Re: [Boost-users] Newbie question

Stopping the execution from outside is very fragile. The main problem is that you don't know, what code is currently executed. A simple idea to accomplish your task is to suspend the thread in which the function executes. That will put the thread into the waiting state. Than you can interrupt this waiting thread. But it is possible, that your code ends up in a deadlock, because if the interrupted thread is not going to free all resources used (like mutex locks or whatever) the code might deadlock. Another point here is that you might likely produce race conditions. If you suspend the thread which currently executes a critical region it might not end up writing all data, so that if interrupt is issued the written objects might have a corrupted state. On the other hand if you have access to mutex variable (or whatever synchronization primitive, which controls the critical region access within the function to be cancelled) you can lock it and issue the thread interruption. This will ensure that the function to be cancelled has exited the critical region and is either waiting to acquire lock (can be cancelled) or will enter the wait for lock and cancelled than. But that is not all ;) Actually, at that point the function which issued the thread interrupt must wait until the other function is cancelled (because interruption can only happen if the thread is in the waiting state) and the interruption exception (if using boost::thread_interrupt) is not propagated outside the cancelled thread. Therefore, a wrapper function should be introduced which catches the exception and notifies the cancellation requestor about successful cancellation. Here another mutex+condition variable+flag tuple can be used. Additionally, the canceller should ensure that the function did not exit before (otherwise it will block indefinitely=>use the flag) and wait for notification of condition variable with timeouts. If you are interested I can summarize it in pseudo-code. To get back to your post: exception !!management!! does not matter. Exception SAFETY matters. You must ensure that this ported to C++ function automatically free all acquired resources or unlocks other mutex etc., when an exception is initiated. But I would be really VERY careful with proposed solution. If someone changes the function the entire approach might be broken. So don't interrupt smth, what was not designed for interruption ;) Regards, Ovanes On Mon, Dec 7, 2009 at 6:43 PM, Vadim Ryvchin wrote:

There is no exception management in the function. Its code based on C and migrated to C++. I can wrapper it with try catch.

Vadim.

*From:* boost-users-bounces@lists.boost.org [mailto: boost-users-bounces@lists.boost.org] *On Behalf Of *Ovanes Markarian *Sent:* Monday, December 07, 2009 19:34

*To:* boost-users@lists.boost.org *Subject:* Re: [Boost-users] Newbie question

Is that function exception-safe?

Regards,

Ovanes

On Mon, Dec 7, 2009 at 6:00 PM, Vadim Ryvchin wrote:

I can’t change the function so it will check the token, that is my main problem. I want to be able to stop the execution from outside the function.

Thank you anyway,

Vadim.

*From:* boost-users-bounces@lists.boost.org [mailto: boost-users-bounces@lists.boost.org] *On Behalf Of *Ovanes Markarian *Sent:* Monday, December 07, 2009 18:09 *To:* boost-users@lists.boost.org *Subject:* Re: [Boost-users] Newbie question

I would suggest to introduce a Cancellation Token as parameter to both functions. If one function becomes ready it sets the cancelation flag to true, so that other function can check it and exit. In the most simple case it can be a volatile boolean value passed by reference to both functions.

Hope that helps,

Ovanes

On Mon, Dec 7, 2009 at 1:44 PM, Vadim Ryvchin wrote:

Hello,

I have a problem with interruption of threads. I have one function that I want to run in two different threads with different parameters. When one of the function's instances finishes it stops the other instance. I can't change the function's code, but I can write wrapper to it. My problem is in stopping the other thread. Using timed_join I find which thread is finished, but I can't find a good solution for stopping the second one. How should I use interrupt method correctly or suggest me for correct thread stopping function. BTW, I can't just leave the other thread to run, it's very high CPU consumer.

Thank you all.

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