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Subject: Re: [OMPI users] MPI_Send, MPI_Recv problem on Mac and Linux
From: Jeffrey Squyres (jsquyres_at_[hidden])
Date: 2012-04-17 16:15:46

Sorry for the delay in replying; I was out last week.

MPI_SEND and MPI_RECV take pointers to the buffer to send and receive, respectively.

When you send a scalar variable, like an int, you get the address of the buffer via the & operator (e.g., MPI_Send(&i, ...)). When you send a new'ed/malloc'ed array, you only need to send the pointer value -- i.e., the address pointing to the buffer. Don't send the address of the pointer, because then you're telling MPI to overwrite the pointer itself. I.e.,:

  work = new char[...];
  MPI_Send(work, ...)


  work = new char[...];
  MPI_Send(&work, ...);

More below.

On Apr 11, 2012, at 3:03 PM, Peter Sels wrote:

> Probably a buffer overrun or so, but I cannot see where.

The buffer overrun is where you specify &array in your MPI_SEND/MPI_RECV calls.

> On linux I get: Segmentation fault (11)
> Increasing the length gives more problems...
> How can I get this code stable?
> What am I doing wrong?
> Is there a maximum length to MPI messages?


> For sending a string, do I use MPI_CHARACTER or MPI_BYTE or ...?


> How come I cannot assert that my messages end in '\0' when received?
> And how come that when I print them, I also get a segmentation fault?

I think these two issues are symptoms of (work) vs. (&work), from above.

> Can I send two subsequent messages using MPI_Send, or do I have to do


> the first as MPI_Isend and then do a MPI_Wait before the next
> MPI_Send?...

You can do multiple Isend's and then a Waitall, if you want.

> Why do I not find code online for receiving the length first and then
> allocating a buffer of this size and then receiving the next message?

I don't know. Perhaps you didn't google enough? :-)

FWIW, the new MPI-3 functions MPI_MPROBE and MPI_IMPROBE will help with unknown-length messages, too. We have that implemented on the Open MPI SVN trunk, but they are not yet available in a stable release. They'll debut in OMPI v1.7.

Jeff Squyres
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