Re: gthread-win32.c

[Sebastian Wilhelmi <wilhelmi ira uka de>]

Hi Kaz,

> > 234                if (retval == WAIT_TIMEOUT)
> > 235                  {
> > 236                    EnterCriticalSection (&cond->lock);
> 
> Beware that EnterCriticalSection can throw a low memory structured
> exception, if it needs to allocate the internal auto-reset event
> and the CreateEvent() fails. As a hack, it's possible to create an
> auto-reset event, and store its handle into the critical section.
> Been there, done that. On NT4 there is a variant called
> InitializeCriticalSectionAndSpincount which will preallocate
> the event if the spincount is 0x80000000 - 0x7FFFFFFF.

In general we don't care much for out-of-mem in GLib. So if
EnterCriticalSection fails, system memory is low? Or does that happen from
time to time without real memory shortage. If CreateEvent() would fail at the
same place, then we don't care about EnterCriticalSection throwing whatever.
If however CreateEvent() wouldn't fail, then we might consider taking
provision for that case. Ok, it might be unwanted, that the program fails
without notice, so is there a simple way (a la win32_check_for_error) to catch
those strcutured exeptions and make a nice error message before quiting?

> > 237                    g_ptr_array_remove (cond->array, event);
> > 238
> > 239                    /* In the meantime we could have been signaled,
> > so we must again
> > 240                     * wait for the signal, this time with no
> > timeout, to reset it */
> > 241                    win32_check_for_error (WAIT_FAILED !=
> > WaitForSingleObject (event, 0));
> > 242
> > 243                    LeaveCriticalSection (&cond->lock);
> > 244                  }
> >
> > Should this not change retval?  If the object has been signalled, then
> > it should return successful.  Otherwise, the event which caused the
> > object to be signalled is lost.
> 
> Absolutely.  There are two requirements: one is that if a timeout and
> signal happen about the same time, we want to make sure everything is
> in a sane state. Then there is a secondary requirement that it would be
> nice if the timeout would avoid ``eating'' the signal, because the
> signal is more important. You can dispense with the second requirement,
> but then the application programmer must treat every timeout as a
> potential signal: record that the timeout happened, then re-test
> the predicate and do what is necessary, then act on the timeout.

That's true, so I already changed it.

> > 462                retval->thread = (HANDLE) _beginthreadex (NULL,
> > stack_size, g_thread_proxy,
> > 463 wilhelmi 1.1 retval, 0, &ignore);
> >
> > Interesting.  Why not use CreateThread?
> 
> Because _beginthreadex uses CreateThread to routes your thread through
> an internal threading function which sets up and cleans up
> thread-specific storage related to Microsoft's C library. The cleanup
> happens when your thread terminates by returning from its threading
> function. But what if two separate libraries want to attach thread
> specific storage to a thread?
> 
> It's all because of Microsoft's brain-damaged thread local storage
> implementation, which doesn't have destructor functions unlike the
> POSIX one. In the POSIX world, your software component can attach
> thread-specific data to a thread without the need to override the
> thread startup function. When the thread terminates, a registered
> handler associated with the thread specific key will be invoked which
> can clean up the thread specific resources. So a large number of
> program modules can independently attach thread specific resources to a
> thread.

Ah, that's a very plausible explanation. I only read the text on the MS-page
and they just say:

  Note: For an executable file linked with LIBCMT.LIB, do not call the
  Win32 ExitThread API; this prevents the run-time system from reclaiming
  allocated resources. _endthread and _endthreadex reclaim allocated
  thread resources and then call ExitThread.

Which of course in retrospect seems quite similiar to your explanation, but
when I first read it, it sounded more like "do what we say, it's for your own
good, just don't ask why"

Thanks,
Sebastian
-- 
Sebastian Wilhelmi
mailto:wilhelmi ira uka de
http://goethe.ira.uka.de/~wilhelmi