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# 同时调试父进程和子进程 ## 例子 #include <stdio.h> #include <stdlib.h> int main(void) { pid_t pid; pid = fork(); if (pid < 0) { exit(1); } else if (pid > 0) { printf("Parent\n"); exit(0); } printf("Child\n"); return 0; } ## 技巧 在调试多进程程序时,gdb默认只会追踪父进程的运行,而子进程会独立运行,gdb不会控制。以上面程序为例: (gdb) start Temporary breakpoint 1 at 0x40055c: file a.c, line 7. Starting program: /data2/home/nanxiao/a Temporary breakpoint 1, main () at a.c:7 7 pid = fork(); (gdb) n 8 if (pid < 0) (gdb) Child 12 else if (pid > 0) (gdb) 14 printf("Parent\n"); (gdb) Parent 15 exit(0); 可以看到当单步执行到第8行时,程序打印出“Child” ,证明子进程已经开始独立运行。 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。仍以上面程序为例: (gdb) set detach-on-fork off (gdb) start Temporary breakpoint 1 at 0x40055c: file a.c, line 7. Starting program: /data2/home/nanxiao/a Temporary breakpoint 1, main () at a.c:7 7 pid = fork(); (gdb) n [New process 1050] 8 if (pid < 0) (gdb) 12 else if (pid > 0) (gdb) i inferior Num Description Executable 2 process 1050 /data2/home/nanxiao/a * 1 process 1046 /data2/home/nanxiao/a (gdb) n 14 printf("Parent\n"); (gdb) n Parent 15 exit(0); (gdb) [Inferior 1 (process 1046) exited normally] (gdb) The program is not being run. (gdb) i inferiors Num Description Executable 2 process 1050 /data2/home/nanxiao/a * 1 <null> /data2/home/nanxiao/a (gdb) inferior 2 [Switching to inferior 2 [process 1050] (/data2/home/nanxiao/a)] [Switching to thread 2 (process 1050)] #0 0x00007ffff7af6cad in fork () from /lib64/libc.so.6 (gdb) bt #0 0x00007ffff7af6cad in fork () from /lib64/libc.so.6 #1 0x0000000000400561 in main () at a.c:7 (gdb) n Single stepping until exit from function fork, which has no line number information. main () at a.c:8 8 if (pid < 0) (gdb) 12 else if (pid > 0) (gdb) 17 printf("Child\n"); (gdb) Child 18 return 0; (gdb) 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。 这个命令目前Linux支持,其它很多操作系统都不支持,使用时请注意。参见[gdb手册](https://sourceware.org/gdb/onlinedocs/gdb/Forks.html) 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 0x40059c: file a.c, line 7. Starting program: /data2/home/nanxiao/a Temporary breakpoint 1, main () at a.c:7 7 pid = fork(); (gdb) n [New process 26597] Child 可以看到打印出了“Child”,证明子进程也在运行了。 参见[gdb手册](https://sourceware.org/gdb/onlinedocs/gdb/All_002dStop-Mode.html#All_002dStop-Mode) ## 贡献者 nanxiao