Conditional compilation in C

The preprocessor can include or drop whole regions based on what is defined.

#include <stdio.h>

#define FEATURE_LEVEL 2

void report(void) {
#if defined(__linux__)
    printf("linux\n");
#elif defined(_WIN32)
    printf("windows\n");
#else
    printf("somewhere else\n");
#endif

#if FEATURE_LEVEL >= 2
    printf("extended features\n");
#endif

#if __STDC_VERSION__ >= 201112L
    printf("C11 or newer\n");
#endif
}

How it works

1. #ifdef tests whether a macro exists at all.
2. #if evaluates an integer constant expression.
3. Predefined macros identify the platform and the standard.

Keywords and builtins used here

• report
• void

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Step 2 of 2 in Headers & conditions, step 9 of 12 in Preprocessor & headers.

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