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>