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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)

          int MPI_Gather_init(const void *sendbuf, int sendcount, 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_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

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

   Fortran 2008 Syntax
          USE mpi_f08

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

          MPI_Igather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
                  root, comm, request, ierror)
              TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
              TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
              INTEGER, INTENT(IN) :: sendcount, 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_Gather_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
                  root, comm, info, request, ierror)
              TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
              TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
              INTEGER, INTENT(IN) :: sendcount, 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

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).

       • info : Info (handle, persistent only).

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  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:

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

       • 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.

       • MPI_ERRORS_RETURN Returns an error code to the application.

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

       • MPI_Comm_create_errhandler then MPI_Comm_set_errhandlerMPI_File_create_errhandler then MPI_File_set_errhandlerMPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_initMPI_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_Gatherv

COPYRIGHT

       2003-2025, The Open MPI Community

                                                  Feb 17, 2025                                MPI_GATHER_INIT(3)