Hi, There doesn't seems to be an elgant way to do an in place all_reduce on array right now (Although I only stumbled on the issue after I moved to openmpi 1.3.2). That is, if all_reduce_impl is called with (in_values == out_values) MPI is allowed to compain with a MPI_ERR_BUF. Of course, one can call all_reduce with static_cast(MPI_IN_PLACE) but I do not find that solution very elegant. Would it be a problem to modify all_reduce_impl into: template void all_reduce_impl(const communicator& comm, const T* in_values, int n, T* out_values, Op /*op*/, mpl::true_ /*is_mpi_op*/, mpl::true_ /*is_mpi_datatype*/) { if (in_values == out_values) { in_values = static_cast(MPI_IN_PLACE); } BOOST_MPI_CHECK_RESULT(MPI_Allreduce, (const_cast(in_values), out_values, n, boost::mpi::get_mpi_datatype<T>(*in_values), (is_mpi_op::op()), comm)); } It worked for me, and if there is an agreement I can propose a patch. Regards, -- Alain Miniussi http://www.oca.eu/alainm Equipe CRIMSON https://crimson.oca.eu +33 492 00 30 09