rwlock(3C) Standard C Library Functions rwlock(3C)NAME
rwlock, rwlock_init, rwlock_destroy, rw_rdlock, rw_wrlock, rw_tryrd‐
lock, rw_trywrlock, rw_unlock - multiple readers, single writer locks
SYNOPSIS
cc -mt [ flag... ] file...[ library... ]
#include <synch.h>
int rwlock_init(rwlock_t *rwlp, int type, void * arg);
int rwlock_destroy(rwlock_t *rwlp);
int rw_rdlock(rwlock_t *rwlp);
int rw_wrlock(rwlock_t *rwlp);
int rw_unlock(rwlock_t *rwlp);
int rw_tryrdlock(rwlock_t *rwlp);
int rw_trywrlock(rwlock_t *rwlp);
DESCRIPTION
Many threads can have simultaneous read-only access to data, while only
one thread can have write access at any given time. Multiple read
access with single write access is controlled by locks, which are gen‐
erally used to protect data that is frequently searched.
Readers/writer locks can synchronize threads in this process and other
processes if they are allocated in writable memory and shared among
cooperating processes (see mmap(2)), and are initialized for this pur‐
pose.
Additionally, readers/writer locks must be initialized prior to use.
rwlock_init() The readers/writer lock pointed to by rwlp is initialized
by rwlock_init(). A readers/writer lock is capable of having several
types of behavior, which is specified by type. arg is currently not
used, although a future type may define new behavior parameters by way
of arg.
The type argument can be one of the following:
USYNC_PROCESS The readers/writer lock can synchronize threads in
this process and other processes. The readers/writer
lock should be initialized by only one process. arg
is ignored. A readers/writer lock initialized with
this type, must be allocated in memory shared between
processses, i.e. either in Sys V shared memory (see
shmop(2)) or in memory mapped to a file (see
mmap(2)). It is illegal to initialize the object this
way and to not allocate it in such shared memory.
USYNC_THREAD The readers/writer lock can synchronize threads in
this process, only. arg is ignored.
Additionally, readers/writer locks can be initialized by allocation in
zeroed memory. A type of USYNC_THREAD is assumed in this case. Multiple
threads must not simultaneously initialize the same readers/writer
lock. And a readers/writer lock must not be re-initialized while in use
by other threads.
The following are default readers/writer lock initialization (intra-
process):
rwlock_t rwlp;
rwlock_init(&rwlp, NULL, NULL);
or
rwlock_init(&rwlp, USYNC_THREAD, NULL);
or
rwlock_t rwlp = DEFAULTRWLOCK;
The following is a customized readers/writer lock initialization
(inter-process):
rwlock_init(&rwlp, USYNC_PROCESS, NULL);
Any state associated with the readers/writer lock pointed to by rwlp
are destroyed by rwlock_destroy() and the readers/writer lock storage
space is not released.
rw_rdlock() gets a read lock on the readers/writer lock pointed to by
rwlp. If the readers/writer lock is currently locked for writing, the
calling thread blocks until the write lock is freed. Multiple threads
may simultaneously hold a read lock on a readers/writer lock.
rw_tryrdlock() trys to get a read lock on the readers/writer lock
pointed to by rwlp. If the readers/writer lock is locked for writing,
it returns an error; otherwise, the read lock is acquired.
rw_wrlock() gets a write lock on the readers/writer lock pointed to by
rwlp. If the readers/writer lock is currently locked for reading or
writing, the calling thread blocks until all the read and write locks
are freed. At any given time, only one thread may have a write lock on
a readers/writer lock.
rw_trywrlock() trys to get a write lock on the readers/writer lock
pointed to by rwlp. If the readers/writer lock is currently locked for
reading or writing, it returns an error.
rw_unlock() unlocks a readers/writer lock pointed to by rwlp, if the
readers/writer lock is locked and the calling thread holds the lock for
either reading or writing. One of the other threads that is waiting for
the readers/writer lock to be freed will be unblocked, provided there
is other waiting threads. If the calling thread does not hold the lock
for either reading or writing, no error status is returned, and the
program's behavior is unknown.
RETURN VALUES
If successful, these functions return 0. Otherwise, a non-zero value is
returned to indicate the error.
ERRORS
The rwlock_init() function will fail if:
EINVAL type is invalid.
The rw_tryrdlock() or rw_trywrlock() functions will fail if:
EBUSY The reader or writer lock pointed to by rwlp was already
locked.
These functions may fail if:
EFAULT rwlp or arg points to an illegal address.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │MT-Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOmmap(2), attributes(5)NOTES
These interfaces also available by way of:
#include <thread.h>
If multiple threads are waiting for a readers/writer lock, the acquisi‐
tion order is random by default. However, some implementations may bias
acquisition order to avoid depriving writers. The current implementa‐
tion favors writers over readers.
SunOS 5.11 14 May 1998 rwlock(3C)
