[Xcb] XCB on *BSD: many pthread-related problems

[Jamey Sharp]

First off, thanks for testing XCB on non-Linux. :-)

On Fri, Nov 03, 2006 at 09:46:57PM +0100, Diego 'Flameeyes' Pettenò wrote:
> So, I've been trying to find why X didn't start after having it built fine 
> with xcb enabled... and I've seen there's a little big mess with it on 
> FreeBSD (and DragonFly and OpenBSD at least, not sure on NetBSD, but it's 
> likely to be the same).

I'm very surprised that you're having trouble on FreeBSD, because I
believe Eric Anholt has been testing this code on that OS, and I'm sure
he'd have complained at me if he'd seen this problem.

I'm also not convinced I understand the symptom you're seeing. Your X
server surely starts just fine, right? (Try just running `X` from the
console: you should see the lovely root weave.) I assume the problem is
that your applications fail to load and startx or xdm or whatever you're
using kills the X server as a result.

Anyway, I'll address your actual problem; I just want to understand the
symptom too.

(fascinating background snipped)

> libX11 wasn't threaded previously, but libxcb and libxcb-xlib are now. The 
> result is that programs linking to libX11 will now need to use -pthread to 
> finalise the link, or they won't work.

Actually, libX11 and XCB still aren't (exclusively) threaded libraries.
They're meant to work with both multi-threaded and single-threaded
programs. So I suspect you have the wrong fix.

Josh has been planning to write what I think is the correct fix for your
problem. It's worrisome that he was inspired to do this because the Hurd
needs it, but we'll let that pass... ;-)

On Linux and some other platforms, libc provides weak symbols for the
pthread functions we're using. If you don't link a pthread library, then
calls to those functions go to the libc versions, which do nothing.
Otherwise, the calls go to the real thread implementation, which
actually acquires a lock or whatever. Currently, we rely on this
behavior.

What Josh planned to do is embed our own no-op weak symbols in XCB on
platforms where libc doesn't provide them. I think only ELF supports
weak symbols, so on non-ELF platforms we may need a third solution, but
I don't think that's an issue yet.

libX11 has code for this. See where configure.ac defines thrstubs, and
see src/UIThrStubs.c. (And why isn't this code enough for your system?
Pure XCB apps would fail, but Xlib apps ought to still work.)

If you wanted to try your hand at such a patch for XCB, it'd be a big
help. Josh is quite busy...

--Jamey
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