SYNTAX

MPI_Neighbor_alltoallw <#mpi-neighbor-alltoallw> is a generalized collective operation in which all processes send data to and receive data from all neighbors. It adds flexibility to MPI_Neighbor_alltoallv <#mpi-neighbor-alltoallv> by allowing the user to specify the datatype of individual data blocks (in addition to displacement and element count). Its operation can be thought of in the following way, where each process performs 2n (n being the number of neighbors in the topology of communicator comm) independent point-to-point communications. The neighbors and buffer layout are determined by the topology of comm.

MPI_Cart_get(comm, maxdims, dims, periods, coords);
for (dim = 0, i = 0 ; dim < dims ; ++dim) {
    MPI_Cart_shift(comm, dim, 1, &r0, &r1);
    MPI_Isend(sendbuf + sdispls[i]  * extent(sendtype),
              sendcount, sendtypes[i], r0, ..., comm, ...);
    MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
              recvcount, recvtypes[i], r0, ..., comm, ...);
    ++i;
    MPI_Isend(sendbuf + sdispls[i] * extent(sendtype),
              sendcount, sendtypes[i], r1, ..., comm, &req[i]);
    MPI_Irecv(recvbuf + rdispls[i] * extent(recvtype),
              recvcount, recvtypes[i], r1, ..., comm, ...);
    ++i;
}
MPI_Wait_all (...);
MPI_Comm_size(comm, &n);
for (i = 0, i < n; i++)
  MPI_Send(sendbuf + sdispls[i], sendcounts[i],
       sendtypes[i], i, ..., comm);
for (i = 0, i < n; i++)
  MPI_Recv(recvbuf + rdispls[i], recvcounts[i],
       recvtypes[i], i, ..., comm);

Process j sends the k-th block of its local sendbuf to neighbor k, which places the data in the j-th block of its local recvbuf.

When a pair of processes exchanges data, each may pass different element count and datatype arguments so long as the sender specifies the same amount of data to send (in bytes) as the receiver expects to receive.

Note that process i may send a different amount of data to process j than it receives from process j. Also, a process may send entirely different amounts and types of data to different processes in the communicator.

The MPI_IN_PLACE option for sendbuf is not meaningful for this operation

The specification of counts, types, and displacements should not cause any location to be written more than once.

All arguments on all processes are significant. The comm argument, in particular, must describe the same communicator on all processes.

The offsets of sdispls and rdispls are measured in bytes. Compare this to MPI_Neighbor_alltoallv <#mpi-neighbor-alltoallv>, where these offsets are measured in units of sendtype and recvtype, respectively.

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init <#mpi-init>/MPI_Init_thread <#mpi-init-thread>, after MPI_Finalize <#mpi-finalize>, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler <#mpi-comm-set-errhandler> on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn <#mpi-comm-spawn>/MPI_Comm_spawn_multiple <#mpi-comm-spawn-multiple>. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

MPI applications can also implement their own error handlers by calling:

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

See the Error Handling section of the MPI-3.1 standard for more information.

See also:

2003-2026, The Open MPI Community