On Sun, 24 May 2020 at 18:55, Florian Weimer wrote:
>
> * Thomas Rodgers:
>
> > + static __gthread_t
> > + _S_get_tid() noexcept
> > + {
> > +#ifdef __GLIBC__
> > + // For the GNU C library pthread_self() is usable without linking to
> > + // libpthread.so but returns 0, so we cannot use it in single-threaded
> > + // programs, because this_thread::get_id() != thread::id{} must be
> > true.
> > + // We know that pthread_t is an integral type in the GNU C library.
> > + if (!__gthread_active_p())
> > + return 1;
> > +#endif
> > + return __gthread_self();
> > + }
>
> This comment seems outdated or incomplete. pthread_self returns a
> proper pointer since glibc 2.27, I believe.
The comment is copied from the <thread> header, and dates from 2015.
> I'm also not sure how the difference is observable for the libstdc++
> implementation. Late loading of libpthread isn't quite supported.
It's nothing to do with late loading. A single threaded program that
doesn't create any threads and doesn't link to libpthread can still
expect std::this_thread::get_id() != std::thread::id() to be true in
the main (and only) thread. If pthread_self() returns 0, and
thread::id() default constructs with a value of 0, then we can't
distinguish "the main thread" from "not a thread".
But I do see a non-zero value from glibc now, which is great. I'll add
it to my TODO list to remove that workaround from <thread>.
