[Xmonad] xmobar-0.7rc1 testers needed

Spencer Janssen sjanssen at cse.unl.edu
Tue Jul 17 13:54:38 EDT 2007

On Tuesday 17 July 2007 12:27:38 Andrea Rossato wrote:
> Hi,
> today I added XEvents handling to xmobar, a status bar you should know
> I'm writing.
> Now, this is more difficult of what I first thought, since xmobar makes
> heavy use of Haskell threads.
> Since we need to call XNextEvent, it seems, as far as I can
> understand, that this is going to block every other Haskell threads,
> unless the threaded RTS is used and/or (?) forkOS (which requires
> anyway the threaded RTS) is used instead of forkIO.

Here is the relevant source from X11:
foreign import ccall safe "HsXlib.h XNextEvent"
        nextEvent    :: Display -> XEventPtr  -> IO ()

The "safe" bit means that the FFI call will run in it's own OS thread -- it 
shouldn't block any Haskell threads.  However, I believe that Xlib has 
internal locking that will cause XNextEvent to block all other Xlib calls.  
If forkOS changes the behavior here, we most likely have a mismarked "unsafe" 
function in X11.

> While this is working fine on my system, I have a system with an old
> Xorg build (I cannot compile X11-1.2.2 and X11-extras on that
> machine), which seem to block all threads no matter what I do. This is
> not a valuable test for me, so I need testers to confirm or refute my
> personal test.

I believe this is XNextEvent blocking other threads.  I've got an idea for a 
solution, expect another email shortly :).

Spencer Janssen

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