无法从TCP套接字读取

Question

I have a C++ client using QT and a JAVA server and I have successfully written from the client to the server but I cannot write from the server to the client. My code:

QString
Client::readTCP ( )
{
    socketTCP->waitForReadyRead();

    QTextStream in (socketTCP);

    return in.readAll() ;
}

// Later on

qDebug() << Client::readTCP();

But no matter what method I choose I can't get a response from the server. The server code is as follows:

DataOutputStream  output = new DataOutputStream (SOCKET.getOutputStream());

output.writeBytes ( "myString" );

ANSWER:

It works either because I changed in.readAll() to in.readLine() or it is because I waited a couple seconds after the server started before sending a message.

Answer 1

The QTextStream::readAll function attempts to read the entire contents of the stream. Either this message is or isn't the entire contents of the stream.

If this message isn't the entire contents of the stream, then it should not return. It would be a serious error if readAll returned only a part of the contents of the stream despite the fact that it's specified to return all the contents.

If this is the entire contents of the stream, then the server is broken. If it doesn't close the socket, how can the client know it's received the entire contents? Unless there's some other way to indicate end of message, it has to be indicated by closing the stream, and you don't show the server closing the stream.

I'll repeat the advice I always give when I see problems like this -- do not ever implement a network protocol until you specify that protocol in a protocol specification. Otherwise, it's not possible to fix problems where the server and client disagree because there's no way to know which end is right. Here, the server and client disagree over how the end of a message is to be marked, and without a protocol specification to refer to, there's no way to know which end to fix.

If you had a protocol specification, you could just look at the section that explains how the ends of messages are marked and detected. Then you could fix whichever end doesn't follow the specification. (Or, if the specification doesn't say how, then fix the specification! Clearly, this has to happen somehow and it's the specification's job to explain how.)

Answer 2

In Java, after sending data to buffer, flush it. Output streams have flush() method which forces any data left in stream to be written/sent. Try that if using readAll() on client side.

Also, readLine() is advised if you know how much data will be sent. You can loop trough in.readLine() until it gets null. Also readLine() will remove any \\n or \\r\\n.

Answer 3

I have no experience with Qt so I cannot say if you are reading correctly from the socket, but with Java I use PrintStream 's method println when sending text, and a VS C++ Client receives it just fine using recv .

Also, you may want to check if the packet is actually sent over the socket using Wireshark.