Conditional inclusion

 

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Technical Specifications

 

Preprocessor

#if#ifdef#ifndef#else#elif#endif

#define#undef#,## operators

#include

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#pragma

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The preprocessor supports conditional compilation of parts of a source file. This behavior is controlled by #if, #else, #elif, #ifdef, #ifndef and #endif directives.

Syntax

#if expression
#ifdef identifier
#ifndef identifier
#elif expression
#else
#endif

Explanation

The conditional preprocessing block starts with #if, #ifdef or #ifndef directive, then optionally includes any number of #elif directives, then optionally includes at most one #else directive and is terminated with the #endif directive. Any inner conditional preprocessing blocks are processed separately.

Each of #if, #elif, #else, #ifdef and #ifndef directives control a code block until the first #elif, #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 and #elif directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent #else or #elif directive (if any) is processed. In the former case, the code block controlled by the #else directive is unconditionally compiled. In the latter case, the #elif directive acts as if it were a #if directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent #elif and #else directives. The conditional preprocessing block is terminated by the #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​.

In context of a preprocessor directive, a __has_c_attribute expression detects whether a given attribute token is supported and its supported version. See Attribute testing.

(since C23)

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.

#ifdef, #ifndef

Checks if the identifier was defined using #define directive.

#ifdef identifier is essentially equivalent to #if defined identifier.

#ifndef identifier is essentially equivalent to #if !defined identifier.

Example

Run this code

#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
}

Output:

1: yes
2: yes
3: 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
DR 412	C89	failed #elif's expression was required to be valid	failed #elif is skipped

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

See also

C++ documentation for Conditional inclusion