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MPI_Reduce(3)			      MPI			 MPI_Reduce(3)

NAME
       MPI_Reduce -  Reduces values on all processes to	a single value

SYNOPSIS
       int MPI_Reduce(MPICH2_CONST void	*sendbuf, void *recvbuf, int count, MPI_Datatype datatype,
		      MPI_Op op, int root, MPI_Comm comm)

INPUT PARAMETERS
       sendbuf
	      -	address	of send	buffer (choice)
       count  -	number of elements in send buffer (integer)
       datatype
	      -	data type of elements of send buffer (handle)
       op     -	reduce operation (handle)
       root   -	rank of	root process (integer)
       comm   -	communicator (handle)

OUTPUT PARAMETER
       recvbuf
	      -	address	of receive buffer (choice, significant only at root )

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-
       safe.

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
       Fortran.

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.

ERRORS
       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 com-
       municators),  MPI_File_set_errhandler  (for files), and MPI_Win_set_er-
       rhandler	(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	 when-
       ever possible.

       MPI_SUCCESS
	      -	No error; MPI routine completed	successfully.
       MPI_ERR_COMM
	      -	 Invalid communicator.	A common error is to use a null	commu-
	      nicator in a call	(not even allowed in MPI_Comm_rank ).
       MPI_ERR_COUNT
	      -	Invalid	count argument.	 Count arguments must be non-negative;
	      a	count of zero is often valid.
       MPI_ERR_TYPE
	      -	Invalid	datatype argument.  May	be an uncommitted MPI_Datatype
	      (see MPI_Type_commit ).
       MPI_ERR_BUFFER
	      -	Invalid	buffer pointer.	 Usually a null	buffer	where  one  is
	      not valid.
       MPI_ERR_BUFFER
	      -	 This error class is associcated with an error code that indi-
	      cates that two buffer arguments are aliased ; that is,  the  de-
	      scribe 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 For-
	      tran, the	MPI Forum adopted the more restrictive requirements of
	      Fortran).

LOCATION
       src/mpi/coll/reduce.c

				   9/20/2012			 MPI_Reduce(3)

NAME | SYNOPSIS | INPUT PARAMETERS | OUTPUT PARAMETER | THREAD AND INTERRUPT SAFETY | NOTES FOR FORTRAN | NOTES ON COLLECTIVE OPERATIONS | ERRORS | LOCATION

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