Understanding Semaphore Example

[KingDavid]

Hi,
I don't understand some details about the Semaphores Example QSemaphore.

For simplicity I'll paste the example code:

Global Variables

const int DataSize = 100000;
 
const int BufferSize = 8192;
char buffer[BufferSize];
 
QSemaphore freeBytes(BufferSize);
QSemaphore usedBytes;

const int DataSize = 100000;

const int BufferSize = 8192;
char buffer[BufferSize];

QSemaphore freeBytes(BufferSize);
QSemaphore usedBytes;

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Producer Class

class Producer : public QThread
{
public:
    void run() Q_DECL_OVERRIDE
    {
        qsrand(QTime(0,0,0).secsTo(QTime::currentTime()));
        for (int i = 0; i < DataSize; ++i) {
            freeBytes.acquire();
            buffer[i % BufferSize] = "ACGT"[(int)qrand() % 4];
            usedBytes.release();
        }
    }
};

class Producer : public QThread
{
public:
    void run() Q_DECL_OVERRIDE
    {
        qsrand(QTime(0,0,0).secsTo(QTime::currentTime()));
        for (int i = 0; i < DataSize; ++i) {
            freeBytes.acquire();
            buffer[i % BufferSize] = "ACGT"[(int)qrand() % 4];
            usedBytes.release();
        }
    }
};

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Consumer Class

class Consumer : public QThread
{
    Q_OBJECT
public:
    void run() Q_DECL_OVERRIDE
    {
        for (int i = 0; i < DataSize; ++i) {
            usedBytes.acquire();
            fprintf(stderr, "%c", buffer[i % BufferSize]);
            freeBytes.release();
        }
        fprintf(stderr, "\n");
    }
 
signals:
    void stringConsumed(const QString &text);
 
protected:
    bool finish;
};

class Consumer : public QThread
{
    Q_OBJECT
public:
    void run() Q_DECL_OVERRIDE
    {
        for (int i = 0; i < DataSize; ++i) {
            usedBytes.acquire();
            fprintf(stderr, "%c", buffer[i % BufferSize]);
            freeBytes.release();
        }
        fprintf(stderr, "\n");
    }

signals:
    void stringConsumed(const QString &text);

protected:
    bool finish;
};

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The questions are:

• does freeBytes(BufferSize) reserves BufferSize resources or not?
• how the consumer is blocked waiting for the usedBytes semaphore to be released?
• The example say "Once the producer has put one byte in the buffer, freeBytes.available() is BufferSize - 1 and usedBytes.available() is 1" why the freeBytes is decremented and the usedBytes semaphore is incremented?