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Locks and Holds: The Basics of Synchronization


Listing One


class Lock; // this is the basic lock

class AutoLock {
public:
    AutoLock(Lock &arg) : lock_(arg)
    {
        lock_.lock();
    }

    ~AutoLock()
    {
        lock_.unlock();
    }
    
protected:
    Lock &lock_;
};

// ... used like this ...
    {
        AutoLock al(myLock);
        call();
    }

Listing Two


class AutoUnlock {
public:
    AutoUnlock(Lock &arg) : lock_(arg)
    {
        lock_.unlock();
    }

    ~AutoUnlock()
    {
        lock_.lock();
    }
    
protected:
    Lock &lock_;
};

// ... used like this ...
    {
        AutoLock al(myLock);
        for(...) {
            SomeClass variable;
            ...
            {
                AutoUnlock au(myLock);
                long_operation();
            }
            ...
        }
    }

Listing Three


class Hold {
public:
    Hold() : count_(0), waiting_(0)
    { 
        pthread_mutex_init(&mutex_, NULL);
        pthread_cond_init(&cond_, NULL);
    }
    
    ~Hold()
    {
        pthread_mutex_destroy(&mutex_);
        pthread_cond_destroy(&cond_);
    }

    void getHold() 
    {
        pthread_mutex_lock(&mutex_);
        ++count_;
        pthread_mutex_unlock(&mutex_);
    }

    void releaseHold() 
    {
        pthread_mutex_lock(&mutex_);
        if (--count_ == 0 && waiting_ != 0)
            pthread_cond_broadcast(&cond_);
        pthread_mutex_unlock(&mutex_);
    }

    void wait()
    {
        pthread_mutex_lock(&mutex_);
        while(count_ != 0) {
            ++waiting_;
            pthread_cond_wait(&cond_, &mutex_);
            --waiting_;
        }
        pthread_mutex_unlock(&mutex_);
    }

protected:
    pthread_mutex_t mutex_; // access to count_
    pthread_cond_t cond_; // signalled when count_ drops to 0
    int count_; // count of holds
    int waiting_; // count of threads waiting for release

private: // prevent the copy constructors and assignments
    Hold(const Hold&);
    void operator=(const Hold&);
};


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