内存排查 valgrind

1 概述

在用C/C++编程的时候，经常会出现下面三种内存问题：

内存泄漏
悬挂指针
多次释放同一块内存

本系列文章简要介绍排查这三个问题的工具和方法，先看看Valgrind

2 Valgrind

Valgrind是一款可以监测内存使用情况、监测内存泄漏的工具。对于一些规模不是很大的应用程序，Valgrind是一把利器。

3 内存泄漏监测

3.1 示例代码

 1: int main()
 2: {
 3:     char *p = malloc(sizeof(char) * 10);
 4:     if (p == NULL) {
 5:         return 0;
 6:     }
 7: 
 8:     *p++ = ‘a‘;
 9:     *p++ = ‘b‘;
10: 
11:     printf("%s\n", *p);
12: 
13:     return 0;
14: }

3.2 编译它

1: gcc -g -o core1 core1.c

3.3 用Valgrind监测进程的内存泄漏

1: valgrind --leak-check=yes --show-reachable=yes ./core

Valgrind的输出为为：

 1: ==25500== Memcheck, a memory error detector
 2: ==25500== Copyright (C) 2002-2009, and GNU GPL‘d, by Julian Seward et al.
 3: ==25500== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
 4: ==25500== Command: ./core1
 5: ==25500==
 6: ==25500== Conditional jump or move depends on uninitialised value(s)
 7: ==25500==    at 0x36A104546A: vfprintf (in /lib64/libc-2.12.so)
 8: ==25500==    by 0x36A104EAC9: printf (in /lib64/libc-2.12.so)
 9: ==25500==    by 0x40055D: main (core1.c:13)
10: ==25500==
11: (null)
12: ==25500==
13: ==25500== HEAP SUMMARY:
14: ==25500==     in use at exit: 10 bytes in 1 blocks
15: ==25500==   total heap usage: 1 allocs, 0 frees, 10 bytes allocated
16: ==25500==
17: ==25500== 10 bytes in 1 blocks are definitely lost in loss record 1 of 1
18: ==25500==    at 0x4A0515D: malloc (vg_replace_malloc.c:195)
19: ==25500==    by 0x400515: main (core1.c:5)
20: ==25500==
21: ==25500== LEAK SUMMARY:
22: ==25500==    definitely lost: 10 bytes in 1 blocks
23: ==25500==    indirectly lost: 0 bytes in 0 blocks
24: ==25500==      possibly lost: 0 bytes in 0 blocks
25: ==25500==    still reachable: 0 bytes in 0 blocks
26: ==25500==         suppressed: 0 bytes in 0 blocks
27: ==25500==
28: ==25500== For counts of detected and suppressed errors, rerun with: -v
29: ==25500== Use --track-origins=yes to see where uninitialised values come from
30: ==25500== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 6 from 6)

可以看到，Valgrind提示在第五行分配的内存未被释放

4 悬挂指针

4.1 示例代码

 1: struct elem {
 2:     int     a;
 3:     double  b;
 4: };
 5: 
 6: int main()
 7: {
 8:     struct elem *e = malloc(sizeof(struct elem));
 9:     if (e == NULL) {
10:         return 0;
11:     }
12: 
13:     e->a = 10;
14:     e->b = 10.10;
15: 
16:     double *xx = &e->b;
17: 
18:     printf("%f\n", *xx);
19: 
20:     free(e);
21: 
22:     printf("%f\n", *xx);
23: 
24:     return 0;
25: }

4.2 Valgrind运行结果

同样用-g编译后valgrind运行的结果：

 1: [cobbliu@MacBook]$ valgrind --leak-check=yes --show-reachable=yes ./core2
 2: ==26148== Memcheck, a memory error detector
 3: ==26148== Copyright (C) 2002-2009, and GNU GPL‘d, by Julian Seward et al.
 4: ==26148== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
 5: ==26148== Command: ./core2
 6: ==26148==
 7: 10.100000
 8: ==26148== Invalid read of size 8
 9: ==26148==    at 0x4005CA: main (core2.c:26)
10: ==26148==  Address 0x502a048 is 8 bytes inside a block of size 16 free‘d
11: ==26148==    at 0x4A04D72: free (vg_replace_malloc.c:325)
12: ==26148==    by 0x4005C5: main (core2.c:24)
13: ==26148==
14: 10.100000
15: ==26148==
16: ==26148== HEAP SUMMARY:
17: ==26148==     in use at exit: 0 bytes in 0 blocks
18: ==26148==   total heap usage: 1 allocs, 1 frees, 16 bytes allocated
19: ==26148==
20: ==26148== All heap blocks were freed -- no leaks are possible
21: ==26148==
22: ==26148== For counts of detected and suppressed errors, rerun with: -v
23: ==26148== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 6 from 6)

可以看到在free(e)后，指针xx成为了悬挂指针，此后对xx的读，如果xx指向的内存还未被glibc回收，进程不会core掉。valgrind提示在26行做了对xx的 Invalid read.

5 多次释放同一个指针

5.1 示例代码

 1: int main()
 2: {
 3:     char *p = malloc(sizeof(char) * 10);
 4:     if (p == NULL) {
 5:         return 0;
 6:     }
 7: 
 8:     char *q = p;
 9: 
10:     *p++ = ‘a‘;
11:     *p++ = ‘b‘;
12: 
13:     printf("%s\n", *p);
14: 
15:     free(p);
16:     free(q);
17:     return 0;
18: }

5.2 Valgrind 监测

 1: [cobbliu@MacBook]$ valgrind --leak-check=yes --show-reachable=yes ./core1
 2: ==26874== Memcheck, a memory error detector
 3: ==26874== Copyright (C) 2002-2009, and GNU GPL‘d, by Julian Seward et al.
 4: ==26874== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
 5: ==26874== Command: ./core1
 6: ==26874==
 7: ==26874== Conditional jump or move depends on uninitialised value(s)
 8: ==26874==    at 0x36A104546A: vfprintf (in /lib64/libc-2.12.so)
 9: ==26874==    by 0x36A104EAC9: printf (in /lib64/libc-2.12.so)
10: ==26874==    by 0x4005B5: main (core1.c:15)
11: ==26874==
12: (null)
13: ==26874== Invalid free() / delete / delete[]
14: ==26874==    at 0x4A04D72: free (vg_replace_malloc.c:325)
15: ==26874==    by 0x4005C1: main (core1.c:17)
16: ==26874==  Address 0x502a042 is 2 bytes inside a block of size 10 alloc‘d
17: ==26874==    at 0x4A0515D: malloc (vg_replace_malloc.c:195)
18: ==26874==    by 0x400565: main (core1.c:5)
19: ==26874==
20: ==26874==
21: ==26874== HEAP SUMMARY:
22: ==26874==     in use at exit: 0 bytes in 0 blocks
23: ==26874==   total heap usage: 1 allocs, 2 frees, 10 bytes allocated
24: ==26874==
25: ==26874== All heap blocks were freed -- no leaks are possible
26: ==26874==
27: ==26874== For counts of detected and suppressed errors, rerun with: -v
28: ==26874== Use --track-origins=yes to see where uninitialised values come from
29: ==26874== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 6 from 6)

可以看到，valgrind提示在17行有一个Invalid Free()

6 Valgrind的优缺点

valgrind默认使用memcheck工具做内存监测。

6.1 Advantages

用valgrind监测内存泄漏，不用重新编译应用程序，不用重新链接应用程序，不用对应用进程做任何修改。如果想查看详细的出错信息，只需要在编译时加上-g选项。

6.2 Disadvantages

不管valgrind在使用memcheck工具监测内存时，它会接管应用程序，并且读取应用程序可执行文件和库文件中的debug信息来显示详细的出错位置。当valgrind启动后，应用进程实际上在valgrind的虚拟环境中执行，valgrind会将每行代码传递给memcheck工具，memcheck工具再加入自己的调试信息，之后再将合成的代码真正运行。memcheck工具在应用进程每个防存操作和每个变量赋值操作时加入额外的统计代码，通常情况下，使用memcheck工具后应用程序的运行时间会比原生代码慢大约10-50倍。

其次，对于一些不停机运行的服务器程序的内存问题，valgrind无能为力。不仅仅是因为valgrind无法使之停止，还有可能是因为服务器进程本身就被设计为申请一些生命周期与进程生命周期一样长的内存，永远不释放，这些内存会被valgrind报泄漏错误。

再次，valgrind对多线程程序支持得不够好。在多线程程序执行时，valgrind在同一时刻只让其中一个线程执行，它不会充分利用多核的环境。在用valgrind运行您的多线程程序时，您的宝贵程序的运行情况可能跟不使用valgrind的运行情况千差万别。

7 Valgrind的其他工具

除了memcheck工具外，valgrind工具包还有一些别的好用的工具

7.1 Cachegrind

这个工具模拟 CPU中的一级缓存I1,D1和L2二级缓存，能够精确地指出程序中 cache的丢失和命中。如果需要，它还能够为我们提供cache丢失次数，内存引用次数，以及每行代码，每个函数，每个模块，整个程序产生的指令数。这对优化程序有很大的帮助。详情见这里

7.2 Callgrind

Callgrind收集程序运行时的一些数据，函数调用关系等信息，还可以有选择地进行cache 模拟。在运行结束时，它会把分析数据写入一个文件。callgrind_{annotate可以把} 这个文件的内容转化成可读的形式。详情见这里

7.3 Helgrind

它主要用来检查C/C++多线程程序（使用POSIX线程）中出现的同步问题。Helgrind 寻找内存中被多个线程访问，而又没有一贯加锁的区域，这些区域往往是线程之间失去同步的地方，而且会导致难以发掘的错误。Helgrind实现了名为"Eraser" 的竞争检测算法，并做了进一步改进，减少了报告错误的次数。详情见这里

7.4 DRD

这也使一款多线程程序监测工具，它提供的监测信息比Helgrind更丰富。详情见这里

7.5 Massif

堆栈分析器，它能测量程序在堆栈中使用了多少内存。告诉我们堆块，堆管理块和栈的大小。对于那些被应用进程释放但是还没有交还给操作系统的内存，memcheck是监测不出来的，而Massif能有效第监测到这类内存。详情见这里

7.6 DHAT

这个工具能详细地显示应用进程如何使用堆栈，以使用户更好地评估程序的设计。详情见这里

8 参考

http://www.valgrind.org/