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Section: 33.6.4.2.2 [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:
33.6.4 [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?
33.6.4.2.2 [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
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:
- 33.6.4 [thread.mutex.requirements]/24: NAD. If the precondition fails the program has undefined behaviour and therefore an implementation may throw an exception already.
- 33.6.4.2.2 [thread.mutex.class]/3 bullet 2: Already addressed by issue 893.
- 33.6.4.2.2 [thread.mutex.class]/3 proposed addition: NAD. This is already covered by the mutex requirements, which have ownership as a Precondition.
Proposed resolution: