NAME
rwlock
, rw_init
,
rw_init_flags
, rw_enter
,
rw_exit
, rw_enter_read
,
rw_enter_write
,
rw_exit_read
, rw_exit_write
,
rw_assert_wrlock
,
rw_assert_rdlock
,
rw_assert_anylock
,
rw_assert_unlocked
,
rw_status
,
RWLOCK_INITIALIZER
,
rrw_init
, rrw_init_flags
,
rrw_enter
, rrw_exit
,
rrw_status
—
interface to read/write
locks
SYNOPSIS
#include
<sys/rwlock.h>
void
rw_init
(struct
rwlock *rwl, const char
*name);
void
rw_init_flags
(struct
rwlock *rwl, const char
*name, int
flags);
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_anylock
(struct
rwlock *rwl);
void
rw_assert_unlocked
(struct
rwlock *rwl);
int
rw_status
(struct
rwlock *rwl);
RWLOCK_INITIALIZER
(const
char *name);
void
rrw_init
(struct
rrwlock *rrwl, const char
*name);
void
rrw_init_flags
(struct
rrwlock *rrwl, const char
*name, int
flags);
int
rrw_enter
(struct
rrwlock *rrwl, int
flags);
void
rrw_exit
(struct
rrwlock *rrwl);
int
rrw_status
(struct
rrwlock *rrwl);
DESCRIPTION
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_init_flags
()
macro is similar to rw_init
(), but it additionally
accepts a bitwise OR of the following flags:
RWL_DUPOK
- Prevents witness(4) from logging when a thread acquires more than one lock of this lock type.
RWL_IS_VNODE
- Make witness(4) ignore
lock order issues between this lock type and any other lock type tagged
with the
RWL_IS_VNODE
flag. RWL_NOWITNESS
- Instructs witness(4) to ignore this lock.
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. RW_DUPOK
- Prevents witness(4), for
just this
rw_enter
(), from logging when this thread already has a lock of this lock type.
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.
The
rw_assert_wrlock
(),
rw_assert_rdlock
(),
rw_assert_anylock
(),
and
rw_assert_unlocked
()
functions check the status rwl, panicking if it is not
write-, read-, any-, or unlocked, respectively.
rw_status
returns the current state of the
lock.
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.
CONTEXT
rw_init
(),
rw_init_flags
(), rrw_init
()
and rrw_init_flags
() can be called during autoconf,
from process context, or from interrupt context.
All other functions can be called during autoconf or from process context.
RETURN VALUES
rw_enter
and
rrw_enter
return 0 on success, or an
errno(2) style value on
failure.
rw_status
and
rrw_status
return the 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.
SEE ALSO
HISTORY
The rwlock
functions first appeared in
OpenBSD 3.5.
AUTHORS
The rwlock
functions were written by
Artur Grabowski
<art@openbsd.org>.