On Mar 25, 2007, at 7:54 PM, Peter Dimov wrote:
Howard Hinnant wrote:
What has me worried about adding try/timed_join to N2184 is actually N2178! :-)
:-)
But that is why N2178 also has pthread_*join2_np. The amount of work that is required from a pthread implementation to make pthread_cancel C++ friendly is much more than what's needed to provide the extensions.
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.
I just got permission to quote Hans Boehm here, since he put it much better than I could in a post to POSIX-C++:
"To me, this reaction makes perfect sense in many contexts, but I'm not sure it makes sense here. I do not work with the people at HP who would have to fix this. But what I've heard in the past is that there is enough dissatisfaction with the fact that cancellation & C++ basically doesn't work that most people would like to see this fixed. The hassle involved in doing the work is probably less than that of listening to the continuous complaints.
My impression is that this is probably one of the more superficial problems with C++ & pthreads, but it seems to be the one that draws the most complaints. And implementers would like guidance as to the correct solution."
It seems very likely to me that once threads are officially recognized by the C++ standard, the customers will demand unified C/C++ cancelation, and do that loudly. So regardless of what the POSIX committee says, pthread_cancel will start throwing something that looks like an exception to C++ code, earlier or later. We should be prepared for that.
<grin> Yeah, glad you're listening to POSIX-C++. Hans speaks quite elegantly. I definitely would like to allow pthread_cancel to throw an exception. I suspect this will even become the norm on 64 bit platforms. But I'm still very worried about 32 bit platforms and I don't think it will be the norm there. There's so many backwards compatibility problems. I agree with you that there are good use cases for try/timed_join. And for the 32 bit systems I'm worried about, they can be added no cost to the N2184 model. For the 64 bit systems, your pthread extension functions look like a good route to pursue for keeping these zero-cost. Note that simply allowing pthread_cancel to throw an exception doesn't make C and C++ cancellation portably interoperable. So we've still got a problem there. Indeed maybe even a bigger one: fclose is allowed to throw! :-\ -Howard