N1963: unclear specification of mtx_trylock on non-recursive mutexes

Submitter: Torvald Riegel triegel@redhat.com, Martin Sebor msebor@redhat.com
Submission Date: 2015-09-25


The specification of mtx_trylock if applied to a non-recursive mutex is not clear. Whereas it is spelled out for mtx_lock that a thread must not attempt to lock a non-recursive mutex more than once, there is no such requirement for mtx_trylock. The existing wording for mtx_trylock could be understood as requiring mtx_trylock to fail; however, that would defeat the purpose of separating recursive from non-recursive mutexes because it would require implementations to track which thread owns the mutex.

(It might also be good if the standard would define what recursive locking actually is, but this is outside of the focus of this paper.)

Suggested Technical Corrigendum

The standard should specify the requirement for mtx_lock explicitly for mtx_trylock as well. Specifically, add the following sentence to

If the mutex is non-recursive, it shall not be locked by the calling thread.