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Some platforms, particularly Windows, do not have a single method that can release both a held reader lock and a held writer lock; instead, a separate method (ReleaseSRWLockShared or ReleaseSRWLockExclusive) must be called in each case. This may also be necessary to back MutexRW with a shared_mutex in C++14; the current language proposal includes both an unlock() and a shared_unlock() method.
This directory contains interfaces and implementations that isolate the rest of the package from platform details. Code in the rest of the package includes "port.h" from this directory. "port.h" in turn includes a platform specific "port_<platform>.h" file that provides the platform specific implementation. See port_posix.h for an example of what must be provided in a platform specific header file.