Is there a built in swap function in C?

Question

Is there any built in swap function in C which works without using a third variable?

Answer 1

I believe I've come up with a type-agnostic function for swapping any two values in standard C, though since I'm fairly new to the language I may have overlooked something. It uses the XOR swap algorithm, and I'm sure it could be optimized more, but it works as long as the two values point to the same number of bytes, specified by the 3rd argument:

void swapn(void *a, void *b, size_t n) {
    if (a == b) {
        return;
    }

    size_t i;
    char *x = (char *)a,
        *y = (char *)b;

    for (i = 0; i < n; i++) {
        *x ^= *y;
        *y ^= *x;
        *x ^= *y;
        x++;
        y++;
    }
}

Example usage:

// swap two integers
int x = 5,
    y = 30;

printf("%d\t%d\n", x, y);

swapn(&x, &y, sizeof(int));

printf("%d\t%d\n\n", x, y);

// swap two floats
float a = 9.23f,
    b = 6.83f;

printf("%.2f\t%.2f\n", a, b);

swapn(&a, &b, sizeof(float));

printf("%.2f\t%.2f\n\n", a, b);

// swap two doubles
double p = 4.7539,
    q = 0.9841;

printf("%.4f\t%.4f\n", p, q);

swapn(&p, &q, sizeof(double));

printf("%.4f\t%.4f\n\n", p, q);

// swap two chars
char m = 'M',
    n = 'n';

printf("%c\t%c\n", m, n);

swapn(&m, &n, sizeof(char));

printf("%c\t%c\n\n", m, n);

// swap two strings of equivalent length
char s[] = "Hello",
    t[] = "World";

printf("%s\t%s\n", s, t);

swapn(s, t, sizeof(s));

printf("%s\t%s\n\n", s, t);

The output is:

5   30
30  5

9.23    6.83
6.83    9.23

4.7539  0.9841
0.9841  4.7539

M   n
n   M

Hello   World
World   Hello