----- Original Message ----- > From: "Marc Lehmann" <schm...@schmorp.de>
> 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
