QTcpServer newConnection signal not emitted when QMainWindow is instantiated
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Hi. I have a QTcpServer object instantiated inside a derived QMainWindow class.
After connecting the newConnection() signal to a slot in the QMainWindow class and calling listen(QHostAddress::Any, 2021), the newConnection signal should be emitted once a client connects. This does however not happen - A client using QTcpSocket can connect to the server, but the newConnection signal is not emitted by the server.I'm guessing this is a eventloop issue. Even removing the server from the QMainWindow derived class and placing it in main does not work if the QMainWindow derived class is still being instantiated from main as well.
I would greatly appreciate some help on this topic.
Please see the attached code below://main.cpp #include "netmansim2_class.h" #include <QApplication> int main(int argc, char *argv[]) { QApplication a(argc, argv); NetmanSim2_class w; w.show(); return a.exec(); } //netmansim2_class.h namespace Ui { class NetmanSim2_class; } class NetmanSim2_class : public QMainWindow { Q_OBJECT public: explicit NetmanSim2_class(QWidget *parent = 0); ~NetmanSim2_class(); private slots: void slotServerConnected(); private: Ui::NetmanSim2_class *ui; QTcpServer* server; }; //netmansim2_class.cpp #include "netmansim2_class.h" #include "ui_netmansim2_class.h" NetmanSim2_class::NetmanSim2_class(QWidget *parent) : QMainWindow(parent), ui(new Ui::NetmanSim2_class) { ui->setupUi(this); server = new QTcpServer(); connect(server, SIGNAL(newConnection()), this, SLOT(slotServerConnected())); if (server->listen(QHostAddress::Any, 2021)) qDebug() << "Server listening on port " << 2021; } void NetmanSim2_class::slotServerConnected() { qDebug() << "Server connected"; } NetmanSim2_class::~NetmanSim2_class() { delete ui; }
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I found the solution:
Due to a stackoverflow upon initial project setup, I added the following to the .pro file:
QMAKE_LFLAGS += /STACK:256000000Removing one 0 from the size resulted in the server working correctly.
Why would a large stack size break the eventloop though? I have used the same stack size for other projects without problems.
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@jsulm The connected() signal is emitted by the client upon connection. Here is a broken-down example of the client code which is built and run in a different project:
//.h class tcp_client_class : public QObject { Q_OBJECT public: tcp_client_class(); private: QTcpSocket tcpClientSocket; private slots: void slotRxReady(void); void slotHandleTcpError(const QAbstractSocket::SocketError socketError); void slotTcpConnectSuccess(void); } //.cpp tcp_client_class::tcp_client_class(): QObject() { tcpClientSocket.setSocketOption(QAbstractSocket::LowDelayOption,1); connect(&tcpClientSocket, SIGNAL(connected()), this, SLOT(slotTcpConnectSuccess())); connect(&tcpClientSocket, SIGNAL(readyRead()), this, SLOT(slotRxReady())); connect(&tcpClientSocket, SIGNAL(error(QAbstractSocket::SocketError)), this, SLOT(slotHandleTcpError(const QAbstractSocket::SocketError))); tcpClientSocket.connectToHost("127.0.0.1", 2021); } void tcp_client_class::slotTcpConnectSuccess() { emit sigSocketConnected(); qDebug() << "TCP Client Interface: TCP Socket connection success"; }
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@izkb mmh, can you connect to ther fortune-server example with your client?
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Hi and welcome to devnet,
From what I can see, you try to bind to localhost while listening on any of the network interfaces. You should replace
QHostAddress::Any
byQHostAddress::LocalHost
.In any case, a call to QTcpServer::serverAddress should give you a clue about which address to use to connect to sour server object.
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Hi @SGaist
Replacing QHostAddress::Any with QHostAddress::LocalHost and printing server->serverAddress displays 127.0.0.1 which is the address to which the client is connecting and as mentioned the client connects successfully, the problem is on the server side which does not register a new connection.The problem must be on the eventloop and possibly results from the server being contained in a QMainWindow object, since the FortuneServer example derives a QDialog class and works on my system. Are there any known issues of this sort with QMainWindow?
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@izkb I'm quite sure the issue isn't QMainWindow as it has an event loop as well. It doesn't matter whether it is QMainWindow or QDialog. As long as you don't block the event loop it should work.
What else are you doing in your server? Do you by any chance blocking the event loop? -
@jsulm
The server code provided is the only code I am using at the moment and as far as I know nothing there should block the eventloop...
Could it not be that QMainWindow creates separate threads which might break the inner workings of the QTcpServer class? -
@izkb If you think, creating the server in the constructor of your mainclass is the issue, why don't you move that away than?
#include <QTimer> server = Q_NULLPTR; connect(server, QTcpServer::newConnection, this, &NetmanSim2_class::slotServerConnected); QTimer::singleShot(0,this,[=]{ server = new QTcpServer(this); if(!server->listen()){ qDebug() << "Unable to open Server"; }else qDebug() << "Server Read for Connection"; });
The QTimer of 0ms means will be executed next eventloop cycle.
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One thing that is missing in your code: acceptError handling. That might provide additional information about what is happening.