Number of bytes receives not the same after each cycle

[kuzulis]

Foo::readyRead()
{
    static const int expectedPacketSize = 9; // your 9 bytes
    if (port->bytesAvailable() < expectedPacketSize)
        return; // do nothing
    QByteArray data = port->read(expectedPacketSize);
 
    // do domething processing of your packet
}

Foo::readyRead()
{
    static const int expectedPacketSize = 9; // your 9 bytes
    if (port->bytesAvailable() < expectedPacketSize)
        return; // do nothing
    QByteArray data = port->read(expectedPacketSize);
 
    // do domething processing of your packet
}

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Of course, I'm skip the parsing and synchronization of incoming packets from an incoming data stream.