next_permutation

void swap(void *src, void *dst, size_t size)
{
    char tmp;
    char *sp = (char *)src;
    char *dp = (char *)dst;

    do {
        tmp = *sp;
        *sp++ = *dp;
        *dp++ = tmp;
    } while (--size);
}

void reverse(void *base, size_t nmemb, size_t size)
{
    char *first = (char *)base;
    char *last = (char *)base + (nmemb - 1) * size;

    if (nmemb == 0)
        return;

    for (; first < last; first += size, last -= size) {
        swap(first, last, size);
    }
}

_Bool next_permutation(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *))
{
    char *first = (char *)base;
    char *last = (char *)base + (nmemb - 1) * size;
    char *i = last;

    for (;;) {
        i -= size;
        if (compar(i, i + size) < 0) {
            char *j = last;
            while (compar(i, j) >= 0) {
                j -= size;
            }
            swap(i, j, size);
            reverse(i + size, (last - i) / size, size);
            return 1;
        }
        if (i == first) {
            reverse(base, nmemb, size);
            return 0;
        }
    }
}