plucky (3) MPI_Scan.openmpi.3.gz
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 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.
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 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_thread, after
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 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_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 was called on that communicator. If called on a
window or file, acts as if 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 then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler
Note that MPI does not guarantee that an MPI program can continue past an error.
See the MPI man page for a full list of MPI error codes.
See the Error Handling section of the MPI-3.1 standard for more information.
SEE ALSO:
• MPI_Exscan
• MPI_Op_create
• MPI_Reduce
COPYRIGHT
2003-2025, The Open MPI Community
Feb 17, 2025 MPI_SCAN(3)