memcmp

int memcmp(const void * s1, const void * s2,size_t n);

int memcmp(const void* s1, const void* s2,size_t n)
{
    const unsigned char *p1 = s1, *p2 = s2;
    while(n--)
        if( *p1 != *p2 )
            return *p1 - *p2;
        else
            p1++,p2++;
    return 0;
}