C:/rwlock/rwlock.h

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/* ****************************************************************************
 * Owner:               Evgeniy Makeev
 * Created by:  Evgeniy Makeev
 * ****************************************************************************
 */
#ifndef __RW_LOCK_H_Included__
#define __RW_LOCK_H_Included__ 

#include "atomic_util.h"
#include "system_abstraction.h"

/*---------------------------------*/
/* Written by Evgeniy Makeev, 1998 */
/*---------------------------------*/
//
// Multiple readers - one writer interface class.
// Implements multiple readers one writer mechanism.
// Optimizes readers speed, provides fast readers
// synchronization when no writers are waiting.
// Useful when data is needed to be written/changed
// only occasionally but it is read often by multiple
// threads.
// 

class NonblockingRWLock
{
public:
        static const char *name() { return "Nonblocking_Reader_Lock"; }
        NonblockingRWLock() : readers(0), writers(0) {}
        virtual ~NonblockingRWLock() {}
//
// Must be called at the beginning of any read/data access.
//
        void enter_read()
        {
                atomic_inc(&readers);
                if(writers > 0)
                {
                        atomic_dec(&readers);
                        l.lock();
                        atomic_inc(&readers);
                        l.unlock();
                }
        }

//
// Must be called at the end of any read/data access.
//
        void leave_read()
        {
                atomic_dec(&readers);
        }

        //
        // Must be called at the beginning of a write operation
        //
        void enter_write()
        {
                l.lock();
                atomic_inc(&writers);
                while(readers) thread_yield();
        }
                
        //
        // Must be called at the end of a write operation
        //
        void leave_write()
        {
                atomic_dec(&writers);
                l.unlock();
        }

private:
        volatile int readers;
        volatile int writers;
        SystemLock l;
};

#endif // __RW_LOCK_H_Included__

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