2014-05-05

debugging programs with gdb

Features

let program run by your order
set breakpoints and let program stop
check values when program stop
change environment of program

Basic

add -g option to compile, e.g: gcc -g -Wall -o test test.c

run

gdb ./test   <--- start gdb debug
l            <--- list source code
break 5      <--- break at 5th line
break fun1   <--- break at func1 (function name)
info break   <--- show all break info
r            <--- run ./test
n            <--- execute next line
c            <--- continue 
p i          <--- print value of i
bt           <--- backtrace of the stack (same as info stack)
finish       <--- exit function
q            <--- exit gdb

Advance

modify program running environment

(gdb) set args -f -i /tmp/rssp.pid  
(gdb) show args  
Argument list to give program being debugged when it is started is "-f -i /tmp/rssp.pid".

debug running programm

find PID by command pgrep, then using `gdb <program> PID` start debugging  
using command `gdb <program>`，then using `attach PID` to start debugging, `detach` to cancel.

info command

info address -- Describe where symbol SYM is stored  
info args -- Argument variables of current stack frame  
info bookmarks -- Status of user-settable bookmarks  
info breakpoints -- Status of specified breakpoints (all user-settable breakpoints if no argument)  
info checkpoints -- IDs of currently known checkpoints  
info classes -- All Objective-C classes  
info common -- Print out the values contained in a Fortran COMMON block  
info files -- Names of targets and files being debugged  
info float -- Print the status of the floating point unit  
info frame -- All about selected stack frame  
info functions -- All function names  
info line -- Core addresses of the code for a source line  
info locals -- Local variables of current stack frame  
info macro -- Show the definition of MACRO  
info macros -- Show the definitions of all macros at LINESPEC  
info mem -- Memory region attributes  
info os -- Show OS data ARG  
info pretty-printer -- GDB command to list all registered pretty-printers  
info proc -- Show /proc process information about any running process  
info program -- Execution status of the program  
info signals -- What debugger does when program gets various signals  
info source -- Information about the current source file  
info sources -- Source files in the program  
info stack -- Backtrace of the stack  
info symbol -- Describe what symbol is at location ADDR  
info threads -- Display currently known threads  
info tracepoints -- Status of specified tracepoints (all tracepoints if no argument)  
info watchpoints -- Status of specified watchpoints (all watchpoints if no argument)

break command

break file:linenumber  
break file:function  
break *address  
break ... if <condition>  

ofcourse, you can clear or disable or enable the breakpoint, 

clear
clear <function>
clear <filename:function>
clear <linenum>
clear <filename:linenum>

delete [breakpoints] [range...]

disable [breakpoints] [range...]

enable [breakpoints] [range...]

watch

watch <expr>
rwatch <expr>
awatch <expr>
info watchpoints

catch point
catch , event list as below:

1).throw
2).catch
3).exec
4).fork
5).vfork
6).load
7).unload

condition
signal

thread

- info threads 显示当前可调试的所有线程，每个线程会有一个GDB为其分配的ID，后面操  
               作线程的时候会用到这个ID。 前面有*的是当前调试的线程。
- thread ID 切换当前调试的线程为指定ID的线程。
- break thread_test.c:123 thread all 在所有线程中相应的行上设置断点
- thread apply ID1 ID2 command 让一个或者多个线程执行GDB命令command。 
- thread apply all command 让所有被调试线程执行GDB命令command。
- set scheduler-locking off|on|step   
  估计是实际使用过多线程调试的人都可以发现，在使用step或者continue命令调试当前被  
  调试线程的时候，其他线程也是同时执行的，怎么只让被调试程序执行呢？通过这个命令  
  就可以实现这个需求。
  + off 不锁定任何线程，也就是所有线程都执行，这是默认值。   
  + on只有当前被调试程序会执行。 
  + step 在单步的时候，除了next过一个函数的情况(熟悉情况的人可能知道，这其实是一  
    个设置断点然后continue的行为)以外，只有当前线程会执行。

example:

(gdb) r
Starting program: /root/thread
[New Thread 1073951360 (LWP 12900)]
[New Thread 1082342592 (LWP 12907)]---以下三个为新产生的线程
[New Thread 1090731072 (LWP 12908)]
[New Thread 1099119552 (LWP 12909)]

查看线程：使用info threads可以查看运行的线程。 

(gdb) info threads
  4 Thread 1099119552 (LWP 12940)   0xffffe002 in ?? ()
  3 Thread 1090731072 (LWP 12939)   0xffffe002 in ?? ()
  2 Thread 1082342592 (LWP 12938)   0xffffe002 in ?? ()
* 1 Thread 1073951360 (LWP 12931)   main (argc=1, argv=0xbfffda04) at thread.c:21
(gdb)

注意，行首的蓝色文字为gdb分配的线程号，对线程进行切换时，使用该该号码，而不是
上文标出的绿色数字。

另外，行首的红色星号标识了当前活动的线程

切换线程：使用 thread THREADNUMBER 进行切换，THREADNUMBER 为上文提到的线程号。
下例显示将活动线程从 1 切换至 4。 

(gdb) info threads
   4 Thread 1099119552 (LWP 12940)   0xffffe002 in ?? ()
   3 Thread 1090731072 (LWP 12939)   0xffffe002 in ?? ()
   2 Thread 1082342592 (LWP 12938)   0xffffe002 in ?? ()
* 1 Thread 1073951360 (LWP 12931)   main (argc=1, argv=0xbfffda04) at thread.c:21
(gdb) thread 4
[Switching to thread 4 (Thread 1099119552 (LWP 12940))]#0   0xffffe002 in ?? ()
(gdb) info threads
* 4 Thread 1099119552 (LWP 12940)   0xffffe002 in ?? ()
   3 Thread 1090731072 (LWP 12939)   0xffffe002 in ?? ()
   2 Thread 1082342592 (LWP 12938)   0xffffe002 in ?? ()
   1 Thread 1073951360 (LWP 12931)   main (argc=1, argv=0xbfffda04) at thread.c:21
(gdb)

打印线程信息:

usage:
    break <linespec> thread <threadno>
    break <linespec> thread <threadno> if ...

example:
(gdb) break frik.c:13 thread 28 if bartab > lim

例子:

(gdb) r
Starting program: /home/dennis/work/test/test 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff77f5700 (LWP 5073)]
thr_fun_1: 9
[New Thread 0x7ffff6ff4700 (LWP 5074)]
thr_fun_2: 69
[New Thread 0x7ffff67f3700 (LWP 5075)]
 >> func_1
 >> func_2
 >> func_3
 >> func_4
 >> func_5
thr_fun_3: 99
thr_fun_1: 8
thr_fun_3: 98
^C
Program received signal SIGINT, Interrupt.
0x00007ffff7bc566b in pthread_join (threadid=140737345705728, thread_return=0x0) at pthread_join.c:92
92    pthread_join.c: No such file or directory.
(gdb) break main.c:45 thread 4
Breakpoint 1 at 0x400849: file main.c, line 45.
(gdb) c
Continuing.
thr_fun_3: 97
thr_fun_1: 7
thr_fun_2: 68
[Switching to Thread 0x7ffff67f3700 (LWP 5075)]

Breakpoint 1, thr_fun_3 (parm=0x0) at main.c:45
45            sleep(1);
(gdb) l
40    void* thr_fun_3 (void *parm)
41    {
42        int count = 100;
43        while (count--) {
44            printf("%s: %d\n",__FUNCTION__, count);
45            sleep(1);
46        }
47        return NULL;
48    }
49    
(gdb) print count
$1 = 97
(gdb)

stack info
memory of code

info line test.c:func

disassemble func
view values of running data

(gdb) p file::variable
(gdb) p function::variable

(gdb) p *array@len

view memory
examine
auto display
display
set print option
history
view register

info registers
info all-registers
info registers
change program value

(gdb) print x=4
(gdb) set var width=47
jump

run shell command

(gdb) shell
view all backtrace of all thread: thread apply all bt, short type t a a bt
set print elements 1024, if want unlimited, use set print elements 0, use
for print std::string contents. Type help set print elements for more
x/20xb &arr, print 20 bytes start from address of variable arr, help with help x
dump binary memory filename start_addr end_addr dump memory to files
ref : https://sourceware.org/gdb/onlinedocs/gdb/Dump_002fRestore-Files.html

Matrix207's Blog

debugging programs with gdb

Features

Basic

Advance

Reference