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Subject: Re: [boost] [thread] Can Boost.Thread use Boost.Atomic without falling on a compatibility issue?
From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2013-01-13 12:38:37

On Jan 13, 2013, at 9:57 AM, Vicente J. Botet Escriba <vicente.botet_at_[hidden]> wrote:

> This algorithm is efficient but I suspect that this algorithm is not always correct for the same reasons the Boost.Thread current algorithm is not. But on which platforms the libc++ implementation is correct? Howard, could you comment if you are reading this post.

Yes, libc++ is currently using double-checked locking for call_once. The rationale is two-fold:

1. It is actually correct for those platforms I need to target today.
2. A correct implementation using atomic_load with memory_order_acquire wasn't available when I wrote call_once. <atomic> appeared in libc++ about 2 years later. It needed compiler support which was slow in coming.

There are a couple of places in libc++ that need to be retrofitted to use <atomic>. In the cases where that retrofit is in a header (i.e. call_once), the retrofit will currently need to be protected by __has_feature(cxx_atomic) as clang shuts this feature off in C++03 mode.


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