2nd occurrence of signal not being handled?

Sat Dec 19 15:30:00 GMT 2009

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According to Mark Geisert on 12/18/2009 10:23 PM:
> Or maybe there's some subtle
> mistake in my use of the signal functions. Any advice would be appreciated.
>> ..mark
>> #include <setjmp.h>
> #include <signal.h>
> #include <stdio.h>
>> volatile int step = 0;
> sigjmp_buf trapoline;
>> void
> segv_handler(int sig)
> {
> printf("case %d: SIGSEGV handled\n", step++);
> siglongjmp(trapoline, step);

Within this handler, SIGSEGV is still blocked. Why? because...
> }
>> int
> main()
> {
> struct sigaction sa;
>> sa.sa_handler = segv_handler;
> sa.sa_flags = 0;

...you didn't request SA_NODEFER. But calling siglongjmp from within the
handler only restores the original mask (and thus unblocking SIGSEGV), if...
> sigemptyset(&sa.sa_mask);
>> sigaction(SIGSEGV, &sa, NULL);
>> switch(sigsetjmp(trapoline, 0)) {

...you call sigsetjmp with a non-zero savemask argument. Therefore...
> case 0:
> printf("case %d reached\n", step);
> printf("case %d: %08X\n", *(int *) 42);
> ++step;
>> case 1:

...when you get here, your signal mask still includes SIGSEGV...
> printf("case %d reached\n", step);
> printf("case %d: %08X\n", *(int *) 42);

...and causing a fault when SIGSEGV is blocked is fatal. Meanwhile, it
didn't help that your program has undefined behavior: since you didn't
pass enough arguments to printf, the program is allowed to do whatever it
wants.
> ++step;
>> default:
> printf("case %d reached\n", step);
> break;
> }
>> return 0;
> }

And as far as I can tell, your problem is not cygwin-specific.
- --
Don't work too hard, make some time for fun as well!
Eric Blake ebb9@byu.net
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