[MPI3 Fortran] Variable length argument lists

[Lionel, Steve]

Fortran does not have variable-length argument lists.  It does have
optional arguments.  The only context in Fortran where one sees anything
that looks like variable argument lists is in the MAX and MIN intrinsics
which have an unspecified number of arguments designated A1, A2, A3,
etc.  This cannot be expressed in Fortran source. As best as I can tell,
the proper action is to remove the ",..." from the callback signature.
I would guess that this is intended to pass some sort of user context to
the callback routine, which is a helpful idea.  You could define an
optional CONTEXT argument of type C_PTR, but this would require existing
applications to be rebuilt using a module providing an explicit
interface.  Possibly more trouble than it's worth.

It is also not correct to say "you can pass as many/few arguments as you
want".   While this may work with some implementations, with others it
does not.


Steve Lionel
Developer Products Division
Intel Corporation
Nashua, NH 




> -----Original Message-----
> From: mpi3-fortran-bounces at lists.mpi-forum.org [mailto:mpi3-fortran-
> bounces at lists.mpi-forum.org] On Behalf Of Jeff Squyres
> Sent: Tuesday, October 21, 2008 7:23 AM
> To: MPI-3 Fortran working group
> Subject: [MPI3 Fortran] Variable length argument lists
> Importance: Low
> 
> Fortran experts --
> 
> The MPI 2.1 spec lists some Fortran callback signatures as having
> variable arguments.  I do believe that this is a holdover from a long,
> long time ago, and I believe that whoever wrote this section simply
> copied the C prototypes.  If the spec is wrong, I'd like to get it
> fixed during the ongoing MPI-2.1 process.  See MPI-2.1 p266 line 29,
> or http://www.mpi-forum.org/docs/mpi21-report-bw/node183.htm#Node183.
> 
> The Fortran callback signature is expressed as:
> 
> SUBROUTINE COMM_ERRHANDLER_FN(COMM, ERROR_CODE, ...)
> INTEGER COMM, ERROR_CODE
> 
> 1. Does Fortran have variable length argument lists (akin to C
> varargs)?  If so, was it introduced in a specific version of Fortran?
> (I realize that F77 compilers do no checking and you can pass as many/
> few arguments as you want)
> 
> 2. If Fortran has variable length argument lists, what is the correct
> syntax to express them?  Is "..." correct?  Or is there a different
> way?
> 
> Thanks.
> 
> --
> Jeff Squyres
> Cisco Systems
> 
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