Using QtSerialBus (5.7.0 at first, then I tried 5.7.1 too) and taking the CAN-bus example from Qt I have modified sendMessage() method simulating a heavy-download. I send the same frame i times consecutively (being i >1000 per example). I tried this:

for(int i=0; i<m_ui->loopBox->currentData().toInt(); i++){
    while(m_canDevice->framesToWrite() > 0){/*do nothing*/}
    m_canDevice->writeFrame(frame);
}

But I still getting "No buffer space available" errors, resulting on packets loss.
If I print framesToWrite() it always returns me 0.
The workaround I do:

while(!m_canDevice->writeFrame(frame)){}

Which isn't the best way as I'm forcing the error to occur ("No buffer space available") but I send as fast as possible without loosing packets.

The documentation specifies:

qint64 QCanBusDevice::framesToWrite() const
Returns the number of frames waiting to be written.

So why it doesn't work as I'm expecting?

My configuration:

Socketcan backend, 1MB/s bitrate, 4B data frame