QIODevice read() problem (reads more than maxSize)

[m15ch4]

Hello.

I'm writing an application that captures audio from microphone input using QtMultimedia (QAudioInput). Based on the "spectrum" demo from http://qt.gitorious.org/qt/qt/trees/4.7/demos/spectrum I wrote "audioDataReady()" slot:

void Engine::audioDataReady()
{
    Q_ASSERT(0 == m_bufferPosition);
    const qint64 bytesReady = m_audioInput->bytesReady();
    const qint64 bytesSpace = m_buffer.size() - m_dataLength;
    const qint64 bytesToRead = qMin(bytesReady, bytesSpace);
    const qint64 bytesRead = m_audioInputIODevice->read(
                                       m_buffer.data() + m_dataLength,
                                       bytesToRead);
 
    if (bytesRead) {
        m_dataLength += bytesRead;
        qDebug() << "bytesReady:" << bytesReady << "| bytesSpace:" << bytesSpace << "| bytesToRead:" << bytesToRead << "| bytesRead:" << bytesRead;                    // added line 
        emit dataLengthChanged(dataLength());
    }
 
    if (m_buffer.size() == m_dataLength)
        stopRecording();
}

void Engine::audioDataReady()
{
    Q_ASSERT(0 == m_bufferPosition);
    const qint64 bytesReady = m_audioInput->bytesReady();
    const qint64 bytesSpace = m_buffer.size() - m_dataLength;
    const qint64 bytesToRead = qMin(bytesReady, bytesSpace);
    const qint64 bytesRead = m_audioInputIODevice->read(
                                       m_buffer.data() + m_dataLength,
                                       bytesToRead);

    if (bytesRead) {
        m_dataLength += bytesRead;
        qDebug() << "bytesReady:" << bytesReady << "| bytesSpace:" << bytesSpace << "| bytesToRead:" << bytesToRead << "| bytesRead:" << bytesRead;                    // added line 
        emit dataLengthChanged(dataLength());
    }

    if (m_buffer.size() == m_dataLength)
        stopRecording();
}

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First few lines of output are:
bytesReady: 680 | bytesSpace: 160000 | bytesToRead: 680 | bytesRead: 1700
bytesReady: 680 | bytesSpace: 158300 | bytesToRead: 680 | bytesRead: 1360
bytesReady: 1020 | bytesSpace: 156940 | bytesToRead: 1020 | bytesRead: 1700
...

The question is: why QIODevice::read ( char * data, qint64 maxSize ) reads more bytes than second argument tells to read.

P.S. After 10 seconds of recording, "spectrum" demo exits with "Buffer overflow" error what is normal I think because read() method tries to read eg. 1700 bytes but there is only eg. 1000 bytes free.