.. _Reader_Writer_Mutexes: Reader Writer Mutexes ===================== Mutual exclusion is necessary when at least one thread *writes* to a shared variable. But it does no harm to permit multiple readers into a protected region. The reader-writer variants of the mutexes, denoted by ``_rw_`` in the class names, enable multiple readers by distinguishing *reader locks* from *writer locks.* There can be more than one reader lock on a given mutex. Requests for a reader lock are distinguished from requests for a writer lock via an extra boolean parameter in the constructor for ``scoped_lock``. The parameter is false to request a reader lock and true to request a writer lock. It defaults to ``true`` so that when omitted, a ``spin_rw_mutex`` or ``queuing_rw_mutex`` behaves like its non-``_rw_`` counterpart.
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