The C Preprocessor
In C and C++, the language supports a simple macro preprocessor. Source lines that should be handled by the preprocessor, such as #define and #include are referred to as preprocessor directives.
Another C construct, the #pragma directive, is used to instruct the compiler to use pragmatic or implementation-dependent features. Two notable users of this directive are OpenMP and OpenACC.
Syntactic constructs similar to C's preprocessor directives, such as C#'s #if, are also typically called "directives", although in these cases there may not be any real preprocessing phase involved.
Read more about this topic: Directive (programming)
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