Re: Use system extensions conditionally?

[Russ Allbery]

Eric Blake <address@hidden> writes:

> On the other hand, BSD's funopen() is exposed in <stdio.h>, which IS a
> standard header; but the name funopen() is not in the list of reserved
> name patterns in POSIX.  Therefore, if BSD wants to comply with POSIX,
> then when _POSIX_C_SOURCE is defined before including <stdio.h>, then
> funopen() must NOT be visible to a user of <stdio.h>.  Now, not everyone
> cares about being strictly POSIX-compliant, so the default behavior,
> when you don't request extensions, might have names leaked into the
> environment.  But on the converse side, when you KNOW you want to use
> extensions, you are best off to explicitly request those extensions.
> And if you are going to use non-POSIX extensions, you might as well use
> all extensions on all platforms, all in one go.

Of course, one of the drawbacks is that you have an all-or-nothing
situation.  Either you have to stick to only POSIX interfaces, or you have
to give the local system carte blanche to stomp all over your namespace,
possibly resulting in code breakage down the line when some new local
extension is introduced that conflicts with your code.

This of course isn't a problem that Autoconf can solve.  It's basically
inherent in how the feature test macros work.  But it's worth being aware
of.  All namespace bets are essentially off once you enable system
extensions.

-- 
Russ Allbery (address@hidden)              <http://www.eyrie.org/~eagle/>