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Subject: [Boost-mpi] Boost::mpi status error() uninitialized on non-blocking request from irecv
From: Frederik Heber (frederik.heber_at_[hidden])
Date: 2016-04-25 08:36:18

Hey there,

The documentation of boost::mpi says the following about the mpi::status
object returned from a wait_all(..) on a prior stash of irecv()s, i.e.
non-blocking receives (version 1.55, taken from boost/mpi/status.hpp, did
not check later version as revision history is unchanged since 1.36):

> This structure contains status information about messages that
> have been received (with @c communicator::recv) or can be received
> (returned from @c communicator::probe or @c
> communicator::iprobe). It permits access to the source of the
> message, message tag, error code (rarely used), or the number of
> elements that have been transmitted.

Hence, calling error() should give me the error_code returned by MPI_Irecv.
However, this "rarely used" is a strange formulation ...

Thus, we take the example contained in the tutorial on non-blocking
communication, modify it slightly to store the resulting status objects,
and inspect their error() return values.

    #include <boost/mpi.hpp>
    #include <iostream>
    #include <string>
    #include <boost/serialization/string.hpp>
    #include <boost/mpi.hpp>
    namespace mpi = boost::mpi;

    int main()
      mpi::environment env;
      mpi::communicator world;

      if (world.rank() == 0) {
        mpi::request reqs[2];
        std::string msg, out_msg = "Hello";
        reqs[0] = world.isend(1, 0, out_msg);
        reqs[1] = world.irecv(1, 1, msg);
        mpi::wait_all(reqs, reqs + 2);
        std::cout << msg << "!" << std::endl;
      } else {
        mpi::request reqs[2];
        std::string msg, out_msg = "world";
        reqs[0] = world.isend(0, 1, out_msg);
        reqs[1] = world.irecv(0, 0, msg);
        std::vector<mpi::status> status;
        mpi::wait_all(reqs, reqs + 2, std::back_inserter(status));
        std::cout << "error code: send - " << status[0].error() << ",
receive - "
            << status[1].error() << std::endl;
        std::cout << msg << ", ";

      return 0;

The result then is

    error code: send - 0, receive - 331828272

I.e. the error code of isend's status is fine, while the one of irecv's
status just shows a probably uninitialized value.

I know that boost::mpi internally checks the return code from MPI via its
macro BOOST_MPI_CHECK_RESULT and it would throw an exception in case of an
error. However, this can be switched off via BOOST_NO_EXCEPTIONS.
Therefore, I would feel better if I were able to check that what I just
received is really ok.

Is this a bug (at least in the documentation) or am I missing something

Kind regards,

Frederik Heber

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