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905. Mutex specification questions

Section: 32.6.4.2.2 [thread.mutex.class] Status: Dup Submitter: Herb Sutter Opened: 2008-09-18 Last modified: 2016-01-28

Priority: Not Prioritized

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Duplicate of: 893

Discussion:

A few questions on the current WP, N2723:

32.6.4 [thread.mutex.requirements]/24 says an expression mut.unlock() "Throws: Nothing." I'm assuming that, per 99 [res.on.required], errors that violate the precondition "The calling thread shall own the mutex" opens the door for throwing an exception anyway, such as to report unbalanced unlock operations and unlocking from a thread that does not have ownership. Right?

32.6.4.2.2 [thread.mutex.class]/3 (actually numbered paragraph "27" in the WP; this is just a typo I think) says

The behavior of a program is undefined if:

• it destroys a mutex object owned by any thread,
• a thread that owns a mutex object calls lock() or try_lock() on that object, or
• a thread terminates while owning a mutex object.

As already discussed, I think the second bullet should be removed, and such a lock() or try_lock() should fail with an exception or returning false, respectively.

A potential addition to the list would be

• a thread unlocks a mutex it does not have ownership of.

but without that the status quo text endorses the technique of the program logically transferring ownership of a mutex to another thread with correctness enforced by programming discipline. Was that intended?

[ Summit: ]

Two resolutions: "not a defect" and "duplicate", as follows:

• 32.6.4 [thread.mutex.requirements]/24: NAD. If the precondition fails the program has undefined behaviour and therefore an implementation may throw an exception already.
• 32.6.4.2.2 [thread.mutex.class]/3 bullet 2: Already addressed by issue 893⁽ⁱ⁾.
• 32.6.4.2.2 [thread.mutex.class]/3 proposed addition: NAD. This is already covered by the mutex requirements, which have ownership as a Precondition.

Proposed resolution: