NO IMAGE

1. 預設設定下,在除錯多程序程式時GDB只會除錯主程序。但是GDB(>V7.0)支援多程序的分別以及同時除錯,換句話說,GDB可以同時除錯多個程式。只需要設定follow-fork-mode(預設值:parent)和detach-on-fork(預設值:on)即可。

      follow-fork-mode  detach-on-fork   說明
parent                   on               只除錯主程序(GDB預設)
child                     on               只除錯子程序
parent                   off              同時除錯兩個程序,gdb跟主程序,子程序block在fork位置
child                     off              同時除錯兩個程序,gdb跟子程序,主程序block在fork位置

   設定方法:set follow-fork-mode [parent|child]   set detach-on-fork [on|off]

   查詢正在除錯的程序:info inferiors
   切換除錯的程序: inferior <infer number>
   新增新的除錯程序: add-inferior [-copies n] [-exec executable] ,可以用file executable來分配給inferior可執行檔案。
   其他:remove-inferiors infno, detach inferior

2. GDB預設支援除錯多執行緒,跟主執行緒,子執行緒block在create thread。
   查詢執行緒:info threads
   切換除錯執行緒:thread <thread number>

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”);

  }

輸出:

[[email protected] 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


除錯:
1. 除錯主程序,block子程序。

(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. 切換到子程序:

(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. 設斷點繼續除錯主程序,主程序產生兩個子執行緒:

(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. 切換到主程序中的子執行緒,注意:執行緒2為前面產生的子程序

(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