Re: wait and spurious wakeups

Lew <>
Thu, 29 Nov 2007 20:28:07 -0500
<> wrote:

On 29 Nov, 12:54, Patricia Shanahan <> wrote:

Daniel Pitts wrote:


 From the documentation on wait(int):
"A thread can also wake up without being notified, interrupted, or
timing out, a so-called spurious wakeup. While this will rarely occur in
practice, applications must guard against it by testing for the
condition that should have caused the thread to be awakened, and
continuing to wait if the condition is not satisfied. In other words,
waits should always occur in loops, like this one:"

OK, I concede it is documented, at least if one treats the "behaves like
wait(0)" comment as overriding the direct description of wait(). I'm
still curious about how the thread becomes runnable when there is no
timer and no activity on the monitor.


Yes, so am I. All I get is "it is something to do with the OS, read
Brian's book". Well that book is well and truely on my list now, but I
wonder if anyone apart from Brian actually knows the specific
technical reason.

JCIP says, on p. 300,

"When control re-enters the code calling /wait/, it has reacquired the lock associated with the condition queue. Is the condition predicate now true? Maybe. It might have been true at the time the notifying thread called /notifyAll/, but could have become false again by the time /you/ reacquire the lock. ... Or maybe it hasn't been true at all since you called /wait/. You don't know why another thread called /notify/ or /notifyAll/; maybe it was because /another/ condition predicate associated with the same condition queue became true."

(emphasis original)

All a return from wait() guarantees is that you have the lock, not that the
condition has changed.


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