Linux调试器gdb和cgdb的使用【Ubuntu】

一、样例代码

// mycmd.c
#include <stdio.h>int Sum(int s, int e)
{int result = 0;for(int i = s; i <= e; i++){result += i;} return result;
} 
int main()
{int start = 1;int end = 100;printf("I will begin\n");int n = Sum(start, end);printf("running done, result is: [%d-%d]=%d\n", start, end, n);return 0;
}

二、预备

• 程序的发布方式有两种， debug 模式和 release 模式， Linux gcc/g++ 出来的二进制程序，默认是 release 模式。
• 要使用gdb调试，必须在源代码生成二进制程序的时候, 加上 -g 选项，如果没有添加，程序无法被编译

$ gcc mycmd.c -o mycmd  # 默认模式，不支持调试
$ file mycmd	  
mycmd: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically 
linked, interpreter /lib64/ld-linux-x86-64.so.2, 
BuildID[sha1]=82f5cbaada10a9987d9f325384861a88d278b160, for GNU/Linux 
3.2.0, not stripped$ gcc mycmd.c -o mycmd -g  # debug模式
$ file mycmd	
mycmd: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically 
linked, interpreter /lib64/ld-linux-x86-64.so.2,
BuildID[sha1]=3d5a2317809ef86c7827e9199cfefa622e3c187f, for GNU/Linux 
3.2.0, with debug_info, not stripped

三、常见使用

• 开始： gdb binFile
• 退出： ctrl + d 或 quit 调试命令

1、cgdb调试操作

按下ESC键进入代码窗口，此时可以上下浏览代码，并且可以进行一系列的操作：

按键	操作
空格键	设置或者取消断点;
o	查看代码所在的文件;
/ 或者 ?	在代码中搜索字符串;
-	缩小代码窗口;
+	扩大代码窗口;
gg	光标移动到文件头部;
GG	光标移动到文件尾部;
ctrl + b	代码向上翻一页;
ctrl + u	代码向上翻半页;
ctrl + f	代码向下翻一页;
ctrl + d	代码向下翻半页;

按下i键回到调试窗口，进入调试模式，使用的调试指令与GDB几乎一样！

2、gdb调试操作

命令	作用	样例
list/l	显⽰源代码，从上次位置开始，每次列出	list/l 10
list/l 函数名	列出指定函数的源代码	list/l main
list/l ⽂件名:行号	列出指定⽂件的源代码	list/l mycmd.c:1
r/run	从程序开始连续执⾏	run
n/next	单步执行，不进⼊函数内部	next
s/step	单步执行，进⼊函数内部	step
break/b [⽂件名:]行号	在指定⾏号设置断点	break 10 break test.c:10
break/b 函数名	在函数开头设置断点	break main
info break/b	查看当前所有断点的信息	info break
finish	执⾏到当前函数返回，然后停⽌	finish
print/p 表达式	打印表达式的值	print start+end
p 变量	打印指定变量的值	p x
set var 变量=值	修改变量的值	set var i=10
continue/c	从当前位置开始连续执⾏程序	continue
delete/d breakpoints	删除所有断点	delete breakpoints
delete/d breakpoints n	删除序号为n的断点	delete breakpoints 1
disable breakpoints	禁⽤所有断点	disable breakpoints
enable breakpoints	启⽤所有断点	enable breakpoints
info/i breakpoints	查看当前设置的断点列表	info breakpoints
display 变量名	跟踪显⽰指定变量的值（每次停止时）	display x
undisplay 编号	取消对指定编号的变量的跟踪显示	undisplay 1
until X⾏号	执⾏到指定⾏号	until 20
backtrace/bt	查看当前执⾏栈的各级函数调⽤及参数	backtrace
info/i locals	查看当前栈帧的局部变量值	info locals
quit	退出GDB调试器	quit

• -断点(breakpoints) -行号(n)

四、常见技巧

📌

1、 安装cgdb:

• 上面的基本调试还是麻烦，虽然是黑屏，但是还是想看到代码调试
推荐安装cgdb:
• Ubuntu: sudo apt-get install -y cgdb
• Centos: sudo yum install -y cgdb

2、watch

执行时监视一个表达式（如变量）的值。如果监视的表达式在程序运行期间的值发生变化，GDB 会暂
停程序的执行，并通知使用者

(gdb) l main
11
12 			return result;
13 		}
14
15 		int main()
16 		{
17 		int start = 1;
18 		int end = 100;
19 		printf("I will begin\n");
20 		int n = Sum(start, end);
(gdb) b 20
Breakpoint 1 at 0x11c3: file mycmd.c, line 20.
(gdb) info b
Num	  Type	 	Disp	 Enb	 Address	 			What
1 	breakpoint  keep 	  y 	0x00000000000011c3 	in main at mycmd.c:20 
(gdb) r
Starting program: /home/whb/test/test/mycmd
I will beginBreakpoint 1, main () at mycmd.c:20
20 				int n = Sum(start, end);
(gdb) s
Sum (s=32767, e=-7136) at mycmd.c:5
5 		{
(gdb) n
6 			int result = 0;
(gdb) watch result
Hardware watchpoint 2: result
(gdb) c
Continuing.Hardware watchpoint 2: resultOld value = -6896
New value = 0
Sum (s=1, e=100) at mycmd.c:7
7 			for(int i = s; i <= e; i++)
(gdb) c
Continuing.Hardware watchpoint 2: resultOld value = 0
New value = 1
Sum (s=1, e=100) at mycmd.c:7
7 			for(int i = s; i <= e; i++)
(gdb) c
Continuing.Hardware watchpoint 2: resultOld value = 1
New value = 3
Sum (s=1, e=100) at mycmd.c:7
7 			for(int i = s; i <= e; i++)
(gdb) c
Continuing.Hardware watchpoint 2: resultOld value = 3
New value = 6
Sum (s=1, e=100) at mycmd.c:7
7 			for(int i = s; i <= e; i++)
(gdb) info b
Num 	Type 	Disp 	Enb 	Address 			What
1 	breakpoint 	keep 	y 	0x00005555555551c3 	in main at mycmd.c:20breakpoint already hit 1 time
2 	hw watchpoint keep y resultbreakpoint already hit 4 times
(gdb) d 2
(gdb) info b
Num 	Type 	Disp 	Enb 	Address 		What
1 	breakpoint 	keep 	y 	0x00005555555551c3 	in main at mycmd.c:20breakpoint already hit 1 time
(gdb) finish
Run till exit from #0 	Sum (s=1, e=100) at mycmd.c:7
0x00005555555551d2 in main () at mycmd.c:20
20 			int n = Sum(start, end);
Value returned is $1 = 5050

📌 注意：

• 如果你有一些变量不应该修改，但是你怀疑它修改导致了问题，你可以watch它，如果变
  化了，就会通知你.

3、set var确定问题原因

更改一下标志位，假设我们想得到 +-result

// mycmd.c
#include <stdio.h>int flag = 0; // 故意错误
//int flag = -1;
//int flag = 1;int Sum(int s, int e)
{int result = 0;for(int i = s; i <= e; i++){result += i;}return result*flag;
} int main()
{int start = 1;int end = 100;printf("I will begin\n");int n = Sum(start, end);printf("running done, result is: [%d-%d]=%d\n", start, end, n);return 0;
}
(gdb) l main
15
16			return result*flag;
17 			}
18
19			 int main()
20 			{
21 				int start = 1;
22 				int end = 100;
23 				printf("I will begin\n");
24 				int n = Sum(start, end);
(gdb) b 24
Breakpoint 1 at 0x11ca: file mycmd.c, line 24.
(gdb) r
Starting program: /home/whb/test/test/mycmd
I will beginBreakpoint 1, main () at mycmd.c:24
24 				int n = Sum(start, end);
(gdb) n
25 				printf("running done, result is: [%d-%d]=%d\n", start, end,
n);
(gdb) n
running done, result is: [1-100]=0 # 这⾥结果为什么是0？
26 				return 0;
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/whb/test/test/mycmd
I will beginBreakpoint 1, main () at mycmd.c:24
24 				int n = Sum(start, end);
(gdb) s
Sum (s=32767, e=-7136) at mycmd.c:9
9 		{
(gdb) n
10 			int result = 0;
(gdb) n
11 			for(int i = s; i <= e; i++)
(gdb)
13 				result += i;
(gdb)
11 			for(int i = s; i <= e; i++)
(gdb)
13 				result += i;
(gdb) until 14
Sum (s=1, e=100) at mycmd.c:16
16 			return result*flag;
(gdb) p result
$1 = 5050
(gdb) p flag
$2 = 0
(gdb) set var flag=1 	# 更改flag的值，确认是否是它的原因
(gdb) p flag
$3 = 1
(gdb) n
17 			}
(gdb) n
main () at mycmd.c:25
25 			printf("running done, result is: [%d-%d]=%d\n", start, end,
n);
(gdb) n
running done, result is: [1-100]=5050 # 是它的原因
26 				return 0;

4、条件断点

添加条件断点

(gdb) l main
11
12 			return result;
13 		}
14
15 		int main()
16 		{
17 			int start = 1;
18 			int end = 100;
19 			printf("I will begin\n");
20 			int n = Sum(start, end);
(gdb) b 20
Breakpoint 1 at 0x11c3: file mycmd.c, line 20.
(gdb) r
Starting program: /home/whb/test/test/mycmd
I will beginBreakpoint 1, main () at mycmd.c:20
20 				int n = Sum(start, end);
(gdb) s
Sum (s=32767, e=-7136) at mycmd.c:5
5 		{
(gdb) n
6 			int result = 0;
(gdb) n
7 			for(int i = s; i <= e; i++)
(gdb) n
9 				result += i;
(gdb) display i
1: i = 1
(gdb) n
7 			for(int i = s; i <= e; i++)
1: i = 1
(gdb) n
9 				result += i;
1: i = 2
(gdb) n
7 			for(int i = s; i <= e; i++)
1: i = 2
(gdb) n
9 				result += i;
1: i = 3
(gdb)
7 			for(int i = s; i <= e; i++)
1: i = 3
(gdb) info b
Num 	Type 	Disp 	Enb 	Address 			What
1 	breakpoint 	keep 	y 		0x00005555555551c3 	in main at mycmd.c:20breakpoint 	already hit 1 time
(gdb) b 9 if i == 30 	# 9是⾏号，表⽰新增断点的位置
Breakpoint 2 at 0x555555555186: file mycmd.c, line 9.
(gdb) info b
Num 	Type 	Disp 	Enb 	Address 			What
1 	breakpoint 	keep 	y 		0x00005555555551c3 	in main at mycmd.c:20breakpoint 	already hit 1 time
2 	breakpoint 	keep 	y 		0x0000555555555186 	in Sum at mycmd.c:9stop only if i == 30
(gdb) finish
Run till exit from #0 Sum (s=1, e=100) at mycmd.c:7Breakpoint 2, Sum (s=1, e=100) at mycmd.c:9
9 				result += i;
1: i = 30
(gdb) finish
Run till exit from #0 Sum (s=1, e=100) at mycmd.c:9
0x00005555555551d2 in main () at mycmd.c:20
20 				int n = Sum(start, end);
Value returned is $1 = 5050

给已经存在的端点新增条件

(gdb) l main
11
12 			return result;
13 		}
14
15 		int main()
16 		{
17 			int start = 1;
18 			int end = 100;
19 			printf("I will begin\n");
20 			int n = Sum(start, end);
(gdb) b 20
Breakpoint 1 at 0x11c3: file mycmd.c, line 20.
(gdb) r
Starting program: /home/whb/test/test/mycmd
I will beginBreakpoint 1, main () at mycmd.c:20
20 			int n = Sum(start, end);
(gdb) s
Sum (s=32767, e=-7136) at mycmd.c:5
5 		{
(gdb) n
6 			int result = 0;
(gdb) n
7 			for(int i = s; i <= e; i++)
(gdb) n
9 				result += i;
(gdb)
7 			for(int i = s; i <= e; i++)
(gdb)
9 				result += i;
(gdb)
7 			for(int i = s; i <= e; i++)
(gdb)
9 				result += i;
(gdb)
7 			for(int i = s; i <= e; i++)
(gdb) b 9 # 我们在第9⾏新增⼀个断点，⽤来开始测试
Breakpoint 2 at 0x555555555186: file mycmd.c, line 9.
(gdb) info b
Num Type 		Disp 	Enb 	Address 			What
1 	breakpoint 	keep 	y 		0x00005555555551c3 	in main at mycmd.c:20breakpoint 	already hit 1 time
2 	breakpoint 	keep 	y	 	0x0000555555555186 	in Sum at mycmd.c:9
(gdb) nBreakpoint 2, Sum (s=1, e=100) at mycmd.c:9
9 				result += i;
(gdb) n
7 			for(int i = s; i <= e; i++)
(gdb) nBreakpoint 2, Sum (s=1, e=100) at mycmd.c:9
9 				result += i;
(gdb) condition 2 i==30 #给2号断点，新增条件i==30
(gdb) info b
Num Type 		Disp 	Enb Address 			What
1 	breakpoint 	keep 	y 	0x00005555555551c3 	in main at mycmd.c:20breakpoint already hit 1 time
2 	breakpoint 	keep 	y 	0x0000555555555186 	in Sum at mycmd.c:9stop only if i==30breakpoint already hit 2 times
(gdb) n
7 			for(int i = s; i <= e; i++)
(gdb) n
9 				result += i;
(gdb) c
Continuing.
Breakpoint 2, Sum (s=1, e=100) at mycmd.c:9
9 				result += i;
(gdb) p i
$1 = 30
(gdb) p result
$2 = 435

📌 注意：

• 条件断点添加常见两种方式：1. 新增 2. 给已有断点追加
• 注意两者的语法有区别，不要写错了。
• 新增： b 行号/文件名:行号/函数名 if i == 30(条件)
• 给已有断点追加：condition 2 i==30， 其中2是已有断点编号，没有if