RWLOCK(9) BSD Kernel Developer's Manual RWLOCK(9)
rwlock, rw_init, rw_enter, rw_exit, rw_enter_read, rw_enter_write,
rw_exit_read, rw_exit_write, rw_assert_wrlock, rw_assert_rdlock,
rw_assert_unlocked, rw_status, RWLOCK_INITIALIZER, rrw_init, rrw_enter,
rrw_exit, rrw_status -- interface to read/write locks
rw_init(struct rwlock *rwl, const char *name);
rw_enter(struct rwlock *rwl, int flags);
rw_exit(struct rwlock *rwl);
rw_enter_read(struct rwlock *rwl);
rw_enter_write(struct rwlock *rwl);
rw_exit_read(struct rwlock *rwl);
rw_exit_write(struct rwlock *rwl);
rw_assert_wrlock(struct rwlock *rwl);
rw_assert_rdlock(struct rwlock *rwl);
rw_assert_unlocked(struct rwlock *rwl);
rw_status(struct rwlock *rwl);
RWLOCK_INITIALIZER(const char *name);
rrw_init(struct rrwlock *rrwl, const char *name);
rrw_enter(struct rrwlock *rrwl, int flags);
rrw_exit(struct rrwlock *rrwl);
rrw_status(struct rrwlock *rrwl);
The rwlock set of functions provides a multiple-reader, single-writer
locking mechanism to ensure mutual exclusion between different threads.
Read locks can be acquired while the write lock is not held, and may
coexist in distinct threads at any time. A write lock, however, can only
be acquired when there are no read locks held, granting exclusive access
to a single thread.
The rw_init() function is used to initiate the lock pointed to by rwl.
The name argument specifies the name of the lock, which is used as the
wait message if the thread needs to sleep.
The rw_enter() function acquires a lock. The flags argument specifies
what kind of lock should be obtained and also modifies the operation.
The possible flags are:
RW_READ Acquire a shared lock.
RW_WRITE Acquire an exclusive lock.
RW_DOWNGRADE Safely release an exclusive lock and acquire a
shared lock without letting other exclusive locks
RW_INTR When waiting for a lock, allow signals to inter-
rupt the sleep.
RW_NOSLEEP Do not wait for busy locks, fail with EBUSY
RW_SLEEPFAIL Wait for busy locks, but do not obtain them, fail
with EAGAIN instead.
The rw_exit() function is used to release a held lock.
The rw_enter_read() function acquires a read lock, sleeping if necessary.
The rw_enter_write() function acquires a write lock, sleeping if neces-
The rw_exit_read() function releases a read lock.
The rw_exit_write() function releases a write lock.
The rw_assert_wrlock(), rw_assert_rdlock(), and rw_assert_unlocked()
functions check the status rwl, panicking if it is not write-, read-, or
rw_status returns the current state of the lock:
RW_WRITE Lock is write locked by the calling thread.
RW_WRITE_OTHER Lock is write locked by a different thread.
RW_READ Lock is read locked. The current thread may be one
of the threads that has it locked.
0 Lock is not locked.
A lock declaration may be initialised with the RWLOCK_INITIALIZER()
macro. The name argument specifies the name of the lock, which is used
as the wait message if the thread needs to sleep.
The rrwlock functions support recursive write locking by the same
process. They otherwise behave the same as their rwlock counterparts.
rw_init() and rrw_init() can be called during autoconf, from process con-
text, or from interrupt context.
All other functions can be called during autoconf or from process con-
The rwlock functions first appeared in OpenBSD 3.5.
The rwlock functions were written by Artur Grabowski <email@example.com>.
BSD July 9, 2014 BSD