QTcpServer: newConnection not emitted, but QTcpServer::waitForNewConnection works

[Moritz Spiller]

Hi,
I'm using Qt 5.15 on Windows 10 to create a simple TCP-Server that will receive data from a raspberry pi acting as client (I previously used a python program to receive data from the pi, so I know that the code on the pi works).

The code for my new qt application is based on QT's fortune server example. The problem is that neither the QTcpServer::newConnection, nor the QTcpSocket::readyRead signals are emitted. However, if I'm using the blocking versions QTcpServer::waitForNewConnection and QTcpSocket::waitForReadyRead, everything works fine and I can read data from the stream. I suspected that there is no event loop running, but an implemented QPushButton works fine.

Can anyone help and guide me towards the problem, any hint is appreciated?

My Code:

wifistream.h

#ifndef WIFISTREAM_H
#define WIFISTREAM_H

#include <QObject>
#include <QString>
#include <QVector>
#include <QtNetwork>

QT_BEGIN_NAMESPACE
class QLabel;
class QTcpServer;
QT_END_NAMESPACE

class wifiStream : public QObject
{
    Q_OBJECT

public:
    explicit wifiStream();
    bool getStatus();

private slots:
    void accept_connection();
    void read();

private:
    void initServer();
    void write();
    void disconnect();

    QTcpServer *tcpServer = nullptr;
    QTcpSocket *clientConnection = nullptr;
    QVector<QString> fortunes;
};

#endif // WIFISTREAM_H

wifistream.cpp

#include "wifistream.h"
#include <QtWidgets>
#include <QtCore>

wifiStream::wifiStream()
{
    initServer();

    qDebug() << connect(tcpServer, &QTcpServer::newConnection, this, &wifiStream::accept_connection);

//    tcpServer->waitForNewConnection(10000);
}

void wifiStream::initServer()
{
    QString ipAddress;
    QList<QHostAddress> ipAddressesList = QNetworkInterface::allAddresses();

    // use the first non-localhost IPv4 address
    for (int i = 0; i < ipAddressesList.size(); ++i) {
        if (ipAddressesList.at(i) != QHostAddress::LocalHost &&
            ipAddressesList.at(i).toIPv4Address() &&
            ipAddressesList.at(i).toString().contains("137")) {
            ipAddress = ipAddressesList.at(i).toString();
            break;
        }
    }

    // if we did not find one, use IPv4 localhost
    if (ipAddress.isEmpty())
        ipAddress = QHostAddress(QHostAddress::LocalHost).toString();

    tcpServer = new QTcpServer(this);
    if (!tcpServer->listen(QHostAddress(ipAddress), 80)) {
        return;
    }

    qDebug() << ipAddress;
    qDebug() << tcpServer->serverPort();
    qDebug() << tcpServer->isListening();
    qDebug() << tcpServer->serverAddress();
}

//! [4]
void wifiStream::accept_connection()
{
    qDebug() << "new connection";

    clientConnection = tcpServer->nextPendingConnection();

    qDebug() << clientConnection->peerName();
    qDebug() << clientConnection->peerAddress();

//    connect(clientConnection, &QAbstractSocket::connected,
//            this, &wifiStream::read);
    connect(clientConnection, &QAbstractSocket::disconnected,
            clientConnection, &QObject::deleteLater);
    qDebug() << connect(clientConnection, &QAbstractSocket::readyRead,
            this, &wifiStream::read);

//    clientConnection->waitForReadyRead(30000);
    qDebug() << "done waiting for read";
}

void wifiStream::disconnect()
{
    clientConnection->disconnectFromHost();
}

void wifiStream::read()
{
    for (;;) {
        //qDebug() <<"here C1 bytes = " << clientConnection->bytesAvailable();
        QDataStream in(clientConnection);
        //in.setVersion(QDataStream::Qt_5_15);

        if (clientConnection->bytesAvailable() != 0) {
            qDebug() << "Bytes available : " << clientConnection->bytesAvailable();
            QString nextFortune;
            in >> nextFortune;
            qDebug() << nextFortune;
            char* b = new char[clientConnection->bytesAvailable()];
            qDebug() << in.readRawData(b, clientConnection->bytesAvailable());
            qDebug() << b;
        }
    }
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QObject>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

    void gen_data();

private:
    Ui::MainWindow *ui;

signals:
    void new_sample(const int &sample);

private slots:
    void on_testButton_clicked();
};
#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <chrono>
#include <thread>
#include "cbuffer.h"
#include "processing.h"
#include "wifistream.h"
#include <QPushButton>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    wifiStream s;
}

The processing and cbuffer classes are not used in this version of the code.

And to add everythng relevant, main.cpp

#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

[mrjj]

Hi
You code looks good and structure so I wonder if its just a paste error.
You show.

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    wifiStream s;
}

Here "s" is a local variable that is deleted as soon as ctor ends. So using async calls would never be
able to work as it's dead long before.

I expect something like
wifiStream *s = new wifiStream;

as to have it live longer that constructor of MainWindow.

[J.Hilk]

@Moritz-Spiller so, why the infinite loop in read ?

void wifiStream::read()
{
    for (;;) {

that is blocking the event loop and prevent signals from being processed. Remove it, and check if you're suddenly seeing debug prints ;)

[Moritz Spiller]

@mrjj thanks a lot, this was the problem. I was not aware of that..

[Moritz Spiller]

@J-Hilk thanks for your comment. I put that to read continuously from the buffer, but of course it is not necessary as soon as the signal/slot mechansim works.