Re: [boost] [thread] 2 questions about thread interruption

2016-04-19 20:36 GMT+02:00 Vicente J. Botet Escriba < vicente.botet@wanadoo.fr>:

Le 18/04/2016 23:50, Andrzej Krzemienski a écrit :

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2016-04-18 23:42 GMT+02:00 Andrzej Krzemienski :

Hi,

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I have two questions connected to thread interruption in Boost.Thread.

*1.* The docs say that interrupt_and_join_if_joinable is defined as follows:

``` struct interrupt_and_join_if_joinable{

void operator()(thread& t) { t.interrupt(); if (t.joinable()) { t.join(); } }};

```

Sorry, let me continue with the question. Is there a reason why t.interrupt() is called without checking if t is joinable? This looks uncomfortable to me that I would be potentially calling interrupt() on a not-a-thread.

Interrupting a not-a-thread should do nothing. I will add this to the reference documentation. However nothing prevents to implement it this way

void operator()(thread& t) { if (t.joinable()) { t.interrupt(); t.join(); } }};

I will do it.

Hi Vicente, thanks for the reply.

*2.* When I make a call to this_thread::wait_for(),

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Do you mean sleep_for here?

Yes :) sleep_for(). But in general, my concern applies to any operation that can potentially block.

blocking for a long

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time in thread t2, and the instant later I call t2.interrupt() from another thread t1. Do I have to wait the long time until wait_for() stops blocking t2, or does thread_interrupted get called 'immediately' after interrupt() is called from t1?

It depends. If sleep_for is implemented using nanosleep()/pthread_delay_np() there is no way to throw the thread_interrupted exception. However, if it is implemented waiting on a condition variable wait_for, then the exception will be throw before the duration elapses.

Currently, sleep_for make use of nanosleep/pthread_delay_np when available. The advantage been that the sleep is steady/monotonic.

So, I understand that this means that the interface (as opposed to the current or any future implementation) can only guarantee that in the worst case thread_interrupted is called *after* the operation unblocks the thread. Does the same guarantee apply to other blocking operations on mutex and or conditional variables?

*3.* Does the support for interruption points require the other

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synchronization operations (like locks on mutexes) to be slower (in order to monitor interruption requests)?

Not for locks and mutex, but yes it will be a little bit slower for condition variables, as we need to store the condition variable on which a thread could be waiting for.

I've on my TODO list a task to build interuptible threads on top of raw threads, but this will need to have also specific condition variables :(

Perhaps I owe you some background on these questions. I wanted to use boost::thread in place of std::thread in my job just because of this interruption mechanism that I would otherwise need to implement manually. But other colleagues raised concerns about what guarantees are really offered by the interruption mechanism. Regards, &rzej