RWLOCK(9)

NAME
rwlock, rw_init, rw_enter, rw_exit, rw_enter_read, rw_enter_write, rw_exit_read, rw_exit_write - interface to read/write locks

SYNOPSIS
#include <sys/rwlock.h>
void
rw_init(struct rwlock *rwl, const char *name);
int
rw_enter(struct rwlock *rwl, int flags);
void
rw_exit(struct rwlock *rwl);
void
rw_enter_read(struct rwlock *rwl);
void
rw_enter_write(struct rwlock *rwl);
void
rw_exit_read(struct rwlock *rwl);
void
rw_exit_write(struct rwlock *rwl);
void
rw_assert_wrlock(struct rwlock *rwl);
void
rw_assert_rdlock(struct rwlock *rwl);
void
rw_assert_unlocked(struct rwlock *rwl);

DESCRIPTION
The rwlock set of functions provides a multiple-reader, single-writer locking mechanism to ensure mutual exclusion between different processes.

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 process 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 in between.
RW_INTR
    When waiting for a lock, allow signals to interrupt the sleep.
RW_NOSLEEP
    Do not wait for busy locks, fail with EBUSY instead.
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 necessary.

The rw_exit_read() function releases a read lock.

The rw_exit_write() function releases a write lock.

Read locks can be acquired while the write lock is not held, and may coexist in distinct processes at any time. A write lock, however, can only be acquired when there are no read locks held, granting exclusive access to a single process.

The rw_assert_wrlock(), rw_assert_rdlock(), and rw_assert_unlocked() functions check the status rwl, panicking if it is not write-, read-, or unlocked, respectively.

SEE ALSO
lockmgr(9), mutex(9), spl(9)

HISTORY
The rwlock functions first appeared in OpenBSD 3.5.

AUTHORS
The rwlock functions were written by Artur Grabowski <art@openbsd.org>.

CAVEATS
While it is safe to sleep with an rwlock held, they cannot be used in an interrupt handler as an rwlock is bound to a process.