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Subject: Re: [OMPI users] Difficulty with MPI_Unpack
From: Prentice Bisbal (prentice_at_[hidden])
Date: 2010-02-08 10:33:34

Jed Brown wrote:
> On Sun, 07 Feb 2010 22:40:55 -0500, Prentice Bisbal <prentice_at_[hidden]> wrote:
>> Hello, everyone. I'm having trouble packing/unpacking this structure:
>> typedef struct{
>> int index;
>> int* coords;
>> }point;
>> The size of the coords array is not known a priori, so it needs to be a
>> dynamic array. I'm trying to send it from one node to another using
>> MPI_Pack/MPI_Unpack as shown below. When I unpack it, I get this error
>> when unpacking the coords array:
>> [fatboy:07360] *** Process received signal ***
>> [fatboy:07360] Signal: Segmentation fault (11)
>> [fatboy:07360] Signal code: Address not mapped (1)
>> [fatboy:07360] Failing at address: (nil)
> Looks like b_point.coords = NULL. Has this been allocated on rank=1?

Yep, that was the problem. I left that out. I can't believe I overlooked
something so obvious. Thanks for the code review. Thanks to Brian
Austin, too, who also found that mistake.

> You might need to use MPI_Get_count to decide how much to allocate.
> Also, if you don't have a convenient upper bound on the size of the
> receive buffer, you can use MPI_Probe followed by MPI_Get_count to
> determine this before calling MPI_Recv.

Thanks for the tip. I'll take a look at those functions.