . . . . . . . . . . . . . . . . . 147
7.5 Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
7.6 Floating-point environment . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
_
7.6.1 TheFENV ACCESS pragma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.6.2 Floating-point exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
7.6.3 Rounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
7.6.4 Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
7.7 Characteristics of floating types . . . . . . . . . . . . . . . . . . . . . . . . 157
7.8 Format conversion of integer types . . . . . . . . . . . . . . . . . . . . 158
7.8.1 Macros for format specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
7.8.2 Functions for greatest-width integer types . . . . . . . . . . . . . . . . . . . . 159
7.9 Alternative spellings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
7.10 Sizes of integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
7.11 Localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
7.11.1 Locale control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
7.11.2 Numeric formatting convention inquiry . . . . . . . . . . . . . . . . . . . . . . 164
7.12 Mathematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
7.12.1 Treatment of error conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
_
7.12.2 TheFP CONTRACT pragma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
7.12.3 Classification macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
7.12.4 Trigonometric functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
7.12.5 Hyperbolic functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
7.12.6 Exponential and logarithmic functions . . . . . . . . . . . . . . . . . . . . . . 177
7.12.7 Power and absolute-value functions . . . . . . . . . . . . . . . . . . . . . . . . 180
7.12.8 Error and gamma functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
7.12.9 Nearest integer functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
7.12.10 Remainder functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
7.12.11 Manipulation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
7.12.12 Maximum, minimum, and positive difference functions . . . . . . . . . . . . 187
7.12.13 Floating multiply-add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
7.12.14 Comparison macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
7.13 Nonlocal jumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
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© ISO/IEC 2018– All rights reserved ISO/IEC 9899:2018 (E)
7.13.1 Save calling environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
7.13.2 Restore calling environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
7.14 Signal handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
7.14.1 Specify signal handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
7.14.2 Send signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
7.15 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
7.16 Variable arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
7.16.1 Variable argument list access macros . . . . . . . . . . . . . . . . . . . . . . . . 197
7.17 Atomics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
7.17.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
7.17.2 Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
7.17.3 Order and consistency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
7.17.4 Fences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
7.17.5 Lock-free property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
7.17.6 Atomic integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
7.17.7 Operations on atomic types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
7.17.8 Atomic flag type and operations . . . . . . . . . . . . . . . . . . . . . . . . . . 208
7.18 Boolean type and values . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
7.19 Common definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
7.20 Integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
7.20.1 Integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
7.20.2 Limits of specified-width integer types . . . . . . . . . . . . . . . . . . . . . . 213
7.20.3 Limits of other integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
7.20.4 Macros for integer constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
7.21 Input/output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
7.21.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
7.21.2 Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
7.21.3 Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
7.21.4 Operations on files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
7.21.5 File access functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
7.21.6 Formatted input/output functions . . . . . . . . . . . . . . . . . . . . . . . . . 225
7.21.7 Character input/output functions . . . . . . . . . . . . . . . . . . . . . . . . . 241
7.21.8 Direct input/output functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
7.21.9 File positioning functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
7.21.10 Error-handling functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
7.22 General utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
7.22.1 Numeric conversion functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
7.22.2 Pseudo-random sequence generation functions . . . . . . . . . . . . . . . . . 253
7.22.3 Memory management functions . . . . . . . . . . . . . . . . . . . . . . . . . . 254
7.22.4 Communication with the environment . . . . . . . . . . . . . . . . . . . . . . 256
Contents vii
7.22.5 Searching and sorting utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
7.22.6 Integer arithmetic functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
7.22.7 Multibyte/wide character conversion functions . . . . . . . . . . . . . . . . . 261
7.22.8 Multibyte/wide string conversion functions . . . . . . . . . . . . . . . . . . . 262
_
7.23 Noreturn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
7.24 String handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
7.24.1 String function conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
7.24.2 Copying functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
7.24.3 Concatenation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
7.24.4 Comparison functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
7.24.5 Search functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
7.24.6 Miscellaneous functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271
7.25 Type-generic math . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
7.26 Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
7.26.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
7.26.2 Initialization functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
7.26.3 Condition variable functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
7.26.4 Mutex functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
7.26.5 Thread functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
7.26.6 Thread-specific storage functions . . . . . . . . . . . . . . . . . . . . . . . . . . 282
7.27 Date and time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
7.27.1 Components of time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
7.27.2 Time manipulation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
7.27.3 Time conversion functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
7.28 Unicode utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
7.28.1 Restartable multibyte/wide character conversion functions . . . . . . . . . . 293
7.29 Extended multibyte and wide character utilities . . . . . . . . . . . . . . . 296
7.29.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296
7.29.2 Formatted wide character input/output functions . . . . . . . . . . . . . . . . 296
7.29.3 Wide character input/output functions . . . . . . . . . . . . . . . . . . . . . . 308
7.29.4 General wide string utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
7.29.4.1 Wide string numeric conversion functions . . . . . . . . . . . . . . . 312
7.29.4.2 Wide string copying functions . . . . . . . . . . . . . . . . . . . . . . 315
7.29.4.3 Wide string concatenation functions . . . . . . . . . . . . . . . . . . 316
7.29.4.4 Wide string comparison functions . . . . . . . . . . . . . . . . . . . . 316
7.29.4.5 Wide string search functions . . . . . . . . . . . . . . . . . . . . . . . 318
7.29.4.6 Miscellaneous functions . . . . . . . . . . . . . . . . . . . . . . . . . 321
7.29.5 Wide character time conversion functions . . . . . . . . . . . . . . . . . . . . . 321
7.29.6 Extended multibyte/wide character conversion utilities . . . . . . . . . . . . 322
7.29.6.1 Single-byte/wide character conversion functions . . . . . . . . . . . 322
viii Contents
© ISO/IEC 2018– All rights reserved ISO/IEC 9899:2018 (E)
7.29.6.2 Conversion state functions . . . . . . . . . . . . . . . . . . . . . . . . 323
7.29.6.3 Restartable multibyte/wide character conversion functions . . . . . 323
7.29.6.4 Restartable multibyte/wide string conversion functions . . . . . . . 325
7.30 Wide character classification and mapping utilities . . . . . . . . . . . . 327
7.30.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327
7.30.2 Wide character classification utilities . . . . . . . . . . . . . . . . . . . . . . . . 327
7.30.2.1 Wide character classification functions . . . . . . . . . . . . . . . . . 327
7.30.2.2 Extensible wide character classification functions . . . . . . . . . . . 330
7.30.3 Wide character case mapping utilities . . . . . . . . . . . . . . . . . . . . . . . 331
7.30.3.1 Wide character case mapping functions . . . . . . . . . . . . . . . . . 331
7.30.3.2 Extensible wide character case mapping functions . . . . . . . . . . 331
7.31 Future library directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
7.31.1 Complex arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . 333
7.31.2 Character handling . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
7.31.3 Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
7.31.4 Floating-point environment . . . . . . . . . . . . . . . . . . . . . . . 333
7.31.5 Format conversion of integer types . . . . . . . . . . . . . . . . 333
7.31.6 Localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
7.31.7 Signal handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
7.31.8 Atomics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
7.31.9 Boolean type and values . . . . . . . . . . . . . . . . . . . . . . 333
7.31.10 Integer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
7.31.11 Input/output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
7.31.12 General utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
7.31.13 String handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
7.31.14 Date and time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
7.31.15 Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
7.31.16 Extended multibyte and wide character utilities . . . . . . . . . . 334
7.31.17 Wide character classification and mapping utilities . . . . . . . . 334
Annex A (informative) Language syntax summary 335
Annex B (informative) Library summary 347
Annex C (informative) Sequence points 367
Annex D (normative) Universal character names for identifiers 368
Annex E (informative) Implementation limits 369
Annex F (normative) IEC 60559 floating-point arithmetic 371
Annex G (normative) IEC 60559-compatible complex arithmetic 389
Contents ix
Annex H (informative) Language independent arithmetic 399
Annex I (informative) Common warnings 403
Annex J (informative) Portability issues 404
Annex K (normative) Bounds-checking interfaces 425
Annex L (normative) Analyzability 474
Annex M (informative) Change History 476
Bibliography 479
Index 480
x Contents
© ISO/IEC 2018– All rights reserved ISO/IEC 9899:2018 (E)
Foreword
1 ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that
are member 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. In the field of information technology, ISO and IEC have established a joint
technical committee, ISO/IEC JTC 1.
2 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 accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
3 Attention is drawn to the possibility that some of the elements of this document may be the subject
of patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent
rights. Details of any patent rights identified during the development of the document will be in the
Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents).
4 Any trade name used in this document is information given for the convenience of users and does
not constitute an endorsement.
5 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 the
following URL: www.iso.org/iso/foreword.html.
6 This document was prepared by Technical Committee ISO/IEC JTC 1, Information technology, Sub-
committee SC 22, Programming languages, their environments and system software interfaces.
7 This fourth edition cancels and replaces the third edition, ISO/IEC 9899:2011, which has been
technically revised. It also incorporates the Technical Corrigendum ISO/IEC 9899:2011/Cor 1:2012.
8 There are no major changes in this edition, only technical corrections and clarifications.
9 A complete change history can be found in Annex M.
Foreword xi
Introduction
1 With the introduction of new devices and extended character sets, new features could be added to
this document. Subclauses in the language and library clauses warn implementors and programmers
of usages which, though valid in themselves, could conflict with future additions.
2 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 [7.31]) is discouraged.
3 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).
4 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 implementors. Annexes define optional features, provide additional
information and summarize the information contained in this document. A bibliography lists
documents that were referred to during the preparation of this document.
5 The language clause (Clause 6) is derived from "The C Reference Manual".
6 The library clause (Clause 7) is based on the 1984 /usr/group Standard.
7 The Working Group responsible for this document (WG 14) maintains a site on the World Wide Web
athttp://www.open-std.org/JTC1/SC22/WG14/ containing ancillary information that may be of
interest to some readers such as a Rationale for many of the decisions made during its preparation
and a log of Defect Reports and Responses.
xii Introduction
© ISO/IEC 2018– All rights reserved ISO/IEC 9899:2018 (E)
INTERNATIONAL STANDARD ©ISO/IEC ISO/IEC 9899:2018
Programming languages — C
1. Scope
1 This document specifies the form and establishes the interpretation of programs written in the C
1)
programming language. 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 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.
1)
This document 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. Annex J gives an overview of portability issues that a C program might
encounter.
§ 1 General 1
2. Normative references
1 The following documents are referred to in the text in such a way that some or all of 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.
2 ISO/IEC 2382:2015, Information technology — Vocabulary. Available from the ISO online browsing
platform at http://www.iso.org/obp
3 ISO 4217, Codes for the representation of currencies and funds
4 ISO 8601, Data elements and interchange formats — Information interchange — Representation of dates and
times
ISO/IEC 10646, Information technology — Universal Coded Character Set (UCS). Available from the
ISO/IEC Information Technology Task Force (ITTF) web site at http://isotc.iso.org/livelink/
_
livelink/fetch/2000/2489/Ittf Home/PubliclyAvailableStandards.htm
IEC 60559:1989, Binary floating-point arithmetic for microprocessor systems (previously designated
IEC 559:1989)
ISO 80000–2, Quantities and units — Part 2: Mathematical signs and symbols to be used in the natural
sciences and technology
2 General § 2
© ISO/IEC 2018– All rights reserved ISO/IEC 9899:2018 (E)
3. Terms, definitions and symbols
1 For the purposes of this document, the terms and definitions given in ISO/IEC 2382, ISO 80000–2,
and the following apply.
2 ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available athttps://www.iso.org/obp
— IEC Electropedia: available athttp://www.electropedia.org/
3 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
1 access (verb)
⟨execution-time action⟩ to read or modify the value of an object
2 Note 1 to entry: Where only one of these two actions is meant, "read" or "modify" is used.
3 Note 2 to entry: "Modify" includes the case where the new value being stored is the same as the previous value.
4 Note 3 to entry: Expressions that are not evaluated do not access objects.
3.2
1 alignment
requirement that objects of a particular type be located on storage boundaries with addresses that
are particular multiples of a byte address
3.3
1 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
1 behavior
external appearance or action
3.4.1
1 implementation-defined behavior
unspecified behavior where each implementation documents how the choice is made
2 Note 1 to entry: J.3 gives an overview over properties of C programs that lead to implementation-defined behavior.
3 EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer
is shifted right.
3.4.2
1 locale-specific behavior
behavior that depends on local conventions of nationality, culture, and language that each implemen-
tation documents
§ 3.4.2 General 3
© ISO/IEC 2018– All rights reserved
2 Note 1 to entry: J.4 gives an overview over properties of C programs that lead to locale-specific behavior.
3 EXAMPLE An example of locale-specific behavior is whether theislower function returns true for characters other than
the 26 lowercase Latin letters.
3.4.3
1 undefined behavior
behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which
this document imposes no requirements
2 Note 1 to entry: Possible 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).
3 Note 2 to entry: J.2 gives an overview over properties of C programs that lead to undefined behavior.
4 EXAMPLE An example of undefined behavior is the behavior on integer overflow.
3.4.4
1 unspecified behavior
behavior, that results from the use of an unspecified value, or other behavior upon which this
document provides two or more possibilities and imposes no further requirements on which is
chosen in any instance
2 Note 1 to entry: J.1 gives an overview over properties of C programs that lead to unspecified behavior.
3 EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.
3.5
1 bit
unit of data storage in the execution environment large enough to hold an object that can have one
of two values
2 Note 1 to entry: It need not be possible to express the address of each individual bit of an object.
3.6
1 byte
addressable unit of data storage large enough to hold any member of the basic character set of the
execution environment
2 Note 1 to entry: It is possible to express the address of each individual byte of an object uniquely.
3 Note 2 to entry: 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.
3.7
1 character
⟨abstract⟩ member of a set of elements used for the organization, control, or representation of data
3.7.1
1 character
single-byte character
⟨C⟩ bit representation that fits in a byte
3.7.2
1 multibyte character
sequence of one or more bytes representing a member of the extended character set of either the
source or the execution environment
2 Note 1 to entry: The extended character set is a superset of the basic character set.
4 General § 3.7.2
© ISO/IEC 2018– All rights reserved ISO/IEC 9899:2018 (E)
3.7.3
1 wide character
_
value representable by an object of type wchar t, capable of representing any character in the
current locale
3.8
1 constraint
restriction, either syntactic or semantic, by which the exposition of language elements is to be
interpreted
3.9
1 correctly rounded result
representation in the result format that is nearest in value, subject to the current rounding mode, to
what the result would be given unlimited range and precision
2 Note 1 to entry: In this document, when the words "correctly rounded" are not immediately followed by "result", this is the
intended usage.
3.10
1 diagnostic message
message belonging to an implementation-defined subset of the implementation’s message output
3.11
1 forward reference
reference to a later subclause of this document that contains additional information relevant to this
subclause
3.12
1 implementation
particular set of software, running in a particular translation environment under particular con-
trol options, that performs translation of programs for, and supports execution of functions in, a
particular execution environment
3.13
1 implementation limit
restriction imposed upon programs by the implementation
3.14
1 memory location
either an object of scalar type, or a maximal sequence of adjacent bit-fields all having nonzero width
2 Note 1 to entry: Two threads of execution can update and access separate memory locations without interfering with each
other.
3 Note 2 to entry: A bit-field and an adjacent non-bit-field member are in separate memory locations. The same applies to
two bit-fields, if one is declared inside a nested structure declaration and the other is not, or if the two are separated by a
zero-length bit-field declaration, or if they are separated by a non-bit-field member declaration. It is not safe to concurrently
update two non-atomic bit-fields in the same structure if all members declared between them are also (nonzero-length)
bit-fields, no matter what the sizes of those intervening bit-fields happen to be.
4 EXAMPLE A structure declared as
struct {
char a;
int b:5, c:11,:0, d:8;
struct { int ee:8; } e;
}
§ 3.14 General 5
contains four separate memory locations: The member a, and bit-fields d and e.ee are each separate memory locations,
and can be modified concurrently without interfering with each other. The bit-fieldsb andc together constitute the fourth
memory location. The bit-fieldsb andc cannot be concurrently modified, butb anda, for example, can be.
3.15
1 object
region of data storage in the execution environment, the contents of which can represent values
2 Note 1 to entry: When referenced, an object can be interpreted as having a particular type; see 6.3.2.1.
3.16
1 parameter
formal parameter
DEPRECATED: formal argument
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
3.17
1 recommended practice
specification that is strongly recommended as being in keeping with the intent of the standard, but
that might be impractical for some implementations
3.18
1 runtime-constraint
requirement on a program when calling a library function
2 Note 1 to entry: Despite the similar terms, a runtime-constraint is not a kind of constraint as defined by 3.8, and need not be
diagnosed at translation time.
3 Note 2 to entry: Implementations that support the extensions in Annex K are required to verify that the runtime-constraints
for a library function are not violated by the program; see K.3.1.4.
4 Note 3 to entry: Implementations that support Annex L are permitted to invoke a runtime-constraint handler when they
perform a trap.
3.19
1 value
precise meaning of the contents of an object when interpreted as having a specific type
3.19.1
1 implementation-defined value
unspecified value where each implementation documents how the choice is made
3.19.2
1 indeterminate value
either an unspecified value or a trap representation
3.19.3
1 unspecified value
valid value of the relevant type where this document imposes no requirements on which value is
chosen in any instance
2 Note 1 to entry: An unspecified value cannot be a trap representation.
6 General § 3.19.3
© ISO/IEC 2018– All rights reserved ISO/IEC 9899:2018 (E)
3.19.4
1 trap representation
an object representation that need not represent a value of the object type
3.19.5
1 perform a trap
interrupt execution of the program such that no further operations are performed
2 Note 1 to entry: In this document, when the word "trap" is not immediately followed by "representation", this is the
2)
intended usage.
3 Note 2 to entry: Implementations that support Annex L are permitted to invoke a runtime-constraint handler when they
perform a trap.
3.20
1 ⌈x⌉
ceiling ofx
the least integer greater than or equal tox
2 EXAMPLE ⌈2.4⌉ is3,⌈−2.4⌉ is−2.
3.21
1 ⌊x⌋
floor ofx
the greatest integer less than or equal tox
2 EXAMPLE ⌊2.4⌋ is2,⌊−2.4⌋ is−3.
2)
For example, "Trapping or stopping (if supported) is disabled . . . " (F.8.2). Note that fetching a trap representation might
perform a trap but is not required to (see 6.2
...