C Preprocessor - Phases - Conditional Compilation

Conditional Compilation

The #if, #ifdef, #ifndef, #else, #elif and #endif directives can be used for conditional compilation.

#if VERBOSE >= 2 print("trace message"); #endif

Most compilers targeting Microsoft Windows implicitly define _WIN32. This allows code, including preprocessor commands, to compile only when targeting Windows systems. A few compilers define WIN32 instead. For such compilers that do not implicitly define the _WIN32 macro, it can be specified on the compiler's command line, using -D_WIN32.

#ifdef __unix__ /* __unix__ is usually defined by compilers targeting Unix systems */ # include #elif defined _WIN32 /* _Win32 is usually defined by compilers targeting 32 or 64 bit Windows systems */ # include #endif

The example code tests if a macro __unix__ is defined. If it is, the file is then included. Otherwise, it tests if a macro _WIN32 is defined instead. If it is, the file is then included.

A more complex #if example can use operators, for example something like:

#if !(defined __LP64__ || defined __LLP64__) || defined _WIN32 && !defined _WIN64 // we are compiling for a 32-bit system #else // we are compiling for a 64-bit system #endif

Translation can also be caused to fail by using the #error directive:

#if RUBY_VERSION == 190 # error 1.9.0 not supported #endif

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