From: Dries Kimpe <Dries.Kimpe@XXXXXXXXXXXXXX>
Date: January 12, 2008 1:49:40 AM CST
To: mpi-21@XXXXXXXXXXXXX
Subject: Re: [mpi-21] Proposal EH2: add const keyword to the C bindings
Reply-To: mpi-21@XXXXXXXXXXXXX

A similar problem (and maybe more frequent) will be with the MPI callback
functions (e.g. MPI_Op)

If we really want to implement const in all places where it makes sense,
-- for reasons of consistency and to help users avoid incorrect code --
we will break a lot of code:

For example: (MPI_Op_create)

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

Since a should only be read, to be consistent, void * a should actually be
const void * a; 

Making this modification would surely help catch bugs.

Also -- not related to the const issue --  it is rather strange that len
is passed by a pointer, since 'len' isn't meant to be modified?  Applying
the rule would yield 'const int * len', which is a rather strange (and
inefficient) way  to pass an integer. 

The attribute copy function is immune:
there the original attribute is passed by value (casted
to void *), and hence doesn't need an added const; 
(Well one could argue that it does need a const, but it will be casted to
something else anyway)

   Dries