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Re: Is `std::ios_base::xalloc' free from data races?

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Am 24.06.2012 09:03, schrieb Ai Azuma:
>
> Hi folks,
>
> I have a question about the data-race-freedom of
> `std::ios_base::xalloc'.
>
> `std::ios_base::xalloc' has to read and modify an internal counter
> object that is shared among threads. So, the implementation has to
> synchronize the operations on the counter object, otherwise it may
> introduce data races.
>
> I believe that the standard wording implicitly specifies `xalloc' is
> free from data races, because the internally used shared counter
> object "is not visible to users and is protected against data
> races" (quoted from 17.6.5.9/7).
>
> However, `std::ios_base::index' is declared to be `static int' (marked
> with "exposition only") in 27.5.3 (`std::ios_base' definition), and
> 27.5.3.5/1 says that `xalloc' returns `index++'. Therefore,
> straightforward reading of these paragraphs falls into the conclusion
> that concurrent invocations of `xalloc' may introduce data races.


I agree with your analysis that the current specification may
introduce data races. There already exists a library issue submission
request (LWG 2143 "ios_base::xalloc should be thread-safe") not yet
visible in the official list.

> Which is the intent of the standard, then?
>
> If the former interpretation (i.e., `xalloc' is free from data races)
> is the intent of the standard, some editorial modifications (e.g.,
> declaring `std::ios_base::index' to be `static std::atomic_int'
> instead of `static int' in 27.5.3) would make the wording much
> clearer.
>
> I would like to hear from experts. Thank you in advance for your help.


I think we miss normative wording that provides the necessary
guarantee. We better should add explicit normative wording to that
effect, IMO.

HTH & Greetings from Bremen,

Daniel Krügler




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Thread

Is `std::ios_base::xalloc' free from data races? Ai Azuma<ai.azuma@gmail.com> - 2012-06-24 00:03 -0700
  Re: Is `std::ios_base::xalloc' free from data races? Daniel Krügler <daniel.kruegler@googlemail.com> - 2012-06-24 23:48 -0700
    Re: Is `std::ios_base::xalloc' free from data races? Ai Azuma<ai.azuma@gmail.com> - 2012-06-25 08:45 -0700

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