gdb

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了解gdb用法

更新記錄

item note
20170105 第一版

目錄

gdb指令

指令皆可以使用help去查訊
ex.help break

cmd note
l (list) 顯示內容
b (breakpoint) 設定斷點
r (run執行) 會執行到斷點停止
n (next) 執行下一步，若遇到function會當成一個
step 單步執行，遇到function會進入單步執行
q (quit) 離開
display a 設定變數a，每次顯示內容
info display 查看display設定內容
info break 查看break設定內容
{disable/enable} breakpoints 1 {關閉/開啟} 第1個斷點
{disable/enable} display 1 {關閉/開啟} 第1個變數

gdb

example

gcc要帶-g參數(-g: Debugging Options)

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[root@localhost tt]# cat test2.c 
#include <stdio.h>

int main()
{
	int a,b,c;
	a = 5;
	b = 10;
	b +=a;
	c = b+a;
	return 0;
}
[root@localhost tt]# gcc -Wall -g test2.c

test

  • 使用gdb帶起執行檔

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    gdb a.out

  • list顯示程式內容 (或則用 l)
    list 5 or list 10

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    (gdb) l
    1	#include <stdio.h>
    2	
    3	int main()
    4	{
    5		int a,b,c;
    6		a = 5;
    7		b = 10;
    8		b +=a;
    9		c = b+a;
    10		return 0;

  • 設定中斷點(breakpoint)

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    (gdb) b 2
    Breakpoint 1 at 0x4004da: file test2.c, line 2.

  • 執行(run)

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    (gdb) r
    Starting program: /root/tt/a.out 
    Missing separate debuginfos, use: dnf debuginfo-install glibc-2.23.1-8.fc24.x86_64

    Breakpoint 1, main () at test2.c:6
    6		a = 5;

  • 查看數值
    未設定值之前a為0(line 6),設定值之後為5(linie 7)
    n (next)，執行下一行
    p (print),打印變數內容

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    Breakpoint 1, main () at test2.c:6
    6		a = 5;
    (gdb) p a
    $1 = 0
    (gdb) n
    7		b = 10;
    (gdb) p a
    $2 = 5

  • 執行到結束
    c (continue)執行到結束

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    (gdb) c
    Continuing.

    Breakpoint 2, main () at test2.c:10
    10		return 0;
    (gdb) print a
    $16 = 5
    (gdb) print b
    $17 = 15
    (gdb) print c
    $18 = 20
    (gdb)

display

  • display
    display a : 設定變數a，每次顯示內容
    info display: 查看全部的display內容

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    (gdb) r
    The program being debugged has been started already.
    Start it from the beginning? (y or n) y
    Starting program: /root/tt/a.out 

    Breakpoint 1, main () at test2.c:6
    6		a = 5;
    2: b = 0
    (gdb) n
    7		b = 10;
    2: b = 0
    (gdb) n
    8		b +=a;
    2: b = 10
    (gdb) n
    9		c = b+a;
    2: b = 15
    (gdb) info display
    Auto-display expressions now in effect:
    Num Enb Expression
    2:   y  b
    (gdb)

  • disalbe display
    disable display 2
    關閉顯示第2個變數(即變數c)

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    (gdb) info display
    Auto-display expressions now in effect:
    Num Enb Expression
    1:   y  b
    2:   y  c
    (gdb) disable display 2
    (gdb) info display
    Auto-display expressions now in effect:
    Num Enb Expression
    1:   y  b
    2:   n  c
    (gdb)

關閉斷點

  • disable breakpoints 1
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    (gdb) info break
    Num     Type           Disp Enb Address            What
    1       breakpoint     keep y   0x00000000004004da in main at test2.c:6
    	breakpoint already hit 1 time
    (gdb) disable breakpoints 1
    (gdb) info break
    Num     Type           Disp Enb Address            What
    1       breakpoint     keep n   0x00000000004004da in main at test2.c:6
    	breakpoint already hit 1 time
    (gdb) r
    Starting program: /root/tt/a.out 
    [Inferior 1 (process 12772) exited normally]
    (gdb) r
    Starting program: /root/tt/a.out 
    [Inferior 1 (process 12773) exited normally]
    (gdb) c
    The program is not being run.
    (gdb) r
    Starting program: /root/tt/a.out 
    [Inferior 1 (process 12778) exited normally]
    (gdb)

gdb正在執行的程式

  • 先取得程式的pid
  • 開啟gdb
  • attach pid
    進入process pid的gdb
  • detach
    離開process pid的gdb

參考