GDB多进程调试

内容来自牛客C++高薪面试项目2.5节，视频链接：https://www.nowcoder.com/study/live/504/2/5

命令简介

使用GDB 调试的时候，GDB 默认只能跟踪一个进程，可以在fork 函数调用之前，通过指令设置GDB 调试工具跟踪父进程或者是跟踪子进程，默认跟踪父进程。

• 设置调试父进程或者子进程：set follow-fork-mode [parent（默认）| child]
• 设置调试模式：set detach-on-fork [on | off]
  • 默认为on，表示调试当前进程的时候，其它的进程继续运行
  • 如果为off，调试当前进程的时候，其它进程被GDB 挂起。
• 查看调试的进程：info inferiors
• 切换当前调试的进程：inferior id
• 使进程脱离GDB 调试：detach inferiors id

实例

• 编译文件：gcc hello.c -o hello -g
• 启动GDB调试：gdb hello
• 分别在父进程和子进程里面打断点，并查看断点信息：

  (gdb) b 10
  Breakpoint 1 at 0x4006b9: file hello.c, line 10.
  (gdb) b 20
  Breakpoint 2 at 0x40070b: file hello.c, line 20.
  (gdb) i b
  Num     Type           Disp Enb Address            What
  1       breakpoint     keep y   0x00000000004006b9 in main at hello.c:10
  2       breakpoint     keep y   0x000000000040070b in main at hello.c:20
  

• 启动调试程序，默认是对父进程代码进行的调试，使用 set follow-fork-mode child切换到子进程：

  (gdb) r
  Starting program: /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
  begin
  [Attaching after process 21220 fork to child process 21220]
  [New inferior 3 (process 21220)]
  [Detaching after fork from parent process 21219]
  [Inferior 2 (process 21219) detached]
  我是父进程：pid = 21219, ppid = 20241
  i = 0
  [Switching to process 21220]
  
  Breakpoint 2, main () at hello.c:20
  20              printf("我是子进程：pid = %d, ppid = %d\n", getpid(), getppid());
  (gdb) i = 1
  i = 2
  i = 3
  i = 4
  i = 5
  i = 6
  i = 7
  i = 8
  i = 9
  n
  我是子进程：pid = 21220, ppid = 1
  23              for(j = 0; j < 10; j++) {
  (gdb) n
  24                  printf("j = %d\n", j);
  (gdb) n
  j = 0
  25                  sleep(1);
  (gdb) n
  23              for(j = 0; j < 10; j++) {
  (gdb) n
  24                  printf("j = %d\n", j);
  (gdb) c
  Continuing.
  j = 1
  j = 2
  j = 3
  j = 4
  j = 5
  j = 6
  j = 7
  j = 8
  j = 9
  [Inferior 3 (process 21220) exited normally]
  

• 在对子进程进行调试时，父进程是会继续运行的，使用 set detach-on-fork off挂起父进程：

  (gdb) set detach-on-fork off
  (gdb) r
  Starting program: /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
  begin
  [Attaching after process 22326 fork to child process 22326]
  [New inferior 4 (process 22326)]
  [Switching to process 22326]
  
  Breakpoint 2, main () at hello.c:20
  20              printf("我是子进程：pid = %d, ppid = %d\n", getpid(), getppid());
  Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
  (gdb) n
  我是子进程：pid = 22326, ppid = 22325
  23              for(j = 0; j < 10; j++) {
  (gdb) n
  24                  printf("j = %d\n", j);
  (gdb) n
  j = 0
  25                  sleep(1);
  (gdb) n
  23              for(j = 0; j < 10; j++) {
  (gdb) n
  24                  printf("j = %d\n", j);
  (gdb) c
  Continuing.
  j = 1
  j = 2
  j = 3
  j = 4
  j = 5
  j = 6
  j = 7
  j = 8
  j = 9
  [Inferior 4 (process 22326) exited normally]
  

• 使用 info inferiors查看当前调试的进程，并使用 inferior id切换到父进程，注意这里的id是 info inferiors显示出来的num：

  (gdb) r
  Starting program: /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
  begin
  [Attaching after process 22853 fork to child process 22853]
  [New inferior 5 (process 22853)]
  Reading symbols from /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello...done.
  [Switching to process 22853]
  
  Breakpoint 2, main () at hello.c:20
  20              printf("我是子进程：pid = %d, ppid = %d\n", getpid(), getppid());
  Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
  (gdb) info inferiors 
    Num  Description       Executable  
  * 5    process 22853     /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
    4    process 22852     /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
    3    process 22325     /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
    2    <null>            /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
    1    <null>            /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
  (gdb) n
  我是子进程：pid = 22853, ppid = 22852
  23              for(j = 0; j < 10; j++) {
  (gdb) inferior 4
  [Switching to inferior 4 [process 22852] (/home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello)]
  [Switching to thread 3 (process 22852)] 
  #0  0x00007ffff7ad2a02 in fork () from /lib64/libc.so.6
  (gdb) n
  Single stepping until exit from function fork,
  which has no line number information.
  
  Breakpoint 1, main () at hello.c:10
  10              printf("我是父进程：pid = %d, ppid = %d\n", getpid(), getppid());
  (gdb) info inferiors 
    Num  Description       Executable  
    5    process 22853     /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
  * 4    process 22852     /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
    3    process 22325     /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
    2    <null>            /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello 
    1    <null>            /home/wzy1999/cpp/nowcoder/chapter02/lesson03-05/hello
  

• 使用 detach inferiors id可以使进程脱离GDB调试，和continue的效果类似，进程执行完之后，Description会变成 <null>