用C实现的memmove [英] memmove implementation in C

问题描述

有人能帮助我了解memmove与如何用C实现的我只有一个特殊情况吧？

Can some one help me to understand how memmove is implemented in C. I have only one special condition right ?

if((src<dst)&&((src+sz) > dst))

copy from the back

另外它依赖于堆栈增长方式？

Also does it depend on the way stack grows ?

推荐答案

在数学上，你不必担心是否重叠的。如果的src 小于 DST ，刚刚从高端副本。如果的src 比 DST 时，从一开始就只是复制。

Mathematically, you don't have to worry about whether they overlap at all. If src is less than dst, just copy from the end. If src is greater than dst, just copy from the beginning.

如果的src 和 DST 都是平等的，只是退出立竿见影。

If src and dst are equal, just exit straight away.

这是因为你的案件之一：

That's because your cases are one of:

1) <-----s----->                start at end of s
                 <-----d----->

2) <-----s----->                start at end of s
            <-----d----->

3) <-----s----->                no action
   <-----d----->

4)          <-----s----->       start at beginning of s
   <-----d----->

5)               <-----s----->  start at beginning of s
   <-----d----->

即使没有重叠，这将仍然正常工作，并简化您的条件。

Even if there's no overlap, that will still work fine, and simplify your conditions.

如果你有一个更有效的方式来复制比向前向后那么，是的，你应该检查重叠，以确保您尽可能使用更有效的方法。换句话说，上述更改选项1，从一开始复制。

If you have a more efficient way to copy forwards than backwards then, yes, you should check for overlap to ensure you're using the more efficient method if possible. In other words, change option 1 above to copy from the beginning.

然而，在这种情况下，你可能也发现它的速度更快复制两次，一次是从取值来那么一些暂时性的内存从暂时存储器 D 但是这完全取决于你的各自的算法。

However, in that case, you may also find that it's faster to copy twice, once from s to some temporary memory then from that temporary memory to d but that depends entirely on your respective algorithms.

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