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Subject: Re: [OMPI users] Can't compile C++ program with extern "C" { #include mpi.h }
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2008-02-08 12:06:07


On Feb 7, 2008, at 7:03 PM, Adam C Powell IV wrote:

> But then, why wouldn't programs expect to be able to include C headers
> in a C++ extern C block?

But that's exactly the issue: mpi.h is not a C header. It is mandated
by the MPI standard to be both a C and a C++ header file. We (Open
MPI) can't change the filename or the languages where it can be used.

> Or rather, why shouldn't they be able to do so
> with mpi.h -- or hdf5.h, which isn't mpi.h

Both mpi.h and hdf5.h do the right things with regards to extern
"C" {}. So they are C++-safe.

> -- when numerous other C
> header files allow it, possibly including other MPI implementations?

Are you sure of that? It is my [possibly flawed] understanding that
HDF5 can be compiled with or without MPI support. If that is right,
it is therefore possible that Salome has been developed/tested/used
with serial HDF5, and this issue never came up.

Can you get Salome to compile with an HDF5 that was built with support
for another MPI implementation?

> After all, it's called mpi.h not mpi.hh or .hxx or mpi_cxx.h, right?
> And isn't the patched version cleaner, in that it separates the C and
> C++ prototypes into different #ifdef/#define regions?

My objection to your patch is twofold:

1. OMPI is not doing anything wrong, and I'm reluctant to "fix"
something that is not wrong without a very good reason. To be blunt:
one open source application that is coded wrong (and can easily be
fixed) is not a very good reason, IMHO.

2. Your patch still requires the application to use an OMPI-specific
#define. That doesn't seem like a good idea.

Put differently: from your description, Salome is a non-conformant MPI
application and therefore may or may not work with any given MPI.
Instead of having each MPI that Salome (HDF5) fails to compile with
provide a workaround, the [much] more portable solution is to have
Salome fix their #includes. This will allow Salome to compile against
any MPI implementation.

I'm clearly not the only OMPI developer, though, so if the community
decides to accept your patch, I certainly won't prevent anyone from
doing so. These are just my opinions. :-)

Have you talked to the Salome developers? Wouldn't an upstream fix
alleviate the need for you to put in an Open MPI-specific patch (or
any patch at all)? Alternatively, have you talked to the HDF5
maintainers to see if they can remove <mpi.h> from their public header
files?

-- 
Jeff Squyres
Cisco Systems