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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. 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.
type may 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.
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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):
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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.
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