PTHREAD_RWLOCK_RDLOCK(3)  OpenBSD Programmer's 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 *lock);

     int
     pthread_rwlock_tryrdlock(pthread_rwlock_t *lock);

DESCRIPTION
     The pthread_rwlock_rdlock() function acquires a read lock on lock provid-
     ed that lock is not presently held for writing and no writer threads are
     presently blocked on the lock.  If the read lock cannot be immediately
     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.

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 lock for writing.

     [EINVAL]      The value specified by lock 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 OpenBSD 2.5.

OpenBSD 2.6                     August 4, 1998                               2
