I'm adding support for the MPI-3 neighborhood collectives to mpi4py. By mistake, I called a neighbor collective on COMM_SELF, and got a segfault. After running under valgrind, I get the trace below. It seems that MPICH (running 3.0.4) is not checking the communicators for a topology being attached. This should be fixed in MPIR_Topo_canon_nhb_count() at src/mpi/topo/topoutil.c, adding a check after the following line: topo_ptr = MPIR_Topology_get(comm_ptr); BTW, the same kind of check should also be added to MPIR_Topo_canon_nhb(). ==14696== Invalid read of size 4 ==14696== at 0xDE0ED39: MPIR_Topo_canon_nhb_count (topoutil.c:283) ==14696== by 0xDFA870A: MPIR_Ineighbor_allgather_default (inhb_allgather.c:50) ==14696== by 0xDFA8B8B: MPIR_Ineighbor_allgather_impl (inhb_allgather.c:98) ==14696== by 0xDFAE27C: MPIR_Neighbor_allgather_default (nhb_allgather.c:37) ==14696== by 0xDFAE350: MPIR_Neighbor_allgather_impl (nhb_allgather.c:58) ==14696== by 0xDFAE918: PMPI_Neighbor_allgather (nhb_allgather.c:155) ==14696== by 0xDAD7B77: __pyx_pw_6mpi4py_3MPI_9Intracomm_25Neighbor_allgather (mpi4py.MPI.c:87767) ==14696== by 0x31784DD280: PyEval_EvalFrameEx (in /usr/lib64/libpython2.7.so.1.0) ==14696== by 0x31784DCEF0: PyEval_EvalFrameEx (in /usr/lib64/libpython2.7.so.1.0) ==14696== by 0x31784DDCBE: PyEval_EvalCodeEx (in /usr/lib64/libpython2.7.so.1.0) ==14696== by 0x317846DA36: ??? (in /usr/lib64/libpython2.7.so.1.0) ==14696== by 0x3178449C0D: PyObject_Call (in /usr/lib64/libpython2.7.so.1.0) ==14696== Address 0x0 is not stack'd, malloc'd or (recently) free'd -- 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