1. 默认设置下，在调试多进程程序时GDB只会调试主进程。但是GDB（>V7.0）支持多进程的
分别以及同时
调试，换句话说，GDB可以同时调试多个程序。只需要设置follow-fork-mode(默认值：parent)和detach-on-fork（默认值：on）即可。

 
  
  
  
  follow-fork-mode 
  detach-on-fork 
  
  说明
 
 
 
  
  
  
  
  
  
  
  
  
  
  
  
  
   
  
  on 
  
  
  
  
  
  
  
  
  
  
  
  
  
  只调试主进程（GDB默认）
 
 child 
   
  
  
   
   
  
  
   
   
   
  
   
  on 
  
  
  
  
  
  
  
  
  
  
  
  
  
  只调试子进程
 
 parent 
  
  
  
  
  
  
  
  
  
   
  
   
   
  
  off 
  
  
  
  
  
  
  
  
  
  
  
  
  同时调试两个进程，gdb跟主进程，子进程block在fork位置
 
 child 
   
  
  
  
  
  
  
  
   
   
  
   
   
  
  off  
  
  
  
  
  
  
  
  
  
  
  
  同时调试两个进程，gdb跟子进程，主进程block在fork位置
  
  设置方法：set follow-fork-mode [parent|child]  


 

 

 

 


2. GDB默认支持调试多线程，跟主线程，子线程block在create thread。

 

 


例程：


 #include <stdio.h>
 
 #include <pthread.h>
 

 void processA();
 
 void processB();
 
 void * processAworker(void *arg);
 

 int main(int argc, const char *argv[])
 
  
  {
 
  
  int pid;
 

  
  pid = fork();
 

  
  if(pid != 0)
 
  
  
  
  processA();
 
  
  else
 
  
  
  
  processB();
 

  
  return 0;
 
  
  }
 

 void processA()
 
  
  {
 
  
  pid_t pid = getpid();
 
  
  char prefix[] = "ProcessA: ";
 
  
  char tprefix[] = "thread ";
 
  
  int tstatus;
 
  
  pthread_t pt;
 

  
  printf("%s%lu %s\n", prefix, pid, "step1");
 

  
  tstatus = pthread_create(&pt, NULL, processAworker, NULL);
 
  
  if( tstatus != 0 )
 
  
  
  
  {
 
  
  
  
  printf("ProcessA: Can not create new thread.");
 
  
  
  
  }
 
  
 
  
  processAworker(NULL);
 
  
  sleep(1);
 
  
  }
 

 void * processAworker(void *arg)
 
  
  {
 
  
  pid_t pid = getpid();
 
  
  pthread_t tid = pthread_self();
 
  
  char prefix[] = "ProcessA: ";
 
  
  char tprefix[] = "thread ";
 

  
  printf("%s%lu %s%lu %s\n", prefix, pid, tprefix, tid, "step2");
 
  
  printf("%s%lu %s%lu %s\n", prefix, pid, tprefix, tid, "step3");
 

  
  return NULL;
 
  
  }
 

 void processB()
 
  
  {
 
  
  pid_t pid = getpid();
 
  
  char prefix[] = "ProcessB: ";
 
  
  printf("%s%lu %s\n", prefix, pid, "step1");
 
  
  printf("%s%lu %s\n", prefix, pid, "step2");
 
  
  printf("%s%lu %s\n", prefix, pid, "step3");
 

  
  }
 
 
输出：


 [cnwuwil@centos c-lab]$ ./test
 
 

 ProcessA: 802 step1
 
 ProcessB: 803 step1
 
 ProcessB: 803 step2
 
 ProcessB: 803 step3
 
 ProcessA: 802 thread 3077555904 step2
 
 ProcessA: 802 thread 3077555904 step3
 
 ProcessA: 802 thread 3077553008 step2
 
 ProcessA: 802 thread 3077553008 step3
  

调试：

1. 调试主进程，block子进程。


 (gdb) set detach-on-fork off
 
 (gdb) show detach-on-fork
 
 Whether gdb will detach the child of a fork is off.
 
 (gdb) catch fork
 
 Catchpoint 1 (fork)
 
 (gdb) r
 
 [Thread debugging using libthread_db enabled]
 

 Catchpoint 1 (forked process 3475), 0x00110424 in __kernel_vsyscall ()
 
 Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.47.el6.i686
 
 (gdb) break test.c:14
 
 Breakpoint 2 at 0x8048546: file test.c, line 14.
 
 (gdb) cont
 
 [New process 3475]
 
 [Thread debugging using libthread_db enabled]
 

 Breakpoint 2, main (argc=1, argv=0xbffff364) at test.c:14
 
 Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.47.el6.i686
 
 (gdb) info inferiors
 
  
  Num 
  Description 
  
  
  
  
  
  Executable 
  
  
  
  
  
  
 
  
  2 
  
  
  process 3475 
  
  
  
  
  /home/cnwuwil/labs/c-lab/test
 
 * 1 
  
  
  process 3472 
  
  
  
  
  /home/cnwuwil/labs/c-lab/test
 
 
2. 切换到子进程：


 (gdb) inferior 2
 
 [Switching to inferior 2 [process 3475] (/home/cnwuwil/labs/c-lab/test)]
 
 [Switching to thread 2 (Thread 0xb7fe86c0 (LWP 3475))]
 
 #0 
  0x00110424 in ?? ()
 
 (gdb) info inferiors
 
  
  Num 
  Description 
  
  
  
  
  
  Executable 
  
  
  
  
  
  
 
 * 2 
  
  
  process 3475 
  
  
  
  
  /home/cnwuwil/labs/c-lab/test
 
  
  1 
  
  
  process 3472 
  
  
  
  
  /home/cnwuwil/labs/c-lab/test
 
 (gdb) inferior 1
 
 [Switching to inferior 1 [process 3472] (/home/cnwuwil/labs/c-lab/test)]
 
 [Switching to thread 1 (Thread 0xb7fe86c0 (LWP 3472))]
 
 #0 
  main (argc=1, argv=0xbffff364) at test.c:14
 
 (gdb) info inferiors
 
  
  Num 
  Description 
  
  
  
  
  
  Executable 
  
  
  
  
  
  
 
  
  2 
  
  
  process 3475 
  
  
  
  
  /home/cnwuwil/labs/c-lab/test
 
 * 1 
  
  
  process 3472 
  
  
  
  
  /home/cnwuwil/labs/c-lab/test
 
 
3. 设断点继续调试主进程，主进程产生两个子线程：


 (gdb) break test.c:50
 
 Breakpoint 3 at 0x804867d: file test.c, line 50. (2 locations)
 
 (gdb) cont
 
 ProcessA: 3472 step1
 
 [New Thread 0xb7fe7b70 (LWP 3562)]
 
 ProcessA: 3472 thread 3086911168 step2
 

 Breakpoint 3, processAworker (arg=0x0) at test.c:50
 
 (gdb) info inferiors
 
  
  Num 
  Description 
  
  
  
  
  
  Executable 
  
  
  
  
  
  
 
  
  2 
  
  
  process 3475 
  
  
  
  
  /home/cnwuwil/labs/c-lab/test
 
 * 1 
  
  
  process 3472 
  
  
  
  
  /home/cnwuwil/labs/c-lab/test
 
 (gdb) info threads
 
  
  3 Thread 0xb7fe7b70 (LWP 3562) 
  0x00110424 in __kernel_vsyscall ()
 
  
  2 Thread 0xb7fe86c0 (LWP 3475) 
  0x00110424 in ?? ()
 
 * 1 Thread 0xb7fe86c0 (LWP 3472) 
  processAworker (arg=0x0) at test.c:50
 
 
4. 切换到主进程中的子线程，注意：线程2为前面产生的子进程


 (gdb) thread 3
 
 [Switching to thread 3 (Thread 0xb7fe7b70 (LWP 3562))]#0 
  0x00110424 in __kernel_vsyscall ()
 
 (gdb) cont
 
 ProcessA: 3472 thread 3086911168 step3
 
 ProcessA: 3472 thread 3086908272 step2
 
 [Switching to Thread 0xb7fe7b70 (LWP 3562)]
 

 Breakpoint 3, processAworker (arg=0x0) at test.c:50
 
 (gdb) info threads
 
 * 3 Thread 0xb7fe7b70 (LWP 3562) 
  processAworker (arg=0x0) at test.c:50
 
  
  2 Thread 0xb7fe86c0 (LWP 3475) 
  0x00110424 in ?? ()
 
  
  1 Thread 0xb7fe86c0 (LWP 3472) 
  0x00110424 in __kernel_vsyscall ()
 
 (gdb) thread 1