pthread_rwlock_init(pthread_rwlock_t *restrict rwlock, const pthread_rwlockattr_t *restrict attr); pthread_rwlock_init( ) function shall allocate any resources required to use the read-write lock referenced by rwlock and initializes the lock to an unlocked state with attributes referenced by attr. If attr is NULL, the default read-write lock attributes shall be 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 reinitialized. If successful, the pthread_rwlock_init( ) function shall return zero; otherwise, an error number shall be returned to indicate the error. pthread_rwlock_init( ) function shall fail if: [EAGAIN] The system lacked the necessary resources (other than memory) to initialize another read-write lock. [ENOMEM] Insufficient memory exists to initialize the read-write lock. [EPERM] The caller does not have the privilege to perform the operation. pthread_rwlock_init( ) may fail if: [EBUSY] The implementation has detected an attempt to reinitialize the object referenced by rwlock, a previously initialized but not yet destroyed read-write lock. [EINVAL] The value specified by attr is invalid.