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       MPI_Win_lock -  Begin an RMA access epoch at the target process.


       int MPI_Win_lock(int lock_type, int rank, int assert, MPI_Win win)


              -  Indicates  whether other processes may access the target window at the same time
              (if MPI_LOCK_SHARED ) or not ( MPI_LOCK_EXCLUSIVE )
       rank   - rank of locked window (nonnegative integer)
       assert - Used to optimize this call; zero may be used as a default.  See notes. (integer)
       win    - window object (handle)


       The name of this routine is misleading.  In  particular,  this  routine  need  not  block,
       except when the target process is the calling process.

       Implementations  may  restrict  the  use of RMA communication that is synchronized by lock
       calls to windows in memory allocated by MPI_Alloc_mem .  Locks can be used  portably  only
       in such memory.

       The  assert  argument  is  used  to  indicate  special  conditions  for  the fence that an
       implementation may use to optimize the MPI_Win_lock operation.  The value zero  is  always
       correct.   Other  assertion  values  may be or'ed together.  Assertions that are valid for
       MPI_Win_lock are:

              - no other process holds, or will attempt to acquire a conflicting lock, while  the
              caller  holds  the window lock. This is useful when mutual exclusion is achieved by
              other means, but the coherence operations that may be  attached  to  the  lock  and
              unlock calls are still required.


       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.


       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.


       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
       communicators),  MPI_File_set_errhandler  (for files), and MPI_Win_set_errhandler (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 whenever possible.

              - No error; MPI routine completed successfully.
              -  Invalid  source or destination rank.  Ranks must be between zero and the size of
              the  communicator  minus  one;  ranks  in  a  receive  (  MPI_Recv  ,  MPI_Irecv  ,
              MPI_Sendrecv , etc.) may also be MPI_ANY_SOURCE .

              - Invalid MPI window object
              - Other error; use MPI_Error_string to get more information about this error code.

                                            11/9/2015                             MPI_Win_lock(3)