Conditional Compilation
The #if, #ifdef, #ifndef, #else, #elif and #endif directives can be used for conditional compilation.
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.
The example code tests if a macro __unix__ is defined. If it is, the file _WIN32 is defined instead. If it is, the file
A more complex #if example can use operators, for example something like:
Translation can also be caused to fail by using the #error directive:
Read more about this topic: C Preprocessor, Phases
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