valgrind 内存泄露检查

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/eric/workspace/valgrind/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/eric/workspace/valgrind/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/eric/workspace/valgrind/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/eric/workspace/valgrind/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/eric/workspace/valgrind/test/a.out)