strrev equivallent

#include <string.h>
#define SWAP(T, a, b) \
    do { T save = (a); (a) = (b); (b) = save; } while (0)
char *reverse_string(char *s)
{
    size_t len = strlen(s);
    if (len > 1) {
        char *a = s;
        char *b = s + len - 1;
        for (; a < b; ++a, --b)
            SWAP(char, *a, *b);
    }
    return s;
}