Re: wait and spurious wakeups

Patricia Shanahan <>
Thu, 29 Nov 2007 21:07:40 -0800
Lew wrote: 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.

I think the true spurious wake-up question is whether wait can return
without timeout or lock acquisition. In some contexts, including the one
described at the start of this thread, if wait has acquired the lock
then the resource is available.

In practice, I always use the "while" loop because it is definitely more
robust. Using "if" breaks if the program changes in any of several ways.

My real question is whether there can be a truly spurious wake-up
without a notifyAll or timeout. Is it real, or is it an urban legend?


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