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From: Victor A. Wagner Jr. (vawjr_at_[hidden])
Date: 2006-01-14 16:43:31

At 01:02 2006-01-14, Robin Boerdijk wrote:
>I don't agree with the rationale for excluding a Windows event
>object-like construct from the Boost thread package. The rationale
>justifies the exclusion based on the "Concurrent Programming Concepts"
>paper by Per Brich Hansen in which event variables are dismissed as
>unsafe. The fundamental problem of event variables according to
>Hansen's paper is that,
>"if the sender causes [signals] the event before the receiver waits for
>it, the receiver will remain delayed until the next event is caused
>However, Windows event objects don't work like this. From the API
>"When the state of an auto-reset event object is signaled, it remains
>signaled until a single waiting thread is released; the system then
>automatically resets the state to nonsignaled. If no threads are
>waiting, the event object's state remains signaled."
>In other words, the receiver will _not_ remain delayed if the sender
>signals the event before the receiver waits for it.

I think this the a case of usurping names of things that had
definitions and changing them (MS in this case)
and event _used to be_ (well back in the '70s when Brinch was at
Caltech and we were down in OC writing real-time systems for
minicomputers) a semaphore that released ALL waiting entities (I
guess MS calls them pulse events now).
They aren't dangerous at all, you simply have to KNOW what you're playing with.
There are many different policies (in the Alexandrescu terminology)
one can have regarding semaphores, IMO these are only a couple of
them. Unfortunatly, since I've gotten out of the architecture/design
of OSs people seem to have started assigning names (not always
ensuring consistancy) to some of these flavors (as we called them) of

>Based on this observation, I think the reference to Hansen's paper
>should be removed from the rationale. It does not apply to Windows
>event objects and I had to spend $10 to download the paper from the ACM
>website and find this out.
>Furthermore, according to the rationale, "Experienced programmers using
>the Windows platform today report that event variables are a continuing
>source of errors". Are there any links to documents that describe these
>problems? Are these problems perhaps related to the PulseEvent()
>method, which does cause the behaviour Hansen described but has been
>deprecated (the API documentation states "This function is unreliable
>and should not be used. It exists mainly for backward compatibility").
>In short, are Windows event objects still considered harmful?

personally I don't regard _any_ of the semaphores as "harmful"
provided the user understands what s/he is using.

>Robin Boerdijk
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Victor A. Wagner Jr.
The five most dangerous words in the English language:
               "There oughta be a law"

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