std::recursive_mutex::unlock (3) - Linux Manuals
void unlock(); (since C++11)
Unlocks the mutex if its level of ownership is 1 (there was exactly one more call to lock() than there were calls to unlock() made by this thread), reduces the level of ownership by 1 otherwise.
The mutex must be locked by the current thread of execution, otherwise, the behavior is undefined.
This operation synchronizes-with (as defined in std::memory_order) any subsequent lock operation that obtains ownership of the same mutex.
unlock() is usually not called directly: std::unique_lock and std::lock_guard are used to manage exclusive locking.
This section is incomplete
Reason: no example
lock (public member function)
try_lock (public member function)