Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!sun-barr!ames!uhccux!munnari.oz.au!labtam!davidp
From: davidp@labtam.oz (David Purdue)
Newsgroups: comp.windows.x
Subject: Re: SysV, signals and X
Message-ID: <1796@labtam.oz>
Date: 21 Sep 89 02:50:22 GMT
References: <2400@goanna.oz>
Organization: Labtam Information Systems Pty Ltd, Melbourne, Australia
Lines: 38

From article <2400@goanna.oz>, by dmh@goanna.oz (Darren Hosking):
> Due to the nature of System V signal handling it is possible for system
> calls within an X library (specifically Xlib) to be interrupted, ie the
> system call that was interrupted will return an error (-1) and errno will
> be set to EINTR.
> 
> How can you use signal() and your own signal handlers without the client
> generating an error message and exiting (because of the interrupted syscall)?
> 
> For example, if you want to run an infinite loop of X calls (drawing
> rectangles or whatever) which is stopped by a SIGALRM and a signal handler,
> how can you guarantee that system calls within Xlib (or other X libraries)
> will not be interrupted and abort?

I just came across this problem while trying to do a benchmark
with xbench. The situation is exactly as in the example above.

What I do is this: I set up the signal handling routine using
setsig(2), then around the call to Xlib in the relevant loop
I put sighold(2) and sigrelse(2). Thus signal handling is turned
off while X does its stuff.

I have found that the system call overhead for doing this is
not prohibitive, and it does not affect the xbench results
because xbench always completes its operation before checking
if the alarm went off anyway.

						DavidP
====
Mr. David Purdue        Phone ISD: +61 3 587 1444  Fax: +61 3 580 5581
Labtam Information          Telex: LABTAM AA33550
  Systems Pty Ltd,
43 Malcolm Road,     ACSnet/CSnet: davidp@labtam.oz
BRAESIDE,                    ARPA: davidp%labtam.oz@uunet.uu.net
VICTORIA, 3195.             JANET: davidp@oz.labtam
AUSTRALIA                    UUCP: ...!uunet!munnari!labtam.oz!davidp

"Maybe I think too much." - Paul Simon