Date: Sun, 30 Mar 2003 19:22:55 -0800 (PST) From: =?ISO-8859-1?Q?Mikko_Ty=F6l=E4j=E4rvi?= <mbsd@pacbell.net> To: Sean Hamilton <sh@bel.bc.ca> Cc: hackers@freebsd.org Subject: Re: wait()/alarm() race condition Message-ID: <20030330191611.J1122@atlas.home> In-Reply-To: <007e01c2f730$4b5863d0$0300000a@slugabed.org> References: <001101c2f71d$8d9e4fb0$0300000a@slugabed.org> <20030331023856.GL74971@dan.emsphone.com> <007e01c2f730$4b5863d0$0300000a@slugabed.org>
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On Sun, 30 Mar 2003, Sean Hamilton wrote: > Dan Nelson wrote: > | Just make sure your signal handler has the SA_RESTART flag unset > | (either via siginterrupt() if the handler was installed with signal(), > | or directly if the signal was installed with sigaction() ), and the > | signal will interrupt the wait() call. > > Er, I think you've missed my problem. Or I'm not getting your solution. > > I'm concerned about this order of events: > > - alarm() > - wait() returns successfully > - if (alarmed...) [false] > - SIGALRM is delivered, alarmed = true > - loop > - wait() waits indefinitely > > This is incredibly unlikely to ever happen, but it's irritating me somewhat > that the code isn't airtight. Bad design. Surely there is some atomic means > of setting a timeout on a system call. My stock solution to this kind of problem is to turn those pesky signals into I/O and use an old fashioned select() loop to handle them; create a pipe(2), let signal handlers write one-byte "messages" (the signal number) into the pipe and then use select() to dequeue the events (signals) from the pipe. Select() has a timeout parameter you can play with to your hearts content, and provided you don't overflow the pipe, no events will get lost. You'd have to install a hander for SIGCHLD, of course. $.02, /Mikko
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