QFile bytesAvailable() returns always 0

LubomirBogdanov

Hello everyone,
I'm trying to write a simple sequential program controlling an embedded device (through a device file "/dev/usbtmc0" in Linux). I don't want to use signals and slots. The device doesn't return new line or carriage return. And here comes the problem ... My code is this (in a separate thread):

void osc::run(){
    QString line;

    dev_file_stream.setDevice(&dev_file);
    dev_file_stream << "*IDN?\n";
    dev_file_stream.flush();
    qDebug() << "waiting reply ...";
    dev_file.waitForReadyRead(-1);
    qDebug() << "reply received!";
    quint64 bytes = dev_file.bytesAvailable();
    qDebug() << "bytes: " << bytes;
    line = dev_file_stream.read(bytes);
    //line = dev_file_stream.readLine();
    qDebug() << "line: " << line;
}

If I use this code, bytesAvailable( ) returns always 0.
If I use readLine( ), reading becomes terribly slow (5 or 6 seconds) probably because the reply doesn't have a '\n' or a '\r'.
If I use readLine(10), reading becomes fast but I'm missing chars because the replies are of different length (20, 30, 40, etc).

Any help is appreciated!

Regards,
L. B.

p.s. I've been struggling with this problem for years, and have always been forced to write complex signal-slot programs.

JonB

@LubomirBogdanov
Firstly, what Qt classes are your dev_file_stream and dev_file?

Your situation may (well) be "messed up" by your use of dev_file.bytesAvailable(); when you have it "wrapped" inside a dev_file_stream. Assuming that is some kind of "buffer", it looks like the buffer has filled (from readyRead() signal?) with bytes it reads from dev_file. That would mean dev_file.bytesAvailable() now returns 0 while dev_file_stream.read() has bytes in it available to read. And using readLine() when your device does not send \n is not "useful".

Start by just getting rid of your dev_file_stream wrapper level. Use only the dev_file. Depending on its type, use just read & write, don't worry about << convenience. That ought work!? Also try dev_file.readAll() without worrying about what dev_file.bytesAvailable() returns. At least use this to verify what is going on.

Of course, nothing will work "quickly" if your device buffers its output, or buffers it for some seconds before flushing.

I will just say that the signals and slots are pretty easy to work with. waitForReadyRead() is just some Qt code wrapping around the signals with a blocking loop.

LubomirBogdanov

Hi, sorry forgot to include the types:

    QFile dev_file;
    QTextStream dev_file_stream;

OK, so I tried as you said, but still the readAll( ) API returns after a few seconds ... Here is the new code:

void osc::run(){
    QByteArray line;

    dev_file.write("*IDN?\n");
    line = dev_file.readAll();
    qDebug() << "line: " << line;
}

Regards,
L. B.

JonB

@LubomirBogdanov
readAll() returns whatever is available at that instant. You can't put it immediately after a write(), the device will not have had time to respond. Why have you removed your dev_file.waitForReadyRead(-1)? Or do it better with readyRead signal?

LubomirBogdanov

Well, I tried with waitForReadyRead( ) but reading still is slow (5 seconds to get the reply):

void osc::run(){
    QByteArray line;

    dev_file.write("*IDN?\n");
    dev_file.waitForReadyRead(-1);
    line = dev_file.readAll();

    qDebug() << "line: " << line;
}

However, now I read in the Qt documentation:

"Unlike other QIODevice implementations, such as QTcpSocket, QFile does not emit the aboutToClose(), bytesWritten(), or readyRead() signals. This implementation detail means that QFile is not suitable for reading and writing certain types of files, such as device files on Unix platforms"

So, I guess this is not the way to do it ...