The preprocessing of C source code is done before it is compiled by the compiler. The Preprocessor directives are the command used for preprocessing which starts with "#" symbol. The list of preprocessor directives that C language offers is given below:

1. Macro i.e. #define
2. Header file inclusion i.e. #include.
3. Conditional compilation i.e. #ifdef, #endif, #if, #else, #ifndef.
4. Other directives i.e. #undef, #pragma, #undef.

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

   #include <stdio.h>
    #define height 150
    #define number 8.78
    #define letter 'D'
    #define lettersequence "PQRS"


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 lettersequence : %s \n", lettersequence);



}

OUTPUT:

value of height : 150
value of number : 8.780000
value of letter : D
value of lettersequence : PQRS