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

LAM/MPI 7.1.4                                       6/24/2006                                   MPI_Allreduce(3)