mpi multiple init finalize

Question

Assuming I have good reason to do the following (I think I have), how to make it works?

#include "mpi.h"

int main( int argc, char *argv[] )
{
    int myid, numprocs;

    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&myid);
    // ...
    MPI_Finalize();


    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&myid);
    // ...
    MPI_Finalize();

    return 0;
}

I got the error:

--------------------------------------------------------------------------
Calling any MPI-function after calling MPI_Finalize is erroneous.
The only exceptions are MPI_Initialized, MPI_Finalized and MPI_Get_version.
--------------------------------------------------------------------------
*** An error occurred in MPI_Init
*** after MPI was finalized
*** MPI_ERRORS_ARE_FATAL (your MPI job will now abort)
[ange:13049] Abort after MPI_FINALIZE completed successfully; not able to guarantee     that all other processes were killed!

The reason to do that:

I've Python wrapping around C++ code. Some wrapped class have constructor that call MPI_Init, and destructor that call MPI_Finalize. I would like to be able in Python to freely create, delete re-create the Python object that wrap this C++ class. The ultimate goal is to create a webservice entirely in Python, that import the Python C++ exstension once, and execute some Python code given the user request.

EDIT: I think I'll refactor the C++ code to give possibility to not MPI_Init and MPI_Finalize in constructor and destructor, so it's possible to do it exactly one time in the Python script (using mpi4py).

Answer 1

You've basically got the right solution, so I'll just confirm. It is in fact erroneous to call MPI_Init and MPI_Finalize multiple times, and if you have an entity that calls these internally on creation/destruction, then you can only instantiate that entity once. If you want to create multiple instances, you'll need to change the entity to do one of the following:

1. Offer an option to not call Init and Finalize that the user can set externally
2. Use MPI_Initialized and MPI_Finalized to decide whether it needs to call either of the above