How a server can send a message to all of the clients (threads)?

Bamshad

Hello,

I work on a sever-client project where the server should support multiple clients. Therefore, I used QThread to create a thread for each of the clients. The server works properly and it can handle the clients. For example, it can echo the received data from clientX to clientX.

However, I want the server to echo the received message from one of the clients to all of them. For this objective, I make the list of clients with QVector and try to send the message to all (like bellow):

// In the serverSocketClass:
void serverSocketClass::incomingConnection(qintptr socketDescriptor) {
clientSocketClass *thread = new clientSocketClass(socketDescriptor, this);
mThreads.push_back(thread); // Here I make the list of clients.
thread->start();
}

//In the clientSocketClass:

void clientSocketClass::readyReadSlot()
{
QByteArray Data = clientSocket->readAll();
for (int i =0;i<cmThreads.length();i++) {
mThreads[i]->write(Data);
}
}

But, I face with one of the following errors:

If first client connect and do not send any messages and then second client connect and send a message:

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QNativeSocketEngine(0x7f2f20002ef0), parent's thread is clientSocketClass(0x5628b63a6f50), current thread is clientSocketClass(0x7f2f4c005620)

If first client connect and send a message then second client connect and send a message:

QSocketNotifier: Socket notifiers cannot be enabled or disabled from another thread

I hope I could explain the situation well.

jsulm

@Bamshad said in How a server can send a message to all of the clients (threads)?:

mThreads[i]->write(Data);

Don't do this! You're accessing clients which live in other threads!
Emit a signal from the thread which wants to broadcast to other clients. All clients should connect to this signal and send the data from their slot.

kshegunov

https://wiki.qt.io/WIP-How_to_create_a_simple_chat_application

Bamshad

@jsulm In C I use something like this:

for (int j=serverFd+1; j<=fdMax; j++) {
send(j,receiveData,strlen(receiveData), 0))
}

Because socket descriptor is one of the arguments of send(), I can send the data where I want. But, here wirte() has not any argument except the data. So, how we can tell it to send the data to other sockets?

aha_1980

@Bamshad said in How a server can send a message to all of the clients (threads)?:

So, how we can tell it to send the data to other sockets?

By using Signals & Slots, as pointed out by @jsulm and @kshegunov already. You can add arbitrary parameters to a signal and evaluate from the slot.

Regards

Bamshad

@aha_1980 Hi, As far as I understood the signal should emits from the readyReadSlot() of the clinetSocketClass. But, where should be the receiver slot that sends the data to all? I have two class:

serverSocketClass where I create the threads.
clientSocketClass where have the run() function and create the new QTcpSocket.

In both of them, when I want to send to all I receive the following error:

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QNativeSocketEngine(0x7f2f20002ef0), parent's thread is clientSocketClass(0x5628b63a6f50), current thread is clientSocketClass(0x7f2f4c005620)

So, where is the place for function sendToAll()?

jsulm

@Bamshad said in How a server can send a message to all of the clients (threads)?:

So, where is the place for function sendToAll()?

Nowhere. It should be a slot. You connect a signal to that slot using queued connection type.
Instead of calling sendToAll() directly on objects which are in OTHER threads you emit a signal and then Qt will take care to call all connected slots in their threads.

You should really take a look at the example @kshegunov gave you...