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.TH "MPI_EXSCAN_INIT" "3" "May 30, 2025" "" "Open MPI"
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.INDENT 3.5
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.sp
\fI\%MPI_Exscan\fP, \fI\%MPI_Iexscan\fP \- Computes an exclusive scan (partial
reduction)
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
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.nf
.ft C
#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)
.ft P
.fi
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.SS Fortran Syntax
.INDENT 0.0
.INDENT 3.5
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.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
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
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.SS Fortran 2008 Syntax
.INDENT 0.0
.INDENT 3.5
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.nf
.ft C
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
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.fi
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.SH INPUT PARAMETERS
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.IP \(bu 2
\fBsendbuf\fP: Send buffer (choice).
.IP \(bu 2
\fBcount\fP: Number of elements in input buffer (integer).
.IP \(bu 2
\fBdatatype\fP: Data type of elements of input buffer (handle).
.IP \(bu 2
\fBop\fP: Operation (handle).
.IP \(bu 2
\fBcomm\fP: Communicator (handle).
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.SH OUTPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBrecvbuf\fP: Receive buffer (choice).
.IP \(bu 2
\fBrequest\fP: Request (handle, non\-blocking only).
.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
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.SH DESCRIPTION
.sp
\fI\%MPI_Exscan\fP is used to perform an exclusive prefix reduction on data
distributed across the calling processes. The operation returns, in the
\fIrecvbuf\fP of the process with rank i, the reduction (calculated
according to the function \fIop\fP) of the values in the \fIsendbuf\fPs of
processes with ranks 0, …, i\-1. Compare this with the functionality of
\fI\%MPI_Scan\fP, 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 \fI\%MPI_Reduce\fP\&.
.sp
The value in \fIrecvbuf\fP on process 0 is undefined and unreliable as
\fIrecvbuf\fP is not significant for process 0. The value of \fIrecvbuf\fP on
process 1 is always the value in \fIsendbuf\fP on process 0.
.SH USE OF IN-PLACE OPTION
.sp
The \(gain place’ option for intracommunicators is specified by passing
MPI_IN_PLACE in the \fIsendbuf\fP argument. In this case, the input data is
taken from the receive buffer, and replaced by the output data.
.sp
Note that MPI_IN_PLACE is a special kind of value; it has the same
restrictions on its use as MPI_BOTTOM.
.sp
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.
.SH NOTES
.sp
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’ \fIrecvbuf\fP are degenerate.
Therefore, all processes, including 0 and 1, must provide the same \fIop\fP\&.
.sp
It can be argued, from a mathematical perspective, that the definition
of \fI\%MPI_Exscan\fP 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 \fIop\fP functions as
there is no way for MPI to “know” the unit value for these custom
operations.
.SH NOTES ON COLLECTIVE OPERATIONS
.sp
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 \fI\%MPI_Abort\fP 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.
.sp
The reason for this is the performance problems in ensuring that all
collective routines return the same error value.
.SH ERRORS
.sp
Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
.sp
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
\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
Model exclusively) the error raises the initial error handler. The initial
error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
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.
.sp
Open MPI includes three predefined error handlers that can be used:
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.IP \(bu 2
\fBMPI_ERRORS_ARE_FATAL\fP
Causes the program to abort all connected MPI processes.
.IP \(bu 2
\fBMPI_ERRORS_ABORT\fP
An error handler that can be invoked on a communicator,
window, file, or session. When called on a communicator, it
acts as if \fI\%MPI_Abort\fP was called on that communicator. If
called on a window or file, acts as if \fI\%MPI_Abort\fP 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.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
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MPI applications can also implement their own error handlers by calling:
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.IP \(bu 2
\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
.IP \(bu 2
\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
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Note that MPI does not guarantee that an MPI program can continue past
an error.
.sp
See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
.sp
See the Error Handling section of the MPI\-3.1 standard for
more information.
.sp
\fBSEE ALSO:\fP
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.IP \(bu 2
\fI\%MPI_Op_create\fP
.IP \(bu 2
\fI\%MPI_Reduce\fP
.IP \(bu 2
\fI\%MPI_Scan\fP
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.SH COPYRIGHT
2003-2025, The Open MPI Community
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