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MPI_Allreduce(3)		      MPI		      MPI_Allreduce(3)

NAME
       MPI_Allreduce -	Combines values	from all processes and distributes the
       result back to all processes

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

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

OUTPUT PARAMETER
       recvbuf
	      -	starting address of receive buffer (choice)

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_ERR_BUFFER
	      -	Invalid	buffer pointer.	 Usually a null	buffer	where  one  is
	      not valid.
       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_OP
	      -	 Invalid  operation.  MPI operations (objects of type MPI_Op )
	      must either be one of the	predefined operations (e.g., MPI_SUM )
	      or created with MPI_Op_create .

       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 ).

LOCATION
       src/mpi/coll/allreduce.c

				   9/20/2012		      MPI_Allreduce(3)

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

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