在前面的文章中， 我们简单了解了valgrind工具的用途以及安装， 以便大家能进行实际操作。 在本文中， 我们通过实例来看看如何利用valgrind来定位内存泄漏问题。 先看程序：
#include <stdio.h> #include <stdlib.h> char* getMemory() { char *p = (char *)malloc(30); return p; } int main() { char *p = getMemory(); p = NULL; return 0; }
只要是懂一点C/C++的人， 就很容易看出上述程序有内存泄漏， 我们用valgrind工具来检测下：
[root@xxx ~/valgrind-3.8.1/bin]# ./valgrind --tool=memcheck --leak-check=yes --show-reachable=yes ./a.out ==19226== Memcheck, a memory error detector ==19226== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al. ==19226== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info ==19226== Command: ./a.out ==19226== ==19226== ==19226== HEAP SUMMARY: ==19226== in use at exit: 30 bytes in 1 blocks ==19226== total heap usage: 1 allocs, 0 frees, 30 bytes allocated ==19226== ==19226== 30 bytes in 1 blocks are definitely lost in loss record 1 of 1 ==19226== at 0x4C278FE: malloc (vg_replace_malloc.c:270) ==19226== by 0x4005B5: getMemory() (in /root/valgrind-3.8.1/bin/a.out) ==19226== by 0x4005CC: main (in /root/valgrind-3.8.1/bin/a.out) ==19226== ==19226== LEAK SUMMARY: ==19226== definitely lost: 30 bytes in 1 blocks ==19226== indirectly lost: 0 bytes in 0 blocks ==19226== possibly lost: 0 bytes in 0 blocks ==19226== still reachable: 0 bytes in 0 blocks ==19226== suppressed: 0 bytes in 0 blocks ==19226== ==19226== For counts of detected and suppressed errors, rerun with: -v ==19226== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 6 from 6) [root@xxx ~/valgrind-3.8.1/bin]#
我们可以很清楚地看到， 在getMemory调用malloc的那里， 有内存泄漏。最左边的19226表示进程号！
不过， 这样看着也挺蛋疼的， 如果代码过多， 肉眼据不太好分析了， 能不能把内存泄漏的代码行给找出来呢？ 当然能！ 回想一下我们之前介绍过得core dump定位到代码行的问题， 两个必要条件是: 编译时必须有-g参数； 编译后不能strip. 我们一起再看看：
[root@xxx ~/valgrind-3.8.1/bin]# g++ -g test.cpp [root@xxx ~/valgrind-3.8.1/bin]# [root@xxx ~/valgrind-3.8.1/bin]# ./valgrind --tool=memcheck --leak-check=yes --show-reachable=yes ./a.out ==20448== Memcheck, a memory error detector ==20448== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al. ==20448== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info ==20448== Command: ./a.out ==20448== ==20448== ==20448== HEAP SUMMARY: ==20448== in use at exit: 30 bytes in 1 blocks ==20448== total heap usage: 1 allocs, 0 frees, 30 bytes allocated ==20448== ==20448== 30 bytes in 1 blocks are definitely lost in loss record 1 of 1 ==20448== at 0x4C278FE: malloc (vg_replace_malloc.c:270) ==20448== by 0x4005B5: getMemory() (test.cpp:5) ==20448== by 0x4005CC: main (test.cpp:11) ==20448== ==20448== LEAK SUMMARY: ==20448== definitely lost: 30 bytes in 1 blocks ==20448== indirectly lost: 0 bytes in 0 blocks ==20448== possibly lost: 0 bytes in 0 blocks ==20448== still reachable: 0 bytes in 0 blocks ==20448== suppressed: 0 bytes in 0 blocks ==20448== ==20448== For counts of detected and suppressed errors, rerun with: -v ==20448== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 6 from 6) [root@xxx ~/valgrind-3.8.1/bin]#
好了， 泄漏的代码行出来了。 这里， 我顺便说说以往介绍过得addr2line命令， 如果用addr2line -e a.out 0x4005B5, 也是能得出代码行的。
fix后的代码如下：
#include <stdio.h> #include <stdlib.h> char* getMemory() { char *p = (char *)malloc(30); return p; } int main() { char *p = getMemory(); if(p != NULL) { free(p); p = NULL; } return 0; }
我们再用valgrind工具检测一下：
[root@xxx ~/valgrind-3.8.1/bin]# g++ -g test.cpp [root@xxx ~/valgrind-3.8.1/bin]# [root@xxx ~/valgrind-3.8.1/bin]# ./valgrind --tool=memcheck --leak-check=yes --show-reachable=yes ./a.out ==21033== Memcheck, a memory error detector ==21033== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al. ==21033== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info ==21033== Command: ./a.out ==21033== ==21033== ==21033== HEAP SUMMARY: ==21033== in use at exit: 0 bytes in 0 blocks ==21033== total heap usage: 1 allocs, 1 frees, 30 bytes allocated ==21033== ==21033== All heap blocks were freed -- no leaks are possible ==21033== ==21033== For counts of detected and suppressed errors, rerun with: -v ==21033== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 6) [root@xxx ~/valgrind-3.8.1/bin]#
可见， 没有内存泄漏了。 爽爽哒
涛歌依旧 认证博客专家 排名第一 点链接学人工智能 公众号免费领资料 ❤️零基础入门进阶人工智能 ❤️欢迎关注涛哥公众号，免费领海量学习资料。涛哥：毕业后就职于华为和腾讯。微信：ai_taogeyijiu
