When & how to use them
The C Preprocessor is a simple macro-expander that is run on source code files before passing them to the compiler. Lines that begin with the hash symbol '#' are directives to the C preprocessor.
When you create a macro you assign a name to a C expression. You can then use this name in your code just as you would have used the expression. The preprocessor replaces all occurences of that name with the expression.