Reverse string.

reverse.c

#include <stdio.h>
#include <string.h>

void swap(char *x, char *y) {
    char tmp; tmp = *x;
    *x = *y;
    *y = tmp;
}

void *reverse(char *s) {
    char *p = s, *q = s + strlen(s) - 1;
    int t = p - q;
    while ((p - q) * t >= 0) {
        swap(p, q);
        *p++; *q--;
    }
    return s;
}