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NAME

       MPI_Barrier -  Blocks until all process have reached this routine.

SYNOPSIS

       #include <mpi.h>
       int MPI_Barrier(MPI_Comm comm)

INPUT PARAMETERS

       comm   - communicator (handle)

NOTES

       Blocks  the  caller  until  all  group members have called it; the call
       returns at any process only after all group members  have  entered  the
       call.

USAGE WITH IMPI EXTENSIONS

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

ALGORITHM

       For 4 or less ranks, a linear algorithm is used,  where  rank  0  first
       receives  synchronization  message  from  each other rank.  Rank 0 then
       sends out a synchronization message to each other rank.

       If more than 4 ranks are involved, a tree-based algorithm  is  used  to
       receive  and then send the synchronization messages to and from rank 0.

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.

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_SUCCESS
              - No error; MPI routine completed successfully.
       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.

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

       barrier.c