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Subject: Re: [OMPI users] openmpi MPI_Init doesn't work after fork under gdb
From: Geoffrey Irving (irving_at_[hidden])
Date: 2013-02-21 19:38:48


No, I don't really have any idea what it's doing. You have to add
manual sleeps and attach another instance of gdb if you want to step
through the child, since I also can't get follow-fork-mode child to
work. I only tried that once, and it segfaulted accessing the zero
pointer destroying the stack in the process. I haven't done manual
stepping or printf searches, so I don't know where in MPI_Init it
breaks.

I'm trying to install gdb 7.5.1 now (a nontrivial process on Mac,
unfortunately) to see if that fixes it.

Geoffrey

On Thu, Feb 21, 2013 at 4:28 PM, Ralph Castain <rhc_at_[hidden]> wrote:
> Hmmm...how about that? Yeah, it doesn't work with the devel trunk either - I'd missed that point.
>
> No idea why, I'm afraid - never tried it before. Are you sure it "crashes"? I'm still getting a child status of "0", but no message output. My guess is that the I/O is being lost for some reason.
>
> On Feb 21, 2013, at 4:09 PM, Geoffrey Irving <irving_at_[hidden]> wrote:
>
>> Actually, I don't see it printing "We're an MPI program!" under gdb,
>> which means it isn't working.
>>
>> Geoffrey
>>
>> On Thu, Feb 21, 2013 at 4:07 PM, Ralph Castain <rhc_at_[hidden]> wrote:
>>> Hmmm...works with 1.6.4 for me on Mac 10.8.2:
>>>
>>> Ralphs-iMac:v1.6 rhc$ ./fork-bug
>>> We're an MPI program!
>>> child status = 0
>>> Ralphs-iMac:v1.6 rhc$ gdb ./fork-bug
>>> GNU gdb 6.3.50-20050815 (Apple version gdb-1820) (Sat Jun 16 02:40:11 UTC 2012)
>>> Copyright 2004 Free Software Foundation, Inc.
>>> GDB is free software, covered by the GNU General Public License, and you are
>>> welcome to change it and/or distribute copies of it under certain conditions.
>>> Type "show copying" to see the conditions.
>>> There is absolutely no warranty for GDB. Type "show warranty" for details.
>>> This GDB was configured as "x86_64-apple-darwin"...Reading symbols for shared libraries ... done
>>>
>>> (gdb) r
>>> Starting program: /Volumes/RHCHD/rhc/openmpi/v1.6/fork-bug
>>> Reading symbols for shared libraries ++............................. done
>>> child status = 0
>>>
>>> Program exited normally.
>>> (gdb)
>>>
>>> Afraid I have no idea why you might be hitting the problem, though...
>>>
>>>
>>> On Feb 21, 2013, at 3:50 PM, Geoffrey Irving <irving_at_[hidden]> wrote:
>>>
>>>> The singleton fork/exec itself is fine, since normal MPI programs work
>>>> under gdb (e.g., fork-bug.c without the fork). gdb is has
>>>> follow-fork-mode set to parent, so it's odd that gdb is looking at the
>>>> child process's trickery at all.
>>>>
>>>> I've confirmed that it's still broken under 1.6.4, unfortunately.
>>>>
>>>> Geoffrey
>>>>
>>>> On Thu, Feb 21, 2013 at 3:36 PM, Ralph Castain <rhc_at_[hidden]> wrote:
>>>>> Singletons fork/exec a daemon to support them - my guess is that gdb may not like it on your machine?
>>>>>
>>>>> FWIW - it runs fine for me using the developer's trunk. You might try with 1.6.4 in case it's a bug in 1.6.0
>>>>>
>>>>>
>>>>> On Feb 21, 2013, at 3:18 PM, Geoffrey Irving <irving_at_[hidden]> wrote:
>>>>>
>>>>>> The attached program illustrates the problem. It forks, and the child
>>>>>> calls MPI_Init. This works fine unless I'm inside gdb. Inside gdb,
>>>>>> MPI_Init silently crashes.
>>>>>>
>>>>>> I'm using OpenMPI 1.6.0 on Mac 10.8.2. I'm running the program
>>>>>> directly, not through mpirun.
>>>>>>
>>>>>> Any ideas what might be wrong?
>>>>>>
>>>>>> Thanks,
>>>>>> Geoffrey
>>>>>>
>>>>>> cone:scratch% /usr/local/bin/mpicc -o fork-bug fork-bug.c
>>>>>> cone:scratch% ./fork-bug
>>>>>> We're an MPI program!
>>>>>> child status = 0
>>>>>> cone:scratch% gdb ./fork-bug
>>>>>> gdb ./fork-bug
>>>>>> GNU gdb 6.3.50-20050815 (Apple version gdb-1824) (Thu Nov 15 10:42:43 UTC 2012)
>>>>>> Copyright 2004 Free Software Foundation, Inc.
>>>>>> GDB is free software, covered by the GNU General Public License, and you are
>>>>>> welcome to change it and/or distribute copies of it under certain conditions.
>>>>>> Type "show copying" to see the conditions.
>>>>>> There is absolutely no warranty for GDB. Type "show warranty" for details.
>>>>>> This GDB was configured as "x86_64-apple-darwin"...Reading symbols for
>>>>>> shared libraries ... done
>>>>>>
>>>>>> (gdb) run
>>>>>> Reading symbols for shared libraries ++............................. done
>>>>>> child status = 5
>>>>>>
>>>>>> Program exited normally.
>>>>>> (gdb) cone:scratch%
>>>>>> <fork-bug.c>_______________________________________________
>>>>>> users mailing list
>>>>>> users_at_[hidden]
>>>>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>>>>
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