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From: ballou@brahms.Berkeley.EDU.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: help for routine to handle exit conditions
Message-ID: <1006@cartan.Berkeley.EDU>
Date: Fri, 20-Feb-87 03:38:47 EST
Article-I.D.: cartan.1006
Posted: Fri Feb 20 03:38:47 1987
Date-Received: Sat, 21-Feb-87 04:27:23 EST
References: <972@unc.unc.UUCP>
Sender: news@cartan.Berkeley.EDU
Reply-To: ballou@brahms.Berkeley.EDU (Kenneth R. Ballou)
Distribution: na
Organization: Math Dept. UC Berkeley
Lines: 38

In article <972@unc.unc.UUCP> bmh@unc.UUCP (Brad Hemminger) writes:
>
>  Additionally the Manx signal call only allows you to setup a
>procedure for one time (apparently) as it resets the interrupt
>to the default terminate meaning once your procedure is called.  
>Does anyone know whether this is because of DOS/PC considerations 
>or simply a limitation of Manx's code?

	Remarkably, it is neither.  It is for compatibility with the
UN*X signal function.  UN*X signal resets the signal handler to the default
when the signal is received.  This leads to code sequences such as:

#include <signal.h>

main ()
{
    int my_signal_handler ();

    signal (SIGINT, my_signal_handler);
/* rest of code for main () */
}

int my_signal_handler (signum)
int signum;
{
    signal (signum, my_signal_handler); /* restore action for signal signum */
/* rest of code for my_signal_handler () */
}

	This can lead to interesting race conditions; it is possible for
your process to be sent the same signal twice before it has had a chance
to reset the signal handler for that particular signal.

--------
Kenneth R. Ballou			ARPA:  ballou@brahms.berkeley.edu
Department of Mathematics		UUCP:  ...!ucbvax!brahms!ballou
University of California
Berkeley, California  94720