Date: Sun, 30 Mar 2003 21:36:43 -0600 From: Dan Nelson <dnelson@allantgroup.com> To: Sean Hamilton <sh@bel.bc.ca> Cc: hackers@freebsd.org Subject: Re: wait()/alarm() race condition Message-ID: <20030331033643.GO74971@dan.emsphone.com> 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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In the last episode (Mar 30), Sean Hamilton said: > 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 You can probably do something like "alarm(1);" at the top of your handle_sigalarm function. That way after 60 seconds the alarm will fire every second until cleared. A cleaner solution would be to use ualarm(60000,1000) or setitimer() to do this (replacing the alarm(60) call outside the handler). > 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. -- Dan Nelson dnelson@allantgroup.com
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