What is ## Preprocessor Directive in C?

This C Tutorial explains how Directive “##” Works.

Like “#” construct “##” construct can be used in the replacement text of function like macro or object like macro. This macro combines two tokens on either side of it into a single token called an identifier. Let’s recall that a token is a group of words separated from its adjacent tokens with white spaces. Also, remember that each token thus created must be a legal identifier. Let’s consider a simple implementation of “##” construct in the following prog.

/* 
 * double_##.c -- program exploits ## capability to create new identifiers
 */
/* ## construct concatenates tokens on either side of it */
#include <stdio.h>
 
#define VAL(n) val ## n
#define PRINTSUM(n,m) \
 printf("sum of val" #n " = %d and val" #m " = %d is %d\n", \
         val ## n, val ## m, val ## n + val ## m)
 
int main(void)
{
    int VAL(1) = 10;  /* becomes val1 = 10 */
    int VAL(2) = 20;  /* becomes val2 = 20 */
 
    PRINTSUM(1,2);
 
    return 0;
}

Observe the output below,

sum of val1 = 10 and val2 = 20 is 30

Let’s observe output of above program, just last few lines, generated by preprocessor,
a .i extension file,

int main(void)
{
    int val1 = 10;
    int val2 = 20;
 
    printf("sum of val" "1" " = %d and val" "2" " = %d is %d\n",
            val1, val2, val1 + val2);
 
    return 0;
}

Notice that “#” construct creates string and adajecent strings concatenate into one.

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