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PTHREAD_RWLOCK_RDLOCK(3) BSD Library Functions Manual PTHREAD_RWLOCK_RDLOCK(3) NAME pthread_rwlock_rdlock, pthread_rwlock_tryrdlock -- acquire a read/write lock for reading SYNOPSIS #include <pthread.h> int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock); int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock); DESCRIPTION The pthread_rwlock_rdlock() function acquires a read lock on rwlock, pro-vided provided vided that rwlock is not presently held for writing and no writer threads are presently blocked on the lock. If the read lock cannot be immedi-ately immediately ately acquired, the calling thread blocks until it can acquire the lock. The pthread_rwlock_tryrdlock() function performs the same action, but does not block if the lock cannot be immediately obtained (i.e., the lock is held for writing or there are waiting writers). A thread may hold multiple concurrent read locks. If so, pthread_rwlock_unlock() must be called once for each lock obtained. The results of acquiring a read lock while the calling thread holds a write lock are undefined. IMPLEMENTATION NOTES To prevent writer starvation, writers are favored over readers. RETURN VALUES If successful, the pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions will return zero. Otherwise, an error number will be returned to indicate the error. SEE ALSO pthread_rwlock_init(3), pthread_rwlock_trywrlock(3), pthread_rwlock_unlock(3), pthread_rwlock_wrlock(3) STANDARDS The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions are expected to conform to Version 2 of the Single UNIX Specification (``SUSv2''). ERRORS The pthread_rwlock_tryrdlock() function will fail if: [EBUSY] The lock could not be acquired, because a writer holds the lock or was blocked on it. The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions may fail if: [EAGAIN] The lock could not be acquired, because the maximum number of read locks against lock has been exceeded. [EDEADLK] The current thread already owns rwlock for writing. [EINVAL] The value specified by rwlock is invalid. [ENOMEM] Insufficient memory exists to initialize the lock (applies to statically initialized locks only). HISTORY The pthread_rwlock_rdlock() function first appeared in FreeBSD 3.0. BSD August 4, 1998 BSD |