一 前言

对于C或C++程序员来说，面对的bug很大部分是内存操作问题，这其中比较令人头疼的就是内存泄漏了，虽然我们有valgrind 和AScan等内存问题的检测工具，但是valgrind每次输出一大堆，AScan有时候看输出结果看的是云里雾里的。再说，谁会嫌弃工具箱里面多个工具那。

二 内存泄漏的一般检查2.1 基本准备

内存泄漏问题的检查步骤，对于做过c或c++同学都比较熟悉:

首先通过top或vmstat 、或smem（本次介绍）等工具查看内存情况，看看是否出现了内存泄漏。其次用pidstat 或top指定进程的方式，观察可以进程内存占用情况。用memleak或gdb工具查看内存泄漏。

先上测试代码：

#include <stdio.h>#include <stdlib.h>#include <pthread.h>#include <unistd.h>#define MALLOC_SIZE 256000int *fibo(int *n0, int *n1){    int *v = (int *) malloc(MALLOC_SIZE*sizeof(int));    memset(v, 0, MALLOC_SIZE*sizeof(int));    *v = *n0 + *n1;    return v;}void do_test(){    int n0 = 0;    int n1 = 1;    int *v = NULL;    int n = 2;    for (n = 2; n > 0; n++)     {        v = fibo(&n0, &n1);        n0 = n1;        n1 = *v;        printf("%dth => %lld\n", n, *v);          //free(v)                 sleep(1);            }}int main(void){     printf("pid=%d\n", getpid());     do_test();     return 0;}

程序比较简单，编译运行起来：

gcc memtest.c ; ./a.out

这次用下新工具smem，这是一个python写的小工具，可以统计系统中所有进程占用的物理内存RSS、以及去掉共享内存的PSS、以及程序本身的独占内存USS的情况。

安装：

# centos 下yum install epel-releaseyum install smem python-matplotlib python-tk# ubuntu 下apt-get install smem

常用命令：

-k 带单位显示内存

root@ubuntu-lab:/home/miao# smem -k  PID User     Command                         Swap      USS      PSS      RSS  1009 root     /usr/sbin/cron -f -P               0   304.0K   399.0K     2.9M  1137 root     nginx: master process /usr/        0   196.0K   435.0K     2.1M   931 root     /usr/sbin/irqbalance --fore        0   492.0K   655.0K     4.0M ....

-u -k 带单位显示每个用户的内存占用：

root@ubuntu-lab:/home/miao# smem -u -kUser     Count     Swap      USS      PSS      RSS systemd-timesync     1        0   764.0K     1.1M     6.7M messagebus     1        0   924.0K     1.2M     4.9M systemd-network     1        0     1.7M     2.1M     7.4M syslog       1        0     3.0M     3.1M     6.2M www-data     4        0     2.0M     4.2M    22.4M systemd-resolve     1        0     4.8M     5.8M    12.7M miao         8        0    11.0M    16.9M    49.1M postgres     7        0     9.2M    22.0M    74.5M mysql        1        0    74.0M    74.7M    80.7M root        30        0   260.7M   284.1M   429.5M 

-w -k 显示系统整体内存情况类似free

root@ubuntu-lab:/home/miao# smem -w -kArea                           Used      Cache   Noncache firmware/hardware                 0          0          0 kernel image                      0          0          0 kernel dynamic memory          1.5G       1.3G     268.5M userspace memory             414.0M     191.5M     222.5M free memory                    2.8G       2.8G          0 

-k -s uss -r 按照uss的占用从大到小排序的方式展示内存的占用情况 非常实用

root@ubuntu-lab:/home/miao# smem  -k -s uss -r  PID User     Command                         Swap      USS      PSS      RSS  1298 root     /usr/bin/dockerd -H                  0    74.3M    74.5M    77.9M  1068 mysql    /usr/sbin/mariadbd                 0    74.0M    74.8M    80.7M   939 root     /usr/lib/snapd/snapd               0    44.9M    45.0M    46.7M ....

好了基本命令介绍完毕，那我们来看看如何查看内存是否泄漏吧，因为内存泄漏的程序占用的内存是一直再增加的（这不是废话嘛），这样我们就可以用上面的排序命令只观察上面几个进程了。

watch smem  -k -s uss -r

小技巧，watch加在命令前面，5s执行一次命令，会高亮显示改变的部分。

2.3 memleak检查

在ubuntu下安装memleak竟然很难安装，我用的是最新的服务器版本，后面在centos下安装后测试的：

[root@xxx]# python2 /usr/share/bcc/tools/memleak -p 160399Attaching to pid 160399, Ctrl+C to quit.[17:27:25] Top 10 stacks with outstanding allocations:        5120000 bytes in 5 allocations from stack                fibo+0x1a [a.out]                do_test+0x41 [a.out]                main+0x24 [a.out]                __libc_start_main+0xf5 [libc-2.17.so][17:27:30] Top 10 stacks with outstanding allocations:        10240000 bytes in 10 allocations from stack                fibo+0x1a [a.out]                do_test+0x41 [a.out]                main+0x24 [a.out]                __libc_start_main+0xf5 [libc-2.17.so][17:27:35] Top 10 stacks with outstanding allocations:        15360000 bytes in 15 allocations from stack                fibo+0x1a [a.out]                do_test+0x41 [a.out]                main+0x24 [a.out]                __libc_start_main+0xf5 [libc-2.17.so][17:27:40] Top 10 stacks with outstanding allocations:        19456000 bytes in 19 allocations from stack

fibo 函数出现内存泄漏，把泄漏的字节数都打印了出来，我们改了下代码把free的注释去掉，再用memleak查看等了一会还是没有泄漏信息，说明已经修复了，如下：

[root@xxx]# python2 /usr/share/bcc/tools/memleak -p 165349Attaching to pid 165349, Ctrl+C to quit.[17:35:21] Top 10 stacks with outstanding allocations:[17:35:26] Top 10 stacks with outstanding allocations:[17:35:31] Top 10 stacks with outstanding allocations:[17:35:36] Top 10 stacks with outstanding allocations:

也许你对memleak已经很熟悉了，那来看看gdb查看函数的内存泄漏方法吧，这个方法只是查看具体的一个函数是否存在内存泄漏，一定的场景下还是蛮实用的。 把代码中的 for (n = 2; n > 0; n++) 改成 for (n = 2; n > 0&& n <10; n++)

(gdb) b mainBreakpoint 1 at 0x400739: file memleaktest.c, line 34.(gdb) rStarting program: /home/miaohq/testcode/./a.out Breakpoint 1, main () at memleaktest.c:3434               printf("pid=%d\n", getpid());Missing separate debuginfos, use: debuginfo-install glibc-2.17-325.el7_9.x86_64(gdb) call malloc_stats()Arena 0:system bytes     =          0in use bytes     =          0Total (incl. mmap):system bytes     =          0in use bytes     =          0max mmap regions =          0max mmap bytes   =          0$1 = -136490560(gdb) npid=18197735                    do_test();(gdb) call malloc_stats()Arena 0:system bytes     =          0in use bytes     =          0Total (incl. mmap):system bytes     =          0in use bytes     =          0max mmap regions =          0max mmap bytes   =          0$2 = -136490560(gdb) n2th => 13th => 24th => 35th => 56th => 87th => 138th => 219th => 3436                         return 0;(gdb) call malloc_stats()Arena 0:system bytes     =          0in use bytes     =          0Total (incl. mmap):system bytes     =    8224768in use bytes     =    8224768max mmap regions =          8max mmap bytes   =    8224768$3 = -136490560(gdb) p 256000*4*8$4 = 8192000(gdb) 

Total (incl. mmap):即本程序占用的总内存，看到明显的增加部分即为未释放的内存，程序使用的内存增加：8224768 稍大于 256000*4*8 分配的内存，内存分配需要存储链表还有一些对齐原因所以会多分配些。

free之后的场景：

(gdb) call malloc_stats()Arena 0:system bytes     =          0in use bytes     =          0Total (incl. mmap):system bytes     =          0in use bytes     =          0max mmap regions =          0max mmap bytes   =          0$1 = -136490560(gdb) npid=18340635                    do_test();(gdb) n2th => 13th => 24th => 35th => 56th => 87th => 138th => 219th => 3436                         return 0;(gdb) call malloc_stats()Arena 0:system bytes     =    1159168in use bytes     =          0Total (incl. mmap):system bytes     =    1159168in use bytes     =          0max mmap regions =          1max mmap bytes   =    1028096$2 = -136490560(gdb) 

in use bytes 为0了。