Preprocessor in C

In c programming languages, Preprocessor is a macro processor, processes all the preprocessor directives present in the source code before the compilation. Preprocessor always starts with # sign.
Preprocessor is also called as text substitution tool or CPP.

Preprocessor has following features:

1) Header file inclusion: It includes header files.
2) Macro expansion: It replaces all the macro with the given substituted text.
3) Conditional compilation: allows to add or avoid program part on basis of given condition.
4) Line control: It is called as preprocessor line control directive gives the line number of next source line.

Following are the preprocessor directives:

#define	Substitutes a preprocessor macro
#include	Inserts a particular header from another file
#undef	Undefines a preprocessor macro
#ifdef	if macro is defined, Returns true.
#ifndef	if macro is not defined, Returns true.
#if	Checks if a condition is true in compile time.
#else	The alternative for #if.
#elif	#else and #if in one statement.
#endif	Ends preprocessor conditional.
#error	Prints error message on stderr.
#pragma	Provides extension specific to the compiler.