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NAME

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

SYNOPSIS

       #include <mpi.h>
       int MPI_Allreduce(void *sbuf, void *rbuf, int count,
                        MPI_Datatype dtype, MPI_Op op, MPI_Comm comm)

INPUT PARAMETERS

       sbuf   - starting address of send buffer (choice)
       count  - number of elements in send buffer (integer)
       dtype  - data type of elements of send buffer (handle)
       op     - operation (handle)
       comm   - communicator (handle)

OUTPUT PARAMETER

       rbuf   - starting address of receive buffer (choice)

USAGE WITH IMPI EXTENSIONS

       This function has had the IMPI extensions implemented.  It is legal  to
       call this function on IMPI communicators.

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  (e.g.,
       MPI_ERRORS_RETURN ), then no error may be indicated.

       The reason for this is the performance problems that arise in  ensuring
       that all collective routines return the same error value.

ERRORS

       If an error occurs in an MPI function, the current MPI error handler is
       called to handle it.  By default, this error  handler  aborts  the  MPI
       job.   The  error  handler may be changed with MPI_Errhandler_set ; the
       predefined error handler MPI_ERRORS_RETURN may be used to  cause  error
       values  to  be  returned  (in C and Fortran; this error handler is less
       useful in with the C++ MPI  bindings.   The  predefined  error  handler
       MPI::ERRORS_THROW_EXCEPTIONS  should  be used in C++ if the error value
       needs to be recovered).  Note that MPI does not guarantee that  an  MPI
       program can continue past an error.

       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.   The  C++  bindings  for  MPI do not return error
       values; instead, error values are communicated by  throwing  exceptions
       of  type  MPI::Exception  (but  not  by  default).  Exceptions are only
       thrown if the error value is not MPI::SUCCESS .

       Note that if the MPI::ERRORS_RETURN handler is set in  C++,  while  MPI
       functions  will  return  upon an error, there will be no way to recover
       what the actual error value was.
       MPI_ERR_COMM
              - Invalid communicator.   A  common  error  is  to  use  a  null
              communicator in a call (not even allowed in MPI_Comm_rank ).
       MPI_ERR_OTHER
              - A collective implementation was not able to be located at run-
              time for this communicator.
       MPI_ERR_OTHER
              - A communicator that contains some  non-local  IMPI  procs  was
              used for some function which has not yet had the IMPI extensions
              implemented  yet.   For  example,  most  collectives   on   IMPI
              communicators have not been implemented yet.
       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  .   Additionally,  only  certain
              datatypes  are  alloed  with  given  predefined operations.  See
              MPI-1, section 4.9.2.

MORE INFORMATION

       For more information, please see the official MPI Forum web site, which
       contains  the  text  of  both  the  MPI-1  and  MPI-2 standards.  These
       documents contain detailed information about each MPI function (most of
       which is not duplicated in these man pages).

       http://www.mpi-forum.org/

ACKNOWLEDGEMENTS

       The  LAM Team would like the thank the MPICH Team for the handy program
       to       generate        man        pages        ("doctext"        from
       ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz     ),    the    initial
       formatting, and some initial text for most of the MPI-1 man pages.

LOCATION

       allreduce.c