Conditional Preprocessor :

These are used to perform inclusion of file based on some condition.

Sr. keyword Full Form Usage
1 #ifdef if defined used to check whether a macro is declared previously.
2 #ifndef if not defined used to check whether a macro is not declared previously.
3 #undef undefine used to undefine a previous defined macro.
4 #endif end if used to terminate #ifdef statement.
5 #elif else if used to declare second #ifdef (if first condition check fails).

Syntax of Conditionals and examples :

#ifdef

    #ifdef macro_name
        macro_body
    #endif

    #define SUM +

    #ifdef SUM
        #include<stdio.h>
    #endif

    int main(int argc, char const *argv[])
    {
        int result = 10 SUM 20;

        printf("10 + 20 = %d \n", result);

        return 0;
    }

#undef

Used to undefine existing macro

    #undef macro_name

    #define SUM +

    #ifdef SUM
        #undef SUM
        #define SUM -
    #endif

    int main(int argc, char const *argv[])
    {
        // program will perform subtraction in place of addition.
        int result =  10 SUM 20;
        printf("10 + 20 = %d\n", result);

        return 0;
    }

#ifndef

    #ifndef macro_name

    #ifndef SUM
      #define SUM +
    #endif

    int main(int argc, char const *argv[])
    {
        int result = 10 SUM 20;

        printf("10 + 20 = %d\n", result);

        return 0;
    }

#elif

    #elif condition
        elif_body

    #include<stdio.h>
    #define NUMBER 12

    #if NUMBER > 2
        #define SQUARE(x) x * x     
    #elif NUMBER == 2
        #define SQUARE(x) 2 * x
    #else
        #define SQUARE(x) x
    #endif

    int main(int argc, char const *argv[])
    {
        int result = SQUARE(2);
        printf("Square of 2 is %d\n",result);

        return 0;
    }

Examples of Preprocessor Directives :

    #define CUBE(x) x * x * x

    #ifndef CUBE
        #define CUBE(x) x * x * x
    #endif

    int main(int argc, char const *argv[])
    {
        int result = CUBE(2);
        printf("Cobe of 3 is %d\n", result);

        return 0;
    }

Program to find max of 6 numbers using macro :

#include<stdio.h>
#define MAX(a, b) (a) > (b) ? (a) : (b)

// Note in above case we used round brackets around a and b
// to prevent incorrect substitutuion.

int main(int argc, char const *argv[])
{
    int result = MAX(
            MAX(MAX(1, 2), MAX(4, 4)),
             MAX(MAX(5, 6), MAX(7, 8))
             );

    printf("result : %d\n",result);
    return 0;
}

Pragma preprocessor (IDE Specific) :

    #pragma startup Before
    #pragma exit After

    void Before(){
        printf("I am before main....\n");
    }

    int main(int argc, char const *argv[])
    {
        printf("inside main....\n");
        return 0;
    }

    void After(){
        printf("After main...\n");
    }