Qt create a dialog that waits for a network event - c++

Question

I am writing the client for a client/server application. The clients are supposed to login using a login window. If the login is successful, a "waiting" window appears (this is just a window that contains a label). On the server side there is a barrier that waits for n clients to be logged in; when this happens, a message is broadcasted, the waiting window is supposed to close and a new window appears for every client.
The networking interface is implemented by me, using low-level functions, not the functionality provided by Qt.
The actual waiting loop is something like this:
char buffer[256];
while (strcmp(buffer, "proceed"))
read(sockfd, buffer, 256);
The problem is that if I start this loop in the main thread, the application blocks, for obvious reasons.
How can I make this loop run and not block the application, and close the dialog when it ends?
Later edit: I did also attempt to use QThreads, but, for reasons which I don't fully understand yet, the application still crashes:
class WaitLoop : public QThread {
public:
WaitLoop(NetworkHandler &network) : network(network) {}
private :
NetworkHandler &network;
void run() {
this->network.waitForGameStart();
}
};
In the wait dialog constructor:
WaitLoop *waitLoop = new WaitLoop(network);
connect(waitLoop, SIGNAL(finished()), this, SLOT(gameStartSlot()));
waitLoop->start();
The application still crashes using this approach.

Answer 1

The sanest way to approach this would not be using low-level functions, because you aren't writing in C. Use at least QAbstractSocket to wrap a sockfd. The setSocketDescriptor method lets you do it.
Your code then becomes non-blocking and asynchronous:
class Controller : public QObject {
Q_OBJECT
QStateMachine m_sm;
QState s_init{&m_sm}, s_proceeding{&m_sm};
QAbstractSocket m_socket;
Q_SIGNAL void proceed();
Q_SLOT void onData() {
auto data = m_socket.readAll();
if (data.contains("proceed")) proceed();
}
public:
Controller(QObject * parent = 0) : QObject(parent) {
connect(&m_socket, &QIODevice::readyRead, this, &Controller::onData);
s_init.addTransition(this, &Controller::proceed, &s_proceeding);
m_sm.setInitialState(&s_init);
m_sm.start();
}
bool setup(quintptr fd) {
return m_socket.setSocketDescriptor(fd);
}
};
Through the use of a state machine, it's easy to add more states, react to their transitions (see QState::onEntry signal, etc.), and ensure that the behavior is correct. Fleshing out a UML statechart forces you to think about handling corner cases, etc. See this answer for a full example.