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From: pfalstad@dae.Princeton.EDU (Paul John Falstad)
Newsgroups: comp.lang.c
Subject: Re: Catching Signals in 'C'
Summary: Stop and post before you think.
Message-ID: <2910@idunno.Princeton.EDU>
Date: 28 Sep 90 16:59:08 GMT
References: <1990Sep28.120043.17628@NCoast.ORG> <2901@idunno.Princeton.EDU> <2905@idunno.Princeton.EDU>
Sender: news@idunno.Princeton.EDU
Organization: Princeton University, Princeton, New Jersey
Lines: 69

In article <2905@idunno.Princeton.EDU> subbarao@phoenix.Princeton.EDU (Kartik Subbarao) writes:
>In article <2901@idunno.Princeton.EDU> pfalstad@bow.Princeton.EDU (Paul John Falstad) writes:
>>In article <1990Sep28.120043.17628@NCoast.ORG>, ramsey@NCoast.ORG (Cedric Ramsey) writes:
>>|> main()
>>|> {
>>|>   signal (SIGINT, (*handler1) ());
>>|>   signal (SIGQUIT, (*handler2) ());
>>|>   ... 
>>|> }
>>|> But, when I type a control-c the program handles the signal as
>>|> expected; however, when I type the control-c a second time the program 
>>|> doesn't catch it and simply exits. 
>>I quote from K&R, p. 255:
>>
>>"When a signal 'sig' subsequently occurs, the signal is __restored to its
>>default behavior__; then the signal-handler is called, as if by (*handler)(sig)."
>Ahh... But Paul, don't we want to give him the answer to his question? Too many
>times K & R has been quoted...and yet no real solutions are given.

I did answer his question.  Oh, sorry, you wanted a code fragment I
suppose.  Ok:

void handler1(whatever)
{
	printf("You're not getting out so easily!\n");
	signal(SIGINT,handler1);  /*  (*handler1) () not necessary */
}

This code implements the following sentence.

>>The signal handler should reinstall itself before returning. signal() 

>>apparently does not act this way in my implementation (SunOS)...  Fortunately.
>
>But programming should be PORTABLE, should it not :-)

True.  Why are you telling me this?  My above answer is portable.  I was
just commenting on the fact that SunOS does not implement signal this
way.

>What you should put in your signal handlers (*handler1), (*handler2),
>is a simple 
>signal(SIGINT, SIG_DFL); 
>This will restore the signal to previous status.

Thank you!  You have just found a portable way to implement the bug that
the original poster wanted to fix.  Now it doesn't work on nearly ALL
implementations of C instead of merely ANSI compliant ones.

Next time, read the article before following up to it.

>>Here is the address to complain to:
>Or a simple "F" to alt.dev.null :-)

Why didn't you then?

>(I need a new .signature -- any suggestions?)

Disclaimer: I have for a long time been suffering from a species of brain
injury which I incurred during the rigors of childbirth; and I'd like to
conclude by putting my finger up my nose.

How's that?  ;-)

--
Here is the address to complain to:
pfalstad@phoenix.princeton.edu PLink:HYPNOS GEnie:P.FALSTAD CIS: 70016,1355
That address again,
sync@thumper.princeton.edu PLink:OHS738 GEnie:OHS738 CIS: 4128 143 1234 937