NAME

       MPI_Op_create -  Creates a user-defined combination function handle

SYNOPSIS

       int MPI_Op_create(MPI_User_function *user_fn, int commute, MPI_Op *op)

INPUT PARAMETERS

       user_fn
              - user defined function (function)
       commute
              - true if commutative;  false otherwise. (logical)

OUTPUT PARAMETERS

       op     - operation (handle)

NOTES ON THE USER FUNCTION

       The calling list for the user function type is
       typedef void (MPI_User_function) ( void * a,
       void * b, int * len, MPI_Datatype * );

       where  the  operation  is  b[i]  =  a[i]  op  b[i] , for i=0,...,len-1 .  A pointer to the
       datatype given to the MPI collective computation routine (i.e., MPI_Reduce , MPI_Allreduce
       , MPI_Scan , or MPI_Reduce_scatter ) is also passed to the user-specified routine.

THREAD AND INTERRUPT SAFETY

       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.

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, for example, MPI_ERRORS_RETURN , then no error may be indicated.

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

ERRORS

       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.

       MPI_SUCCESS
              - No error; MPI routine completed successfully.

SEE ALSO

       MPI_Op_free

                                            11/12/2016                           MPI_Op_create(3)