strcpy的是如何实现的？ [英] How is strcpy implemented?

问题描述

我有一个关于使用strcpy的问题。我知道ANSI C标准说：源和目的一定不能重叠，否则该行为是未predictable。我告诉你一件code如我所料，如果是使用Linux下的旧的GNU C编译器编译的作品的。

I have a question about using strcpy. I know the ANSI C standard says : source and destination must not overlap, otherwise the behaviour is unpredictable. I show you a piece of code that works as I expect if it is compiled using an old gnu C compiler under Linux.

#include <string.h>
#include <stdio.h>

char S[80],*P;

int main() {
    strcpy(S,"abcdefghi\r\njklmnopqr\r\nstuvwxyz\r\n");
    for (P=S; P=strchr(P,'\r'); P++) strcpy(P,P+1);
    printf("%s\n",S);
    return 0;
}

这个序列中删除从输入字符串每 \\ r （回车）。我知道（从Kernigham和Ritchie）对于strcpy的一个非常简单的实现如下

This sequence removes every \r (carriage return) from the input string. I know (from Kernigham and Ritchie) that a very simple implementation for strcpy is the following

while (*t++=*s++) ;

现在我编译使用gcc（Gentoo的4.5.4 P1.0，饼0.4.7）我的程序4.5.4，它打印此：

Now I compiled my program using gcc (Gentoo 4.5.4 p1.0, pie-0.4.7) 4.5.4 and it prints this:

abcdefghi
jklmnpqr          <-- missing 'o'
stuvwxxyz         <-- doubled 'x'

我想这编译器（实际上它的库）使用了的strcpy 一个非常复杂的程序，我不明白其中的道理。

I suppose this compiler (in fact its library) uses a very sophisticated sequence for strcpy, and I don't understand the reason.

推荐答案

您被警告不要那样做。其原因是，一个字节用于字节拷贝实际上是相当缓慢的，需要大量的循环通过一个串来获得的。编译器可以轻松地优化这个（例如，通过每次复制一个 INT -sized块，或使用一些特定于平台的parallellization。）

You were warned not to do that. The reason is that a byte-for-byte copy is actually quite slow and requires a lot of looping to get through a string. The compiler can optimize this easily (for instance, by copying an int-sized chunk at a time, or using some platform-specific parallellization.)

但是，如果字符串重叠，那么这些优化对您的数据不再有效的假设。这样一来，他们给你不确定的结果。很可能你的旧有的GCC根本没有做任何这样的优化。

But if the strings overlap, then those optimizations make assumptions about your data that are no longer valid. As a result, they give you unspecified results. It is likely your older GCC simply didn't do any such optimizations.

由于对的strcpy（）的文档表示，不使用重叠的字符串的不的

Since the documentation for strcpy() says not to use overlapping strings, don't.

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