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1494. Term "are serialized" not defined

Section: 32.6.7.2[thread.once.callonce]Status:C++11Submitter: INCITSOpened: 2010-08-25Last modified: 2016-01-28

Priority:Not Prioritized

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Discussion:

Addresses US-190

The term "are serialized" is never defined (32.6.7.2[thread.once.callonce] p. 2).

[Resolution proposed by ballot comment:]

Remove the sentence with "are serialized" fromparagraph 2. Add "Calls tocall_once on the sameonce_flag object shall not introduce data races(17.6.4.8)." to paragraph 3.

[2010-11-01 Daniel translates NB comment into wording]

[2011-02-17: Hans proposes an alternative resolution]

[2011-02-25: Hans, Clark, and Lawrence update the suggested wording]

[2011-02-26 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Change 32.6.7.2[thread.once.callonce] p.2+3 as indicated:

template<class Callable, class ...Args>void call_once(once_flag& flag, Callable&& func, Args&&... args);

[..]

2Effects:Calls tocall_once on the sameonce_flag object are serialized.If there has been a prior effective call tocall_once on the sameonce_flag object, the call tocall_once returns without invokingfunc. If there has been no prior effective call tocall_once on the sameonce_flag object,INVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...) is executed. The call tocall_once is effective if and only ifINVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...) returns without throwing an exception. If an exception is thrown it is propagated to the caller.An execution ofcall_once that does not call itsfunc is a passive execution.An execution ofcall_once that calls itsfunc is an active execution.An active execution shall callINVOKE(decay_copy(std::forward<Callable>(func)),decay_copy(std::forward<Args>(args))...). If such a call tofunc throws an exception,the execution is exceptional, otherwise it is returning.An exceptional execution shall propagate the exception to the caller ofcall_once.Among all executions ofcall_once for any givenonce_flag: at most one shall be a returning execution; if there is a returning execution, it shall be the last active execution; andthere are passive executions only if there is a returning execution.[Note: Passive executions allow other threads to reliably observe the results produced by the earlier returning execution. —end note]

3Synchronization:The completion of an effective call tocall_once on aonce_flag objectsynchronizes with (6.10.2[intro.multithread]) all subsequent calls tocall_once on the sameonce_flag object.For any givenonce_flag: all active executions occur in a total order; completion of an active execution synchronizes with (6.10.2[intro.multithread]) the start of the next one in this total order; and the returning execution synchronizes with the return from all passive executions.