What is preprocessor?

The preprocessor is not a part of c compiler. It is an unique feature of c language. Preprocessor is a program that processes the code before it passes through the compiler. Preprocessor makes the program easy to read, modify, portable and more efficient by providing their tools.

Preprocessor directives-

Directives                       Function
#define          defines a macro substitution
#undef           undefines a macro
#include        specifies a file to be included
#ifdef            tests for macro definition
#endef          specifies the end of #if
#ifendef        tests weather the macro                               
                     is not def
#if              tests a compile time condition
#else       specify alternatives when # if  

                 test fail