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. rwlock_init() The readers/writer lock pointed to by rwlp is initial-
       ized 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. Mul-
       tiple  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, pro-
       vided 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 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.

SunOS 5.11							    14 May 1998 							rwlock(3C)