[Interest] Parsing data from serialport

roland at logikalsolutions.com roland at logikalsolutions.com
Thu Apr 4 20:37:54 CEST 2019

Quoting interest-request at qt-project.org:

> I think the point is that there's little reason to poll the serial port if
> you can react to the event. Exactly what you'd do if you had a network
> socket. Qt already does the heavy lifting for you, so you only need to
> react to the signal and read as much as you want/need. Basically:
>     QSerialPort * port;
>     QObject::connect(port, &QIODevice::readyRead, port, [port] () -> void  {
>         while (port->canReadLine())  {
>             QByteArray data = port->readLine();
>             // emit with data as argument and do the parsing
>         }
>     });
> Whether you have the port in another thread or not is irrelevant in this
> case, either can work fine (unlike your while-sleep loop).

Never ever ever do I/O in the main GUI thread of a Qt application.  
It's a recipe for disaster despite the countless examples you will  
find posted on-line and even in the Qt examples themselves. Those  
database examples are really nice. Try testing them with a million+  
row table.

Real world serial comm is fraught with peril. I've been doing it since  
the days of DOS 3.1.

Before anyone goes running off and using the above lambda, they need  
to consider a few things.

Unless something has dramatically changed in QSerialPort, readyRead is  
only emitted for the first character in the buffer because there is no  
concept of block or packet. A UART handles one byte at a time.

When you get a readyRead and fail to completely empty the buffer,  
that's it. You never get another one.

When QSerialPort was taken from Playground and had all its method  
names changed, there was an odd timing bug too. In a production system  
running multiple ports of embedded target the code loading the next  
byte into the class buffer (perhaps not completely Qt code as it could  
have been in the device driver). The adding of the next byte could  
begin after readLine() determined how many bytes were in the buffer  
but prior to actually pulling them. The new byte got added. One less  
than a full buffer got read and readyRead never fired again because  
the next byte wasn't being written to a pristine empty buffer.

Roland Hughes, President
Logikal Solutions
(630) 205-1593


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