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Date: Sat, 19 Dec 2009 08:30:40 -0700
From: Eric Blake <ebb9 AT byu DOT net>
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To: cygwin AT cygwin DOT com, mark AT maxrnd DOT com
Subject: Re: 2nd occurrence of signal not being handled?
References: <loom DOT 20091219T060807-696 AT post DOT gmane DOT org>
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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 AT byu DOT net
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