pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock or attempt to lock read-write lock object for reading


cc -mt [ flag... ] file... -lpthread [ library... ]
#include <pthread.h>

int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);

int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);


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.


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.


The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions will fail if:
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:
The current thread already owns the read-write lock for writing.
The pthread_rwlock_tryrdlock() function will fail if:
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.


See attributes(5) for descriptions of the following attributes:
Interface Stability Standard
MT-Level MT-Safe


pthread_rwlock_init(3C), pthread_rwlock_wrlock(3C), pthread_rwlockattr_init(3C), pthread_rwlock_unlock(3C), attributes(5), standards(5)
March 23, 2005