On Mar 25, 2007, at 5:53 PM, Emil Dotchevski wrote:
If this is correct, and if N2178 is accepted, then zero-overhead try/ timed join becomes available across the the board. If N2184 is accepted, then zero-overhead try/timed join becomes unavailable across the board, regardless of whether it is possible from a technical point of view.
I've been hesitant to say this, but it may be possible to add try/ timed_join to N2184. I know that it definitely could be added to my pthreads-based implementation of N2184 (well, I haven't actually done it so maybe definitely is too strong - 98% sure). And by added here, I mean at no extra cost. What has me worried about adding try/timed_join to N2184 is actually N2178! :-) I implemented N2184 for one of the "problem platforms" that I mentioned in my post concerning C/C++ cancellation interoperability: Mac OS X. I can not currently implement a pthread_cancel on this platform that will work in C++. This is both good and bad. The bad is obvious: no C/C++ interoperability. The good is that it forced me to not implement std::thread::join in terms of pthread_join. Because thread::join must be a C++ cancellation point, and pthread_join isn't a C++ cancellation point (if you can't use pthread_cancel), then one must implement thread::join in terms of a mutex/cv stored in thread local data (and thread local data is hopefully coming via N2147). The reason for this is that condition is the only trigger I have in the tool box to wake a sleeping thread so it can be canceled. That makes it zero cost to add siblings of thread::join which do try_join (just check the done flag in thread local data), and timed_join (use cv/timed_wait). However if thread::cancel is implemented in terms of pthread_cancel, that means that there is pressure to implement thread::join in terms of pthread_join (because cancellation then works). And there is no pthread_try_join or pthread_timed_join so if these were required at the C++ level, then you've added a feature that adds cost. The other, extremely remote, possibility is to ask the posix group to add pthread_try_join and pthread_timed_join. They might even be willing to do this. However there is just no way we're going to get that committee to mandate that pthread_cancel emits a catchable C++ exception. And in that case pthread_try_join and pthread_timed_join are useless to us anyway. Thread cancellation is at the heart of a lot of these issues! :-\ -Howard