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From: dmh@goanna.oz (Darren Hosking)
Newsgroups: comp.windows.x
Subject: SysV, signals and X
Keywords: X, signals, SysV
Message-ID: <2400@goanna.oz>
Date: 20 Sep 89 07:10:20 GMT
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 34

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.

This is not a problem in _XRead(), which does a read() from the socket to
the server, as the interrupted system call error is explicitly checked for
and the XIOErrorFunction is NOT called if the read() was interrupted.

However, in _XSend, which does a write() to the socket connected to the
server, the interrupted system call error is not checked for and therefore
the XIOErrorFunction will be called if the write() is interrupted.

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?

One solution is to ignore the given signal each time you want to call an
Xlib function and then reinstate it after the call, but the system call
overhead could be substantial.

Any ideas or solutions?

	Thanks in advance, dmh

Darren Hosking				ACSnet: dmh@goanna.oz	 
Department Of Computer Science		ARPA:   dmh%goanna.oz.au@uunet.uu.net
RMIT					CSNET:  dmh%goanna.oz.au@australia
GPO Box 2476V				UUCP:   ...!uunet!goanna.oz.au!dmh
Melbourne Vic., Australia 3001