Description:

The C preprocessor is a "macro processor" that is used automatically
by the C compiler to transform your program before actual compilation.
It is called a macro processor because it allows you to define
"macros", which are brief abbreviations for longer constructs.

The C preprocessor provides four separate facilities that you can use
as you see fit:

- Inclusion of header files. These are files of declarations that can
be substituted into your program.
- Macro expansion. You can define "macros", which are abbreviations
for arbitrary fragments of C code, and then the C preprocessor will
replace the macros with their definitions throughout the program.
- Conditional compilation. Using special preprocessing directives, you
can include or exclude parts of the program according to various
conditions.
- Line control. If you use a program to combine or rearrange source
files into an intermediate file which is then compiled, you can use
line control to inform the compiler of where each source line
originally came from.