DESCRIPTION
The
pthread_rwlock_rdlock() function applies a read lock to the read-write lock referenced by
rwlock. The calling thread acquires the read lock if a writer does not hold the lock and there are no writers blocked on the lock.
The calling thread does not acquire the lock if a writer holds the lock or if writers of higher or equal priority are blocked on the lock; otherwise, the calling thread acquires the lock. If the read lock is not acquired, the calling thread blocks until it can acquire the lock.
A thread can hold multiple concurrent read locks on
rwlock (that is, successfully call the
pthread_rwlock_rdlock() function
n times). If so, the thread must perform matching unlocks (that is, it must call the
pthread_rwlock_unlock() function
n times).
The maximum number of concurrent read locks that a thread can hold on one read-write lock is currently set at 100,000, though this number could change in a future release. There is no imposed limit on the number of different threads that can apply a read lock to one read-write lock.
The
pthread_rwlock_tryrdlock() function applies a read lock like the
pthread_rwlock_rdlock() function, with the exception that the function fails if the equivalent
pthread_rwlock_rdlock() call would have blocked the calling thread. In no case will the
pthread_rwlock_tryrdlock() function ever block. It always either acquires the lock or fails and returns immediately.
Results are undefined if any of these functions are called with an uninitialized read-write lock.
If a signal is delivered to a thread waiting for a read-write lock for reading, upon return from the signal handler the thread resumes waiting for the read-write lock for reading as if it was not interrupted.
RETURN VALUES
If successful, the
pthread_rwlock_rdlock() function returns
0. Otherwise, an error number is returned to indicate the error.
The
pthread_rwlock_tryrdlock() function returns
0 if the lock for reading on the read-write lock object referenced by
rwlock is acquired. Otherwise an error number is returned to indicate the error.
ERRORS
The
pthread_rwlock_rdlock() and
pthread_rwlock_tryrdlock() functions will fail if:
EAGAIN
The read lock could not be acquired because the maximum number of read locks by the current thread for rwlock has been exceeded.
The
pthread_rwlock_rdlock() function will fail if:
EDEADLK
The current thread already owns the read-write lock for writing.
The
pthread_rwlock_tryrdlock() function will fail if:
EBUSY
The read-write lock could not be acquired for reading because a writer holds the lock or a writer with the appropriate priority was blocked on it.