[gmx-developers] broadcast of zero-length arrays

[hess at sbc.su.se]

Hi,

I can remember have such issues before, I think also on an IBM
and discussing this with somebody from IBM.
I thought I had removed all MPI calls with NULL pointers,
but apparently this is not the case.
I committed fixes for nblock_bc in mvdata for 4.0.6 and git master.

Berk

> Hi Brian,
>
> you could help before with a Segfault in the BlueGene MPI layer (in
> scatterv). Do you consider the below described segfault a bug in the MPI
> layer or in Gromacs?
>
> Roland
>
> ---------- Forwarded message ----------
> From: Mark Abraham <Mark.Abraham at anu.edu.au>
> Date: Sun, Nov 22, 2009 at 10:11 PM
> Subject: [gmx-developers] broadcast of zero-length arrays
> To: Gromacs Developers <gmx-developers at gromacs.org>
>
>
> Hi,
>
> During src/gmxlib/mvdata.c bc_grpopts(), my BlueGene/L segfaults during
> the
> broadcasts of the QMMM stuff. The lines that break are attempts to
> broadcast
> arrays of zero length. Adding a check for non-zero length into the
> definition of nblock_bc fixes the problem. Presumably a null pointer is
> being dereferenced inside the MPI library.
>
> I'm not sure whether this observation is indicative of (this version of)
> IBM's MPI library not having implemented the full standard, the standard
> not
> specifying behaviour in this case, or GROMACS not being sufficiently
> defensive. I haven't found anything useful in the MPI documentation I have
> to hand. You could argue cases either way - the implementors of the
> library
> want to avoid such checks to speed performance, and the users of the
> library
> expect it either to take care of such housekeeping for them, or not
> dereference pointers unnecessarily (think buffering)...
>
> Does anyone know what expected behaviour is here?
>
> Cheers,
>
> Mark
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