GDB调试

2017-03-15

文中代码部分及部分讲解转载自陈皓gdb教程相关博客

What is GDB?

GDB, the GNU Project debugger, allows you to see what is going on `inside’ another program while it executes – or what another program was doing at the moment it crashed.
GDB can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:
Start your program, specifying anything that might affect its behavior.
Make your program stop on specified conditions.
Examine what has happened, when your program has stopped.
Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.
The program being debugged can be written in Ada, C, C++, Objective-C, Pascal (and many other languages). Those programs might be executing on the same machine as GDB (native) or on another machine (remote). GDB can run on most popular UNIX and Microsoft Windows variants.

简单来说，GDB就是一个DEBUG的工具

GDB 安装

Linux系统下：
终端运行如下命令

1	sudo apt-get install gdb

Windows系统下：
下载MinGw或者TDM-GCC然后配置一下环境变量就可以在CMD中使用了。
安装完成后输入：

gdb -v

出现如下提示表示安装成功：

GNU gdb (Debian 7.11.1-2) 7.11.1
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word".

编译说明

要使用gdb调试程序，必须在进行编译时（g++或gcc）加入-g选项进行编译：

1	g++ test.cpp -g

运行GDB

linux系统下编译链接默认生成a.out文件，所以在a.out文件所在目录下执行：

gdb a.out

windows系统下默认生成a.exe，同理在a.exe文件所在目录下执行：

gdb a.exe

调试实例及部分简单调试命令

test.c:

#include <stdio.h>
int func(int n) {
    int sum=0,i;
    for(i=1; i<=n; i++) {
        sum+=i;
    }
    return sum;
}
int main() {
    int i;
    long result = 0;
    for(i=1; i<=100; i++) {
        result += i;
    }
    printf("result[1-100] = %d \n", result );
    printf("result[1-250] = %d \n", func(250) );
}

(gdb) b 12     //设置断点，break的缩写，在源程序第12行处。
Breakpoint 1 at 0x6e6: file test.c, line 12.
(gdb) b func    //设置断点，在函数func()入口处。
Breakpoint 2 at 0x8048456: file tst.c, line 3.
(gdb) info b   //查看断点信息
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x00000000000006e6 in main at test.c:12
2       breakpoint     keep y   0x00000000000006b7 in func at test.c:4
(gdb) r   //运行程序，run的缩写
Starting program:
Breakpoint 1, main () at test.c:13
13	    long result = 0;
(gdb) n   //next的缩写，执行下一步
14	    for(i=1; i<=100; i++) {
(gdb) n
15	        result += i;
(gdb) n
14	    for(i=1; i<=100; i++) {
(gdb) n
15	        result += i;
(gdb) c   //继续运行程序，continue命令简写。
Continuing.
result[1-100] = 5050    //程序输出。
Breakpoint 2, func (n=250) at tst.c:5
4                int sum=0,i;
(gdb) n
5                for(i=1; i<=n; i++)
(gdb) p i    //打印变量i的值，print命令简写。
$1 = 134513808
(gdb) n
6                        sum+=i;
(gdb) n
5                for(i=1; i<=n; i++)
(gdb) p sum
$2 = 1
(gdb) n
6                        sum+=i;
(gdb) p i
$3 = 2
(gdb) n
5                for(i=1; i<=n; i++)
(gdb) p sum
$4 = 3
(gdb) bt    //查看函数堆栈。
#0  func (n=250) at tst.c:5
#1  0x080484e4 in main () at tst.c:24
#2  0x400409ed in __libc_start_main () from /lib/libc.so.6
(gdb) finish   //退出函数。
Run till exit from #0  func (n=250) at tst.c:5
0x080484e4 in main () at tst.c:24
24              printf("result[1-250] = %d /n", func(250) );
Value returned is $6 = 31375
(gdb) c    //继续运行。
Continuing.
result[1-250] = 31375    //程序输出。
Program exited with code 027. //程序退出，调试结束。
(gdb) q    //退出gdb。

step（简写为s）命令和next命令都是执行下一步，不同的是step命令会进入函数的内部，而next命令则是会直接执行完函数，然后在该函数的下一步等待
如在上面代码的17行加上func(30)并进行gdb调试；

Breakpoint 1, main () at test.c:17
17	    func(30);
(gdb) n
18	    printf("result[1-100] = %d \n", result );

使用next命令直接跳到18行等待

Breakpoint 1, main () at test.c:17
17	    func(30);
(gdb) s
func (n=30) at test.c:4
4	    int sum=0,i;
(gdb) s
5	    for(i=1; i<=n; i++) {
(gdb) s
6	        sum+=i;
(gdb) s

可以看到，使用s进入到了func函数的内部