MPI_ISCAN(3)					     Open MPI					      MPI_ISCAN(3)

       MPI_Scan	 <#mpi-scan>,  MPI_Iscan,  MPI_Scan_init  <#mpi-scan-init>  -  Computes an inclusive scan (partial
       reduction)

SYNTAX
   C Syntax
	  #include <mpi.h>

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

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

	  int MPI_Scan_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_SCAN(SENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR)
	       <type>  SENDBUF(*), RECVBUF(*)
	       INTEGER COUNT, DATATYPE, OP, COMM, IERROR

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

	  MPI_SCAN_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_Scan(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_Iscan(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_Scan_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).

       • info: Info (handle, persistent only)

OUTPUT PARAMETERS
       • recvbuf: Receive buffer (choice).

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

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

DESCRIPTION
       MPI_Scan <#mpi-scan> is used to perform an inclusive  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
       (inclusive).  The  type	of  operations supported, their semantics, and the constraints on send and receive
       buffers are as for MPI_Reduce <#mpi-reduce>.

EXAMPLE
       This example uses a user-defined operation to produce a segmented scan.	A segmented scan takes, as  input,
       a  set  of values and a set of logicals, where the logicals delineate the various segments of the scan. For
       example,

	  values     v1	     v2	     v3	     v4	     v5	     v6	     v7	     v8
	  logicals   0	     0	     1	     1	     1	     0	     0	     1
	  result     v1	   v1+v2     v3	   v3+v4  v3+v4+v5   v6	   v6+v7     v8

       The result for rank j is thus the sum v(i) + … + v(j), where i is the lowest rank such that for	all  ranks
       n, i <= n <= j, logical(n) = logical(j). The operator that produces this effect is

		[ u ]	  [ v ]	    [ w ]
		[   ]  o  [   ]	 =  [	]
		[ i ]	  [ j ]	    [ j ]

	  where

	      ( u + v if i = j w = ( ( v if i != j

       Note that this is a noncommutative operator. C code that implements it is given below.

	  typedef struct {
		  double val;
		  int log;
	  } SegScanPair;

	  /*
	   * the user-defined function
	   */
	  void segScan(SegScanPair *in, SegScanPair *inout, int *len,
		  MPI_Datatype *dptr)
	  {
		  int i;
		  SegScanPair c;

		  for (i = 0; i < *len; ++i) {
			  if (in->log == inout->log)
				  c.val = in->val + inout->val;
			  else
				  c.val = inout->val;

			  c.log = inout->log;
			  *inout = c;
			  in++;
			  inout++;
		  }
	  }

       Note  that  the	inout  argument	 to the user-defined function corresponds to the right-hand operand of the
       operator. When using this operator, we must be careful to specify that it  is  noncommutative,  as  in  the
       following:

	  int			  i, base;
	  SeqScanPair	  a, answer;
	  MPI_Op	  myOp;
	  MPI_Datatype	  type[2] = {MPI_DOUBLE, MPI_INT};
	  MPI_Aint		  disp[2];
	  int			  blocklen[2] = {1, 1};
	  MPI_Datatype	  sspair;

	  /*
	   * explain to MPI how type SegScanPair is defined
	   */
	  MPI_Get_address(a, disp);
	  MPI_Get_address(a.log, disp + 1);
	  base = disp[0];
	  for (i = 0; i < 2; ++i)
		  disp[i] -= base;
	  MPI_Type_struct(2, blocklen, disp, type, &sspair);
	  MPI_Type_commit(&sspair);

	  /*
	   * create the segmented-scan user-op
	   * noncommutative - set commute (arg 2) to 0
	   */
	  MPI_Op_create((MPI_User_function *)segScan, 0, &myOp);
	  ...
	  MPI_Scan(a, answer, 1, sspair, myOp, comm);

USE OF IN-PLACE OPTION
       When  the  communicator	is an intracommunicator, you can perform a scanning operation in place (the output
       buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of  the	sendbuf	 argument.
       The input data is taken from the receive buffer and replaced by the output data.

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_Exscan <#mpi-exscan>

	  • MPI_Op_create <#mpi-op-create>

	  • MPI_Reduce <#mpi-reduce>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					      MPI_ISCAN(3)
