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

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