valgrind 的使用及错误信息分析


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作者联系方式:YU TAO <yut616 at sohu dot com>

这里记录一下使用valgrind查找你的应用程序中的各种潜在的错误信息,并举例说明。

经常使用valgrind查找一下你的代码的内存有关错误,对移植到嵌入系统后的系统稳定性来说有着重要的意义。

usage

  • x86 平台
  • 先编译你自己的应用程序
  • 命令行:
    • valgrind --log-file=1 --tool=memcheck ./a.out

error specification

一、有malloc,但未free

  • code
    #include <stdio.h>
    #include <stdlib.h>

    void main()
    {
    char *p = malloc(20);
    sprintf(p, "%s", "test");
    fprintf(stderr, "p:%s/n", p);
    }
  • 分析:
    • 文件后部,总体来看,有确定无疑的lost 20字节。如下:
      ==26512== LEAK SUMMARY:
      ==26512== definitely lost: 20 bytes in 1 blocks.
    • 在文件之前描述的内容,细节可以看出,有1个malloc,但未去free。如下:
      ==26512== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 11 from 1)
      ==26512== malloc/free: in use at exit: 20 bytes in 1 blocks.
      ==26512== malloc/free: 1 allocs, 0 frees, 20 bytes allocated.

二、free一个未malloc的指针

  • code
    void main()
    {
    char p[] = "hello";
    fprintf(stderr, "p:%s/n", p);
    free(p);
    }
  • 分析:
    • 文件后部,总体来看,有1个错误,0个malloc,1个free。如下:
      ==26786== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 11 from 1)
      ==26786== malloc/free: in use at exit: 0 bytes in 0 blocks.
      ==26786== malloc/free: 0 allocs, 1 frees, 0 bytes allocated.
    • 在文件之前描述的内容,细节可以看出,有一个无效的free。如下:
      ==26786== Invalid free() / delete / delete[]
      ==26786== at 0x402265C: free (vg_replace_malloc.c:323)
      ==26786== by 0x804841F: main (in /home/yutao/test/a.out)

三、stack中,无效的读取,不会提示出错

  • code
    void main()
    {
    char p[8] = "hello"; //p在栈上, "hello"在常量区
    fprintf(stderr, "p10:%c/n", p[10]);
    }
  • 分析:
    • 读取stack栈中的内容,不会提示错误
      ==27452== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 11 from 1)
      ==27452== malloc/free: in use at exit: 0 bytes in 0 blocks.
      ==27452== malloc/free: 0 allocs, 0 frees, 0 bytes allocated.
  • 其为局部变量,存贮在stack中,由于p[10]未超出栈的长度,所以没提示出错?

四、heap堆中,无效的读取,会提示出错

  • code
    void main()
    {
    char *p = malloc(8);
    fprintf(stderr, "p10:%c/n", p[10]);
    free(p);
    }
  • 分析:
    • 读取heap堆中的内容,会提示错误,提示无效的1个字节的读取,位置是malloc的8字节的后面第2字节
      ==27744== Invalid read of size 1
      ==27744== at 0x804842A: main (in /home/yutao/test/a.out)
      ==27744== Address 0x4190032 is 2 bytes after a block of size 8 alloc'd
      ==27744== at 0x4022AB8: malloc (vg_replace_malloc.c:207)
      ==27744== by 0x8048420: main (in /home/yutao/test/a.out)
      ==27744==
      ==27744== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 11 from 1)
      ==27744== malloc/free: in use at exit: 0 bytes in 0 blocks.
      ==27744== malloc/free: 1 allocs, 1 frees, 8 bytes allocated.
    • malloc的数据在heap堆中。

五、stack上的,无效的写

  • code
    void main()
    {
    char p[8] = "hello";
    p[10]='a';
    }
  • 分析:
    • 运行时会出错,会打印Backtrace
      *** stack smashing detected ***: ./a.out terminated
      ======= Backtrace: =========
      /lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x48)[0x412d138]
      /lib/tls/i686/cmov/libc.so.6(__fortify_fail+0x0)[0x412d0f0]
      ./a.out[0x80483d6]
      /lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe0)[0x4056450]
      ./a.out[0x8048331]
      ======= Memory map: ========
      04000000-0401a000 r-xp 00000000 08:06 682589 /lib/ld-2.7.so
    • 总体,会提示有1个错误,内容是5个malloc,0个free
      ==27918== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 13 from 1)
      ==27918== malloc/free: in use at exit: 892 bytes in 5 blocks.
      ==27918== malloc/free: 5 allocs, 0 frees, 892 bytes allocated.
      ==27918== LEAK SUMMARY:
      ==27918== definitely lost: 0 bytes in 0 blocks.
      ==27918== possibly lost: 0 bytes in 0 blocks.
      ==27918== still reachable: 892 bytes in 5 blocks.
    • 文件前面的错误信息是,有一个无效read,读了4个字节。
      ==27918== Invalid read of size 4
      ==27918== at 0x40151F3: (within /lib/ld-2.7.so)
      ==27918== by 0x4005C69: (within /lib/ld-2.7.so)
      ==27918== by 0x4007A97: (within /lib/ld-2.7.so)
      ==27918== by 0x4011543: (within /lib/ld-2.7.so)
      ==27918== by 0x400D5D5: (within /lib/ld-2.7.so)
      ==27918== by 0x4010F5D: (within /lib/ld-2.7.so)
      ==27918== by 0x414E291: (within /lib/tls/i686/cmov/libc-2.7.so)
      ==27918== by 0x400D5D5: (within /lib/ld-2.7.so)
      ==27918== by 0x414E454: __libc_dlopen_mode (in /lib/tls/i686/cmov/libc-2.7.so)
      ==27918== by 0x412A4D8: (within /lib/tls/i686/cmov/libc-2.7.so)
      ==27918== by 0x412A4D8: (within /lib/tls/i686/cmov/libc-2.7.so)
      ==27918== by 0x412A669: backtrace (in /lib/tls/i686/cmov/libc-2.7.so)
      ==27918== by 0x40A3B91: (within /lib/tls/i686/cmov/libc-2.7.so)
      ==27918== Address 0x4190038 is 16 bytes inside a block of size 19 alloc'd
    • 这个提示的错误信息和代码中的信息,没很好的匹配上,但可以知道是已经出错了。

六、heap堆上,无效的写

  • code
    void main()
    {
    char *p = malloc(8);
    p[10]='a';
    free(p);
    }
  • 分析:
    • 程序运行不会出错
    • 写heap堆中的内容,log会提示错误,提示无效的1个字节的写,位置是malloc的8字节的后面第2字节
      ==28351== Invalid write of size 1
      ==28351== at 0x80483CA: main (in /home/yutao/test/a.out)
      ==28351== Address 0x4190032 is 2 bytes after a block of size 8 alloc'd
      ==28351== at 0x4022AB8: malloc (vg_replace_malloc.c:207)
      ==28351== by 0x80483C0: main (in /home/yutao/test/a.out)

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