Read (many) values from QTcpSocket (fast)

Question

I'm using a measurement device which sends (binary) float values using a tcp socket with up to 70 kHz.

My goal is to read these values as fast as possible and use them in other parts of my program.

Till now I'm able to extract value by value using a QTcpSocket and QDataStream:

First I create the socket and connect the stream to it

mysock = new QTcpSocket(this);
mysock->connectToHost(ip, port);
QDataStream stream(mysock);
stream.setByteOrder(QDataStream::LittleEndian);
stream.setFloatingPointPrecision(QDataStream::SinglePrecision);

Then I read from the socket and write the stream data to my float value

while(true) //only for test purpose (dont stop reading)
if (mysock->waitForReadyRead())
{
    while (mysock->bytesAvailable() >= 6)
    {
        QByteArray a = mysock->read(6); //each value sent is 6 bytes long
        stream.skipRawData(2); //first 2 bytes don't belong to the number
        float result;
        stream >> result;
        //qDebug()<<result;
    }
}

When I measure the iteration frequency of the while(true) loop I'm able to achieve about 30 kHz. Reading multiple values per read I can reach up to 70 Khz. (Not taking other calculations into account which might slow me down)

My questions are:

• If I read multiple values at once, how do I extract these values from the QDataStream? I need a 6 bytes spacing with only 4 bytes containing the value.

Answer: In my case there is 2 bytes (trash) followed by a known number of values, for example 4 bytes for a float, 4 bytes for another float, 2 bytes for an uint16.

stream >> trashuint16 >> resultfloat1 >> resultfloat2 >> resultuint16

• Expands 1: I can configure my device to send different values of different type (int, float) which need to be written to different variables.

Answer: Same.

• Is there a more efficient way to read many values from a QTcpSocket?

Answer: Anwered in the comments.

Update (to answer some questions):

• Max rate in Bytes: 70 kHz x 6 Byte (for one value) = 420 kB/s (Doesnt seem that much :))

Update 2

• New Question: When i start a transaction (using stream.startTransaction) I would like to know whats inside that stream in binary code.
• I dont understand how QDataStream::startTransaction works. How many bytes will be read? what happens with the data I dont extract using >>?

I've tried the following:

if (mysock->waitForReadyRead())
{
    stream.startTransaction();

    char *c = new char[40];
    stream.readRawData(c, 40);    //I want to know whats really inside    
    QByteArray a(c);
    qDebug() << a <<stream.status();
    if (!stream.commitTransaction())
        break;
}

Doing this again and again, I'll sometimes get status = -1 (read too much) and sometimes not. How do I get the "size" of the stream?

Answer 1

Your code has couple mistakes.

You are doing direct reading from socket when in the same time you are using QDataStream. This can break stuff.

Also your code is assuming that your application will receive data in same chunks as it was sent by other end. You do not have such warranty! It may happen that you will receive chunk data which are ending in middle of your frame. It works just by pure luck or you are ignoring some bugs of your application.

This should go like this:

while(true)
if (mysock->waitForReadyRead()) // IMO doing such loop is terrible approach
// but this is Out of the scope of question, so ignoring that
{
    while (true)
    {
        stream.startTransaction();
        float result;
        qint32 somedata
        stream >> somedata >> result; // I do not know binary format your application is using

        if (!in.commitTransaction())
            break;

        AddDataToModel(result, somedata);
    } 
}

---

Edit:

From comment:

Please correct me if I'm wrong, but if I want 2 bytes to be discarded I need to do "stream >> someint(2 byte) >> somefloat(4 byte)"? How can I handle many values in stream?

qint16 toBeDiscarded;
float value; 
// note stream.setFloatingPointPrecision(QDataStream::SinglePrecision); 
// is needed to read float as 32 bit floating point number

stream >> toBeDiscarded >> value;
ProcessValue(value);