QSerialPort buffers more bytes than readBufferSize - qt

Question

I have a microcontroller that sends stuff to an embedded Linux device over RS485 that shows up as a /dev/ttysomething. I use the QSerialPort class (from Qt 5.15.2) to read received data.
This basically works.
As a test, I send a block of 100 bytes, approximately once a second (in practice probably slightly slower than that). The value of the first byte of the first block is 0, then the value of each successive byte increases by 1. The receiving side expects that and checks whether something unexpected is received. It does this also once per second.
// In the header:
QTimer timer;
QSerialPort serialPort;
MyValues* myValues; // Contains some Q_PROPERTYs for displaying some values on the screen
// In the cpp:
void MyStuff::setup()
{
serialPort.setPortName("/dev/ttymxc2");
serialPort.setBaudRate(QSerialPort::Baud57600);
serialPort.setReadBufferSize(1);
if (!serialPort.open(QIODevice::ReadWrite))
{
qDebug() << "Cannot serialPort.open: " << serialPort.error();
return;
}
timer.setInterval(1000);
timer.setSingleShot(false);
QObject::connect(&timer, &QTimer::timeout, &timer, [=]()
{
// Display how many times the timer elapsed
myValues->setSendCount(myValues->sendCount() + 1);
{
auto info = qDebug().nospace();
info << "Available = " << serialPort.bytesAvailable() << ", ";
info << "Expected = ";
printHexByte(info, myValues->expectedValue());
}
QByteArray readData = serialPort.readAll();
if (serialPort.error() == QSerialPort::ReadError)
{
qDebug() << "serialPort.error is ReadError: " << serialPort.errorString();
}
else
{
auto line = qDebug().nospace();
line << "Read " << readData.length() << " bytes: ";
for (int i = 0; i < readData.length(); i++)
{
unsigned char actualValue = readData[i];
if (actualValue == myValues->expectedValue())
{
printHexByte(line, actualValue);
line << " ";
}
else
{
myValues->setMismatchCount(myValues->mismatchCount() + 1);
line << "\nMismatch: Expected = ";
printHexByte(line, myValues->expectedValue());
line << ", Actual = ";
printHexByte(line, actualValue);
line << "\n";
myValues->setExpectedValue(actualValue);
}
myValues->setExpectedValue(myValues->expectedValue() + 1);
}
}
});
timer.start();
}
What I would expect:
Assuming that the microcontroller sending the block and the reading happen sufficiently out-of-phase so that we don't read in the middle of a block.
Since the read buffer size is set to 1, I would expect that there is only ever one byte up for grabs, with the 99 other bytes of the block being discarded. So each time the timer elapses, one byte is available, serialPort.readAll() reads 1 byte and it's either the first or last byte of the block.
What actually happens:
Each time the timer elapses, one byte is available, serialPort.readAll() reads 1 byte, but its value is always the previous byte's value + 1, so no bytes are discarded.
So I was wondering: Where are the bytes buffered? And how do I prevent it?
Because if my program reading the data happens to go slower than the microcontroller sending it, I don't want everything to get out of sync and blow when the system eventually runs out of memory after running for hours, I'd rather detect dropped bytes through checksums and be notified of the problem immediately when it happens.