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.

[edit] Syntax

[edit] 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.

[edit] Conditional evaluation

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