多进程调试

        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。

#include <stdio.h>
#include <unistd.h>

int main()
...{
    if(fork() == 0)
    ...{
        printf("Child Process say hello to you! ");
        sleep(5);
        exit(0);
    }
    else
        printf("Parent Process, Hello ");

    return 0;
}

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

1. 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）
2. 运行gdb，attach 子进程ID

orisun@zcypc:~$ ./fork &
[1] 13294
orisun@zcypc:~$ ps -ef|grep fork
       790 1 0 09:00 ? 00:00:01 dbus-daemon --system --fork --activation=upstart
orisun 1546 1 0 09:00 ? 00:00:03 //bin/dbus-daemon --fork --print-pid 5 --print-address 7 --session
orisun 12584 1 0 20:23 ? 00:00:12 gedit /home/orisun/fork.c
orisun 13294 13239 0 20:53 pts/0 00:00:00 ./fork
orisun 13295 13294 0 20:53 pts/0 00:00:00 ./fork
orisun 13297 13239 0 20:54 pts/0 00:00:00 grep --color=auto fork
orisun@zcypc:~$ gdb
GNU gdb (Ubuntu/Linaro 7.2-1ubuntu11) 7.2
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
(gdb) attach 13295
Attaching to process 13295
Reading symbols from /home/orisun/fork...done.
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.
done.
Loaded symbols for /lib/i386-linux-gnu/libc.so.6
Reading symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib/ld-linux.so.2
0x00bd8416 in __kernel_vsyscall ()
(gdb) backtrace
#0 0x00bd8416 in __kernel_vsyscall ()
#1 0x0049bfc0 in __nanosleep_nocancel ()
    at ../sysdeps/unix/syscall-template.S:82
#2 0x0049bde2 in __sleep (seconds=<value optimized out>)
    at ../sysdeps/unix/sysv/linux/sleep.c:138
#3 0x08048595 in main () at fork.c:25
(gdb) up 3
#3 0x08048595 in main () at fork.c:25
           while(pause) sleep(1);
(gdb) list
       if((pid=fork())<0){
           perror("fork");
           exit(1);
       }
       if(pid==0){
           while(pause) sleep(1);
           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
               perror("shmat");
               exit(1);
           }
(gdb) break 31
Breakpoint 1 at 0x8048600: file fork.c, line 31.
(gdb) set pause=0
(gdb) continue
Continuing.

Breakpoint 1, main () at fork.c:31
           if(shmdt(arr)==-1){
(gdb) p arr
$1 = 0xb77fb000 "abc"
(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。

orisun@zcypc:~$ gdb ./fork
GNU gdb (Ubuntu/Linaro 7.2-1ubuntu11) 7.2
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/orisun/fork...done.
(gdb) set follow-fork-mode child
(gdb) set detach-on-fork off
(gdb) break 31
Breakpoint 1 at 0x80485e3: file fork.c, line 31.
(gdb) r
Starting program: /home/orisun/fork
[New process 13534]
[Switching to process 13534]

Breakpoint 1, main () at fork.c:31
           if(shmdt(arr)==-1){
(gdb) p arr
$1 = 0xb7ffd000 "abc"
(gdb) c
Continuing.

Program exited normally.
(gdb)