linux 读写锁（写优先）使用方法

http://sourceware.org/ml/glibc-bugs/2008-11/msg00073.html
http://www.nabble.com/-Bug-nptl-7057--New:-pthread-rwlock-does-not-implement-'writer-preferred'-option-td20703744.html
pthread rwlock does not implement 'writer preferred' option
linux 读写锁
定义（全局变量）：
pthread_rwlock_t    rw_lock;
初始化：
{
    pthread_rwlockattr_t attr;
    int ret = 0;
    ret = pthread_rwlockattr_init( &attr );
    if( ret < 0 )
    {
 ...;
        return;
    }
#ifdef HAVE_PTHREAD_RWLOCK_PREFER_WRITER_NP
    pthread_rwlockattr_setkind_np(&attr, PTHREAD_RWLOCK_PREFER_WRITER_NP); // 写优先
#endif
    ret = pthread_rwlockattr_setpshared( &attr, PTHREAD_PROCESS_SHARED );  // 进程内
    if( ret < 0 )
    {
        ...;
        return;
    }
    ret = pthread_rwlock_init( &rw_lock, &attr );
    if( ret < 0 )
    {
        ...;
        return;
    }
    ret = pthread_rwlockattr_destroy( &attr );
    if( ret < 0 )
    {
        ...;
        return;
    }
    return;
}
使用：
    // 写锁
    pthread_rwlock_wrlock(&rw_lock);
    ......
    pthread_rwlock_unlock(&rw_lock);
    // 读锁
    pthread_rwlock_rdlock(& w_lock);
    ......
    pthread_rwlock_unlock(&rw_lock);
另外一个：
  #define   _XOPEN