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From: sivan@bpa_su10.Sbi.Com (Sivan Mahadevan)
Newsgroups: comp.windows.x
Subject: Re: signals using XLib
Message-ID: <120@bpa_su56.bpa_su10.Sbi.Com>
Date: 18 Dec 90 14:29:47 GMT
References: <1990Dec17.134238.26720@nas.nasa.gov>
Organization: BPA - Salomon Brothers, NY
Lines: 41

In article <1990Dec17.134238.26720@nas.nasa.gov>, parlette@nas.nasa.gov (Edward B. Parlette) writes:
> I am writing a program using straight XLib and need to be able
> to break out of the Event loop if I haven't received a keypress
> event within a certain amount of time.  I have used alarm calls
> in the past for this and am wondering if this will cause a problem
> with XLib calls ??  I thought I saw a message on the new that claimed 
> using signals with X was a bad idea but I have forgotten why.  

> kelvin			email:  kelvin@calvin.larc.nasa.gov	
> calvin.larc.nasa.gov

In order to make use of signals in the manner you wish, you need to

	1) Build a version of Xlib that is signal re-entrant
	2) Take special precautions on the type of X calls made from
	   within signal handlers.

Xlib comes with LockDisplay and UnlockDisplay macros, #define'd to be
empty.  If you set macros these to block and unblock signals
respectively (using sigblock() and sigsetmask()), then you have an
Xlib that is re-entrant for a given signal.  However, if you just do
this, then you won't be able to catch any signals when waiting in the
event loop (i.e., in XNextEvent()).  Therefore, make sure you remove
the LockDisplay and UnlockDisplay macros from XNextEvent().  

You have to be careful to not do any "event
management" within signal handlers, because with the above Xlib,
XNextEvent() itself is not re-entrant.  Therefore don't call any
X*Event() routines from within signal handlers and don't call XSync()
or XFlush() either.  The best practice is to do as little X in the
signal handlers as possible, and wait until you get back to main loop
to do the work.




-- 
Sivan Mahadevan
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