Catching Signals in 'C'
Kartik Subbarao
subbarao at phoenix.Princeton.EDU
Sat Sep 29 01:16:23 AEST 1990
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:
>|> Hello peoplekind. I have a question about the behavior of the
>|> signal function. Firstly, I want to trap the control-c, break,
>|> and other interrupt keys the user may use to stop a program.
>|> I did this by ;
>|>
>|> 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.
>
>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 :-)
A solution is suggested in "UNIX programming", by some author whom I've
forgotten (a Howard SAMS publication).
What you should put in your signal handlers (*handler1), (*handler2),
is a simple
signal(SIGINT, SIG_DFL);
/* replace SIGINT by whatever signal you wish to trap */
This will restore the signal to previous status.
>--
>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
Or a simple "F" to alt.dev.null :-)
-Kartik
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