Networking Threading Issue

[Syndacate]

Okay, so I'm having trouble with trying to get networking and threading to play nice together. Without holding up the GUI thread I'm trying to launch a new thread, and on that new thread communicate with a client and do intensive operations.

Originally I had the GUI make a new thread (I made a class which sublcassed QThread) but I kept getting runtime warnings upon write() because I guess the socket is tied to the server which is tied to the original GUI thread, so I was getting an error similar (but with different classes) to this: QObject: Cannot create children for a parent that is in a different thread.
(Parent is server_thread(0x6125e60), parent's thread is QThread(0x282730), current thread is server_thread(0x6125e60)

So I reworked it using the QT fortune server example and I'm still having issues (now I get the warning and it doesn't work at all), if somebody could point me to another example I can use for reference I would appreciate it, my creation is like this:
In GUI I launch the server class with:

server = new rtserver(this);
 
server->listen(QHostAddress::AnyIPv4, quint16(PORT_NUMBER));

server = new rtserver(this);

server->listen(QHostAddress::AnyIPv4, quint16(PORT_NUMBER));

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rtserver subclasses QTcpServer, in there I overrode the incomingConnection() function, and I create the thread in there like this:

st = new server_thread(mw, socket_descriptor, this);
st->start();

st = new server_thread(mw, socket_descriptor, this);
st->start();

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Where MW is just a pointer to the MainWindow I need for the thread to do some calculations.

Then in the server thread which subclasses QThread I simply set the socket descriptor and MainWindow pointer in the c'tor, in run() I have:

mClientSocket = new QTcpSocket(this);
 
(mClientSocket->setSocketDescriptor(mSocketDescriptor)

mClientSocket = new QTcpSocket(this);

(mClientSocket->setSocketDescriptor(mSocketDescriptor)

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If I run it like this, I get the error above upon socket creation and the socket never receives any data.

If I don't pass "this" as a parent to QTcpSocket then simply this always returns 0 (error is gone):

bytes_avail = mClientSocket->bytesAvailable();

bytes_avail = mClientSocket->bytesAvailable();

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Should I or shouldn't I be passing the server thread as a parent to QTcpSocket in my thread? I feel like I should but in the Qt Fortune example they do not.

I've tested the networking code successfully with Qt (and it works with the old method, it responds to the client), but I was getting this parent runtime warning about accessing the socket from a different thread from the Qt library upon calling write() function, so I'm trying to fix that....and I can't!!!

I feel like I'm not setting up the socket correctly anymore, the previous method relied on the GUI calling "nextPendingConnection()" but with the new method that doesn't get called anymore (incomingConnection() is overridden and it just spawns a new thread and starts it, the GUI thread returns after that). I don't think there's any additional code I need in the incomingConnection() override, the fortune example doesn't have any, just makes the thread and starts it, passing in the raw socket descriptor...but this whole 'passing the raw socket descriptor' bit doesn't seem to be working in the new method..

Any advice?

NOTE: Fortune server example I mention: http://qt-project.org/doc/qt-5/qtnet...r-example.html

[anda_skoa]

Do you call exec() in your run() method?

I.e. do you start the thread's event loop?

Cheers,
_

[Syndacate]

Do you call exec() in your run() method?

I.e. do you start the thread's event loop?

Cheers,
_

Thanks for your response.

The only exec() I call is when I exec the app from the main file.

The server_thread subclasses QThread, so after I construct it in the rtserver, I call:

st->start()

st->start()

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I expected start() to exec the thread, no?

The run() function (which in this case is just an event loop) of the server_thread gets called. The first thing it does is use the raw socket descriptor it got in the c'tor to construct a QTcpSocket in itself (so it's part of the new thread), then it goes into an event loop handling requests from client, doing calculations, and writing data back to them. It is most definitely calling run() because the socket gets constructed (which is when it gives the warning now) and printing stuff out, etc. It's just not getting data from the socket anymore.

Since I overrode incomingConnection() in the server, I just pass the raw/native socket descriptor to the new thread and the new thread constructs the QTcpSocket around it...but the read/write doesn't seem to be working anymore and I'm not sure why... The old method called nextPendingConnection() in the original thread to get the QTcpSocket...then I passed that socket over to the server thread. Qt complained about this on the write() function saying that I shouldn't be writing to a QTcpSocket that is on a different thread, so I restructured it like this so that the QTcpSocket is constructed on the spawned thread.

[anda_skoa]

The only exec() I call is when I exec the app from the main file.

Then you are missing it in the thread's run()

The server_thread subclasses QThread, so after I construct it in the rtserver, I call:

st->start()

st->start()

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I expected start() to exec the thread, no?

That starts the thread. The default implementation of run() calls exec() to start the thread's event loop.
Since you overwrote that, you have to start it either by calling the base implementation of run() or calling exec() yourself.

If you don't execute the thread's event loop then the thread will exit as soon as the run method's scope is done.

Cheers,
_