preprocessor is used to modify your program according to the
preprocessor directives in your source code.
Preprocessor directives (such as #define) give the preprocessor
specific instructions on how to modify your source code. The
preprocessor reads in all of your include files and the source code
you are compiling and creates a preprocessed version of your source
This preprocessed version has all of its macros and constant symbols
replaced by their corresponding code and value assignments. If your
source code contains any conditional preprocessor directives (such
as #if), the preprocessor evaluates the condition and modifies your
source code accordingly.
The preprocessor contains many features that are powerful to use,
such as creating macros, performing conditional compilation,
inserting predefined environment variables into your code, and
turning compiler features on and off.
For the professional programmer, in-depth knowledge of the features
of the preprocessor can be one of the keys to creating fast,