C preprocessor is used to modify your program according to the
preprocessor directives in your source code. A preprocessor
directive is a statement (such as #define) that gives the
preprocessor specific instructions on how to modify your source
The preprocessor is invoked as the first part of your compiler
programís compilation step. It is usually hidden from the programmer
because it is run automatically by the compiler.
The preprocessor reads in all of your include files and the source
code you are compiling and creates a preprocessed version of your
source code. This preprocessed version has all of its macros and
constant symbols replaced by their corresponding code and value
If your source code contains any conditional preprocessor directives
(such as #if), the preprocessor evaluates the condition and modifies
your source code accordingly.