1. 使用未初始化的内存
Code :
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#include <stdio.h>
#include <stdlib.h>
int main(void)
{
char *p;
char c = *p;
printf("\n [%c]\n",c);
return 0;
}
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在上面的代码中，我们尝试使用未初始化的指针 ‘p’.
让我们运行Memcheck来看下结果.
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$ valgrind --tool=memcheck ./val
==2862== Memcheck, a memory error detector
==2862== Copyright (C) 2002-2009, and GNU GPL‘d, by Julian Seward et al.
==2862== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==2862== Command: ./val
==2862==
==2862== Use of uninitialised value of size 8
==2862== at 0x400530: main (valgrind.c:8)
==2862==
[#]
==2862==
==2862== HEAP SUMMARY:
==2862== in use at exit: 0 bytes in 0 blocks
==2862== total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==2862==
==2862== All heap blocks were freed -- no leaks are possible
==2862==
==2862== For counts of detected and suppressed errors, rerun with: -v
==2862== Use --track-origins=yes to see where uninitialized values come from
==2862== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 4 from 4)
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从上面的输出可以看到,Valgrind检测到了未初始化的变量，然后给出了警告(上面加粗的几行(译者注：貌似上面没有加粗的)).
2. 在内存被释放后进行读/写
Code :
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#include <stdio.h>
#include <stdlib.h>
int main(void)
{
char *p = malloc(1);
*p = ‘a‘;
char c = *p;
printf("\n [%c]\n",c);
free(p);
c = *p;
return 0;
}
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上面的代码中，我们有一个释放了内存的指针 ‘p’ 然后我们又尝试利用指针获取值.
让我们运行memcheck来看一下Valgrind对这种情况是如何反应的.
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$ valgrind --tool=memcheck ./val
==2849== Memcheck, a memory error detector
==2849== Copyright (C) 2002-2009, and GNU GPL‘d, by Julian Seward et al.
==2849== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==2849== Command: ./val
==2849==
[a]
==2849== Invalid read of size 1
==2849== at 0x400603: main (valgrind.c:30)
==2849== Address 0x51b0040 is 0 bytes inside a block of size 1 free‘d
==2849== at 0x4C270BD: free (vg_replace_malloc.c:366)
==2849== by 0x4005FE: main (valgrind.c:29)
==2849==
==2849==
==2849== HEAP SUMMARY:
==2849== in use at exit: 0 bytes in 0 blocks
==2849== total heap usage: 1 allocs, 1 frees, 1 bytes allocated
==2849==
==2849== All heap blocks were freed -- no leaks are possible
==2849==
==2849== For counts of detected and suppressed errors, rerun with: -v
==2849== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 4 from 4)
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从上面的输出内容可以看到，Valgrind检测到了无效的读取操作然后输出了警告 ‘Invalid read of size 1′.
另注,使用gdb来调试c程序.
3. 从已分配内存块的尾部进行读/写
Code :
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#include <stdio.h>
#include <stdlib.h>
int main(void)
{
char *p = malloc(1);
*p = ‘a‘;
char c = *(p+1);
printf("\n [%c]\n",c);
free(p);
return 0;
}
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在上面的代码中，我们已经为‘p’分配了一个字节的内存,但我们在将值读取到 ‘c’中的时候使用的是地址p+1.
现在我们使用Valgrind运行上面的代码 :
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$ valgrind --tool=memcheck ./val
==2835== Memcheck, a memory error detector
==2835== Copyright (C) 2002-2009, and GNU GPL‘d, by Julian Seward et al.
==2835== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==2835== Command: ./val
==2835==
==2835== Invalid read of size 1
==2835== at 0x4005D9: main (valgrind.c:25)
==2835== Address 0x51b0041 is 0 bytes after a block of size 1 alloc‘d
==2835== at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==2835== by 0x4005C5: main (valgrind.c:22)
==2835==
[]
==2835==
==2835== HEAP SUMMARY:
==2835== in use at exit: 0 bytes in 0 blocks
==2835== total heap usage: 1 allocs, 1 frees, 1 bytes allocated
==2835==
==2835== All heap blocks were freed -- no leaks are possible
==2835==
==2835== For counts of detected and suppressed errors, rerun with: -v
==2835== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 4 from 4)
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同样，该工具在这种情况下也检测到了无效的读取操作.
4. 内存泄露
Code:
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#include <stdio.h>
#include <stdlib.h>
int main(void)
{
char *p = malloc(1);
*p = ‘a‘;
char c = *p;
printf("\n [%c]\n",c);
return 0;
}
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在这次的代码中, 我们申请了一个字节但是没有将它释放.现在让我们运行Valgrind看看会发生什么：
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$ valgrind --tool=memcheck --leak-check=full ./val
==2888== Memcheck, a memory error detector
==2888== Copyright (C) 2002-2009, and GNU GPL‘d, by Julian Seward et al.
==2888== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==2888== Command: ./val
==2888==
[a]
==2888==
==2888== HEAP SUMMARY:
==2888== in use at exit: 1 bytes in 1 blocks
==2888== total heap usage: 1 allocs, 0 frees, 1 bytes allocated
==2888==
==2888== 1 bytes in 1 blocks are definitely lost in loss record 1 of 1
==2888== at 0x4C274A8: malloc (vg_replace_malloc.c:236)
==2888== by 0x400575: main (valgrind.c:6)
==2888==
==2888== LEAK SUMMARY:
==2888== definitely lost: 1 bytes in 1 blocks
==2888== indirectly lost: 0 bytes in 0 blocks
==2888== possibly lost: 0 bytes in 0 blocks
==2888== still reachable: 0 bytes in 0 blocks
==2888== suppressed: 0 bytes in 0 blocks
==2888==
==2888== For counts of detected and suppressed errors, rerun with: -v
==2888== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 4 from 4)
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输出行(上面加粗的部分)显示，该工具能够检测到内存的泄露.
注意: 在这里我们增加了一个选项‘–leak-check=full’来得到内存泄露的详细细节.
