ifndef

#ifndef is used to check whether a particular symbol is NOT defined.

Example

#ifdef USD              // here preprocessor cheking,  if USD is DEFINED OR NOT
#define currency_rate 50     // if it's defined, currency_rate is equals to 50
#endif                  // end of ifdef, these three lines are finished now


#ifndef USD          // here preprocessor cheking,  if USD is NOT DEFINED
#define currency_rate 1  // Since USD is not defined so this statement executes
#endif                // end these three lines of block 

#include<stdio.h>

int main()
{		
	int rs;
	// currency_rate is defined in preprocessor, which is 1
	rs = 10 * currency_rate;   
	printf ("%d\n", rs);
	getchar();
	return 0;
}