Re: [boost] Boost.Threads, N2178, N2184, et al

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-03-22 12:54:19

Not a direct response to Anthony who already knows most of that, just a 
general explanation of the choices behind N2178:

Anthony Williams wrote:

> Sorry, I misread what you wrote above, and looked at the
> /implementation/ of pthread_once2_np. I agree that it's not that hard
> to write call_once in terms of pthread_once2_np, but that isn't what
> POSIX provides.

The idea behind the _POSIX_CXX09_EXTENSIONS part of N2178 is precisely to 
allow a pthread implementor (in the Windows case that's us) to avoid these 
sources of inefficiency. The join2 functions serve as a similar example. On 
Windows, they are trivially implementable with no overhead using 
WaitForSingleObject on the thread HANDLE. On non-extended POSIX, the 
situation is more complicated.

The join2 functions illustrate another point where I believe N2178 is 
superior to N2184: they expose the superior Windows join model at the C++ 
level instead of the more limited pthread_join. It's trivial to make an 
n2184::thread provide the same semantics under Windows, but nobody would be 
able to take advantage of them portably.