MPI_IEXSCAN(3)					     Open MPI					    MPI_IEXSCAN(3)

       MPI_Exscan <#mpi-exscan>, MPI_Iexscan - Computes an exclusive scan (partial reduction)

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Exscan(const void *sendbuf, void *recvbuf, int count,
	       MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

	  int MPI_Iexscan(const void *sendbuf, void *recvbuf, int count,
	       MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
	       MPI_Request *request)

	  int MPI_Exscan_init(const void *sendbuf, void *recvbuf, int count,
	       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_EXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER COUNT, DATATYPE, OP, COMM, IERROR

	  MPI_IEXSCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, REQUEST, IERROR)
	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER COUNT, DATATYPE, OP, COMM, REQUEST, IERROR

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

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

	  MPI_Iexscan(sendbuf, recvbuf, count, datatype, op, comm, request, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: count
	       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_Exscan_init(sendbuf, recvbuf, count, datatype, op, comm, info, request, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: count
	       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: Send buffer (choice).

       • count: Number of elements in input buffer (integer).

       • datatype: Data type of elements of input buffer (handle).

       • op: Operation (handle).

       • comm: Communicator (handle).

OUTPUT PARAMETERS
       • recvbuf: Receive buffer (choice).

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

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

DESCRIPTION
       MPI_Exscan  <#mpi-exscan>  is  used to perform an exclusive prefix reduction on data distributed across the
       calling processes. The operation returns, in the	 recvbuf  of  the  process  with  rank	i,  the	 reduction
       (calculated  according to the function op) of the values in the sendbufs of processes with ranks 0, …, i-1.
       Compare this with the functionality of MPI_Scan <#mpi-scan>, which  calculates  over  the  range	 0,  …,	 i
       (inclusive).  The  type	of  operations supported, their semantics, and the constraints on send and receive
       buffers are as for MPI_Reduce <#mpi-reduce>.

       The value in recvbuf on process 0 is undefined and unreliable as recvbuf is not significant for process	0.
       The value of recvbuf on process 1 is always the value in sendbuf on process 0.

USE OF IN-PLACE OPTION
       The  `in place’ option for intracommunicators is specified by passing MPI_IN_PLACE in the sendbuf argument.
       In this case, the input data is taken from the receive buffer, and replaced by the output data.

       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.

NOTES
       MPI  does  not  specify	which  process computes which operation. In particular, both processes 0 and 1 may
       participate in the computation even  though  the	 results  for  both  processes’	 recvbuf  are  degenerate.
       Therefore, all processes, including 0 and 1, must provide the same op.

       It  can	be  argued,  from  a  mathematical perspective, that the definition of MPI_Exscan <#mpi-exscan> is
       unsatisfactory because the output at process 0  is  undefined.  The  “mathematically  correct”  output  for
       process	0 would be the unit element of the reduction operation. However, such a definition of an exclusive
       scan would not work with user-defined op functions as there is no way for MPI to “know” the unit value  for
       these custom operations.

NOTES ON COLLECTIVE OPERATIONS
       The  reduction  functions of type 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  the
       error  handler  is  changed  from MPI_ERRORS_ARE_FATAL to something else (e.g., 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.

       See also:

	  • MPI_Op_create <#mpi-op-create>

	  • MPI_Reduce <#mpi-reduce>

	  • MPI_Scan <#mpi-scan>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					    MPI_IEXSCAN(3)
