Re: GDB and threads...

[Kevin Buettner <kevinb@cygnus.com>]

On Apr 26,  5:43pm, Jiva DeVoe wrote:

> Does anyone have any tips on debugging multithreaded apps using gdb?
> I have been trying for a couple of months now, and IMO gdb is REALLY
> not very effective at it (at least in my experience.)  I've used
> windows debuggers to do this, and when I would have a fault in a
> subthread, the debugger would actually break *in* that thread.  With
> gdb, it breaks, but it still breaks in the main thread.  I can't even
> get the thread info commands to work.

Jiva,

I constructed a small program with three threads (counting the main
thread) to see if I could reproduce the problem that you describe.
Here's the program:

--- threads-segv.c ---
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>

void *
worker (void *args)
{
  int i;
  char **a = args;

  for (i = 0; i < 5; i++)
    {
      printf ("%c\n", a[i][0]);
      sleep (1);
    }
  return NULL;
}

int
main(void)
{
  pthread_t tid1, tid2;
  char *a1[] = { "a", "b", "c", "d", "e" };
  char *a2[] = { "A", "B", "C", 0, "E" };

  pthread_create (&tid1, NULL, worker, a1);
  pthread_create (&tid2, NULL, worker, a2);


  pthread_join (tid1, NULL);
  pthread_join (tid2, NULL);
  return 0;
}
--- end threads-segv.c ---

I compile it (on my Red Hat 7.0 machine) as follows:

    gcc -Wall -o threads-segv -g threads-segv.c -lpthread

When I run it (without GDB), it terminates (as intended) with a
SIGSEGV.  When I debug it with GDB (using current development
sources), I see the following:

    (gdb) r
    Starting program: /home/kev/ctests/threads-segv 
    [New Thread 1024 (LWP 5462)]
    [New Thread 2049 (LWP 5463)]
    Delayed SIGSTOP caught for LWP 5463.
    [New Thread 1026 (LWP 5464)]
    Delayed SIGSTOP caught for LWP 5464.
    [New Thread 2051 (LWP 5465)]
    Delayed SIGSTOP caught for LWP 5465.
    a
    A
    b
    B
    C
    c
    d

    Program received signal SIGSEGV, Segmentation fault.
    [Switching to Thread 2051 (LWP 5465)]
    0x08048577 in worker (args=0xbffff968) at threads-segv.c:13
    warning: Source file is more recent than executable.

    13            printf ("%c\n", a[i][0]);
    (gdb) print i
    $1 = 3
    (gdb) print a[i]
    $2 = 0x0
    (gdb) info thread
    * 4 Thread 2051 (LWP 5465)  0x08048577 in worker (args=0xbffff968)
	at threads-segv.c:13
      3 Thread 1026 (LWP 5464)  0x400f3241 in __libc_nanosleep ()
       from /lib/libc.so.6
      2 Thread 2049 (LWP 5463)  0x40113b97 in __poll (fds=0x804b684, nfds=1, 
	timeout=2000) at ../sysdeps/unix/sysv/linux/poll.c:63
      1 Thread 1024 (LWP 5462)  0x40067e75 in __sigsuspend (set=0xbffff7c8)
	at ../sysdeps/unix/sysv/linux/sigsuspend.c:45
    (gdb) bt
    #0  0x08048577 in worker (args=0xbffff968) at threads-segv.c:13
    #1  0x4002bc8e in pthread_start_thread_event (arg=0xbf5ffc00) at manager.c:274
    (gdb) 

So, with my admittedly simple test case, I've been unable to reproduce
the problems that you've described.

Have you tried the current development sources?  If not, see

    http://sources.redhat.com/gdb/#download

and read the section about downloading the development version.

Also, it would be useful if you'd send us your own testcase(s) which
illustrate the problems that you're seeing.

Kevin