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
