A preprocessor Is a software program that processes a source file before sending it to be complied .inclusion of header files,macro expansion ,conditional compilation,and line control are all possible with the preprocessor.
Preprocessing directives are lines in your program that start with # . The # is followed by an identifier that is the directive name. For example, #define is the directive that defines a macro. Whitespace is also allowed before and after the # .
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.