复制字节很简单,但要考虑到当源内存块和目的内存块重叠,且目的指针在源指针前,此时从前向后复制,目的块元素会被覆盖,此时选择从后向前复制
void* memmove(void* dst, const void* src, size_t count)//函数允许内存地址重叠
//所以要判断地址是否重叠,
// 判断首元素,如果源指针在目的指针前,则从前往后复制
//如果源指针在目的指针后且长度重叠,为避免后面被覆盖,选择从后往前复制
{
void* ret = dst;
if (dst <= src || (char*)dst >= ((char*)src + count)) //判断首元素指针,如果dst在src前,则从后往前复制,这样可以避免后面被覆盖
{
/*
* Non-Overlapping Buffers非重叠缓冲区
* copy from lower addresses to higher addresses从低地址复制到高地址
*/
while (count--)
{
*(char*)dst = *(char*)src;
dst = (char*)dst + 1;
src = (char*)src + 1;
}
}
else //如果src在dst前,则从前往后复制
{
/*
* Overlapping Buffers重叠缓冲区
* copy from higher addresses to lower addresses从高地址复制到低地址
*/
dst = (char*)dst + count - 1;
src = (char*)src + count - 1;
while (count--) {
*(char*)dst = *(char*)src;
dst = (char*)dst - 1;
src = (char*)src - 1;
}
}
return(ret);
}