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The pthread_rwlock_init() function initializes the read-write lock referenced by rwlock with the attributes referenced by attr.
If attr is NULL, the default read-write lock attributes are used; the effect is the same as passing the address of a default read-write lock attributes object. Once initialized,
the lock can be used any number of times without being re-initialized. Upon successful initialization, the state of the read-write lock becomes initialized and unlocked. Results are undefined if pthread_rwlock_init() is called specifying an already initialized read-write lock. Results are undefined if a read-write lock is used without first being initialized.
If the pthread_rwlock_init() function fails, rwlock is not initialized and the contents of rwlock are undefined.
The pthread_rwlock_destroy() function destroys the read-write lock object referenced by rwlock and releases any resources used by the lock. The effect
of subsequent use of the lock is undefined until the lock is re-initialized by another call to pthread_rwlock_init(). An implementation may cause pthread_rwlock_destroy()
to set the object referenced by rwlock to an invalid value. Results are undefined if pthread_rwlock_destroy() is called when any thread holds rwlock. Attempting to destroy an uninitialized read-write lock results in undefined behaviour. A destroyed read-write lock object can be re-initialized using pthread_rwlock_init(); the results of otherwise referencing the read-write lock object after it has been destroyed are undefined.
In cases where default read-write lock attributes are appropriate, the macro PTHREAD_RWLOCK_INITIALIZER can be used to initialize read-write locks that are
statically allocated. The effect is equivalent to dynamic initialization by a call to pthread_rwlock_init() with the parameter attr specified as NULL, except
that no error checks are performed.
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