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Subject: Re: [OMPI users] mpi.h macro naming
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2008-02-20 08:15:27

On Feb 19, 2008, at 12:52 PM, Ben Allan wrote:

> Is there some subtle reason that ompi's mpi.h
> leaves the following macros both
> unguarded with an ifndef and un-prefixed with OMPI_ ?

Mainly because they come from standard autoconf/configure tests (e.g.,
AC_CHECK_SIZEOF), meaning that we (OMPI) aren't setting those macros
directly -- the built-in AC test is doing it. Indeed, OMPI's
configure script doesn't have [public] access to the value that it
sets -- AC_CHECK_SIZEOF's public interface sets a #define in the
output header file.

The #defines that are mpi.h are limited to the ones that we need for
that file itself. More specifically: the majority of the #define's
that are generated via OMPI's configure are not in mpi.h.

> This produces considerable amounts of compiler whinage for other
> codes that include mpi.h. As always, extraneous whinage makes real
> errors harder to find. (And yes, those other codes also need
> *their* definitions of HAVE_LONG_LONG, etc properly protected).
> And of course who knows how the answer was defined for any given
> unprotected appearance of these macros?
> /* Define to 1 if the system has the type `long long'. */
> /* The size of a `bool', as computed by sizeof. */
> #undef SIZEOF_BOOL
> /* The size of a `int', as computed by sizeof. */
> #undef SIZEOF_INT

Our assumption was that if some other package defined these values,
they would either likely be coming from the same standard autoconf
tests or use the same #define conventions as the autoconf tests. As
such, the values that they are #defined to would be the same (and
compilers don't whine about multiple #defines of the same macro to the
same value -- they only whine if the values are different).

I'm assuming that you're seeing cases where the defined values are
different. Isn't that a bit scary? E.g., why would OMPI be reporting
a different value for SIZEOF_BOOL and SIZEOF_INT than what is in your
app's header files?

> If it's simply a matter of developer hours, I can post a patch
> somewhere to address this. It appears that of these, only
> sizeof_int affects more than a few source files.

There's two places that would need to be changed:

- the relevant parts of OMPI's configure script to *also* define an
OMPI_* equivalent of the macro (which will sometimes mean extracting
non-public information from the Autoconf tests -- usually a risky
proposition because Autoconf can change their internals at any time).
The only safe way I can think of would be to AC_TRY_RUN and write the
#define'd value out to a temp file. This, of course, won't work for
cross-compiling environments, though.

- modify to use the new OMPI_* macros.

Keep in mind that mpi.h only has a small subset of the #defines from
OMPI's configure script. opal_config.h (and internal OMPI file that
is not installed) has *all* the #defines; that's what's used to
compile the OMPI code base. mpi.h replicates a small number of these
defines that are used by OMPI's public interface.

Jeff Squyres
Cisco Systems