• 内存排查 valgrind

    内存问题排查工具 --- 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
", *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
", *xx);
19: 
20:     free(e);
21: 
22:     printf("%f
", *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
", *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。callgrindannotate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

    Author: Cobbliu

    Created: 2015-04-14 Tue 01:19

    Emacs 24.4.1 (Org mode 8.2.10)

    Validate

     
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  • 原文地址：https://www.cnblogs.com/Leo_wl/p/4423926.html
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