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### 3030. Who shall meet the requirements of `try_lock`?

Section: 33.6.6 [thread.lock.algorithm] Status: C++20 Submitter: Jonathan Wakely Opened: 2017-11-07 Last modified: 2021-02-25

Priority: 0

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Discussion:

33.6.6 [thread.lock.algorithm] says:

> "If a call to `try_lock()` fails, `unlock()` shall be called for all prior arguments and there shall be no further calls to `try_lock()`."

We try to use "shall" for requirements on the user (e.g. as in the previous paragraph) which is absolutely not what is meant here.

[2017-11 Albuquerque Wednesday night issues processing]

Moved to Ready

[2018-3-17 Adopted in Jacksonville]

Proposed resolution:

This wording is relative to N4700.

1. Change 33.6.6 [thread.lock.algorithm] as indicated:

   >     template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);

   > > -1- Requires: […]

   > > -2- Effects: Calls `try_lock()` for each argument in order beginning with the first until all arguments have been processed or a call to `try_lock()` fails, either by returning `false` or by throwing an exception. If a call to `try_lock()` fails, `unlock()` shall be is called for all prior arguments and there shall be with no further calls to `try_lock()`.

   > > […]

   >     template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);

   > > -4- Requires: […]

   > > -5- Effects: All arguments are locked via a sequence of calls to `lock()`, `try_lock()`, or `unlock()` on each argument. The sequence of calls shall does not result in deadlock, but is otherwise unspecified. [Note: A deadlock avoidance algorithm such as try-and-back-off must be used, but the specific algorithm is not specified to avoid over-constraining implementations. — end note] If a call to `lock()` or `try_lock()` throws an exception, `unlock()` shall be is called for any argument that had been locked by a call to `lock()` or `try_lock()`.