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)