Catching Signals in 'C'
Paul John Falstad
pfalstad at dae.Princeton.EDU
Sat Sep 29 02:59:08 AEST 1990
In article <2905 at idunno.Princeton.EDU> subbarao at phoenix.Princeton.EDU (Kartik Subbarao) writes:
>In article <2901 at idunno.Princeton.EDU> pfalstad at bow.Princeton.EDU (Paul John Falstad) writes:
>>In article <1990Sep28.120043.17628 at NCoast.ORG>, ramsey at 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 at phoenix.princeton.edu PLink:HYPNOS GEnie:P.FALSTAD CIS: 70016,1355
That address again,
sync at thumper.princeton.edu PLink:OHS738 GEnie:OHS738 CIS: 4128 143 1234 937
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