MPI_IGATHERV(3)                    Open MPI                    MPI_IGATHERV(3)


       MPI_Gatherv, MPI_Igatherv, MPI_Gatherv_init - Gathers varying amounts
       of data from all processes to the root process

SYNTAX
   C Syntax

          #include <mpi.h>

          int MPI_Gatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
              void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype,
              int root, MPI_Comm comm)

          int MPI_Igatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
              void *recvbuf, const int recvcounts[], const int displs[], MPI_Datatype recvtype,
              int root, MPI_Comm comm, MPI_Request *request)

          int MPI_Gatherv_init(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
              void *recvbuf, const int recvcounts[], const int displs[], 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_GATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
                  DISPLS, RECVTYPE, ROOT, COMM, IERROR)
              <type>  SENDBUF(*), RECVBUF(*)
              INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
              INTEGER RECVTYPE, ROOT, COMM, IERROR

          MPI_IGATHERV(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
                  DISPLS, RECVTYPE, ROOT, COMM, REQUEST, IERROR)
              <type>  SENDBUF(*), RECVBUF(*)
              INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
              INTEGER RECVTYPE, ROOT, COMM, REQUEST, IERROR

          MPI_GATHERV_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNTS,
                  DISPLS, RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR)
              <type>  SENDBUF(*), RECVBUF(*)
              INTEGER SENDCOUNT, SENDTYPE, RECVCOUNTS(*), DISPLS(*)
              INTEGER RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax

          USE mpi_f08

          MPI_Gatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
                  recvtype, root, comm, ierror)
              TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
              TYPE(*), DIMENSION(..) :: recvbuf
              INTEGER, INTENT(IN) :: sendcount, recvcounts(*), displs(*), root
              TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
              TYPE(MPI_Comm), INTENT(IN) :: comm
              INTEGER, OPTIONAL, INTENT(OUT) :: ierror

          MPI_Igatherv(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
                  recvtype, root, comm, request, ierror)
              TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
              TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
              INTEGER, INTENT(IN) :: sendcount, root
              INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*), displs(*)
              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_Gatherv_init(sendbuf, sendcount, sendtype, recvbuf, recvcounts, displs,
                  recvtype, root, comm, info, request, ierror)
              TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
              TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
              INTEGER, INTENT(IN) :: sendcount, root
              INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*), displs(*)
              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 : Starting address of send buffer (choice).

       o sendcount : Number of elements in send buffer (integer).

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

       o

         recvcounts
                Integer array (of length group size) containing the number of
                elements that are received from each process (significant only
                at root).

       o

         displs Integer array (of length group size). Entry i specifies the
                displacement relative to recvbuf at which to place the
                incoming data from process i (significant only at root).

       o

         recvtype
                Datatype of recv buffer elements (significant only at root)
                (handle).

       o root : Rank of receiving process (integer).

       o comm : Communicator (handle).

       o info : Info (handle, persistent only).

OUTPUT PARAMETERS

       o

         recvbuf
                Address of receive buffer (choice, significant only at root).

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

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

DESCRIPTION
       MPI_Gatherv extends the functionality of MPI_Gather by allowing a
       varying count of data from each process, since recvcounts is now an
       array. It also allows more flexibility as to where the data is placed
       on the root, by providing the new argument, displs.

       The outcome is as if each process, including the root process, sends a
       message to the root,

          MPI_Send(sendbuf, sendcount, sendtype, root, ...);

       and the root executes n receives,

          MPI_Recv(recvbuf + disp[i] * extent(recvtype), recvcounts[i],
                   recvtype, i, ...);

       Messages are placed in the receive buffer of the root process in rank
       order, that is, the data sent from process j is placed in the jth
       portion of the receive buffer recvbuf on process root. The jth portion
       of recvbuf begins at offset displs[j] elements (in terms of recvtype)
       into recvbuf.

       The receive buffer is ignored for all nonroot processes.

       The type signature implied by sendcount, sendtype on process i must be
       equal to the type signature implied by recvcounts[i], recvtype at the
       root. 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, as
       illustrated in Example 2, below.

       All arguments to the function are significant on process root, while on
       other processes, only arguments sendbuf, sendcount, sendtype, 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 written more than once. Such a call is
       erroneous.

       Example 1: Now have each process send 100 ints to root, but place each
       set (of 100) stride ints apart at receiving end. Use MPI_Gatherv and
       the displs argument to achieve this effect. Assume stride >= 100.

          MPI_Comm comm;
          int gsize, sendarray[100];
          int root, *rbuf, stride;
          int *displs, i, rcounts;
          ...

          MPI_Comm_size(comm, &gsize);
          rbuf = (int)malloc(gsize * stride * sizeof(int));
          displs = (int)malloc(gsize * sizeof(int));
          rcounts = (int )malloc(gsize * sizeof(int));

          for (i=0; i<gsize; ++i) {
            displs[i] = i * stride;
            rcounts[i] = 100;
          }
          MPI_Gatherv(sendarray, 100, MPI_INT, rbuf, rcounts, displs, MPI_INT,
                      root, comm);

       Note that the program is erroneous if stride < 100.

       Example 2: Same as Example 1 on the receiving side, but send the 100
       ints from the 0th column of a 100 150 int array, in C.

          MPI_Comm comm;
          int gsize, sendarray[100][150];
          int root, *rbuf, stride;
          MPI_Datatype stype;
          int displs,i, rcounts;
          ...

          MPI_Comm_size(comm, &gsize);
          rbuf = (int )malloc(gsize * stride * sizeof(int));
          displs = (int)malloc(gsize * sizeof(int));
          rcounts = (int )malloc(gsize * sizeof(int));

          for (i=0; i<gsize; ++i) {
            displs[i] = i * stride;
            rcounts[i] = 100;
          }

          // Create datatype for 1 column of array
          MPI_Type_vector(100, 1, 150, MPI_INT, &stype);
          MPI_Type_commit( &stype );
          MPI_Gatherv(sendarray, 1, stype, rbuf, rcounts, displs, MPI_INT,
                      root, comm);

       Example 3: Process i sends (100-i) ints from the ith column of a 100 x
       150 int array, in C. It is received into a buffer with stride, as in
       the previous two examples.

          MPI_Comm comm;
          int gsize, sendarray[100][150], *sptr;
          int root, *rbuf, stride, myrank;
          MPI_Datatype stype;
          int displs, i, rcounts;
          ...

          MPI_Comm_size(comm, &gsize);
          MPI_Comm_rank( comm, &myrank );
          rbuf = (int)malloc(gsize * stride * sizeof(int));
          displs = (int)malloc(gsize * sizeof(int));
          rcounts = (int )malloc(gsize * sizeof(int));

          for (i=0; i<gsize; ++i) {
            displs[i] = i * stride;
            rcounts[i] = 100-i; // note change from previous example
          }

          // Create datatype for the column we are sending
          MPI_Type_vector(100-myrank, 1, 150, MPI_INT, &stype);
          MPI_Type_commit( &stype );
          // sptr is the address of start of "myrank" column
          sptr = &sendarray[0][myrank];
          MPI_Gatherv(sptr, 1, stype, rbuf, rcounts, displs, MPI_INT,
                      root, comm);

       Note that a different amount of data is received from each process.

       Example 4: Same as Example 3, but done in a different way at the
       sending end. We create a datatype that causes the correct striding at
       the sending end so that we read a column of a C array.

          MPI_Comm comm;
          int gsize, sendarray[100][150], *sptr;
          int root, *rbuf, stride, myrank, disp[2], blocklen[2];
          MPI_Datatype stype, type[2];
          int displs, i, rcounts;
          ...

          MPI_Comm_size(comm, &gsize);
          MPI_Comm_rank(comm, &myrank );
          rbuf = (int )alloc(gsize * stride * sizeof(int));
          displs = (int )malloc(gsize * sizeof(int));
          rcounts = (int)malloc(gsize * sizeof(int));

          for (i=0; i<gsize; ++i) {
            displs[i] = i* stride;
            rcounts[i] = 100-i;
          }
          // Create datatype for one int, with extent of entire row
          disp[0] = 0;
          disp[1] = 150 * sizeof(int);
          type[0] = MPI_INT;
          type[1] = MPI_UB;
          blocklen[0] = 1;
          blocklen[1] = 1;

          MPI_Type_struct( 2, blocklen, disp, type, &stype );
          MPI_Type_commit(&stype );
          sptr = &sendarray[0][myrank];
          MPI_Gatherv(sptr, 100-myrank, stype, rbuf, rcounts, displs, MPI_INT,
                      root, comm);

       Example 5: Same as Example 3 at sending side, but at receiving side we
       make the stride between received blocks vary from block to block.

          MPI_Comm comm;
          int gsize, sendarray[100][150], *sptr;
          int root, *rbuf, *stride, myrank, bufsize;
          MPI_Datatype stype;
          int *displs, i, *rcounts, offset;
          ...

          MPI_Comm_size( comm, &gsize);
          MPI_Comm_rank( comm, &myrank );
          de = (int )malloc(gsize * sizeof(int));
          ...
          // stride[i] for i = 0 to gsize-1 is set somehow

          // set up displs and rcounts vectors first
          displs = (int)malloc(gsize * sizeof(int));
          rcounts = (int )malloc(gsize * sizeof(int));
          offset = 0;

          for (i=0; i<gsize; ++i) {
            displs[i] = offset;
            offset += stride[i];
            rcounts[i] = 100-i;
          }

          // the required buffer size for rbuf is now easily obtained
          bufsize = displs[gsize-1]+rcounts[gsize-1];
          rbuf = (int )malloc(bufsize * sizeof(int));
          // Create datatype for the column we are sending
          MPI_Type_vector(100-myrank, 1, 150, MPI_INT, &stype);
          MPI_Type_commit( &stype );
          sptr = &sendarray[0][myrank];
          MPI_Gatherv(sptr, 1, stype, rbuf, rcounts, displs, MPI_INT,
                      root, comm);

       Example 6: Process i sends num ints from the ith column of a 100 x 150
       int array, in C. The complicating factor is that the various values of
       num are not known to root, so a separate gather must first be run to
       find these out. The data is placed contiguously at the receiving end.

          MPI_Comm comm;
          int gsize, sendarray[100][150], *sptr;
          int root, *rbuf, stride, myrank, disp[2], blocklen[2];
          MPI_Datatype stype,types[2];
          int *displs, i, *rcounts, num;
          ...

          MPI_Comm_size( comm, &gsize);
          MPI_Comm_rank( comm, &myrank );

          // First, gather nums to root
          rcounts = (int )malloc(gsize * sizeof(int));
          MPI_Gather( &num, 1, MPI_INT, rcounts, 1, MPI_INT, root, comm);
          // root now has correct rcounts, using these we set
          // displs[] so that data is placed contiguously (or concatenated) at receive end

          displs = (int)malloc(gsize * sizeof(int));
          displs[0] = 0;
          for (i=1; i<gsize; ++i) {
            displs[i] = displs[i-1]+rcounts[i-1];
          }

          // And, create receive buffer
          rbuf = (int *)malloc(gsize * (displs[gsize-1]+rcounts[gsize-1]) * sizeof(int));
          // Create datatype for one int, with extent of entire row
          disp[0] = 0;
          disp[1] = 150 * sizeof(int);
          type[0] = MPI_INT;
          type[1] = MPI_UB;
          blocklen[0] = 1;
          blocklen[1] = 1;
          MPI_Type_struct(2, blocklen, disp, type, &stype );
          MPI_Type_commit( &stype );
          sptr = &sendarray[0][myrank];
          MPI_Gatherv(sptr, num, stype, rbuf, rcounts, displs, MPI_INT, root, comm);

USE OF IN-PLACE OPTION
       The in-place option operates in the same way as it does for MPI_Gather.
       When the communicator is an intracommunicator, you can perform a gather
       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 sendbuf. In
       this case, sendcount and sendtype are ignored, and the contribution of
       the root process to the gathered vector is assumed to already be in the
       correct place in the receive buffer.

       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 gathers data from all the 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 send buffer argument of the
       processes in the first group must be consistent with the receive buffer
       argument of the root process 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:
          MPI_Gather

COPYRIGHT
       2003-2024, The Open MPI Community

                                 April 8, 2024                 MPI_IGATHERV(3)