Re: libev's child reaping breaks system() function
> > It works fine without multithreading, so how is it libev? > > > I also considered this - as I already mentioned. I didn't get any I explaiend the facts to you, and you can look them up easily online too. If you insist on your non-fatc based opinion, that is your right. > The only solution I'm aware of is using a custom SIGCHLD signal Solution to *what*? > handler and activating an async watcher when the signal handler > gets invoked. This is like simulating an ev_signal. > Is this the way I'm supposed to go under these circumstances? Special requirements that rely on unportable behaviour might require special implementations, yes. This is primarily your problem, the best that libev can do is provide you the ability to do it, which is clearly does. > That's true, there are no segfaults. But imagine libev or some other > library would always "ignore" SIGCHLDs, maybe repeatedly to prevent > the user from fixing the handler. waitpid() wouldn't be able to > reap children anymore. This would be documented waitpid() > behaviour, too. If libev gave you the option to do it either way, that would be fine, no? I fail to see your point. It's not as if libev forced this behaviour on you. > I would think that a library with such side-conditions, effectively > breaking a documented system call is poorly designed. You know that this is a lie, so why do you repeat it? All you do here now is trolling: neither waitpid nor system are broken by libev in any way whatsoever, you know that, and repeatedly claiming the opposite is simply not acceptable. If you want to troll, go elsewhere. > > > Making the mechanism asynchronous/non-blocking, would > > > require major code refactoring and make conceptually > > > simple code dis-proportionally harder to read. > > > > Seems like an empty claim - can you back it up? > > Yes. Usually you're using system() for programs that [...] libev doesn't force you to do that, you can also rovide your own signal handler, _as you well know_. Since you can't back up your claim, I'll consider it dropped. > Let's not talk about the problems you would have when > using that mechanism from a thread that hasn't got the > default loop (the only one supporting signal/child > watchers). There are many well known problems with multihreading - you can't fork and do soemthing nontrivial anymore, system, read, close etc. all suddenly have race conditions, signal handlign is order of magnitudes more complex etc. All you do is whine about that and want libev to somehow solve the problem for you. But the problem is multihreading, and that simply requires harder and more code. Whining doesn't help you, you need to sit down and face the reality: if you use threads, your code gets more complicated. libev can't magically fix that for you, and trolling here by making repeated wrong claims is not going to win me over in any way either. -- The choice of a Deliantra, the free code+content MORPG -==- _GNU_ http://www.deliantra.net ==-- _ generation ---==---(_)__ __ __ Marc Lehmann --==---/ / _ \/ // /\ \/ / schm...@schmorp.de -=/_/_//_/\_,_/ /_/\_\ ___ libev mailing list libev@lists.schmorp.de http://lists.schmorp.de/cgi-bin/mailman/listinfo/libev
Re: libev's child reaping breaks system() function
- Original Message - > From: "Marc Lehmann" > Since you can install your own sigchld handler already, this doesn't > seem > to be so helpful? > I admit, it wouldn't be that useful, but it would be cleaner to use an ev_signal instead of an ev_async+custom signal handler. > > Also system() isn't broken by multithreading - it works fine as > > long as only one thread's executing system() at a time. > > It's the child reaping that breaks it. > > It works fine without multithreading, so how is it libev? > I also considered this - as I already mentioned. I didn't get any feedback from the uClibc people though. But since libev's default child-reaping side-conditions also break waitpid() calls in multithreaded environments, I see the responsibility more on your side. Let's summarize the problem *again*: libev's internally reaps any child process (see childcb() in ev.c). This results in child-reaping race conditions with any concurrent waitpid() call, including system() calls that cannot mask out the signals correctly. If you don't need event-based child termination handling, you can reset SIGCHLD signal handler (presumably to "default" since when "ignoring" the signal, the kernel won't keep zombie-children). But if you still want to use libev for child reaping you can't simply use an ev_signal and do waitpid()s in its handler, e.g. don't reap any child but only children that don't intersect with children reaped concurrently to prevent race conditions. The only solution I'm aware of is using a custom SIGCHLD signal handler and activating an async watcher when the signal handler gets invoked. This is like simulating an ev_signal. Is this the way I'm supposed to go under these circumstances? > So far, you have not explained how libev "breaks" system, btw. On a > naive > implementation, you'd simply get ECHILD, which is documented by POSIX > for > this case, so apparently libev doesn't break anything at all. > > > *any* other waitpid() occurrence by resulting in race > > conditions. waitpid() is even defined as thread-safe. > > The (documented) behaviour of waitpid is not changed by libev though. > That's true, there are no segfaults. But imagine libev or some other library would always "ignore" SIGCHLDs, maybe repeatedly to prevent the user from fixing the handler. waitpid() wouldn't be able to reap children anymore. This would be documented waitpid() behaviour, too. I would think that a library with such side-conditions, effectively breaking a documented system call is poorly designed. > > Making the mechanism asynchronous/non-blocking, would > > require major code refactoring and make conceptually > > simple code dis-proportionally harder to read. > > Seems like an empty claim - can you back it up? Yes. Usually you're using system() for programs that aren't expected to run long enough to cause any trouble. That's why it may well be used in libev watcher handlers. Now if you replace these calls with some implementation that forks and registers an ev_child (you're still interested in the termination status!) you'll get your result in the watcher handler. Since your code might depend on the status, you have to continue in that handler. This complicates otherwise simple code because it cannot be written sequentially anymore. Let's not talk about the problems you would have when using that mechanism from a thread that hasn't got the default loop (the only one supporting signal/child watchers). cheers, Robin -- -- -- managed broadband access -- Travelping GmbH phone: +49-391-8190990 Roentgenstr. 13 fax: +49-391-819099299 D-39108 Magdeburg email: i...@travelping.com GERMANY web: http://www.travelping.com Company Registration: Amtsgericht Stendal Reg No.: HRB 10578 Geschaeftsfuehrer: Holger Winkelmann | VAT ID No.: DE236673780 -- ___ libev mailing list libev@lists.schmorp.de http://lists.schmorp.de/cgi-bin/mailman/listinfo/libev
