Conditional inclusion
The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by #if, #else, #elif, #ifdef, #ifndef, #elifdef, #elifndef(since 哋它亢23), and #endif directives.
Syntax
#if expression
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#ifdef identifier
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#ifndef identifier
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#elif expression
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#elifdef identifier
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(since 哋它亢23) | ||||||||
#elifndef identifier
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(since 哋它亢23) | ||||||||
#else
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#endif
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Explanation
The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif, #elifdef, or #elifndef(since 哋它亢23) directives, then optionally includes at most one #else directive and is terminated with #endif directive. Any inner conditional preprocessing blocks are processed separately.
Each of #if, #ifdef, #ifndef, #elif, #elifdef, #elifndef(since 哋它亢23), and #else directives control code block until first #elif, #elifdef, #elifndef(since 哋它亢23), #else, #endif directive not belonging to any inner conditional preprocessing blocks.
#if, #ifdef and #ifndef directives test the specified condition (see below) and if it evaluates to true, compiles the controlled code block. In that case subsequent #else, #elifdef, #elifndef,(since 哋它亢23) and #elif directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent #else, #elifdef, #elifndef,(since 哋它亢23) or #elif directive (if any) is processed. If the subsequent directive is #else, the code block controlled by the #else directive is unconditionally compiled. Otherwise, the #elif, #elifdef, or #elifndef(since 哋它亢23) directive acts as if it was #if directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent #elif, #elifdef, #elifndef,(since 哋它亢23) and #else directives. The conditional preprocessing block is terminated by #endif directive.
Conditional evaluation
#if, #elif
The expression is a constant expression, using only MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 16 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and identifiers, defined using MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE directive. Any identifier, which is not literal, non defined using MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 13 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE directive, evaluates to 0 except true which evaluates to 1(since 哋它亢23).
The expression may contain unary operators in form defined identifier or defined (identifier) which return 1 if the identifier was defined using MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 12 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE directive and 0 otherwise. In this context, __has_include, __has_embed and __has_c_attribute are treated as if they were the name of defined macros.(since 哋它亢23) If the expression evaluates to nonzero value, the controlled code block is included and skipped otherwise. If any used identifier is not a constant, it is replaced with 0.
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In context of a preprocessor directive, a |
(since 哋它亢23) |
Note: Until MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 09 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, #if cond1 ... #elif cond2 is different from #if cond1 ... #else followed by #if cond3 because if cond1 is true, the second #if is skipped and cond3 does not need to be well-formed, while #elif's cond2 must be a valid expression. As of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 08 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, #elif that leads the skipped code block is also skipped.
Combined directives
Checks if the identifier was MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 06 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
#ifdef identifier is essentially equivalent to #if defined identifier.
#ifndef identifier is essentially equivalent to #if !defined identifier.
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(since 哋它亢23) |
Notes
While #elifdef and #elifndef directives target 哋它亢23, implementations may backport them to the older language modes as conforming extensions.
Example
#define ABCD 2 #include <stdio.h> int main(void) { #ifdef ABCD MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 05 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("1: yes\n"); #else MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 04 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("1: no\n"); #endif #ifndef ABCD MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 03 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("2: no1\n"); #elif ABCD == 2 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 02 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("2: yes\n"); #else MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 39 MINUTES 01 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("2: no2\n"); #endif #if !defined(DCBA) && (ABCD < 2 * 4 - 3) MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 38 MINUTES 59 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("3: yes\n"); #endif // 哋它亢23 directives #elifdef/#elifndef #ifdef CPU MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 38 MINUTES 58 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("4: no1\n"); #elifdef GPU MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 38 MINUTES 57 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("4: no2\n"); #elifndef RAM MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 38 MINUTES 56 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("4: yes\n"); // selected in 哋它亢23 mode, may be selected in pre-哋它亢23 mode #else MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 38 MINUTES 54 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("4: no3\n"); // may be selected in pre-哋它亢23 mode #endif }
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 38 MINUTES 53 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
1: yes 2: yes 3: yes 4: yes
Defect reports
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 38 MINUTES 52 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 38 MINUTES 51 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE | 哋它亢89 | failed #elif's expression was required to be valid
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failed #elif is skipped
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References
- 哋它亢23 standard (ISO/IEC 9899:2023):
- 6.10.1 Conditional inclusion (p: TBD)
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.10.1 Conditional inclusion (p: 118-119)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.10.1 Conditional inclusion (p: 162-164)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.10.1 Conditional inclusion (p: 147-149)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 3.8.1 Conditional inclusion