How differently are Compiler directives and preprocessor directives handled by compiler

In C, I was wondering how differently compiler directives and preprocessor directives are handled/implemented by compiler such as GCC? Thanks!

By compiler directives, they are as in:

OpenMP (Open Multi-Processing) is an application programming interface (API) that supports multi-platform shared memory multiprocessing programming in C, C++, and Fortran on many architectures, including Unix and 开发者_如何学Python Microsoft Windows platforms. It consists of a set of compiler directives, library routines, and environment variables that influence run-time behavior.

The compiler handles preprocessor directives as specified in C99 section 6.10.

Compilers don't handle preprocessor directives; preprocessors do that. Once the preprocessor is done, it gives its output to the compiler, which can interpret compiler directives (like #pragmas) as it sees fit.

As for the preprocessor, see Pete Wilson's answer. As for the #pragma directive, Wikipedia has this to say:

The #pragma directive is a compiler specific directive which compiler vendors may use for their own purposes.