The XOR swap algoritm in C.

swap.c

void swap(int *a, int *b);

int main(int argc, char **argv) {
	int a = 1;
	int b = 2;
	swap(&a, &b);
	return 0;
}

void swap(int *a, int *b) {
	/*
		If the two addresses are the same, the
		algorithm will simply be `*x = *x ^ *x`
		three times, which results in zero and
		the value will be lost.
	*/
	if (a == b) {
		return;
	}
	/*
	1.  *a = *a ^ *b
	2.  *b = *a ^ *b  ->  (*a ^ *b) ^ *b  ->  *a ^ (*b ^ *b)  ->  *a ^ 0  ->  *a
	3.  *a = *a ^ *b  ->  (*a ^ *b) ^ *a  ->  *b ^ (*a ^ *a)  ->  *b ^ 0  ->  *b
	*/
	*a = *a ^ *b;
	*b = *a ^ *b;
	*a = *a ^ *b;
}

/*
As an example, let the variable `a` hold the value 12 and the variable `b` hold the value 7.

    1. a = a ^ b
                      00001100
                    ^ 00000111
                    ----------
                      00001011    the current value of a

    2. b = a ^ b
                      00001011
                    ^ 00000111
                    ----------
                      00001100    the current value of b

    3. a = a ^ b
                      00001011
                    ^ 00001100
                    ----------
                      00000111    the current value of a

The swap finishes with `a` getting the value of 7 and `b` getting the value of 12.
*/