MPI_REDUCE_SCATTER_INIT(3)			     Open MPI				MPI_REDUCE_SCATTER_INIT(3)

       MPI_Reduce_scatter	  <#mpi-reduce-scatter>,	MPI_Ireduce_scatter	   <#mpi-ireduce-scatter>,
       MPI_Reduce_scatter_init - Combines values and scatters the results.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Reduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
	       MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

	  int MPI_Ireduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
	       MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Request *request)

	  int MPI_Reduce_scatter_init(const void *sendbuf, void *recvbuf,
	       const int recvcounts[], MPI_Datatype datatype, MPI_Op op,
	       MPI_Comm comm, MPI_Info info, MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_REDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
		       COMM, IERROR)
	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, IERROR

	  MPI_IREDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
		       COMM, REQUEST, IERROR)
	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, REQUEST, IERROR

	  MPI_REDUCE_SCATTER_INIT(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
		       COMM, INFO, REQUEST, IERROR)
	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, INFO, REQUEST, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Reduce_scatter(sendbuf, recvbuf, recvcounts, datatype, op, comm,
		       ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
	       TYPE(*), DIMENSION(..) :: recvbuf
	       INTEGER, INTENT(IN) :: recvcounts(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: datatype
	       TYPE(MPI_Op), INTENT(IN) :: op
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Ireduce_scatter(sendbuf, recvbuf, recvcounts, datatype, op, comm,
		       request, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: datatype
	       TYPE(MPI_Op), INTENT(IN) :: op
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Reduce_scatter_init(sendbuf, recvbuf, recvcounts, datatype, op, comm,
		       info, request, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*)
	       TYPE(MPI_Datatype), INTENT(IN) :: datatype
	       TYPE(MPI_Op), INTENT(IN) :: op
	       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).

       • recvcounts: Integer array specifying the number of elements in result distributed to each process.  Array
	 must be identical on all calling processes.

       • datatype: Datatype of elements of input buffer (handle).

       • op: Operation (handle).

       • comm: Communicator (handle).

       • info: Info (handle, persistent).

OUTPUT PARAMETERS
       • recvbuf: Starting address of receive buffer (choice).

       • request: Request (handle, non-blocking only).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       MPI_Reduce_scatter  <#mpi-reduce-scatter>  first	 does  an  element-wise	 reduction  on	vector	of count =
       S(i)recvcounts[i] elements in the send buffer defined by sendbuf, count, and datatype. Next, the	 resulting
       vector  of  results  is	split  into  n disjoint segments, where n is the number of processes in the group.
       Segment i contains recvcounts[i] elements.  The ith segment is sent to process i and stored in the  receive
       buffer defined by recvbuf, recvcounts[i], and datatype.

USE OF IN-PLACE OPTION
       When  the  communicator	is  an intracommunicator, you can perform a reduce-scatter operation in-place (the
       output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of the  sendbuf.	In
       this  case, the input data is taken from the top of the receive buffer. The area occupied by the input data
       may be either longer or shorter than the data filled by the output data.

WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
       When the communicator is an inter-communicator, the reduce-scatter operation occurs in two  phases.  First,
       the result of the reduction performed on the data provided by the processes in the first group is scattered
       among  the  processes  in  the  second  group. Then the reverse occurs: the reduction performed on the data
       provided by the processes in the second group is scattered among the processes in  the  first  group.   For
       each group, all processes provide the same recvcounts argument, and the sum of the recvcounts values should
       be the same for both groups.

NOTES ON COLLECTIVE OPERATIONS
       The  reduction  functions  ( MPI_Op ) do not return an error value. As a result, if the functions detect an
       error, all they can do is either call MPI_Abort <#mpi-abort> or silently skip the  problem.  Thus,  if  you
       change the error handler from MPI_ERRORS_ARE_FATAL to something else, for example, MPI_ERRORS_RETURN , then
       no error may be indicated.

       The  reason  for	 this is the performance problems in ensuring that all collective routines return the same
       error value.

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.

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				MPI_REDUCE_SCATTER_INIT(3)
