Re: Setting signal handlers considered unsafe?

What are you actually trying to do here? What did you expect to see?

Do you want to set a handler in the parent? You don't need to do anything fancy. Just set the handler in the parent. You can do it from a sub if you want. Don't do it in a while 1 loop, that's just asking for trouble. You don't need to local it, because it's a forked process at that point, it's totally separate from the child.

Once you go into mult-process and multi-thread, you really have to watch out. Strange error messages that are different every time you run are pretty normal if you don't have a good understanding of the non-sequentiality of these things.

--Pileofrogs

Comment on Re: Setting signal handlers considered unsafe?

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Re^2: Setting signal handlers considered unsafe? by gnosek (Sexton) on Nov 05, 2008 at 20:39 UTC
Hi, First of all, thanks everybody for your numerous replies :) Second, I guess I should have written in my original post. I know a tight loop of setting a local signal handler is silly, just like calling `kill` in another tight loop. I don't do that in my code, really. It's just the simplest way of reproducing the issue. It happens, say, once a week in the real world and once a second in the demo code. Third, I do have a pretty good grasp of multiprocessing issues, including atomicity of signal handlers and (simplifying a bit) non-sharing of address space between different processes. I don't expect to learn why `local $SIG{FOO} = \&bar;` is silly, I expect to learn why it isn't atomic and what I can do about it. I have tried `sigprocmask` together with the totally undocumented `POSIX::SigSet` (does it have something like `->add`?) but I can still easily "crash" perl with this code. Avoiding `local` and setting/resetting the handler "by hand" seemed to decrease the frequency in the real world but the race still exists. Also, `strace` shows that perl internally does mostly the same thing (block signal, call `sigaction`, unblock) when setting the handler so I don't think there's anything more I can do, other than cry for help in the Monastery ;)	[reply]
Re^3: Setting signal handlers considered unsafe? by djp (Hermit) on Nov 06, 2008 at 01:12 UTC
... totally undocumented POSIX::SigSet ... Try POSIX	[reply]
Re^4: Setting signal handlers considered unsafe? by gnosek (Sexton) on Nov 06, 2008 at 09:11 UTC
D'oh! I've read through that page numerous times (and have learnt of `POSIX::SigSet` from there) without ever seeing it. My apologies.	[reply]