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认识linux -- 调试器 - gdb/cgdb的使用

news 2025/11/8 13:35:34

1. 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;
}

2 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

    3 常见使用

    开始： gdb binFile
    退出： ctrl + d 或 quit 调试命令
                   命令                                作用                样例
     list/l
    显⽰源代码，从上次位置开始，每次列出
    10⾏
    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， 逐过程 F10
    		next
       s/step
    单步执⾏，进⼊函数内部, 逐语句 F11
    		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
    查看当前所有断点的信息
    break main
        info break/b
    在函数开头设置断点
    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

    4 常见技巧

            4.1 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:20breakpoint already hit 1 time
(gdb) finish
Run till exit from #0 Sum (s=1, e=100) at mycmd.c:70x00005555555551d2 in main () at mycmd.c:20
20 int n = Sum(start, end);
Value returned is $1 = 5050

    4.2 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 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;

    4.3 条件断点

    4.3.1 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: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:20breakpoint 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
    4.3.2 给已经存在的断点添加条件
    (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:20breakpoint 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

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