常常我们会碰到在多进程程序中,利用GDB无法调试子进程问题。实际上,GDB 没有对多进程程序调试提供直接支持。例如,使用GDB调试某个进程,如果该进程fork了子进程,GDB会继续调试该进程,子进程会不受干扰地运行下去。如果你事先在子进程代码里设定了断点,子进程会收到SIGTRAP信号并终止。那么该如何调试子进程呢?其实我们可以利用GDB的特点或者其他一些辅助手段来达到目的。此外,GDB 也在较新内核上加入一些多进程调试支持。
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#include <stdio.h>
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#include <unistd.h>
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int main()
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...{
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if(fork() == 0)
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...{
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printf("Child Process say hello to you! ");
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sleep(5);
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exit(0);
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}
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else
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printf("Parent Process, Hello ");
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return 0;
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}
方法一:
ubuntu默认情况下你在gdb中使用attach id是权限不够的,所以你需要:sudo chmod +s /usr/bin/gdb
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在父子进程中加一句while(pause) sleep(1);,让其暂停,然后后台运行程序,通过ps获取子进程的PID。(如下所示,ps显示的./fork有两个,orisun是进程的有效用户,第一个数字是进程ID,第二个数字是其父进程ID)
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运行gdb,attach 子进程ID
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orisun@zcypc:~$ ./fork &
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[1] 13294
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orisun@zcypc:~$ ps -ef|grep fork
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790 1 0 09:00 ? 00:00:01 dbus-daemon --system --fork --activation=upstart
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orisun 1546 1 0 09:00 ? 00:00:03 //bin/dbus-daemon --fork --print-pid 5 --print-address 7 --session
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orisun 12584 1 0 20:23 ? 00:00:12 gedit /home/orisun/fork.c
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orisun 13294 13239 0 20:53 pts/0 00:00:00 ./fork
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orisun 13295 13294 0 20:53 pts/0 00:00:00 ./fork
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orisun 13297 13239 0 20:54 pts/0 00:00:00 grep --color=auto fork
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orisun@zcypc:~$ gdb
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GNU gdb (Ubuntu/Linaro 7.2-1ubuntu11) 7.2
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Copyright (C) 2010 Free Software Foundation, Inc.
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License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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This is free software: you are free to change and redistribute it.
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There is NO WARRANTY, to the extent permitted by law. Type "show copying"
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and "show warranty" for details.
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This GDB was configured as "i686-linux-gnu".
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For bug reporting instructions, please see:
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<http://www.gnu.org/software/gdb/bugs/>.
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(gdb) attach 13295
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Attaching to process 13295
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Reading symbols from /home/orisun/fork...done.
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Reading symbols from /lib/i386-linux-gnu/libc.so.6...Reading symbols from /usr/lib/debug/lib/i386-linux-gnu/libc-2.13.so...done.
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done.
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Loaded symbols for /lib/i386-linux-gnu/libc.so.6
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Reading symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.
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Loaded symbols for /lib/ld-linux.so.2
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0x00bd8416 in __kernel_vsyscall ()
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(gdb) backtrace
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#0 0x00bd8416 in __kernel_vsyscall ()
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#1 0x0049bfc0 in __nanosleep_nocancel ()
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at ../sysdeps/unix/syscall-template.S:82
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#2 0x0049bde2 in __sleep (seconds=<value optimized out>)
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at ../sysdeps/unix/sysv/linux/sleep.c:138
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#3 0x08048595 in main () at fork.c:25
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(gdb) up 3
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#3 0x08048595 in main () at fork.c:25
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while(pause) sleep(1);
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(gdb) list
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if((pid=fork())<0){
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perror("fork");
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exit(1);
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}
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if(pid==0){
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while(pause) sleep(1);
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if((arr=shmat(shmid,NULL,0))==(void*)-1){ //子进程中把共享内存映射到本,进行修改
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perror("shmat");
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exit(1);
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}
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(gdb) break 31
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Breakpoint 1 at 0x8048600: file fork.c, line 31.
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(gdb) set pause=0
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(gdb) continue
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Continuing.
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Breakpoint 1, main () at fork.c:31
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if(shmdt(arr)==-1){
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(gdb) p arr
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$1 = 0xb77fb000 "abc"
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(gdb)
方法二:follow-fork-mode
不需要专门加while(pause) sleep(1);这种代码了。
follow-fork-mode的用法为:
set follow-fork-mode [parent|child]
* parent: fork之后继续调试父进程,子进程不受影响。
* child: fork之后调试子进程,父进程不受影响。
因此如果需要调试子进程,在启动gdb后:
(gdb) set follow-fork-mode child
并在子进程代码设置断点。
此外还有detach-on-fork参数,指示GDB在fork之后是否断开(detach)某个进程的调试,或者都交由GDB控制:
set detach-on-fork [on|off]
* on: 断开调试follow-fork-mode指定的进程。
* off: gdb将控制父进程和子进程。follow-fork-mode指定的进程将被调试,另一个进程置于暂停(suspended)状态。
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orisun@zcypc:~$ gdb ./fork
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GNU gdb (Ubuntu/Linaro 7.2-1ubuntu11) 7.2
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Copyright (C) 2010 Free Software Foundation, Inc.
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License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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This is free software: you are free to change and redistribute it.
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There is NO WARRANTY, to the extent permitted by law. Type "show copying"
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and "show warranty" for details.
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This GDB was configured as "i686-linux-gnu".
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For bug reporting instructions, please see:
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<http://www.gnu.org/software/gdb/bugs/>...
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Reading symbols from /home/orisun/fork...done.
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(gdb) set follow-fork-mode child
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(gdb) set detach-on-fork off
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(gdb) break 31
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Breakpoint 1 at 0x80485e3: file fork.c, line 31.
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(gdb) r
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Starting program: /home/orisun/fork
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[New process 13534]
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[Switching to process 13534]
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Breakpoint 1, main () at fork.c:31
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if(shmdt(arr)==-1){
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(gdb) p arr
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$1 = 0xb7ffd000 "abc"
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(gdb) c
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Continuing.
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Program exited normally.
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(gdb)
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