Preprocessor Directive

A pre-processor directory is a set of text which is start with # symbol. When C source code compile then process of compilation get start from pre-processor directory. It means all pre-processor directories compile before actual compilation.

Example

• Header files
• Macro
• Conditional Compilation statements

why do we need pre-processor directives

We use pre-processor directives for a variety of things:

1. Macro replacement: To replace a particular text pattern with another one. This achieves one of several things - replace a text pattern with a constant or replace a text pattern with a parameterized expression - #define

2. Define a variable that the per processor can see - #define

3. Includes source code from external files - #include

4. Conditionally exclude a block of code from the original source code that will be subsequently processed by the compiler. This is done using previously defined pre processor variables - #ifdef or #ifndef

Various Pre-processor Directive

Directive	Description
#define	Substitutes a preprocessor macro.
#include	Inserts a particular header from another file.
#undef	Undefines a preprocessor macro.
#ifdef	Returns true if this macro is defined.
#ifndef	Returns true if this macro is not defined.
#if	Tests if a compile time condition is true.
#else	The alternative for #if.
#elif	#else and #if in one statement.
#endif	Ends preprocessor conditional.
#error	Prints error message on stderr.
#pragma	Issues special commands to the compiler, using a standardized

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