打印分配的字符串时，Valgrind的报告错误

问题描述

在code是在这里：

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

int main()
{
    char* buf = malloc(3);
    strcpy(buf, "hi");
    printf("%s\n", buf);
    free(buf);
}

它编译：

gcc a.c && valgrind ./a.out

该错误信息是在这里：

The error message is here:

==1421== Memcheck, a memory error detector
==1421== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==1421== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright info
==1421== Command: ./a.out
==1421== 
==1421== Invalid read of size 8
==1421==    at 0x4EA96C1: ??? (in /lib/libc-2.14.1.so)
==1421==    by 0x4E92D3B: puts (in /lib/libc-2.14.1.so)
==1421==    by 0x4005BB: main (in /home/peter/a.out)
==1421==  Address 0x51b4040 is 0 bytes inside a block of size 3 alloc'd
==1421==    at 0x4C2740D: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1421==    by 0x400595: main (in /home/peter/a.out)
==1421== 
hi
==1421== 
==1421== HEAP SUMMARY:
==1421==     in use at exit: 0 bytes in 0 blocks
==1421==   total heap usage: 1 allocs, 1 frees, 3 bytes allocated
==1421== 
==1421== All heap blocks were freed -- no leaks are possible
==1421== 
==1421== For counts of detected and suppressed errors, rerun with: -v
==1421== ERROR SUMMARY: 2 errors from 1 contexts (suppressed: 6 from 6)

这也是很奇怪，如果我用下面的（只是多了一个空格）的valgrind报告没有更多的错误：

It is also very strange that valgrind reports no more errors if I use the following (just one more space):

printf("%s \n", buf);

会有人帮帮我吗？

Would anyone please help me?

推荐答案

这是一个错误，但在所有机器上不可重现。

This is a bug, but not reproducible on all machines.

在一些机器，GCC优化简单的的printf（）用，例如，看跌期权（），这可能可能涉及非法读取（或只是Valgrind的是这样认为的）。

On some machines, gcc optimizes simple printf() with, for example, puts(), which could possibly involve invalid read (or just valgrind thinks so).

如果它真的很重要，你可以'复杂'的的printf 格式。之间的空格％S 和 \\ n 会怎么做。

If it really matters, you can 'complicate' the printf format. A space between %s and \n would do.

下面是一个类似的错误：

Here is a similar bug: C strings, strlen and Valgrind

这答案结合的讨论意见。谢谢大家！

This answer combines comments in the discussion. Thank you all!

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