memmem function in c for windows

void *memmem(const void *haystack_start, size_t haystack_len, const void *needle_start, size_t needle_len)
{

    const unsigned char *haystack = (const unsigned char *) haystack_start;
    const unsigned char *needle = (const unsigned char *) needle_start;
    const unsigned char *h = NULL;
    const unsigned char *n = NULL;
    size_t x = needle_len;

    /* The first occurrence of the empty string is deemed to occur at
    the beginning of the string.  */
    if (needle_len == 0)
        return (void *) haystack_start;

    /* Sanity check, otherwise the loop might search through the whole
        memory.  */
     if (haystack_len < needle_len)
       return NULL;

    for (; *haystack && haystack_len--; haystack++) {

        x = needle_len;
        n = needle;
        h = haystack;

        if (haystack_len < needle_len)
            break;

        if ((*haystack != *needle) || ( *haystack + needle_len != *needle + needle_len))
            continue;

        for (; x ; h++ , n++) {
            x--;

            if (*h != *n) 
                break;

           if (x == 0)
            return (void *)haystack;
        }
    }

    return NULL;
}