Editing C preprocessor (section)

=== Conditional compilation ===
[[Conditional compilation]] is supported via the [[if-else]] core directives <code>#if</code>, <code>#else</code>, <code>#elif</code>, and <code>#endif</code> and with contraction directives <code>#ifdef</code> and <code>#ifndef</code> which stand for {{code|#if defined(...)}} and {{code|#if !defined(...)}}, respectively. In the following example code, the <code>printf()</code> call is only included for compilation if <code>VERBOSE</code> is defined.

<syntaxhighlight lang="cpp">
#ifdef VERBOSE
  printf("trace message");
#endif
</syntaxhighlight>

The following demonstrates more complex logic:

<syntaxhighlight lang="cpp">
#if !(defined __LP64__ || defined __LLP64__) || defined _WIN32 && !defined _WIN64
	// code for a 32-bit system
#else
	// code for a 64-bit system
#endif
</syntaxhighlight>