实现方式：
int ho_memcmp(const void *s1, const void *s2, size_t n) {
unsigned char *p1, *p2;
p1 = (unsigned char *)s1;
p2 = (unsigned char *)s2;
char c;
while (!(c = *p1 - *p2) && n--) {
p1++;
p2++;
}
return c;
}
原文：http://my.oschina.net/guonaihong/blog/387702