MPI_Gather(3)                      Open MPI                      MPI_Gather(3)



NAME
       MPI_Gather - Gathers values from a group of processes.


SYNOPSIS
C Syntax
       #include <mpi.h>
       int MPI_Gather(void *sendbuf, int sendcount, MPI_Datatype sendtype,
            void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
            MPI_Comm comm)


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_GATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
                 RECVTYPE, ROOT, COMM, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
            INTEGER   COMM, IERROR


C++ Syntax
       #include <mpi.h>
       void MPI::Comm::Gather(const void* sendbuf, int sendcount,
            const MPI::Datatype& sendtype, void* recvbuf,
            int recvcount, 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).

       recvcount Number of elements for any single receive (integer,
                 significant only at root).

       recvtype  Datatype of recvbuffer elements (handle, significant only at
                 root).

       root      Rank of receiving process (integer).

       comm      Communicator (handle).


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

       IERROR    Fortran only: Error status (integer).


DESCRIPTION
       Each process (root process included) sends the contents of its send
       buffer to the root process. The root process receives the messages and
       stores them in rank order. The outcome is as if each of the n processes
       in the group (including the root process) had executed a call to

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

       and the root had executed n calls to

           MPI_Recv(recfbuf + i * recvcount * extent(recvtype),
                    recvcount, recvtype, i, ...)

       where extent(recvtype) is the type extent obtained from a call to
       MPI_Type_extent().

       An alternative description is that the n messages sent by the processes
       in the group are concatenated in rank order, and the resulting message
       is received by the root as if by a call to MPI_RECV(recvbuf, recvcount
       * n, recvtype, . . . ).

       The receive buffer is ignored for all nonroot processes.

       General, derived datatypes are allowed for both sendtype and recvtype.
       The type signature of sendcount, sendtype on process i must be equal to
       the type signature of recvcount, 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.

       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 and types should not cause any location on
       the root to be written more than once. Such a call is erroneous.

       Note that the recvcount argument at the root indicates the number of
       items it receives from each process, not the total number of items it
       receives.

       Example 1:  Gather 100 ints from every process in group to root.

         MPI_Comm comm;
             int gsize,sendarray[100];
             int root, *rbuf;
             ...
             MPI_Comm_size( comm, &gsize);
             rbuf = (int *)malloc(gsize*100*sizeof(int));
             MPI_Gather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, root, comm);


       Example 2: Previous example modified -- only the root allocates memory
       for the receive buffer.

         MPI_Comm comm;
             int gsize,sendarray[100];
             int root, myrank, *rbuf;
             ...
             MPI_Comm_rank( comm, myrank);
             if ( myrank == root) {
                MPI_Comm_size( comm, &gsize);
                rbuf = (int *)malloc(gsize*100*sizeof(int));
                }
             MPI_Gather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, root, comm);

       Example 3: Do the same as the previous example, but use a derived
       datatype. Note that the type cannot be the entire set of  gsize * 100
       ints since type matching is defined pairwise between the root and each
       process in the gather.

         MPI_Comm comm;
             int gsize,sendarray[100];
             int root, *rbuf;
             MPI_Datatype rtype;
             ...
             MPI_Comm_size( comm, &gsize);
             MPI_Type_contiguous( 100, MPI_INT, &rtype );
             MPI_Type_commit( &rtype );
             rbuf = (int *)malloc(gsize*100*sizeof(int));
             MPI_Gather( sendarray, 100, MPI_INT, rbuf, 1, rtype, root, comm);


USE OF IN-PLACE OPTION
       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 the MPI man page for a full list of MPI error codes.


SEE ALSO
       MPI_Gatherv
       MPI_Scatter
       MPI_Scatterv




1.4.1                            Jan 14, 2010                    MPI_Gather(3)
