[Mpi-forum] overloading MPI_Init etc.

[William Gropp]

The issue is really one of whether the C++ (or Fortran) interfaces call the C MPI interface or not.  There are both reasons to do so and reasons not.  The reasons for is that a developer can write a profiling library that intercepts the C calls; on an implementation that routes everything through C, this simplifies their effort (at some cost in precision of call).

In this case, the user should have make use of the PMPI::Init interface.  

Bill

On Mar 5, 2012, at 4:27 AM, Bronis R. de Supinski wrote:

> 
> Um, it is standard practice when using the profiling
> interface to redefine MPI_* functions. I think this
> question is one for the C and C++ standards, not MPI...
> 
> On Sun, 4 Mar 2012, Jeff Hammond wrote:
> 
>> What was ambiguous to me - looking at 2.6.3-4 - was if using MPI_ was
>> still a protected namespace in C++.  The standard really only says
>> explicitly that it is so in C.  It is only with an indirect argument
>> that mpi.h can define both C and C++ bindings that I can see C++ code
>> not being able to define MPI_ functions.
>> 
>> Note that MyMPI_ was ~exactly~ how I fixed the user's code :-)
>> 
>> Jeff
>> 
>> On Sun, Mar 4, 2012 at 9:39 PM, Fab Tillier <ftillier at microsoft.com> wrote:
>>> I don't think defining one's own mpi.h is an issue from a compliance perspective.  However, overloading MPI_Init definitely is - the standard reserves the MPI_ namespace, so if it isn't in the standard, it's not standard compliant.  Now, if they named it MyMPI_Init, it'd be fine.
>>> 
>>> -Fab
>>> 
>>> Jeff Hammond wrote on Sun, 4 Mar 2012 at 18:22:40
>>> 
>>>> I have a user who insists it is MPI-compliant to do the following:
>>>> 
>>>> user file mpi.h, which is included in the application with #include
>>>> "mpi.h": ================================== #include <mpi.h>
>>>> 
>>>> <snip>
>>>> 
>>>> void MPI_Init(int argc, char* argv[], int& my_rank, int& mpi_size, int&
>>>> Master) {
>>>>   MPI::Init(argc, argv);
>>>>   my_rank = MPI::COMM_WORLD.Get_rank();
>>>>   mpi_size = MPI::COMM_WORLD.Get_size();
>>>>   Master = 0;
>>>> 
>>>>   std::cout << "Hello World! I am " << my_rank << " of " << mpi_size
>>>> << std::endl;
>>>> }
>>>> 
>>>> void MPI_finalize()
>>>> {MPI::Finalize();}
>>>> 
>>>> ==================================
>>>> 
>>>> I believe that it is a terrible practice, if not a violation of the
>>>> MPI standard to:
>>>> (1) define one's own mpi.h, since this may confuse the preprocessor
>>>> (or are there precedence rules in the ANSI C standard?) as to which to
>>>> include first (especially if CC=mpicc or CXX=mpicxx), and
>>>> (2) overload MPI_Init, even if one is using C++ and relying upon C++
>>>> name-mangling to avoid a symbol conflict.
>>>> 
>>>> The user believes that the inability to overload MPI_Init in C++ means
>>>> that MPI is not compliant with the C++ standard.
>>>> 
>>>> Thanks,
>>>> 
>>>> Jeff
>>>> 
>>> 
>>> 
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>> 
>> 
>> 
>> -- 
>> Jeff Hammond
>> Argonne Leadership Computing Facility
>> University of Chicago Computation Institute
>> jhammond at alcf.anl.gov / (630) 252-5381
>> http://www.linkedin.com/in/jeffhammond
>> https://wiki.alcf.anl.gov/old/index.php/User:Jhammond
>> https://wiki-old.alcf.anl.gov/index.php/User:Jhammond
>> 
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William Gropp
Director, Parallel Computing Institute
Deputy Director for Research
Institute for Advanced Computing Applications and Technologies
Paul and Cynthia Saylor Professor of Computer Science
University of Illinois Urbana-Champaign