Perhaps, but my reading is that the predefined Op is applied to the basic type from which I constructed MY_2INT (i.e., MPI_INT). Since MPI_MAXLOC does not apply to MPI_INT, this would be an error. I wouldn't know how to write precise language specifying the behavior that would lead to MPI_MAXLOC working with MY_2INT.

You're right. My comment was erroneous. Jeff -- Jeff Hammond Argonne Leadership Computing Facility University of Chicago Computation Institute [email protected] / (630) 252-5381 http://www.linkedin.com/in/jeffhammond https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond