MPI_GATHER_INIT(3)				     Open MPI					MPI_GATHER_INIT(3)

       MPI_Gather  <#mpi-gather>,  MPI_Igather	<#mpi-igather>,	 MPI_Gather_init  - 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)

	  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 <#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>/MPI_Init_thread <#mpi-init-thread>,
       after MPI_Finalize <#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 <#
       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>/MPI_Comm_spawn_multiple	<#
       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 <#mpi-abort> was called on that communicator. If called
	 on  a window or file, acts as if MPI_Abort <#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	<#mpi-comm-create-errhandler>	  then	   MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	 <#mpi-file-create-errhandler>	   then	    MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       See also:

	  • MPI_Gatherv <#mpi-gatherv>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					MPI_GATHER_INIT(3)
