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 C23), 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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#elifndef identifier
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#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 C23) 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 C23), and #else directives control code block until first #elif, #elifdef, #elifndef (since C23), #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 C23) 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 C23) 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 C23) 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 C23) and #else directives. The conditional preprocessing block is terminated by #endif directive.
Conditional evaluation
#if, #elif
The expression is a constant expression, using only constants and identifiers, defined using #define directive. Any identifier, which is not literal, non defined using #define directive, evaluates to 0.
The expression may contain unary operators in form defined identifier or defined (identifier) which return 1 if the identifier was defined using
#define directive and 0 otherwise. 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 |
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Note: Until DR 412, #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 DR 412, #elif that leads the skipped code block is also skipped.
Combined directives
Checks if the identifier was defined as a macro name.
#ifdef identifier is essentially equivalent to #if defined identifier.
#ifndef identifier is essentially equivalent to #if !defined identifier.
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Notes
While #elifdef and #elifndef directives target C23, implementations may backport them to the older language modes as conforming extensions.
Example
#define ABCD 2 #include <stdio.h> int main(void) { #ifdef ABCD printf("1: yes\n"); #else printf("1: no\n"); #endif #ifndef ABCD printf("2: no1\n"); #elif ABCD == 2 printf("2: yes\n"); #else printf("2: no2\n"); #endif #if !defined(DCBA) && (ABCD < 2*4-3) printf("3: yes\n"); #endif // C23 directives #elifdef/#elifndef #ifdef CPU printf("4: no1\n"); #elifdef GPU printf("4: no2\n"); #elifndef RAM printf("4: yes\n"); // selected in C23 mode, may be selected in pre-C23 mode #else printf("4: no3\n"); // may be selected in pre-C23 mode #endif }
Possible output:
1: yes 2: yes 3: yes 4: yes
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C standards.
| DR | Applied to | Behavior as published | Correct behavior |
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| DR 412 | C89 | failed #elif's expression was required to be valid
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failed #elif is skipped
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References
- C17 standard (ISO/IEC 9899:2018):
- 6.10.1 Conditional inclusion (p: 118-119)
- C11 standard (ISO/IEC 9899:2011):
- 6.10.1 Conditional inclusion (p: 162-164)
- C99 standard (ISO/IEC 9899:1999):
- 6.10.1 Conditional inclusion (p: 147-149)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.8.1 Conditional inclusion