多进程调试

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。

本文介绍的方法能让你把断点设在子进程中，单步查看子进程的运行情况。但问题，如果我想在一次调试中同时在父子进程中设置断点，单步执行该怎么做呢？

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/ipc.h>
#include<sys/shm.h>
#include<sys/types.h>
#include<fcntl.h>
#include<sys/stat.h>

main(){
    int pause=1;        //让进程暂停，以便获得pid
    char *arr;
    int shmid;
    if((shmid=shmget(2012,4,IPC_CREAT|0666))==-1){        //父进程中创建共享内存
        perror("shmget");
        exit(1);
    }
    pid_t pid;
    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);
        }
        arr[0]='a';arr[1]='b';arr[2]='c';arr[3]='\0';
        if(shmdt(arr)==-1){
            perror("shmdt");
            exit(1);
        }
    }
    else{
        while(pause) sleep(1); 
        //sleep(1);
        if((arr=shmat(shmid,NULL,0))==(void*)-1){        //子进程中把共享内存映射到本，读取其内容
            perror("shmat");
            exit(1);
        }
        printf("%s\n",arr);
        if(shmdt(arr)==-1){
            perror("shmdt");
            exit(1);
        }
    }
}

方法一：attach pid

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
102        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
25            while(pause) sleep(1); 
(gdb) list
20        if((pid=fork())<0){
21            perror("fork");
22            exit(1);
23        }
24        if(pid==0){
25            while(pause) sleep(1); 
26            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
27                perror("shmat");
28                exit(1);
29            }
(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
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
31            if(shmdt(arr)==-1){
(gdb) p arr
$1 = 0xb7ffd000 "abc"
(gdb) c
Continuing.

Program exited normally.
(gdb)