Re: Use system extensions conditionally?

[Nick Bowler]

On 2014-03-03 13:50 -0800, Russ Allbery wrote:
> 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.

Even without the use of system extensions Autoconf makes no attempt to
enable standards-conformance mode in compilers, and most of them stomp
all over the application namespace by default.  So all namespace bets
are off with or without AC_ENABLE_SYSTEM_EXTENSIONS.

For a concrete example, try to declare a variable called "unix" on a
typical GNU/Linux system...

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)