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.TH MPI_Exscan 3 "11/12/2019" " " "MPI"
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.SH NAME
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MPI_Exscan \- Computes the exclusive scan (partial reductions) of data on a collection of processes
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.SH SYNOPSIS
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.nf
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int MPI_Exscan(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype,
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MPI_Op op, MPI_Comm comm)
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.fi
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.SH INPUT PARAMETERS
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.PD 0
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.TP
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.B sendbuf
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- starting address of send buffer (choice)
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.PD 1
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.PD 0
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.TP
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.B count
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- number of elements in input buffer (integer)
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.PD 1
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.PD 0
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.TP
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.B datatype
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- data type of elements of input buffer (handle)
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.PD 1
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.PD 0
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.TP
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.B op
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- operation (handle)
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.PD 1
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.PD 0
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.TP
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.B comm
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- communicator (handle)
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.PD 1
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.SH OUTPUT PARAMETERS
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.PD 0
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.TP
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.B recvbuf
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- starting address of receive buffer (choice)
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.PD 1
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.SH NOTES
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.I MPI_Exscan
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is like
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.I MPI_Scan
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, except that the contribution from the
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calling process is not included in the result at the calling process
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(it is contributed to the subsequent processes, of course).
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.SH THREAD AND INTERRUPT SAFETY
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This routine is thread-safe. This means that this routine may be
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safely used by multiple threads without the need for any user-provided
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thread locks. However, the routine is not interrupt safe. Typically,
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this is due to the use of memory allocation routines such as
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.I malloc
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or other non-MPICH runtime routines that are themselves not interrupt-safe.
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.SH NOTES FOR FORTRAN
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All MPI routines in Fortran (except for
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.I MPI_WTIME
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and
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.I MPI_WTICK
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) have
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an additional argument
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.I ierr
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at the end of the argument list.
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.I ierr
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is an integer and has the same meaning as the return value of the routine
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in C. In Fortran, MPI routines are subroutines, and are invoked with the
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.I call
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statement.
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All MPI objects (e.g.,
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.I MPI_Datatype
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,
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.I MPI_Comm
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) are of type
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.I INTEGER
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in Fortran.
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.SH NOTES ON COLLECTIVE OPERATIONS
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The reduction functions (
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.I MPI_Op
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) do not return an error value. As a result,
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if the functions detect an error, all they can do is either call
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.I MPI_Abort
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or silently skip the problem. Thus, if you change the error handler from
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.I MPI_ERRORS_ARE_FATAL
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to something else, for example,
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.I MPI_ERRORS_RETURN
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,
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then no error may be indicated.
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The reason for this is the performance problems in ensuring that
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all collective routines return the same error value.
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.SH ERRORS
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All MPI routines (except
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.I MPI_Wtime
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and
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.I MPI_Wtick
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) return an error value;
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C routines as the value of the function and Fortran routines in the last
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argument. Before the value is returned, the current MPI error handler is
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called. By default, this error handler aborts the MPI job. The error handler
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may be changed with
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.I MPI_Comm_set_errhandler
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(for communicators),
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.I MPI_File_set_errhandler
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(for files), and
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.I MPI_Win_set_errhandler
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(for
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RMA windows). The MPI-1 routine
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.I MPI_Errhandler_set
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may be used but
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its use is deprecated. The predefined error handler
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.I MPI_ERRORS_RETURN
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may be used to cause error values to be returned.
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Note that MPI does
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.B not
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guarentee that an MPI program can continue past
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an error; however, MPI implementations will attempt to continue whenever
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possible.
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.PD 0
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.TP
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.B MPI_SUCCESS
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- No error; MPI routine completed successfully.
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.PD 1
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.PD 0
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.TP
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.B MPI_ERR_COMM
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- Invalid communicator. A common error is to use a null
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communicator in a call (not even allowed in
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.I MPI_Comm_rank
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).
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.PD 1
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.TP
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.B MPI_ERR_COUNT
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- Invalid count argument. Count arguments must be
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non-negative; a count of zero is often valid.
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.PD 1
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.PD 0
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.TP
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.B MPI_ERR_TYPE
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- Invalid datatype argument. Additionally, this error can
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occur if an uncommitted MPI_Datatype (see
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.I MPI_Type_commit
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) is used
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in a communication call.
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.PD 1
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.B MPI_ERR_BUFFER
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- Invalid buffer pointer. Usually a null buffer where
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one is not valid.
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.PD 1
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.B MPI_ERR_BUFFER
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- This error class is associcated with an error code that
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indicates that two buffer arguments are
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.B aliased
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; that is, the
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describe overlapping storage (often the exact same storage). This
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is prohibited in MPI (because it is prohibited by the Fortran
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standard, and rather than have a separate case for C and Fortran, the
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MPI Forum adopted the more restrictive requirements of Fortran).
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.PD 1
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