Reading TCP datagram header by Qt

**Kill3rReaper** · 3rd June 2010, 15:28

Exactly that I doing. I'm using QTcpSocket.
For example: someone sending to me message (4000 chars = 4000 bytes). He sending it in one message, but I receive 3 packets (size of one packet depends for MTU; for example MTU = 1500 - most popular). I have got:
1: 1500 bytes
2: 1500 bytes
3: 1000 bytes
The size of each one is very often changeable, and in different places (routers for example) MTU can be different. I never know how large is packet and when message is complete.
I hope, you're understanding what I mean.

**tbscope** · 3rd June 2010, 15:34

Originally Posted by Kill3rReaper

(4000 chars = 4000 bytes).

Not always true! Check encodings!

He sending it in one message, but I receive 3 packets (size of one packet depends for MTU; for example MTU = 1500 - most popular). I have got:
1: 1500 bytes
2: 1500 bytes
3: 1000 bytes
The size of each one is very often changeable, and in very places (routers for example) MTU can be different. I never know how large is packet and when message is complete.
I hope, you're understanding what I mean.

I do, and no, you're not going to find TCP network infrastructure that sends you a gigabyte in one single packet. That would be silly.
So, what to do...

You need to receive a single file or message but it comes in several packets.
A good protocol tells you when the data ends. In HTML for example, the size of the body is mentioned in the headers. In NNTP, a multiline message ends with \r\n.\r\n.
So, find in the protocol you want to use how the server tells the clients that a specific request has ended.

**Kill3rReaper** · 3rd June 2010, 16:02

Originally Posted by tbscope

Not always true! Check encodings!

I know that - it is wrote in simplification

A protocol of XMPP/Jabber looks like that:

Qt Code:

Switch view

<message from="someone@jabber.org" to="someone2@jabber.org">
  <body>
    Content - message from user to user.
  </body>
</message>

<message from="someone@jabber.org" to="someone2@jabber.org">
  <body>
    Content - message from user to user.
  </body>
</message>

To copy to clipboard, switch view to plain text mode

It looks OK, but what when user writes some of XML code...

Qt Code:

Switch view

<message from="someone@jabber.org" to="someone2@jabber.org">
  <body>
    Content - message from user to user. [and a lot of strings, and now message have got 1500 bytes]
    </body> <--- user sends text, and he wrote to his friend a XML code like that
    </message>
  </body>
</message>

<message from="someone@jabber.org" to="someone2@jabber.org">
  <body>
    Content - message from user to user. [and a lot of strings, and now message have got 1500 bytes]
    </body> <--- user sends text, and he wrote to his friend a XML code like that
    </message>
  </body>
</message>

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Understand what I mean?

**tbscope** · 3rd June 2010, 16:12

In this case, it is very simple.

When reading from the socket, keep collecting the data. Everytime you receive data, check for "</message>" as that is the end of a message.
When you receive "</message>" you can start again waiting for and reading data.

In pseudocode:

Qt Code:

Switch view

slotReadyRead()
{
    data += readAll();
 
    if (data.endsWidht("</message>")
    {
        doSomethingUsefullWith(data);
        data = "";
    }
}

slotReadyRead()
{
    data += readAll();

    if (data.endsWidht("</message>")
    {
        doSomethingUsefullWith(data);
        data = "";
    }
}

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