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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: 2013-02-22 13:55:46


No luck with gdb 7.5.1. I can get it to run, but it hits an internal
error and bails out. Thus, I'm stuck with Apple's 6.3.50. I suppose
I'll try to restructure the app so that I don't have to spawn the mpi
process with a fork from the other one.

Geoffrey

On Thu, Feb 21, 2013 at 4:38 PM, Geoffrey Irving <irving_at_[hidden]> wrote:
> 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]
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>>>>>>
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