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 begin

Breakpoint 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: result

Old 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: result

Old 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: result

Old 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: result

Old 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:20
breakpoint already hit 1 time
2 hw watchpoint keep y result
breakpoint 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:20
breakpoint 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 begin

Breakpoint 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 begin

Breakpoint 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 begin

Breakpoint 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:20
breakpoint 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:20
breakpoint already hit 1 time
2 breakpoint keep y 0x0000555555555186 in Sum at mycmd.c:9
stop only if i == 30
(gdb) finish
Run till exit from #0 Sum (s=1, e=100) at mycmd.c:7

Breakpoint 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 begin

Breakpoint 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:20
breakpoint already hit 1 time
2 breakpoint keep y 0x0000555555555186 in Sum at mycmd.c:9
(gdb) n

Breakpoint 2, Sum (s=1, e=100) at mycmd.c:9
9 result += i;
(gdb) n
7 for(int i = s; i <= e; i++)
(gdb) n

Breakpoint 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:20
breakpoint already hit 1 time
2 breakpoint keep y 0x0000555555555186 in Sum at mycmd.c:9
stop only if i==30
breakpoint 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
————————————————

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原文链接：https://blog.csdn.net/2301_79642159/article/details/145838539