SIGALRM can't be delivered after longjmp from handler?

[David A. Gatwood]

On Wed, 15 Mar 2000, Peter M. Jansson wrote:

>
> I wrote the example that follows to illustrate what I think is a
> problem with signal delivery.  If this program works, then it should run
> in 5 cycles; the first 2 end with the "Timed out" message, while the
> last 3 end with the "Wait interrupted" message.  On my PowerMac 7200
> running 2.2.15pre3, I get 2 "Timed out" cycles, one "Wait interrupted"
> cycle, and then two more "Timed out" cycles.  What I think is going on
> is that, once the SIGALRM handler executes the longjmp, no further
> SIGALRM signals are delivered to the process -- I don't know if this is
> because the signals aren't delivered, or because the setitimer call
> isn't working.  I've observed this example to run correctly on BSD/OS,
> IRIX, and Solaris, and seen it fail on LinuxPPC and Linux x86.

Some OSes automatically reset signal handlers to SIG_DFL (which is SIG_IGN
for alarm) after they receive a signal and some leave the signal hndler in
place.  The BSDs to the latter, and Linux, IIRC, does the former.  You can
fix this by re-installing the handler during the signal handler function,
but the better way is to use sigaction instead of signal, which will give
consistent behaviour on all platforms.


Later,
David


> -------------------------------
>
> #include <signal.h>
> #include <setjmp.h>
> #include <sys/time.h>
> #include <stdio.h>
>
> static jmp_buf env;
> static unsigned int count = 0;
>
> static void
> interrupt()
> {

	signal(SIGALRM, interrupt);

>         printf ("Interrupt: %d\n", count);
>         if (count > 2)
>                 longjmp(env, 1);
> }
>
> main ()
> {
>         struct timeval tv;
>         struct itimerval it, oit;
>
>         while (count++ < 5) {
>                 printf("Cycle %d\n", count);
>
>                 if (setjmp(env)) {
>                         printf ("Wait interrupted\n");
>                 } else {
>                         if (signal(SIGALRM, interrupt) == SIG_ERR)
>                                 perror("signal");
>                         getitimer(ITIMER_REAL, &oit);
>                         timerclear(&it.it_interval);
>                         it.it_value.tv_sec = 2;
>                         it.it_value.tv_usec = 16665;
>                         if (setitimer(ITIMER_REAL, &it, &oit) < 0)
>                         perror("tick");
>                         printf ("Set timer; waiting...\n");
>                         tv.tv_sec = 5;
>                         tv.tv_usec = 0;
>                         select(0, NULL, NULL, NULL, &tv);
>                         printf ("Timed out\n");
>                 }
>         }
> }


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