Fwd: Re: ports/52016: New port: lang/harbour -AClipper-compatible compiler

[Bruce Evans]

On Fri, 9 Jan 2004, Stefan Farfeleder wrote:

> On Sat, Jan 10, 2004 at 02:03:57AM +1100, Bruce Evans wrote:
>
> > - at least old standards seem to permit feature test macros to have no
> >   value (they may be empty, or perhaps more bizarre).  The "+ 0" in the
> >   above is to handle this case, so that feature test with an empty value
> >   work the same as ones with value 0.
>
> [...]
>
> > #if _POSIX_SYNCHRONIZED_IO + 0 > 0
>
> Undefined identifiers are replaced with 0 in a #if directive.  Maybe
> some ancient pre-C90 compilers didn't do that, but I think they can be
> ignored these days.

I know :-), and consider it a style bug to not depend on this feature,
but the above is to handle the case where _POSIX_SYNCHRONIZED_IO is
defined to be <nothing>.  Then "_POSIX_SYNCHRONIZED_IO > 0" would
expand to " > 0", which is a syntax error.  I think this possibility
is fixed in POSIX.1-2001.  It clearly says that the feature test
constants "shall be defined with a value" if they are defined at all.
POSIX.1-1990 seems to only say what happens if they are defined with
a value or are undefined, leaving open the possibility that they are
defined to a non-value.  Hmm, POSIX.1-2001 doesn't seem to require the
value to be integral, except tacitly since they would not be usable
in cpp expressions if they were floating point.

Bruce