QIODevice::bytesAvailable in non sequential mode has a bug as follows

johnco3

Re: How to minimise audio output latency?

I found some references to a bug in this quite old post online. I was wondering if others came across this bug where the bytesAvailable is double. I tracked the bug down to virtual size() call in the QIODevice::bytesAvaiable superclass when not in sequential mode.

/**
 * Return the number of bytes available.
 *
 * <p>Note that there may be an issue with this when the
 * IOIDevice is not sequential due to the requirement
 * that the we must call QIODevice::bytesAvailable which
 * in turn calls size().
 *
 * @return number of bytes available.
 */
qint64
IODeviceSink::bytesAvailable() const {
    const qint64 my_size = mAudioData.size();
    const qint64 builtin_size = QIODevice::bytesAvailable();

    return (my_size + builtin_size);
}

/**
 * Gets the Audio buffer Size.
 *
 * @return size of the audio buffer
 */
qint64
IODeviceSink::size() const
{
    return mAudioData.size();
}

The QIODevice::bytesAvailable documentation indicates that

Subclasses that reimplement this function must call the base implementation in order to include the size of the buffer of QIODevice. Example:

The problem with calling the base class' bytesAvailable is that it in turn calls a virtual size() function if the device is opened in default (non sequential mode). Per:

/*!
    Returns the number of bytes that are available for reading. This
    function is commonly used with sequential devices to determine the
    number of bytes to allocate in a buffer before reading.

    Subclasses that reimplement this function must call the base
    implementation in order to include the size of the buffer of QIODevice. Example:

    \snippet code/src_corelib_io_qiodevice.cpp 1

    \sa bytesToWrite(), readyRead(), isSequential()
*/
qint64 QIODevice::bytesAvailable() const
{
    Q_D(const QIODevice);
    if (!d->isSequential())
        return qMax(size() - d->pos, qint64(0)); <<<< size() is virtual and calls the subclass
    return d->buffer.size() - d->transactionPos;
}