cannot read correct len from socket

[saman_artorious]

How were the first four bytes written to the socket by the server?

By the Client, it was written as such:

void    tcpClient::sendCommand(QString com)
{
    QByteArray data;
 
    data.append(com);      // "47Saman"
 
    if(clientSocket->write (data) == -1)
    {
        qDebug("sendCommand(QString) Failed");
    }
}

void    tcpClient::sendCommand(QString com)
{
    QByteArray data;

    data.append(com);      // "47Saman"

    if(clientSocket->write (data) == -1)
    {
        qDebug("sendCommand(QString) Failed");
    }
}

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[wysota]

If you are writing "47Saman" to the socket, what do you expect to receive as first four bytes on the other end of the communication?

[saman_artorious]

If you are writing "47Saman" to the socket, what do you expect to receive as first four bytes on the other end of the communication?

Wysota, client is sending me a jpeg file. in the server program, I used readAll() to read it all and pixmap it to a label. But, the image does not appear in the label (though the widget background flashes). To solve this, I was told that they would send the size of the jpeg file along with its content. Hence, I need to read the length first, and then realize how many bytes exactly to read.

void MainWindow::readPixmap(QByteArray data)
{
    QPixmap pm;
 
    pm.loadFromData(data);
 
    ui->label->setPixmap(pm);
}

void MainWindow::readPixmap(QByteArray data)
{
    QPixmap pm;

    pm.loadFromData(data);

    ui->label->setPixmap(pm);
}

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[wysota]

Please answer my question.

[saman_artorious]

Please answer my question.

A string of chars.

[wysota]

Then why are you trying to read them as an integer?

[saman_artorious]

Then why are you trying to read them as an integer?

because I have no idea what the length of the integer might be as a character, 2 digits or 5 digits!
that's why I try to read 4 bytes = sizeof(char)

[wysota]

1. sizeof(char) != 4
2. int does not contain decimal digits, it is a binary value ranging from ~ -2^31 to 2^31 (which can hold 10 decimal digits) for a 32 bit integer memory layout
3. reading a value that has not been written cannot end well, can it?
4. you're doing a lot of other errors in your code which completely prove you have no idea what you are doing and just blindly trying to get things working without any understanding what is going on.

Sockets are not magical entities, they do not convert your data, add some artificial data or remove some data -- if you write a sequence of bytes to it, the exact same sequence of bytes will be read by the other end of the communication channel. Don't try to do network programming if you don't understand how it works. Spend two days reading about network programming instead of listening to hints you don't even understand.

[saman_artorious]

Then why are you trying to read them as an integer?

I made a mistake. it is not as chars. it is binary data. So, I need to read the first 4 bytes to get the size of buffer.
The problem is that I cannot simply write an integer to the socket, and read the int 4 bytes in the client.

[wysota]

The problem is that I cannot simply write an integer to the socket, and read the int 4 bytes in the client.

Hmmm... why not?

[saman_artorious]

Then why are you trying to read them as an integer?

I want to read it as binary. appending data length at the beginning with this:

void tcpClient::sendToServer()
{
    QByteArray data;
 
    data.append(5);
    data.append("Saman");
 
    if(clientSocket->write (data) == -1)
    {
        qDebug("write() failed");
    }
}

void tcpClient::sendToServer()
{
    QByteArray data;

    data.append(5);
    data.append("Saman");

    if(clientSocket->write (data) == -1)
    {
        qDebug("write() failed");
    }
}

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in the server, let's read the first 4 bytes, as the size of integer and then read the text.

QByteArray         text;
 
    int length;
 
    clientConnection->read((char*)&length, 4);
 
    qDebug() << length;
 
    text.reserve(length);
 
    clientConnection->read(text.data(), length);
 
    clientConnection->flush();

QByteArray         text;

    int length;

    clientConnection->read((char*)&length, 4);

    qDebug() << length;

    text.reserve(length);

    clientConnection->read(text.data(), length);
    
    clientConnection->flush();

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output:
length read is 1835094789 !

what am I missing?

[Lesiok]

QByteArray::append(5) appends one byte with a value of 5 not integer. Do it in such a way :

clientSocket->write (5);//You are sending int(5) and on other side You can read int
clientSocket->write("Saman");

clientSocket->write (5);//You are sending int(5) and on other side You can read int
clientSocket->write("Saman");

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[saman_artorious]

wrong! that would prompt invalid conversion from int to const char*

[ChrisW67]

QByteArray::append(5) appends one byte with a value of 5 not integer. Do it in such a way :

clientSocket->write (5);//You are sending int(5) and on other side You can read int

clientSocket->write (5);//You are sending int(5) and on other side You can read int

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There's no write(int) function anywhere I can see in the socket/qiodevice interfaces; this won't compile.

[Lesiok]

OK, clientSocket->write(5) is only idea. This code should compile

QByteArray data;
QDataStream m_stream(&data,QIODevice::WriteOnly);
m_stream << 5 << "Saman";
clientSocket->write(data);

QByteArray data;
QDataStream m_stream(&data,QIODevice::WriteOnly);
m_stream << 5 << "Saman";
clientSocket->write(data);

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saman_artorious read more about QDataStream.

[wysota]

saman_artorious read more about QDataStream.

No, please, don't read about QDataStream. Ignore its existance completely for this project.

[saman_artorious]

I did not know about QDataStream.

Now, it reads the integer fine. However, the amount of data it reads does not depend on how much I allocate according to length.
it always reads the whole thing. How can I control it?

void Server::readState()
{
   int len = 0;
 
   QDataStream in(clientConnection);
 
   in >> len;
 
   qDebug() << len;
 
   QByteArray data;
 
   data.reserve(len);
 
   in >> data;         // read 20 chars and not all
 
   qDebug() << data.data();
 
   data.clear();
 
   clientConnection->flush();
}

void Server::readState()
{
   int len = 0;

   QDataStream in(clientConnection);

   in >> len;

   qDebug() << len;

   QByteArray data;

   data.reserve(len);

   in >> data;         // read 20 chars and not all

   qDebug() << data.data();

   data.clear();

   clientConnection->flush();
}

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what I sent is of length 50, but only 20 chars would be read, though, it reads all 50 each time:

void tcpClient::sendToServer()
{
    QByteArray data;
    QDataStream m_stream(&data,QIODevice::WriteOnly);
    m_stream << 20 << "xxxxxxxxxx yyyyyyyyyy wwwwwwwwww zzzzzzzzzz aaaaaaaDD"; //text size = 50
    clientSocket->write(data);
 
    clientSocket->flush();
}

void tcpClient::sendToServer()
{
    QByteArray data;
    QDataStream m_stream(&data,QIODevice::WriteOnly);
    m_stream << 20 << "xxxxxxxxxx yyyyyyyyyy wwwwwwwwww zzzzzzzzzz aaaaaaaDD"; //text size = 50
    clientSocket->write(data);

    clientSocket->flush();
}

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output:
20
xxxxxxxxxx yyyyyyyyyy wwwwwwwwww zzzzzzzzzz aaaaaaaDD

[yeye_olive]

I did not know about QDataStream.

Now, it reads the integer fine. However, the amount of data it reads does not depend on how much I allocate according to length.
it always reads the whole thing. How can I control it?

void Server::readState()
{
   int len = 0;
 
   QDataStream in(clientConnection);
 
   in >> len;
 
   qDebug() << len;
 
   QByteArray data;
 
   data.reserve(len);
 
   in >> data;         // read 20 chars and not all
 
   qDebug() << data.data();
 
   data.clear();
 
   clientConnection->flush();
}

void Server::readState()
{
   int len = 0;

   QDataStream in(clientConnection);

   in >> len;

   qDebug() << len;

   QByteArray data;

   data.reserve(len);

   in >> data;         // read 20 chars and not all

   qDebug() << data.data();

   data.clear();

   clientConnection->flush();
}

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what I sent is of length 50, but only 20 chars would be read, though, it reads all 50 each time:

void tcpClient::sendToServer()
{
    QByteArray data;
    QDataStream m_stream(&data,QIODevice::WriteOnly);
    m_stream << 20 << "xxxxxxxxxx yyyyyyyyyy wwwwwwwwww zzzzzzzzzz aaaaaaaDD"; //text size = 50
    clientSocket->write(data);
 
    clientSocket->flush();
}

void tcpClient::sendToServer()
{
    QByteArray data;
    QDataStream m_stream(&data,QIODevice::WriteOnly);
    m_stream << 20 << "xxxxxxxxxx yyyyyyyyyy wwwwwwwwww zzzzzzzzzz aaaaaaaDD"; //text size = 50
    clientSocket->write(data);

    clientSocket->flush();
}

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output:
20
xxxxxxxxxx yyyyyyyyyy wwwwwwwwww zzzzzzzzzz aaaaaaaDD

Please stop trying things at random, this is getting out of hand. You went ahead with QDataStream; you should have read its documentation. Do you have any idea what the insertions (<<) and extractions (>>) do?

What you really need is read a bit about serialization and deserialization of basic C/C++ types. Forget about the network for now, just pretend you are designing a platform and architecture-independent binary file format in which to save/from which to load the piece of data you are interested in. This has nothing to do with Qt, by the way.

[ChrisW67]

Now, it reads the integer fine. However, the amount of data it reads does not depend on how much I allocate according to length.
it always reads the whole thing. How can I control it?

Yes, it is doing that because that is the correct behaviour for QDataStream. You don't "control it" but you could at least try to understand it and all that has preceded it in this thread.

If you are serious about understanding then try this experiment: using each approach write the data to a file (substitute a QFile for the QTcpSocket). These files represent exactly what you are sending over the wire. Look at the length of the files. Inspect the files byte by byte (a binary editor or Linux od command). Try changing the numbers and see which bytes change. Look at sizeof(int) how many bytes are you trying to read?

[wysota]

I want to read it as binary.

You are always reading it as binary, even if it is textual data. It is just a matter of interpreting the data.

what am I missing?

You are missing a couple of hours of learning about computers, their memory and data layout of different data types.

1835094789d = 0x6D615305 = 0x0553616D (swapped byte order) = "\5Sam" interpreted as text. Does that ring a bell?