Biegral Blog

       继续介绍valgrind的使用， 看程序：

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

int main()
{
	char a[] = "abcdefghijk";
    memcpy(a + 1, a, 5);
	printf("%s\n", a);
	
    return 0;
}

     先看看结果：

[root@xxx ~/valgrind-3.8.1/bin]# g++ -g test.cpp
[root@xxx ~/valgrind-3.8.1/bin]# ./a.out 
aaacdeghijk
[root@xxx ~/valgrind-3.8.1/bin]#

      啊？ 怎么和预期的不一样？  原来， memcpy在拷贝的时候， 是不允许内存重叠的。 如果处理内存重叠的内存考虑， 我们应该用memmove.  看程序：

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

int main()
{
	char a[] = "abcdefghijk";
    memcpy(a + 1, a, 5);
	printf("%s\n", a);
	
	char b[] = "abcdefghijk";
    memmove(b + 1, b, 5);
	printf("%s\n", b);
	
    return 0;
}

     结果为：

[root@xxx ~/valgrind-3.8.1/bin]# g++ -g test.cpp
[root@xxx ~/valgrind-3.8.1/bin]# ./a.out 
aaacdeghijk
aabcdeghijk
[root@xxx ~/valgrind-3.8.1/bin]# 

      用memmove就对了。

      有时候， 在调试的时候， 会发现memcpy的上述诡异现象， 我们用valgrind来看看（针对如上第二个程序）， 如下：

[root@xxx ~/valgrind-3.8.1/bin]# g++ -g test.cpp
[root@xxx ~/valgrind-3.8.1/bin]# 
[root@xxx ~/valgrind-3.8.1/bin]# ./valgrind --tool=memcheck --leak-check=yes --show-reachable=yes ./a.out
==12994== Memcheck, a memory error detector
==12994== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==12994== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==12994== Command: ./a.out
==12994== 
==12994== Source and destination overlap in memcpy(0x7ff0004d1, 0x7ff0004d0, 5)
==12994==    at 0x4C29A9E: memcpy (mc_replace_strmem.c:878)
==12994==    by 0x40066C: main (test.cpp:8)
==12994== 
aabcdeghijk
aabcdeghijk
==12994== 
==12994== HEAP SUMMARY:
==12994==     in use at exit: 0 bytes in 0 blocks
==12994==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==12994== 
==12994== All heap blocks were freed -- no leaks are possible
==12994== 
==12994== For counts of detected and suppressed errors, rerun with: -v
==12994== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 6 from 6)
[root@xxx ~/valgrind-3.8.1/bin]# 

       可以看到， 有内存重叠的问题。 下次我们用memcpy的时候， 也要小心才好。