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MPI_Exscan(3) MPI MPI_Exscan(3)
MPI_Exscan - Computes the exclusive scan (partial reductions) of data
on a collection of processes
int MPI_Exscan(MPICH2_CONST void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype,
MPI_Op op, MPI_Comm comm)
- starting address of send buffer (choice)
count - number of elements in input buffer (integer)
- data type of elements of input buffer (handle)
op - operation (handle)
comm - communicator (handle)
- starting address of receive buffer (choice)
MPI_Exscan is like MPI_Scan , except that the contribution from the
calling process is not included in the result at the calling process
(it is contributed to the subsequent processes, of course).
THREAD AND INTERRUPT SAFETY
This routine is thread-safe. This means that this routine may be
safely used by multiple threads without the need for any user-provided
thread locks. However, the routine is not interrupt safe. Typically,
this is due to the use of memory allocation routines such as malloc or
other non-MPICH runtime routines that are themselves not interrupt-
NOTES FOR FORTRAN
All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have
an additional argument ierr at the end of the argument list. ierr is
an integer and has the same meaning as the return value of the routine
in C. In Fortran, MPI routines are subroutines, and are invoked with
the call statement.
All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in
NOTES ON COLLECTIVE OPERATIONS
The reduction functions ( 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 you change the
error handler from MPI_ERRORS_ARE_FATAL to something else, for example,
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.
All MPI routines (except MPI_Wtime and MPI_Wtick ) return an error
value; C routines as the value of the function and Fortran routines in
the last argument. Before the value is returned, the current MPI error
handler is called. By default, this error handler aborts the MPI job.
The error handler may be changed with MPI_Comm_set_errhandler (for
communicators), MPI_File_set_errhandler (for files), and
MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine
MPI_Errhandler_set may be used but its use is deprecated. The
predefined error handler MPI_ERRORS_RETURN may be used to cause error
values to be returned. Note that MPI does not guarentee that an MPI
program can continue past an error; however, MPI implementations will
attempt to continue whenever possible.
- No error; MPI routine completed successfully.
- Invalid communicator. A common error is to use a null
communicator in a call (not even allowed in MPI_Comm_rank ).
- Invalid count argument. Count arguments must be non-negative;
a count of zero is often valid.
- Invalid datatype argument. May be an uncommitted MPI_Datatype
(see MPI_Type_commit ).
- Invalid buffer pointer. Usually a null buffer where one is
- This error class is associcated with an error code that
indicates that two buffer arguments are aliased ; that is, the
describe overlapping storage (often the exact same storage).
This is prohibited in MPI (because it is prohibited by the
Fortran standard, and rather than have a separate case for C and
Fortran, the MPI Forum adopted the more restrictive requirements