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Pthread Library
HP-UX 11i Version 3: February 2007

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pthread_rwlock_rdlock(), pthread_rwlock_tryrdlock() — lock or attempt to lock a read-write lock for reading


#include <pthread.h>

int pthread_rwlock_rdlock( pthread_rwlock_t *rwlock );

int pthread_rwlock_tryrdlock( pthread_rwlock_t *rwlock );



Pointer to the read-write lock to be locked for reading.


The pthread_rwlock_rdlock() function applies a read lock to the read-write lock object referenced by rwlock. The calling thread shall acquire a read lock if a writer does not hold the lock and there are no writers blocked on the lock. It is unspecified whether the calling thread acquires the lock when a writer does not hold the lock and there are writers waiting for the lock. If a writer holds the lock, the calling thread shall not acquire the read lock. If the read lock is not acquired, the calling thread blocks (that is, it does not return from the pthread_rwlock_rdlock() call) until it can acquire the lock. Results are undefined if the calling thread currently owns a write lock on rwlock.

Implementations shall be allowed to favor writers over readers to avoid writer starvation.

A thread may hold multiple concurrent locks on rwlock (that is, successfully call the pthread_rwlock_rdlock() function n times). If so, the thread must perform the matching unlocks (that is, it must call the pthread_rwlock_unlock() function n times).

The function pthread_rwlock_tryrdlock() applies a read lock as in the pthread_rwlock_rdlock() function with the exception that the function fails if any thread holds a write lock on rwlock or there are writers blocked on rwlock.

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, upon return from the signal handler, the thread shall resume waiting for the read-write lock as if it was not interrupted.


Upon successful completion, pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() return zero. Otherwise, an error number is returned to indicate the error (the errno variable is not set).


If any of the following occur, the pthread_rwlock_tryrdlock() function returns the corresponding error number:


The read-write lock rwlock could not be acquired for reading because a writer holds the lock or was blocked on it.

For each of the following conditions, if the condition is detected, the pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions return the corresponding error number:


The value specified by rwlock does not refer to an initialized read-write lock.


The current thread already owns the read-write lock for writing.


The read lock could not be acquired because the maximum number of read locks for rwlock has been exceeded. This error is not detected on HP-UX.


pthread_rwlock_rdlock() and pthread_rwlock_rdlock() were developed by X/Open.


pthread_rwlock_init(3T), pthread_rwlock_destroy(3T), pthread_rwlock_trywrlock(3T), pthread_rwlock_wrlock(3T), pthread_rwlock_unlock(3T).


pthread_rwlock_rdlock(): X/Open. pthread_rwlock_tryrdlock(): X/Open.

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