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<TITLE>MPI_Reduce</TITLE>
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MPI_Reduce
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Reduces values on all processes to a single value
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Synopsis
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int MPI_Reduce(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype,
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MPI_Op op, int root, MPI_Comm comm)
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Input Parameters
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sendbuf address of send buffer (choice)
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count number of elements in send buffer (integer)
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datatype data type of elements of send buffer (handle)
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op reduce operation (handle)
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root rank of root process (integer)
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comm communicator (handle)
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Output Parameters
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recvbuf address of receive buffer (choice,
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significant only at <tt>root</tt>)
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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 <tt>malloc
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</tt>or other non-MPICH runtime routines that are themselves not interrupt-safe.
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Notes for Fortran
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All MPI routines in Fortran (except for <tt>MPI_WTIME</tt> and <tt>MPI_WTICK</tt>) have
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an additional argument <tt>ierr</tt> at the end of the argument list. <tt>ierr
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</tt>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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<tt>call</tt> statement.
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All MPI objects (e.g., <tt>MPI_Datatype</tt>, <tt>MPI_Comm</tt>) are of type <tt>INTEGER
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</tt>in Fortran.
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Notes on collective operations
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The reduction functions (<tt>MPI_Op</tt>) 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 <tt>MPI_Abort
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</tt>or silently skip the problem. Thus, if you change the error handler from
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<tt>MPI_ERRORS_ARE_FATAL</tt> to something else, for example, <tt>MPI_ERRORS_RETURN</tt>,
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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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Errors
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All MPI routines (except <tt>MPI_Wtime</tt> and <tt>MPI_Wtick</tt>) 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 <tt>MPI_Comm_set_errhandler</tt> (for communicators),
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<tt>MPI_File_set_errhandler</tt> (for files), and <tt>MPI_Win_set_errhandler</tt> (for
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RMA windows). The MPI-1 routine <tt>MPI_Errhandler_set</tt> may be used but
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its use is deprecated. The predefined error handler
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<tt>MPI_ERRORS_RETURN</tt> may be used to cause error values to be returned.
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Note that MPI does not 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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MPI_SUCCESS No error; MPI routine completed successfully.
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MPI_ERR_COMM Invalid communicator. A common error is to use a null
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communicator in a call (not even allowed in <tt>MPI_Comm_rank</tt>).
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MPI_ERR_COUNT Invalid count argument. Count arguments must be
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non-negative; a count of zero is often valid.
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MPI_ERR_TYPE Invalid datatype argument. Additionally, this error can
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occur if an uncommitted MPI_Datatype (see <tt>MPI_Type_commit</tt>) is used
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in a communication call.
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MPI_ERR_BUFFER Invalid buffer pointer. Usually a null buffer where
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one is not valid.
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MPI_ERR_BUFFER This error class is associcated with an error code that
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indicates that two buffer arguments are aliased; 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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