MPI_SCATTERV(3)                    Open MPI                    MPI_SCATTERV(3)


MPI_Scatterv, MPI_Iscatterv, MPI_Scatterv_init - Scatters a buffer in parts to
all tasks in a group.

SYNTAX
   C Syntax

          #include <mpi.h>

          int MPI_Scatterv(const void *sendbuf, const int sendcounts[], const int displs[],
               MPI_Datatype sendtype, void *recvbuf, int recvcount,
               MPI_Datatype recvtype, int root, MPI_Comm comm)

          int MPI_Iscatterv(const void *sendbuf, const int sendcounts[], const int displs[],
               MPI_Datatype sendtype, void *recvbuf, int recvcount,
               MPI_Datatype recvtype, int root, MPI_Comm comm, MPI_Request *request)

          int MPI_Scatterv_init(const void *sendbuf, const int sendcounts[], const int displs[],
               MPI_Datatype sendtype, void *recvbuf, int recvcount,
               MPI_Datatype recvtype, int root, MPI_Comm comm, MPI_Info info, MPI_Request *request)

   Fortran Syntax

          USE MPI
          ! or the older form: INCLUDE 'mpif.h'
          MPI_SCATTERV(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
                       RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR)
               <type>  SENDBUF(*), RECVBUF(*)
               INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
               INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, IERROR

          MPI_ISCATTERV(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
                       RECVCOUNT, RECVTYPE, ROOT, COMM, REQUEST, IERROR)
               <type>  SENDBUF(*), RECVBUF(*)
               INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
               INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, REQUEST, IERROR

          MPI_SCATTERV_INIT(SENDBUF, SENDCOUNTS, DISPLS, SENDTYPE, RECVBUF,
                       RECVCOUNT, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR)
               <type>  SENDBUF(*), RECVBUF(*)
               INTEGER SENDCOUNTS(*), DISPLS(*), SENDTYPE
               INTEGER RECVCOUNT, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax

          USE mpi_f08
          MPI_Scatterv(sendbuf, sendcounts, displs, sendtype, recvbuf, recvcount,
                       recvtype, root, comm, ierror)
               TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
               TYPE(*), DIMENSION(..) :: recvbuf
               INTEGER, INTENT(IN) :: sendcounts(*), displs(*), recvcount, root
               TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
               TYPE(MPI_Comm), INTENT(IN) :: comm
               INTEGER, OPTIONAL, INTENT(OUT) :: ierror

          MPI_Iscatterv(sendbuf, sendcounts, displs, sendtype, recvbuf, recvcount,
                       recvtype, root, comm, request, ierror)
               TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
               TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
               INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), displs(*)
               INTEGER, INTENT(IN) :: recvcount, root
               TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
               TYPE(MPI_Comm), INTENT(IN) :: comm
               TYPE(MPI_Request), INTENT(OUT) :: request
               INTEGER, OPTIONAL, INTENT(OUT) :: ierror

          MPI_scatterv_init(sendbuf, sendcounts, displs, sendtype, recvbuf, recvcount,
                       recvtype, root, comm, info, request, ierror)
               TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
               TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
               INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), displs(*)
               INTEGER, INTENT(IN) :: recvcount, root
               TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
               TYPE(MPI_Comm), INTENT(IN) :: comm
               TYPE(MPI_Info), INTENT(IN) :: info
               TYPE(MPI_Request), INTENT(OUT) :: request
               INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

       o sendbuf: Address of send buffer (choice, significant only at root).

       o sendcounts: Integer array (of length group size) specifying the
         number of elements to send to each processor.

       o displs: Integer array (of length group size). Entry i specifies the
         displacement (relative to sendbuf) from which to take the outgoing
         data to process i.

       o sendtype: Datatype of send buffer elements (handle).

       o recvcount: Number of elements in receive buffer (integer).

       o recvtype: Datatype of receive buffer elements (handle).

       o root: Rank of sending process (integer).

       o comm: Communicator (handle).

       o info: Info (handle, persistent only).

OUTPUT PARAMETERS

       o recvbuf: Address of receive buffer (choice).

       o request: Request (handle, non-blocking only).

       o ierror: Fortran only: Error status (integer).

DESCRIPTION
       MPI_Scatterv is the inverse operation to MPI_Gatherv.

       MPI_Scatterv extends the functionality of MPI_Scatter by allowing a
       varying count of data to be sent to each process, since sendcounts is
       now an array. It also allows more flexibility as to where the data is
       taken from on the root, by providing the new argument, displs.

       The outcome is as if the root executed n send operations,

          MPI_Send(sendbuf + displs[i] * extent(sendtype),
                   sendcounts[i], sendtype, i, ...);

          // and each process executed a receive,

          MPI_Recv(recvbuf, recvcount, recvtype, root, ...)

       The send buffer is ignored for all nonroot processes.

       The type signature implied by sendcount[i], sendtype at the root must
       be equal to the type signature implied by recvcount, recvtype at
       process i (however, the type maps may be different). This implies that
       the amount of data sent must be equal to the amount of data received,
       pairwise between each process and the root. Distinct type maps between
       sender and receiver are still allowed.

       All arguments to the function are significant on process root, while on
       other processes, only arguments recvbuf, recvcount, recvtype, root,
       comm are significant. The arguments root and comm must have identical
       values on all processes.

       The specification of counts, types, and displacements should not cause
       any location on the root to be read more than once.

       Example 1: The reverse of Example 5 in the MPI_Gatherv manpage. We have
       a varying stride between blocks at sending (root) side, at the
       receiving side we receive 100 - i elements into the ith column of a 100
       x 150 C array at process i.

          MPI_Comm comm;
          int gsize,recvarray[100][150],*rptr;
          int root, *sendbuf, myrank, bufsize, *stride;
          MPI_Datatype rtype;
          int i, *displs, *scounts, offset;
          ...
          MPI_Comm_size( comm, &gsize);
          MPI_Comm_rank( comm, &myrank );

          stride = (int *)malloc(gsize*sizeof(int));
          ...
          /* stride[i] for i = 0 to gsize-1 is set somehow
           * sendbuf comes from elsewhere
           */
          ...
          displs = (int *)malloc(gsize*sizeof(int));
          scounts = (int *)malloc(gsize*sizeof(int));
          offset = 0;
          for (i=0; i<gsize; ++i) {
              displs[i] = offset;
              offset += stride[i];
              scounts[i] = 100 - i;
          }
          /* Create datatype for the column we are receiving
           */
          MPI_Type_vector( 100-myrank, 1, 150, MPI_INT, &rtype);
          MPI_Type_commit( &rtype );
          rptr = &recvarray[0][myrank];
          MPI_Scatterv(sendbuf, scounts, displs, MPI_INT,
                       rptr, 1, rtype, root, comm);

       Example 2: The reverse of Example 1 in the MPI_Gather manpage. The root
       process scatters sets of 100 ints to the other processes, but the sets
       of 100 are stride ints apart in the sending buffer. Requires use of
       MPI_Scatterv, where stride >= 100.

          MPI_Comm comm;
          int gsize,*sendbuf;
          int root, rbuf[100], i, *displs, *scounts;

          ...

          MPI_Comm_size(comm, &gsize);
          sendbuf = (int *)malloc(gsize*stride*sizeof(int));
          ...
          displs = (int *)malloc(gsize*sizeof(int));
          scounts = (int *)malloc(gsize*sizeof(int));
          for (i=0; i<gsize; ++i) {
             displs[i] = i*stride;
             scounts[i] = 100;
          }
          MPI_Scatterv(sendbuf, scounts, displs, MPI_INT,
                       rbuf, 100, MPI_INT, root, comm);

USE OF IN-PLACE OPTION
       When the communicator is an intracommunicator, you can perform a
       scatter operation in-place (the output buffer is used as the input
       buffer). Use the variable MPI_IN_PLACE as the value of the root process
       recvbuf. In this case, recvcount and recvtype are ignored, and the root
       process sends no data to itself.

       Note that MPI_IN_PLACE is a special kind of value; it has the same
       restrictions on its use as MPI_BOTTOM.

       Because the in-place option converts the receive buffer into a
       send-and-receive buffer, a Fortran binding that includes INTENT must
       mark these as INOUT, not OUT.

WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
       When the communicator is an inter-communicator, the root process in the
       first group sends data to all processes in the second group. The first
       group defines the root process. That process uses MPI_ROOT as the value
       of its root argument. The remaining processes use MPI_PROC_NULL as the
       value of their root argument. All processes in the second group use the
       rank of that root process in the first group as the value of their root
       argument. The receive buffer argument of the root process in the first
       group must be consistent with the receive buffer argument of the
       processes in the second group.

ERRORS
       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_thread, after 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 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_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:

       o MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI
         processes.

       o MPI_ERRORS_ABORT An error handler that can be invoked on a
         communicator, window, file, or session. When called on a
         communicator, it acts as if MPI_Abort was called on that
         communicator. If called on a window or file, acts as if MPI_Abort was
         called on a communicator containing the group of processes in the
         corresponding window or file. If called on a session, aborts only the
         local process.

       o MPI_ERRORS_RETURN Returns an error code to the application.

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

       o MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

       o MPI_File_create_errhandler then MPI_File_set_errhandler

       o MPI_Session_create_errhandler then MPI_Session_set_errhandler or at
         MPI_Session_init

       o MPI_Win_create_errhandler then MPI_Win_set_errhandler

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

       See the MPI man page for a full list of MPI error codes.

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

       SEE ALSO:

          o MPI_Gather

          o MPI_Gatherv

          o MPI_Scatter

COPYRIGHT
       2003-2024, The Open MPI Community

                                 April 8, 2024                 MPI_SCATTERV(3)