Creates a user-defined combination function handle


#include "mpi.h"
int MPI_Op_create( 
        MPI_User_function *function, 
        int commute, 
        MPI_Op *op )

Input Parameters

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

Output Parameter

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.

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.


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_Errhandler_set; 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.

No error; MPI routine completed successfully.
This error is returned when some part of the MPICH implementation is unable to acquire memory.

See Also