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
>  Microsoft Windows platforms. It
>  consists of a set of **compiler directives**, library routines, and
>  environment variables that influence
>  run-time behavior.
> 
> 
>