Understanding Semaphore Example

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

For simplicity I'll paste the example code:

Global Variables

Code:

---

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

---

Producer Class

Code:

---

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();
        }
    }
};

---

Consumer Class

Code:

---

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;
};

---

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?

Quote:

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Originally Posted by KingDavid

does freeBytes(BufferSize) reserves BufferSize resources or not?

---

Yes, it does.

Quote:

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Originally Posted by KingDavid

how the consumer is blocked waiting for the usedBytes semaphore to be released?

---

usedBytes is initialized with 0 resources (constructor argument default value), any require() on it will block until enough resources have been made available with respective release() calls.

Quote:

---

Originally Posted by KingDavid

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?

---

freeBytes is decremented because acquire() is being called. Its argument has a default value of 1, so it is decremented by 1.
Similar for usedBytes, its release() is called, again default value being 1, so it increments by 1.

Cheers,
_