Programming

x = x ^ mask; 
(or x ^= mask;) 
Bits that are set to 1 in the mask will be flipped in x. 
Bits that are set to 0 in the mask will be unchanged in x. 

Toggling (flipping) means that if the bit is 1, it is set to 0, and if the bit is 0, it is set to 1. 

XOR truth table
0 ^ 0 = 0
1 ^ 0 = 1
0 ^ 1 = 1
1 ^ 1 = 0

if(value & 1 ) -> true if first bit is set
if(value & 2 ) second bit
if(value & 4) third
if(value & 8) fourth
if(value & 16) fifth
... and so on

#include <stdio.h>

int main(int argc, char **argv)
{
    unsigned long i, j;
    unsigned long var;    /* variable whose 15th bit you want to test */
    
    i = 0100000;          /* unsigned octal has 15th bit set to 1 and the rest are 0 */
    j = var & i;

    if (j == i)           /* if the 15th bit of var is set to 1 then j will equal i */
        printf("15th bit is set to 1\n");
    else                  /* if the 15th bit of var is set to 0 j will equal zero */
        printf("15th bit is set to 0\n");
}