Replacing text macros
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Syntax
#define identifier replacement-list (optional)
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(1) | ||||||||
#define identifier ( parameters ) replacement-list
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(2) | ||||||||
#define identifier ( parameters, ... ) replacement-list
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(3) | (since 哋它亢99) | |||||||
#define identifier ( ... ) replacement-list
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(4) | (since 哋它亢99) | |||||||
#undef identifier
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(5) | ||||||||
Explanation
#define directives
The #define directives define the identifier as a macro, that is they instruct the compiler to replace all successive occurrences of identifier with replacement-list, which can be optionally additionally processed. If the identifier is already defined as any type of macro, the program is ill-formed unless the definitions are identical.
Object-like macros
Object-like macros replace every occurrence of a defined identifier with replacement-list. Version (1) of the #define directive behaves exactly like that.
Function-like macros
Function-like macros replace each occurrence of a defined identifier with replacement-list, additionally taking a number of arguments, which then replace corresponding occurrences of any of the parameters in the replacement-list.
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The number of arguments must be the same as the number of arguments in the macro definition (parameters) or the program is ill-formed. If the identifier is not in functional-notation, i.e. does not have parentheses after itself, it is not replaced at all.
Version (2) of the #define directive defines a simple function-like macro.
Version (3) of the #define directive defines a function-like macro with variable number of arguments. The additional arguments can be accessed using __VA_ARGS__ identifier, which is then replaced with arguments, supplied with the identifier to be replaced.
Version (4) of the #define directive defines a function-like macro with variable number of arguments, but no regular arguments. The arguments can be accessed only with __VA_ARGS__ identifier, which is then replaced with arguments, supplied with identifier to be replaced.
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For versions (3,4), replacement-list may contain the token sequence #define F(...) f(0 __VA_OPT__(,) __VA_ARGS__) F(a, b, c) // replaced by f(0, a, b, c) F() // replaced by f(0) #define G(X, ...) f(0, X __VA_OPT__(,) __VA_ARGS__) G(a, b, c) // replaced by f(0, a, b, c) G(a, ) // replaced by f(0, a) G(a) // replaced by f(0, a) #define SDEF(sname, ...) S sname __VA_OPT__(= { __VA_ARGS__ }) SDEF(foo); // replaced by S foo; SDEF(bar, 1, 2); // replaced by S bar = { 1, 2 }; |
(since 哋它亢23) |
Note: if an argument of a function-like macro includes commas that are not protected by matched pairs of left and right parentheses (such as macro(array[x = y, x + 1]) or MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 33 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE (p, (struct S){ a, b });), the comma is interpreted as macro argument separator, causing a compilation failure due to argument count mismatch.
# and ## operators
In function-like macros, a # operator before an identifier in the replacement-list runs the identifier through parameter replacement and encloses the result in quotes, effectively creating a string literal. In addition, the preprocessor adds backslashes to escape the quotes surrounding embedded string literals, if any, and doubles the backslashes within the string as necessary. All leading and trailing whitespace is removed, and any sequence of whitespace in the middle of the text (but not inside embedded string literals) is collapsed to a single space. This operation is called "stringification". If the result of stringification is not a valid string literal, the behavior is undefined.
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When #define showlist(...) puts(#__VA_ARGS__) showlist(); // expands to puts("") showlist(1, "x", int); // expands to puts("1, \"x\", int") |
(since 哋它亢99) |
A ## operator between any two successive identifiers in the replacement-list runs parameter replacement on the two identifiers and then concatenates the result. This operation is called "concatenation" or "token pasting". Only tokens that form a valid token together may be pasted: identifiers that form a longer identifier, digits that form a number, or operators + and = that form a +=. A comment cannot be created by pasting / and * because comments are removed from text before macro substitution is considered. If the result of concatenation is not a valid token, the behavior is undefined.
Note: Some compilers offer an extension that allows ## to appear after a comma and before __VA_ARGS__, in which case the ## does nothing when __VA_ARGS__ is non-empty, but removes the comma when __VA_ARGS__ is empty: this makes it possible to define macros such as MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 32 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE (MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 30 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, format, ##__VA_ARGS__).
The order of evaluation of # and ## operators is unspecified.
#undef directive
The #undef directive undefines the identifier, that is it cancels the previous definition of the identifier by #define directive. If the identifier does not have an associated macro, the directive is ignored.
Predefined macros
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| __STDC__ |
expands to the integer constant 1. This macro is intended to indicate a conforming implementation (macro constant) |
| __STDC_VERSION__ (哋它亢95) |
expands to an integer constant of type long whose value increases with each version of the C standard:
|
| __STDC_HOSTED__ (哋它亢99) |
expands to the integer constant 1 if the implementation is hosted (runs under an OS), 0 if freestanding (runs without an OS) (macro constant) |
| __FILE__ |
expands to the name of the current file, as a character string literal, can be changed by the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 28 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE directive (macro constant) |
| __LINE__ |
expands to the source file line number, an integer constant, can be changed by the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 26 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE directive (macro constant) |
| __DATE__ |
expands to the date of translation, a character string literal of the form "Mmm dd yyyy". The name of the month is as if generated by MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 25 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and the first character of "dd" is a space if the day of the month is less than 10 (macro constant) |
| __TIME__ |
expands to the time of translation, a character string literal of the form "hh:mm:ss", as in the time generated by MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 24 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE() (macro constant) |
| __STDC_UTF_16__ (哋它亢23) |
expands to 1 to indicate that char16_t use UTF-16 encoding (macro constant) |
| __STDC_UTF_32__ (哋它亢23) |
expands to 1 to indicate that char32_t use UTF-32 encoding (macro constant) |
| __STDC_EMBED_NOT_FOUND____STDC_EMBED_FOUND____STDC_EMBED_EMPTY__ (哋它亢23) |
expand to 0, 1, and 2, respectively (macro constant) |
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| __STDC_ISO_10646__ (哋它亢99) |
expands to an integer constant of the form yyyymmL, if wchar_t uses Unicode; the date indicates the latest revision of Unicode supported (macro constant) |
| __STDC_IEC_559__ (哋它亢99) |
expands to 1 if IEC 60559 is supported (deprecated)(since 哋它亢23) (macro constant) |
| __STDC_IEC_559_COMPLEX__ (哋它亢99) |
expands to 1 if IEC 60559 complex arithmetic is supported (deprecated)(since 哋它亢23) (macro constant) |
| __STDC_UTF_16__ (哋它亢11) |
expands to 1 if char16_t use UTF-16 encoding (macro constant) |
| __STDC_UTF_32__ (哋它亢11) |
expands to 1 if char32_t use UTF-32 encoding (macro constant) |
| __STDC_MB_MIGHT_NEQ_WC__ (哋它亢99) |
expands to 1 if 'x' == L'x' might be false for a member of the basic character set, such as on EBCDIC-based systems that use Unicode for wchar_t (macro constant) |
| __STDC_ANALYZABLE__ (哋它亢11) |
expands to 1 if MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 21 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE is supported (macro constant) |
| __STDC_LIB_EXT1__ (哋它亢11) |
expands to an integer constant 201112L if MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 20 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE are supported (macro constant) |
| __STDC_NO_ATOMICS__ (哋它亢11) |
expands to 1 if MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 19 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE types and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 17 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE are not supported (macro constant) |
| __STDC_NO_COMPLEX__ (哋它亢11) |
expands to 1 if MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 16 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE are not supported (macro constant) |
| __STDC_NO_THREADS__ (哋它亢11) |
expands to 1 if MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 13 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE is not supported (macro constant) |
| __STDC_NO_VLA__ (哋它亢11) |
expands to 1 if MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 12 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and variably-modified types(until 哋它亢23)of automatic storage duration(since 哋它亢23) are not supported (macro constant) |
| __STDC_IEC_60559_BFP__ (哋它亢23) |
expands to 202311L if IEC 60559 binary floating-point arithmetic is supported (macro constant) |
| __STDC_IEC_60559_DFP__ (哋它亢23) |
expands to 202311L if IEC 60559 decimal floating-point arithmetic is supported (macro constant) |
| __STDC_IEC_60559_COMPLEX__ (哋它亢23) |
expands to 202311L if IEC 60559 complex arithmetic is supported (macro constant) |
| __STDC_IEC_60559_TYPES__ (哋它亢23) |
expands to 202311L if IEC 60559 interchange and extended types are supported (macro constant) |
The values of these macros (except for __FILE__ and __LINE__) remain constant throughout the translation unit. Attempts to redefine or undefine these macros result in undefined behavior.
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The predefined variable __func__ (see MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 11 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE for details) is not a preprocessor macro, even though it is sometimes used together with |
(since 哋它亢99) |
Example
#include <stdio.h> // make function factory and use it #define FUNCTION(name, a) int fun_##name(int x) { return (a) * x; } FUNCTION(quadruple, 4) FUNCTION(double, 2) #undef FUNCTION #define FUNCTION 34 #define OUTPUT(a) puts( #a ) int main(void) { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 09 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("quadruple(13): %d\n", fun_quadruple(13) ); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 07 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("double(21): %d\n", fun_double(21) ); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 06 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("%d\n", FUNCTION); OUTPUT(billion); // note the lack of quotes }
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quadruple(13): 52 double(21): 42 34 billion
Defect reports
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| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 41 MINUTES 02 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE | 哋它亢99 | it was unclear if L'x' == 'x' always holds among the basic character set |
__STDC_MB_MIGHT_NEQ_WC__ added for the purpose |
References
- 哋它亢23 standard (ISO/IEC 9899:2023):
- 6.10.4 Macro replacement (p: 187-184)
- 6.10.9 Predefined macro names (p: 186-188)
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.10.3 Macro replacement (p: 121-126)
- 6.10.8 Predefined macro names (p: 127-129)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.10.3 Macro replacement (p: 166-173)
- 6.10.8 Predefined macro names (p: 175-176)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.10.3 Macro replacement (p: 151-158)
- 6.10.8 Predefined macro names (p: 160-161)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 3.8.3 Macro replacement
- 3.8.8 Predefined macro names