本文基于最基础的Linux下创建子进程的程序，了解GDB在多线程下的调试。参考链接： 100个gdb小技巧操作系统：Ubuntu 16.04/bash on ubuntu on windows

实验内容

编写一段程序，使用系统调用 fork( )创建两个子进程，在系统中有一个父进程和两个子进程活动。让每个进程在屏幕上显示一个字符；父进程显示字符“a”，子进程分别显示字符“b” 和“c”。试观察记录屏幕上的显示结果，并分析原因。

实现代码
#include  
#include  
int main(){  
int p1,p2;  
while((p1=fork())==-1);
if(p1==0)
putchar('b');
else {
while((p2=fork())==-1);
if(p2==0){
putchar('c');
printf("\n");
}
else putchar('a');
}
return 0; 
}

其中，a、b、c 的显示顺序是随机的，取决与进程的调度顺序。fork()创建进程之后，各个进程的时间片的获得不一定是顺序的，所以输出的顺序会变化。

GDB多进程调试

调试子进程

在调试多进程程序时，gdb 默认会追踪父进程。如果要调试子进程，要使用如下命令：set follow-fork-mode child

(gdb) set follow-fork-mode child
(gdb) start
Temporary breakpoint 1 at 0x40056e: file 1.c, line 5.
Starting program: /root/tst

Temporary breakpoint 1, main () at 1.c:5
5               while((p1=fork())==-1);
(gdb) n
[New process 24]
ac
[Switching to process 24]
main () at 1.c:6
6               if(p1==0)
(gdb)
7                       putchar('b');
(gdb)
16              return 0;
(gdb)
17      }
(gdb)
__libc_start_main (main=0x400566 , argc=1, argv=0x7ffffffde398, init=,
    fini=, rtld_fini=, stack_end=0x7ffffffde388)
    at ../csu/libc-start.c:325
325     ../csu/libc-start.c: No such file or directory.
(gdb)
b[Inferior 2 (process 24) exited normally]
(gdb)
The program is not being run.

可以看到执行到第5行，父进程打印“a” 并创建了子进程 24，子进程打印“c”。正常退出后打印“b”。（但这里是不是只监视了一个子进程24 ？）

同时调试父进程和子进程

如果要同时调试父进程和子进程，可以使用set detach-on-fork off（默认 detach-on-fork 是 on）命令，这样 gdb 就能同时调试父子进程，并且在调试一个进程时，另外一个进程处于挂起状态。在使用set detach-on-fork off命令后，用i inferiors（i 是 info 命令缩写）查看进程状态，可以看到父子进程都在被 gdb 调试的状态，前面显示“*”是正在调试的进程。当父进程退出后，用 inferior infno切换到子进程去调试。

(gdb) set detach-on-fork off
(gdb) start
Reading symbols from /usr/lib/debug/lib/x86_64-linux-gnu/libc-2.23.so...done.
Reading symbols from /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.23.so...done.
Reading symbols from /usr/lib/debug/lib/x86_64-linux-gnu/libc-2.23.so...done.
Reading symbols from /usr/lib/debug/lib/x86_64-linux-gnu/ld-2.23.so...done.
Temporary breakpoint 2 at 0x40056e: main. (3 locations)
Starting program: /root/tst

Thread 1.1 "tst" hit Temporary breakpoint 2, main () at 1.c:5
5               while((p1=fork())==-1);
(gdb) n
[New process 26]
6               if(p1==0)
(gdb) i inferior
  Num  Description       Executable
* 1    process 25        /root/tst
  2    process 23        /root/tst
  3    process 24        /root/tst
  4    process 26        /root/tst
(gdb) n
9                       while((p2=fork())==-1);
(gdb)
[New process 27]
10                      if(p2==0){
(gdb)
14                      else putchar('a');
(gdb)
16              return 0;
(gdb)
17      }
(gdb)
__libc_start_main (main=0x400566 , argc=1, argv=0x7ffffffde398, init=,
    fini=, rtld_fini=, stack_end=0x7ffffffde388)
    at ../csu/libc-start.c:325
325     ../csu/libc-start.c: No such file or directory.
(gdb)
a[Inferior 1 (process 25) exited normally]
(gdb)
The program is not being run.
(gdb) inferior 2
[Switching to inferior 2 [process 23] (/root/tst)]
[Switching to thread 2.1 (process 23)]
#0  0x00007fffff0fc41a in __libc_fork () at ../sysdeps/nptl/fork.c:145
145     ../sysdeps/nptl/fork.c: No such file or directory.
(gdb)
[Switching to inferior 2 [process 23] (/root/tst)]
[Switching to thread 2.1 (process 23)]
#0  0x00007fffff0fc41a in __libc_fork () at ../sysdeps/nptl/fork.c:145
145     in ../sysdeps/nptl/fork.c
.
.
.
168     in ../sysdeps/nptl/fork.c
(gdb) bt
#0  __libc_fork () at ../sysdeps/nptl/fork.c:168
#1  0x0000000000400573 in main () at 1.c:5
(gdb) n
169     in ../sysdeps/nptl/fork.c
.
.
.
(gdb)
264     in ../sysdeps/nptl/fork.c
(gdb)
main () at 1.c:6
6               if(p1==0)
(gdb) i inferior
  Num  Description       Executable
  1                /root/tst
* 2    process 23        /root/tst
  3    process 24        /root/tst
  4    process 26        /root/tst
  5    process 27        /root/tst
(gdb) inferior 3
[Switching to inferior 3 [process 24] (/root/tst)]
[Switching to thread 3.1 (process 24)]
#0  0x00007fffff0fc41a in __libc_fork () at ../sysdeps/nptl/fork.c:145
145     ../sysdeps/nptl/fork.c: No such file or directory.
(gdb) i inferior
  Num  Description       Executable
  1                /root/tst
  2    process 23        /root/tst
* 3    process 24        /root/tst
  4    process 26        /root/tst
  5    process 27        /root/tst
(gdb) n
152     in ../sysdeps/nptl/fork.c
.
.
.
(gdb)
264     in ../sysdeps/nptl/fork.c
(gdb)
main () at 1.c:10
10                      if(p2==0){
(gdb)
11                              putchar('c');
(gdb)
12                              printf("\n");
(gdb)
c
16              return 0;
(gdb)
17      }
(gdb)
__libc_start_main (main=0x400566 , argc=1, argv=0x7ffffffde398, init=,
    fini=, rtld_fini=, stack_end=0x7ffffffde388)
    at ../csu/libc-start.c:325
325     ../csu/libc-start.c: No such file or directory.
(gdb)
[Inferior 3 (process 24) exited normally]
(gdb)
7                       putchar('b');
(gdb)
16              return 0;
(gdb)
17      }
(gdb)
__libc_start_main (main=0x400566 , argc=1, argv=0x7ffffffde398, init=,
    fini=, rtld_fini=, stack_end=0x7ffffffde388)
    at ../csu/libc-start.c:325
325     ../csu/libc-start.c: No such file or directory.
(gdb)
b[Inferior 2 (process 23) exited normally]
(gdb)
The program is not being run.

在使用“set detach-on-fork off”命令后，用“i inferiors”（i是info命令缩写）查看进程状态，可以看到父子进程都在被gdb调试的状态，前面显示“*”是正在调试的进程。当父进程退出后，用“inferior infno”切换到子进程去调试。此外，如果想让父子进程都同时运行，可以使用“set schedule-multiple on”（默认schedule-multiple是off）命令，仍以上述代码为例：

(gdb) set detach-on-fork off
(gdb) set schedule-multiple on
(gdb) start
Temporary breakpoint 1 at 0x40056e: file 1.c, line 5.
Starting program: /root/tst

Temporary breakpoint 1, main () at 1.c:5
5               while((p1=fork())==-1);
(gdb) n
[New process 32]
b6              if(p1==0)
(gdb)
[Inferior 2 (process 32) exited normally]
(gdb)
9                       while((p2=fork())==-1);
(gdb)
[New process 33]
c
10                      if(p2==0){
(gdb)
[Inferior 3 (process 33) exited normally]
(gdb)
14                      else putchar('a');
(gdb)
16              return 0;
(gdb)
17      }
(gdb)
__libc_start_main (main=0x400566 , argc=1, argv=0x7ffffffde398, init=, fini=,
    rtld_fini=, stack_end=0x7ffffffde388) at ../csu/libc-start.c:325
325     ../csu/libc-start.c: No such file or directory.
(gdb)
a[Inferior 1 (process 28) exited normally]
(gdb)
The program is not being run.

父进程Inferior 1(process 28)创建Inferior 2(process 32)打印 b ，并正常退出；再创建Inferior 3(process 33)打印c，并正常退出，之后父进程打印a，退出。

总结

第一次上手GDB调试，有助于对进程控制的理解

justtreee / blog

【操作系统实验笔记】进程控制与GDB多线程调试 #2

实验内容

实现代码

GDB多进程调试

调试子进程

同时调试父进程和子进程

总结