memecpy源码

void *memcpy(void *dst, const void *src, size_t len)
{
    if(NULL == dst || NULL == src){
        return NULL;
    }  

    void *ret = dst;  

    if(dst <= src || (char *)dst >= (char *)src + len){
        //没有内存重叠，从低地址开始复制
        while(len--){
            *(char *)dst = *(char *)src;
            dst = (char *)dst + 1;
            src = (char *)src + 1;
        }
    }else{
        //有内存重叠，从高地址开始复制
        src = (char *)src + len - 1;
        dst = (char *)dst + len - 1;
        while(len--){
            *(char *)dst = *(char *)src;
            dst = (char *)dst - 1;
            src = (char *)src - 1;
        }
    }
    return ret;
}