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.TH "MPI_ISCAN" "3" "May 30, 2025" "" "Open MPI"
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.INDENT 3.5
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.sp
\fI\%MPI_Scan\fP, \fI\%MPI_Iscan\fP, \fI\%MPI_Scan_init\fP \- Computes an inclusive scan
(partial reduction)
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#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)
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
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
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran 2008 Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
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
.ft P
.fi
.UNINDENT
.UNINDENT
.SH INPUT PARAMETERS
.INDENT 0.0
.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).
.IP \(bu 2
\fBinfo\fP: Info (handle, persistent only)
.UNINDENT
.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).
.UNINDENT
.SH DESCRIPTION
.sp
\fI\%MPI_Scan\fP is used to perform an inclusive 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 (inclusive). The type of operations
supported, their semantics, and the constraints on send and receive
buffers are as for \fI\%MPI_Reduce\fP\&.
.SH EXAMPLE
.sp
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,
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.INDENT 3.5
.sp
.nf
.ft C
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
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.sp
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
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.INDENT 3.5
.sp
.nf
.ft C
      [ u ]     [ v ]     [ w ]
      [   ]  o  [   ]  =  [   ]
      [ i ]     [ j ]     [ j ]

where

    ( u + v if i = j w = ( ( v if i != j
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.sp
Note that this is a noncommutative operator. C code that implements it
is given below.
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.INDENT 3.5
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.nf
.ft C
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++;
        }
}
.ft P
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.sp
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:
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.INDENT 3.5
.sp
.nf
.ft C
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);
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.SH USE OF IN-PLACE OPTION
.sp
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 \fIsendbuf\fP
argument. The input data is taken from the receive buffer and replaced
by the output data.
.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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.sp
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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.sp
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_Exscan\fP
.IP \(bu 2
\fI\%MPI_Op_create\fP
.IP \(bu 2
\fI\%MPI_Reduce\fP
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.SH COPYRIGHT
2003-2025, The Open MPI Community
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