DragonFly On-Line Manual Pages
PTHREAD_RWLOCK_RDLOCK(3) DragonFly Library Functions Manual
pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - acquire a read/write
lock for reading
POSIX Threads Library (libpthread, -lpthread)
The pthread_rwlock_rdlock() function acquires a read lock on lock
provided 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
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.
To prevent writer starvation, writers are favored over readers.
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.
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
[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).
The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions are
expected to conform to Version 2 of the Single UNIX Specification
The pthread_rwlock_rdlock() function first appeared in FreeBSD 3.0.
DragonFly 5.9-DEVELOPMENT July 10, 2009 DragonFly 5.9-DEVELOPMENT