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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 ¶ÁдËø
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pthread_rwlock_t    rw_lock;
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{
    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); // дÓÅÏÈ
#endif
    ret = pthread_rwlockattr_setpshared( &attr, PTHREAD_PROCESS_SHARED );  // ½ø³ÌÄÚ
    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;
}
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    // дËø
    pthread_rwlock_wrlock(&rw_lock);
    ......
    pthread_rwlock_unlock(&rw_lock);
    // ¶ÁËø
    pthread_rwlock_rdlock(& w_lock);
    ......
    pthread_rwlock_unlock(&rw_lock);
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  #define   _XOPEN