GDB抓虫之旅（上篇）

本文来自网易云社区。

作者：盛国存

前言

问: gdb是什么？
答: 强大的UNIX下命令行调试工具。问: gdb能干什么？
答: 让你随心所欲的驾驭你的程序；Start、Stop、Examine、Change。问: 我们为什么要学习gdb？
答: 欲善其事，必先利其器；利用gdb进一步的定位程序异常。问: 本次分享的宗旨?
答: gdb的介绍和使用入门，为大家抓虫多提供一个选择。

抓虫从0开始

前期准备

1.包含有调试信息的可执行文件

2.编译时加-g选项即可，不建议开优化选项

GDB的启动

gdb <program>gdb <program> coregdb <program> <PID>
(gdb) file <program>

抓虫流程

实战1 : GDB基础命令的使用

1.1、示例程序(example_1.cpp)

#include <iostream>#include <string>using namespace std;int foo(int m, int n){    return 1;
}int foo(int n){    int result=0;    for (int i=0;i<=n;i++){result+=n;}    return result;
}int main(){    string s1="dafdf";    char * s2;//  s1=s2;int sum =0;    for (int i=1;i<=100;i++){sum+=i;}cout<<"result[1-100]="<<sum<<endl;cout<<"result[1-250]="<<foo(250)<<endl;    return 0;
}

1.2、调试准备编译命令：g++ -g -Wall -o example_1 example_1.cpp

1.3、启动gdb

$ gdb example_1
GNU gdb Red Hat Linux (6.3.0.0-1.96rh)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu"...Using host libthread_db library "/lib64/tls/libthread_db.so.1".

1.4、辅助性命令

(gdb) cd ..
Working directory /home/work/testers/sgc.
(gdb) shell ls
autotest  client  Makefile  spanti  spantispam_if.h  study(gdb) cd study/
Working directory /home/work/testers/sgc/study.
(gdb) pwd
Working directory /home/work/testers/sgc/study.
(gdb) help run
Start debugged program.  You may specify arguments to give it.
Args may include "*", or "[...]"; they are expanded using "sh".
Input and output redirection with ">", "<", or ">>" are also allowed.With no arguments, uses arguments last specified (with "run" or "set args").
To cancel previous arguments and run with no arguments,use "set args" without arguments.

1.5、设置断点命令

//查看源代码信息(gdb) l17                      result+=n;18              }19              return result;2021      }2223      int main()24      {2526              string s1="dafdf";
(gdb) l 11       #include <iostream>2       #include <string>345       using namespace std;6       int foo(int m, int n)7       {89               return 1;10(gdb) l example_1.cpp:1611      }12      int foo(int n)13      {14              int result=0;15              for (int i=0;i<=n;i++)16              {17                      result+=n;18              }19              return result;20(gdb) l foo(int)8
9               return 1;1011      }12      int foo(int n)13      {14              int result=0;15              for (int i=0;i<=n;i++)16              {17                      result+=n;

//设置断点(gdb) b 17Breakpoint 1 at 0x400c07: file example_1.cpp, line 17.(gdb) b main
Breakpoint 2 at 0x400c27: file example_1.cpp, line 26.(gdb) info br
Num Type           Disp Enb Address            What1   breakpoint     keep y   0x0000000000400c07 in foo(int) at example_1.cpp:17
2   breakpoint     keep y   0x0000000000400c27 in main at example_1.cpp:26

1.6、执行控制命令

(gdb) r
Starting program: /home/work/testers/sgc/study/example_1 Breakpoint 2, main () at example_1.cpp:2626              string s1="dafdf";
(gdb) c
Continuing.
result[1-100]=5050Breakpoint 1, foo (n=250) at example_1.cpp:1717                      result+=n;
(gdb) n
15              for (int i=0;i<=n;i++)

1.7、程序信息查看命令

//查看变量信息
(gdb) p result$1 = 250(gdb) p s1$2 = 1431655765(gdb) disp result1: result = 250(gdb) c
Continuing.Breakpoint 1, foo (n=250) at example_1.cpp:1717                      result+=n;1: result = 250(gdb) info locals
i = 1result = 250//查看栈信息
(gdb) bt#0  foo (n=250) at example_1.cpp:17#1  0x0000000000400cc1 in main () at example_1.cpp:38(gdb) info f
Stack level 0, frame at 0x7fbffff8a0:rip = 0x400c07 in foo(int) (example_1.cpp:17); saved rip 0x400cc1called by frame at 0x7fbffff910 source language c++.Arglist at 0x7fbffff890, args: n=250Locals at 0x7fbffff890, Previous frame's sp is 0x7fbffff8a0Saved registers:rbp at 0x7fbffff890, rip at 0x7fbffff898
(gdb) f 0
#0  foo (n=250) at example_1.cpp:17
17                      result+=n;

1.8、修改环境命令

(gdb) set var i=97(gdb) p i
$5 = 97(gdb) print i=98$6 = 98(gdb) ignore 1 300Will ignore next 300 crossings of breakpoint 1.(gdb) finish
Run till exit from #0  foo (n=250) at example_1.cpp:170x0000000000400cc1 in main () at example_1.cpp:3838              cout<<"result[1-250]="<<foo(250)<<endl;Value returned is $8 = 38500
(gdb) quit
The program is running.  Exit anyway? (y or n) y

不要放过core文件

问：Core文件是什么？
答：a disk file containing an image of the process's memory at the time of termination

问：Core的作用？
答：
1、让你在调试时，不用花费大量等待程序出错；
2、让你避免了单步调试的烦恼
3、让你定位错误所在

启动方式

$ gdb <program> core

查看程序信息的常用命令

1、查看栈信息：bt, f n, up/down，info frame

2、查看变量信息：info args|locals

实战2 : core文件调试

2.1、示例程序(crash2.c)

#include <string.h>void Strcpy(char *to , char *from){strcpy(to , from);
}int main(){    char *s = NULL;Strcpy(s, "abcdefg");    return 0;
}

2.2、查看信息

$ gdb ./crash2 core.19562 GNU gdb Red Hat Linux (6.3.0.0-1.96rh)Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu"...Using host libthread_db library "/lib64/tls/libthread_db.so.1".Core was generated by `./crash2'.
Program terminated with signal 11, Segmentation fault.
Reading symbols from /lib64/tls/libc.so.6...done.
Loaded symbols for /lib64/tls/libc.so.6
Reading symbols from /lib64/ld-linux-x86-64.so.2...done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
#0  0x000000302af6f9a4 in strcpy () from /lib64/tls/libc.so.6(gdb) bt
#0  0x000000302af6f9a4 in strcpy () from /lib64/tls/libc.so.6
#1  0x00000000004004c5 in Strcpy (to=0x0, from=0x4005dc "abcdefg") at crash2.c:5
#2  0x00000000004004e5 in main () at crash2.c:10(gdb) f 0
#0  0x000000302af6f9a4 in strcpy () from /lib64/tls/libc.so.6(gdb) up
#1  0x00000000004004c5 in Strcpy (to=0x0, from=0x4005dc "abcdefg") at crash2.c:5
5                       strcpy(to , from);
(gdb) info args
to = 0x0from = 0x4005dc "abcdefg"//至此，已经清楚的发现了问题所在，to指针为空

进阶之多线程程序调试

多线程调试常用命令

$ info <...>  // 强大的查看命令，如info threads
$ attach/detach <pid> // 挂载到进程
$ thread <thread_no> // 切换到线程
$ thread apply <thread_no_list> <cmd>// 对于list中的thread，执行cmd
$ break <linenum> thread <threadno> if ...

实战3 : 多线程程序调试

3.1、正常的示例程序(good_thread.c)

#include <pthread.h>#include <stdio.h>#include <sys/time.h>#include <string.h>#define MAX 10pthread_t thread[2];
pthread_mutex_t mut;int number=0, i;void *thread1()
{        printf ("thread1 : I'm thread 1\n");        for (i = 0; i < MAX; i++){                printf("thread1 : number = %d\n",number);pthread_mutex_lock(&mut);number++;pthread_mutex_unlock(&mut);sleep(2);}        printf("thread1 :主函数在等我完成任务吗？\n");pthread_exit(NULL);
}void *thread2()
{        printf("thread2 : I'm thread 2\n");        for (i = 0; i < MAX; i++){                printf("thread2 : number = %d\n",number);pthread_mutex_lock(&mut);number++;pthread_mutex_unlock(&mut);sleep(3);}        printf("thread2 :主函数在等我完成任务吗？\n");pthread_exit(NULL);
}void thread_create(void)
{        int temp;        memset(&thread, 0, sizeof(thread));          //comment1/*创建线程*/if((temp = pthread_create(&thread[0], NULL, thread1, NULL)) != 0)       //comment2printf("线程1创建失败!\n");        elseprintf("线程1被创建\n");        if((temp = pthread_create(&thread[1], NULL, thread2, NULL)) != 0) //comment3printf("线程2创建失败");        elseprintf("线程2被创建\n");
}void thread_wait(void)
{        /*等待线程结束*/if(thread[0] !=0) {                   //comment4pthread_join(thread[0],NULL);                printf("线程1已经结束\n");}        if(thread[1] !=0) {                //comment5pthread_join(thread[1],NULL);                printf("线程2已经结束\n");}
}int main()
{        /*用默认属性初始化互斥锁*/pthread_mutex_init(&mut,NULL);        printf("我是主函数哦，我正在创建线程，呵呵\n");thread_create();        printf("我是主函数哦，我正在等待线程完成任务阿，呵呵\n");thread_wait();        return 0;
}

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