C Preprocessor Directives

• Before a C program is compiled in a compiler, source code is processed by a program called preprocessor. This process is called preprocessing.

• Commands used in preprocessor are called preprocessor directives and they begin with “#” symbol.
• Below is the list of preprocessor directives that C language offers.

S.no	Preprocessor	Syntax	Description
1	Macro	#define	This macro defines constant value and can be any of the basic data types.
2	Header file inclusion	#include	The source code of the file “file_name” is included in the main program at the specified place
3	Conditional compilation	#ifdef, #endif, #if, #else,  #ifndef	Set of commands are included or excluded in source program before compilation with respect to the condition
4	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

A program in C language involves into different processes.  Below diagram will help you to understand all the processes that a C program comes across.

Example program for #define, #include preprocessors in C:

• #define  -   This macro defines constant value and can be any of the basic data types.
• #include   -   The source code of the file “file_name” is included in the main C program where “#include ” is  mentioned.

#include

#define height 100

#define number 3.14

#define letter 'A'

#define letter_sequence "ABC"

#define backslash_char '\?'

void main()

{

   printf("value of height    : %d \n", height );

   printf("value of number : %f \n", number );

   printf("value of letter : %c \n", letter );

   printf("value of letter_sequence : %s \n", letter_sequence);

   printf("value of backslash_char  : %c \n", backslash_char);

}

Output:

value of height : 100 value of number : 3.140000 value of letter : A value of letter_sequence : ABC value of backslash_char : ?