Re: Use system extensions conditionally?

[Fredrik Tolf]

On Mon, 3 Mar 2014, Eric Blake wrote:
> On 03/01/2014 08:32 PM, Fredrik Tolf wrote:
> > 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.
>
> How do you get epoll() on GNU/Linux?  By including <sys/epoll.h>, but
> that's a header that is not mentioned in POSIX.

True enough, my bad.

I guess my interpretation should be, then, that the default environment is 
not so much about strictly hiding everything which is not in POSIX, but 
rather about not breaking anything which should be valid under POSIX, for 
example by not defining potentially conflicting symbols. That makes sense, 
and also explains why GCC C extensions need not be explicitly hidden in 
the default environment.

> Relying on the "default environment" is risky - something that might be 
> polluted into the namespace today could disappear tomorrow if the system 
> headers get patched to provide less pollution.  Explicitly requesting 
> extensions is the only way to ensure that your program that takes 
> advantage of the extensions will not be broken by changes to the default 
> mode.

Yes, I see your point now, and I think I finally understand why 
AC_USE_SYSTEM_EXTENSIONS is all about.

Thanks a lot for the explanation!

--
Fredrik Tolf