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From: Beman Dawes (bdawes_at_[hidden])
Date: 2007-11-13 12:36:51

Phil Endecott wrote:
> Anthony Williams wrote:
>> The interruption points are:
>> thread::join, thread::timed_join
>> this_thread::sleep
>> condition_variable::wait, condition_variable::timed_wait
>> condition_variable_any::wait, condition_variable_any::timed_wait
>> this_thread::interruption_point
> Am I the only one who thinks that not having cancellation points at
> blocking I/O makes this fairly useless? Perhaps I am! I'm curious to
> hear about use-cases where Anthony or Preston's proposals are appropriate.

The folks in the C++ committee's ad hoc thread working groups have been
discussing this for the last year or so. Several points that came up in
those discussions may be worth repeating:

* For some types of I/O on some operating systems there is simply no way
to interrupt a blocking I/O operation once it has started. Even
rewriting the operating system itself wouldn't help for some hardware.
Rewriting the operating system isn't practical, anyhow.

* Experienced (and respected) users of similar cooperative interrupt
facilities report that while far from perfect, these imperfect
facilities are still far better and more useful than having no interrupt
facilities at all. AFAICR, those comments applied both to cooperative
interruption in general and the problem of blocking I/O in particular.

Very smart and knowledgeable designers have tried for decades to come up
with really good solutions to the cancellation problem in general. They
have all failed, regardless of the language involved. This is one tough
problem. The only solutions that seem to work at all are the limited
cooperative solutions.


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