RWLOCK(3C) Standard C Library Functions RWLOCK(3C)


NAME


rwlock, rwlock_init, rwlock_destroy, rw_rdlock, rw_wrlock, rw_tryrdlock,
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 generally 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
purpose.


Additionally, readers/writer locks must be initialized prior to use. 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() tries 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() tries 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
are 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 readers/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 ALSO


mmap(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
acquisition order is random by default. However, some implementations may
bias acquisition order to avoid depriving writers. The current
implementation favors writers over readers.


December 19, 2013 RWLOCK(3C)