Description

   This routine initializes a read-write lock object with the
   attributes specified by the read-write lock attributes object
   specified in attr. A read-write lock is a synchronization object
   that serializes access to shared information that needs to be
   read frequently and written only occasionally. A thread can
   acquire a read-write lock for shared read access or for exclusive
   write access.

   Upon successful completion of this routine, the read-write lock
   is initialized and set to the unlocked state. If attr is set to
   NULL, the default read-write lock attributes are used; the effect
   is the same as passing the address of a default read-write lock
   attributes object. Once initialized, the lock can be used any
   number of times without being reinitialized.

   Results of calling this routine are undefined if attr specifies
   an already initialized read-write lock or if rwlock is used
   without first being initialized.

   If this routine returns unsuccessfully, rwlock is not initialized
   and the contents of rwlock are undefined.

   A read-write lock is a resource of the process, not part of any
   particular thread. A read-write lock is neither destroyed not
   unlocked automatically when any thread exits. Because read-write
   locks are shared, they may be allocated in heap or static memory,
   but not on a stack.

   In cases where default read-write lock attributes are
   appropriate, you may use the PTHREAD_RWLOCK_INITIALIZER macro
   to statically initialize the lock object without calling this
   routine. The effect is equivalent to dynamic initialization by
   a call to pthread_rwlock_init() with attr specified as NULL,
   except that no error checks are performed. Statically initialized
   read-write locks need not be destroyed using pthread_rwlock_
   destroy().

   Use the PTHREAD_RWLOCK_INITIALIZER macro as follows:

   pthread_rwlock_t rwlock= PTHREAD_RWLOCK_INITIALIZER;