The C preprocessor is a simple text parser/replacer that is run before the actual compilation of the code. Used to extend and ease the use of the C (and later C++) language, it can be used for:
a. Including other files using
b. Define a text-replacement macro using
c. Conditional Compilation using
d. Platform/Compiler specific logic (as an extension of conditional compilation)
Preprocessor statements are executed before your source files are handed to the compiler. They are capable of very low level conditional logic. Since preprocessor constructs (e.g. object-like macros) aren't typed like normal functions (the preprocessing step happens before compilation) the compiler cannot enforce type checks, they should therefore be carefully used.