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NAME

       MPI_Gatherv, MPI_Igatherv - 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)

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

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

C++ Syntax

       #include <mpi.h>
       void MPI::Comm::Gatherv(const void* sendbuf, int sendcount,
            const MPI::Datatype& sendtype, void* recvbuf,
            const int recvcounts[], const int displs[],
            const MPI::Datatype& recvtype, int root) const = 0

INPUT PARAMETERS

       sendbuf   Starting address of send buffer (choice).

       sendcount Number of elements in send buffer (integer).

       sendtype  Datatype of send buffer elements (handle).

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

       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).

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

       root      Rank of receiving process (integer).

       comm      Communicator (handle).

OUTPUT PARAMETERS

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

       request   Request (handle, non-blocking only).

       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 );

         stride = (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 value of the function and
       Fortran routines in the last argument. C++ functions do not return errors. If the  default
       error  handler  is  set  to  MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception
       mechanism will be used to throw an MPI::Exception object.

       Before the error value is returned, the current MPI error handler is called.  By  default,
       this  error  handler aborts the MPI job, except for I/O function errors. The error handler
       may   be   changed   with   MPI_Comm_set_errhandler;   the   predefined   error    handler
       MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not
       guarantee that an MPI program can continue past an error.

SEE ALSO

       MPI_Gather
       MPI_Scatter
       MPI_Scatterv