[Bug libstdc++/54847] --enable-libstdcxx-time=yes doesn't find the functional nanosleep() on darwin

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54847

--- Comment #33 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-10-08 15:55:07 UTC ---
(In reply to comment #31)
> (In reply to comment #30)
> > Can we take a step back and determine what exactly this PR is about?  Several
> > of the comments make incorrect statements about the status quo, so I want to
> > get the report clear.
> > 
> > I don't understand comment 1. 

I meant comment 0 there.

> The struct business is a red-herring and can be ignored. The test case without
> struct
> is compiled by g++ fine on darwin.

Of course, that's what Paolo has been saying.

> > 
> > Contrary to comment 2, the configure check for sched_yield does *not* check
> > _POSIX_TIMERS, so sched_yield should be used on darwin when
> > --enable-libstdcxx-time is used.  Is _GLIBCXX_USE_SCHED_YIELD defined on darwin
> > when the --enable-libstdcxx-time option is used?
> 
> I read libstdc++-v3/acinclude.m4 to imply that the check for sched_yield is
> only used
> when configure is passed --enable-libstdcxx-time=rt, I didn't bother with that
> case on
> darwin because Jeremy said....
> 
>   CLOCK_REALTIME isn't supported in darwin, so I wouldn't expect it to work
> unless you had external implementation for the RT POSIX extension.
> 
> ...off-list.

You read it wrong.

The sched_yield check is done for both --enable-libstdcxx-time=yes and
--enable-libstdcxx-time=rt

So is _GLIBCXX_USE_SCHED_YIELD defined on darwin?

> > That option *doesn't* enable the use of clock_gettime on darwin, because the
> > function isn't available. That's unavoidable.
> > 
> > That option does enable checking for nanosleep, but the check fails on darwin
> > because nanosleep is available but darwin doesn't identify itself as supporting
> > the Timers option (presumably because the rest of the option isn't supported.)
> > 
> > If that's correct, could we change the bug description to be more accurate?
> 
> Done. Note that the man page for darwin nanosleep() can be found at...
> 
> https://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man2/nanosleep.2.html
> 
> and claims...
> 
> The nanosleep() function conforms to IEEE Std 1003.1b-1993 (``POSIX.1'').

The problem isn't finding nanosleep, it's finding what version of POSIX or
X/Open or SUS is supported, so we know what the correct feature test macros is
for nanosleep.

Is _XOPEN_REALTIME defined?  If so, to what value?
What about _XOPEN_VERSION and _POSIX_C_VERSION?

Maybe we can find some other combination of feature test macros that indicate
the POSIX Realtime extensions (POSIX.1b) are supported.