Re: [mpi-21] MPI::BOTTOM

[Dave Goodell <goodell@xxxxxxxxxxx>]

On Nov 27, 2007, at 3:27 PM, Jeff Squyres wrote:

> A user recently raised an issue that I just looked into and  
> discovered a problem with the C++ binding for MPI::BOTTOM.
>
> In the spec, MPI::BOTTOM is defined to be of type (const void*).   
> However, all receive buffers are defined to be of type (void*) --  
> such as for the various flavors of point-to-point receive, the  
> receive buffer for collectives, etc.  This means that you'll get a  
> compiler error when trying to use MPI::BOTTOM as a receive buffer:
>
> bottom.cc:81: error: invalid conversion from ‘const void*’ to ‘void*’
> bottom.cc:81: error:   initializing argument 1 of ‘virtual void  
> MPI::Comm::Bcast(void*, int, const MPI::Datatype&, int) const’
>
> A user can cast away the const-ness of MPI::BOTTOM, but that seems  
> inelegant/wrong.
>
> I don't yet have a solution to this problem; I raise it here so  
> that it gets added to the list of issues to be addressed in MPI-2.1.

Looks like the const is on the wrong side of the declaration.  That  
is, unless my C++ is too rusty it should instead be something like:

namespace MPI {
  ...
  extern void * const BOTTOM;
  ...
}

"const TYPE * FOO" indicates that the data pointed to by FOO is read- 
only.  So "*FOO = BAR;" would be an illegal statement.

"TYPE * const FOO" indicates that the memory holding the value of FOO  
is read-only.  So "FOO = &BAR;" would be an illegal statement.

The latter seems to be what is desired for MPI::BOTTOM: an address  
that cannot be changed but a the data that it references can.

--
Dave Goodell
Argonne National Laboratory