Re: Use system extensions conditionally?

[Fredrik Tolf]

On Thu, 27 Feb 2014, Eric Blake wrote:
> > Isn't it ugly, then, to always enable system extensions, regardless of 
> > environment?
>
> No.  Most programs WANT to blindly turn on all extensions, regardless of
> environment, because it is easier to write programs that can take
> advantage of extensions, regardless of how cavalier the system is about
> namespace pollution even when extensions are not requested.
> [...]
> There is a small set of projects that really DOES want to remain
> completely POSIX-compliant (or even stricter ANSI-C compliant) - those
> programs do NOT want to request extensions by default, so that they
> minimize namespace pollution from system headers.  But they tend to be
> the minority.

Thanks for your reply!

I've been mulling over it for a while now, but I still don't think I quite 
get it. In particular, I'm still not in the clear about what 
AC_USE_SYSTEM_EXTENSIONS really does.

I read what you write kind of as if systems shouldn't expose anything 
non-POSIX as long as it isn't used, but this is clearly not the case; not 
only with funopen() on *BSD, but also with e.g. epoll() on GNU/Linux, or 
just such things as GCC C extensions.

As such, it seems to me that there are basically three different modes of 
operation: Standards-compliant mode, enabled explicitly by setting 
POSIXLY_CORRECT or similar; extensions mode, enabled explicitly by using 
AC_USE_SYSTEM_EXTENSIONS; and also "default" mode, which is what we have 
if we don't do any of those.

Seeing as how there are a lot of extensions available in default mode, I 
don't really see what the formal distinction between default mode and 
extensions mode. I'm willing to buy that there is no formally defined 
distinction between the two, and that the, perhaps arguably sloppy, nature 
of systems development leaves things a bit unclear. But if that is so, 
isn't it still ugly to have to use AC_USE_SYSTEM_EXTENSIONS always, on all 
systems, rather than being left the option of using whatever the system 
considers to be its "default environment"?

--
Fredrik Tolf