1. gdb调试多进程
调试多进程有一种笨方法就是在fork之后子进程执行的代码中sleep一下。然后利用这段时间，通过gdb attach 子进程id。
不过在linux系统上（2.5.46之后），gdb是支持调试调试fork和vfork这种类型的多进程程序的。
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char const *argv[]) {
pid_t pid = fork();
if (pid > 0) {
printf("parent thread\n");
exit(0);
} else if (pid < 0) {
perror("fork failed");
exit(1);
} else {
printf("child thread\n");
}
return 0;
}
下面是具体的调试过程
(gdb) show detach-on-fork
Whether gdb will detach the child of a fork is on.
#这个参数默认是开启状态，意思就是当fork时，子进程默认detach
(gdb) show follow-fork-mode
Debugger response to a program call of fork or vfork is "parent".
(gdb) show follow-exec-mode
Follow exec mode is "same".
(gdb) info inferiors
Num Description Executable
* 1 <null> /root/demo_cpp2/./multi_thread
(gdb) set detach-on-fork off #关闭
(gdb) info inferiors
Num Description Executable
* 1 <null> /root/demo_cpp2/./multi_thread
(gdb) n
The program is not being run.
(gdb) start
Temporary breakpoint 1 at 0x40064c: file multi_thread.c, line 5.
Starting program: /root/demo_cpp2/./multi_thread
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Temporary breakpoint 1, main (argc=1, argv=0x7fffffffe378) at multi_thread.c:5
5 pid_t pid = fork();
Missing separate debuginfos, use: debuginfo-install glibc-2.17-105.el7.x86_64
(gdb) n
[New process 10269]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
6 if (pid > 0) {
Missing separate debuginfos, use: debuginfo-install glibc-2.17-105.el7.x86_64
(gdb) info inferiors
Num Description Executable
2 process 10269 /root/demo_cpp2/./multi_thread
* 1 process 10265 /root/demo_cpp2/./multi_thread
#在这里就看到了所有进程
(gdb) n
7 printf("parent thread\n");
(gdb)
parent thread
8 exit(0);
(gdb) inferior 2
[Switching to inferior 2 [process 10269] (/root/demo_cpp2/./multi_thread)]
[Switching to thread 2 (Thread 0x7ffff7fc8740 (LWP 10269))]
#0 0x00007ffff78bb50c in fork () from /lib64/libc.so.6
# 通过inferior命令可以实现切换
(gdb) n
Single stepping until exit from function fork,
which has no line number information.
main (argc=1, argv=0x7fffffffe378) at multi_thread.c:6
6 if (pid > 0) {
(gdb) n
9 } else if (pid < 0) {
(gdb) n
13 printf("child thread\n");
(gdb) show schedule-multiple
Resuming the execution of threads of all processes is off.
(gdb) set schedule-multiple on # 想让父子进程同时运行就设置成on
(gdb) n
child thread
[Thread 0x7ffff7fc8740 (LWP 10265) exited]
15 return 0;
(gdb)
总结一下，两个关键配置项
- detach-on-fork 设为off，可同时调试父进程和子进程s
- schedule-multiple 设为on，可同时运行父子进程
2. 调试多线程
gdb默认调试多线程，断点断在某个线程上时，其他线程也暂停。单步走时，其他线程也启动。这种场景，可能并不利于我们排查问题。
如果我们指向锁定一个线程来排查问题，那么可以设置scheduler-locking线程锁来将其他线程"冻住"。
set scheduler-locking on 锁定线程
set scheduler-locking off 不锁定线程，默认模式
set scheduler-locking step 单步执行时，锁定其他线程，不适用于continue, until 这些非单步操作
参考：
[0] https://sourceware.org/gdb/onlinedocs/gdb/Forks.html
[1] https://sourceware.org/gdb/onlinedocs/gdb/All_002dStop-Mode.html