QtSerialPort waitForReadyRead, read port error, timeout

CarlStenquist

There seems to be a problem with QtSeriaPort when reading a (USB COM) port. Transmit always works fine. But every so often, I get a read() error. That is; the function waitForReadyRead() returns false. This is even though I know that the data is going across, as I am using a USB bus analyzer. I have captured this several times when waitForReadyRead() times out.

When QtSeriaPort .error() is called at the time above happens, no error code is there (value is zero).

I got around this frustrating problem by prompting the target to retransmit, but this was a tedious workaround for something that should work.

Below is the read code which is called right after a transmit of 1-64 bytes.

USBSER.SYS is the driver that windows is using for the COM port.

@ /* waitForReadyRead() blocks until new data is available for reading and the readyRead()
* signal has been emitted. The function will timeout after msecs milliseconds.
* The function returns true if the readyRead() signal is emitted and there is new
* data available for reading, otherwise it returns false (if an error occurred or the
* operation timed out). */
read_result = port->waitForReadyRead(PORT_RW_TIMEOUT);
if (read_result)
{
response_record = port->readAll();

    /* Make sure ALL data has come in by waiting a few more ms. */
    while (port->waitForReadyRead(PORT_RW_TIMEOUT_DELTA))
    {
        response_record += port->readAll();
    }

...
}
else /* SOMETIMES IT TIMES OUT TO HERE, THOUGH I AM WAITING THREE SECONDS FOR DATA
AND THE BUS ANALYZER SHOWS THE DATA ON THE BUS. /
{
/ Read error, or timed out (read_result was false). */
rr_err_code = RR_ERR_OR_TIMEOUT;
port_error = port->error();
}@
...

Using Qt 5.1 on Win7/64 bit.
Thanks for any insight!

kuzulis

Hi.

Do not use the waitForxx() methods because they still has bugs:

https://bugreports.qt-project.org/browse/QTBUG-33987

Use async approach.

CarlStenquist

Could you please explain more abut this?
Will it be fixed?
I will not be given the time to implement a thread or signal solution currently.

kuzulis

bq. Could you please explain more abut this?

You can try do a workaround, like: https://codereview.qt-project.org/#change,67962

bq. Will it be fixed?
I hope - yes.

CarlStenquist

Above does not seem to do anything to this issue. (Plus the English is hard to understand!)
Implemented my own workaround: If Windows does not cooperate, I ask the target once more for the latest data. (Added a "Repeat Response Request" frame to my protocol.)
Carl Stenquist

kuzulis

You can try this patch: https://codereview.qt-project.org/#change,74396 , for me it works.

This patch still not merged yet. Maybe I will merge some later.. (e.g. after yours confirmation that it works).

I mean that maybe in Qt 5.2.1 it will be fixed.