The C preprocessor is a macro processor that the C compiler uses automatically to alter your program before to compilation. A macro processor is so named because it allows you to define macros, which are short abbreviations for longer structures. Lines in your program that begin with # are preprocessing directives. The # is followed by an identifier, which is the name of the directive. #define, for example, is the directive that defines a macro. Whitespace is also permitted before and after the # symbol.