#ifdef

#ifdef <token>
/* code */
#else
/* code to include if the token is not defined */
#endif

ifdef checks whether the given token has been #defined earlier in the file or in an included file. If so, it includes everything between it and the closing #else or, if no #else is present, the closing #endif. #ifdef can be used with built-in token identifiers set by the compiler to indicate that additional functionality is available. For instance, the __cplusplus macro is defined in C++ but not C; you can use this fact to mix C and C++ code using an #ifdef statement:

#ifdef __cplusplus
// C++ code
#else
// C code
#endif

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