PTHREAD_RWLOCK_RDLOCK(3) BSD 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 ac-
quired, 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.
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).
SEE ALSOpthread_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.
HISTORY
The pthread_rwlock_rdlock() function first appeared in FreeBSD 3.0 and
OpenBSD 2.5.
MirOS BSD #10-current August 4, 1998 1