My understanding of the MPI standard is that datatype committing is only required for communication, but not for calls like MPI_Type_size_x(). I'm testing mpi4py with mpich-3.0.1, and getting the following errors (IIRC, 3.0rc1 did not have this issue) mpi4py.MPI.Exception: Invalid datatype, error stack: PMPI_Type_size_x(125): MPI_Type_size_x(datatype=dtype=USER<vector>, size=0x7fffeb4ef658) PMPI_Type_size_x(98).: Datatype has not been committed mpi4py.MPI.Exception: Invalid datatype, error stack: PMPI_Type_get_extent_x(128): MPI_Type_get_extent_x(datatype=dtype=USER<struct>, lb=0x7fff97046bb0, extent=0x7fff97046bb8) PMPI_Type_get_extent_x(102): Datatype has not been committed Looking more closely to type_size_x.c and type_size.c in src/mpi/datatype/, I see MPID_Datatype_committed_ptr() in the former, but not in the later. IMHO, these checks should be removed from type_size_x.c, type_get_extent_x.c, and type_get_true_extent_x.c (please not I'm not listing get_elements_x.c, this is a different beast) Thanks, -- Lisandro Dalcin --------------- CIMEC (INTEC/CONICET-UNL) Predio CONICET-Santa Fe Colectora RN 168 Km 472, Paraje El Pozo 3000 Santa Fe, Argentina Tel: +54-342-4511594 (ext 1011) Tel/Fax: +54-342-4511169