PTHREAD_RWLOCK_RDLOCK(3) OpenBSD Programmer's Manual PTHREAD_RWLOCK_RDLOCK(3)NAME
pthread_rwlock_rdlock, pthread_rwlock_timedrdlock,
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_timedrdlock(pthread_rwlock_t *lock, const struct timespec
*abstime);
int
pthread_rwlock_tryrdlock(pthread_rwlock_t *lock);
DESCRIPTION
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
lock.
The pthread_rwlock_timedrdlock() function performs the same action, but
will not wait beyond abstime to obtain the lock before returning.
The pthread_rwlock_tryrdlock() function performs the same action as
pthread_rwlock_rdlock(), but does not block if the lock cannot be
immediately obtained (i.e., the lock is held for writing or there are
writers waiting).
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(), pthread_rwlock_timedrdlock(),
and pthread_rwlock_tryrdlock() functions will return zero. Otherwise an
error number will be returned to indicate the error.
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_timedrdlock() function will fail if:
[ETIMEDOUT] The time specified by abstime was reached before the lock
could be obtained.
The pthread_rwlock_rdlock(), pthread_rwlock_timedrdlock(), 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).
SEE ALSOpthread_rwlock_init(3), pthread_rwlock_trywrlock(3),
pthread_rwlock_unlock(3), pthread_rwlock_wrlock(3)STANDARDS
The pthread_rwlock_rdlock(), pthread_rwlock_timedrdlock(), and
pthread_rwlock_tryrdlock() functions are expected to conform to Version 2
of the Single UNIX Specification.
HISTORY
The pthread_rwlock_rdlock() function first appeared in FreeBSD 3.0 and
OpenBSD 2.5. The pthread_rwlock_timedrdlock() function first appeared in
OpenBSD 4.8.
OpenBSD 4.9 May 3, 2010 OpenBSD 4.9
