C - Preprocessor Directives

C - Preprocessor Directives

  • In C Language preprocessor is more or less the own language which can be a very powerful tool to the programmer .
  • All the preprocessor directives or commands begin with “#” .
  • Below is the list of preprocessor directives that C language offers .
C Preprocessor

Preprocessors Syntax Description
Macro #define This macro defines constant value and can be any of the basic data types.
Header file inclusion #include <file_name> The source code of the file “file_name” is included in the main program at the specified place
Conditional compilation #ifdef, #endif, #if, #else, #ifndef Set of commands are included or excluded in source program before compilation with respect to the condition
Other directives #undef, #pragma #undef is used to undefine a defined macro variable. #Pragma is used to call a function before and after main function in a C program
  • It must be the first nonblank character, and for readability, a preprocessor directive should begin in the first column.
  • The following section lists down all the important preprocessor directives :
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 method..
  • Use of the preprocessor in C programming :
    • Programs easier to develop,
    • Easier to read,
    • Easier to modify

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