NAME

       MPI_Gather, MPI_Igather - Gathers values from a group of processes.

SYNOPSIS

C Syntax

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

       int MPI_Igather(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
            void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
            MPI_Comm comm, MPI_Request *request)

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

       MPI_IGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
                 RECVTYPE, ROOT, COMM, REQUEST, IERROR)
            <type>    SENDBUF(*), RECVBUF(*)
            INTEGER   SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
            INTEGER   COMM, REQUEST, 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).

       request   Request (handle, non-blocking only).

       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