" cannot send events to objects owned by another thread". exception in Qt visual studio 2019

[fari35]

I have made a client and a server. In server I have done multi threading. Now I am making a UI for my server program using Qt in visual studio 2019. But now here in my server code after running it when I run my client the server program gives me an exception " cannot send events to objects owned by another thread". I want to make a different thread in server for handling all the clients and my main thread will handle the GUI since the GUI freezes so I need a different thread to handle my clients. I have attached my server and client code below . can anyone tell me what I am doing wrong here?

server2.h

#pragma once

#include <QtWidgets/QMainWindow>
#include "ui_server2.h"
#include <iostream>
#include <WS2tcpip.h>
#include <string>
#include<sstream>
#include<WinSock2.h>
#include <cstdlib>
#include <fstream>

class server2 : public QMainWindow
{
    Q_OBJECT

public:
    server2(QWidget *parent = Q_NULLPTR);
    static DWORD WINAPI receive_req(LPVOID lpParam);
    void handle_gui(SOCKET &listening);
    
private slots:
    void on_connect_clicked();

private:
    Ui::server2Class ui;
    

};

server2.cpp

#include "server2.h"

#include "ui_server2.h"
#pragma once
#include <iostream>
#include <WS2tcpip.h>
#include <string>
#include<sstream>
#include<WinSock2.h>
#include <cstdlib>
#include <fstream>
#include<thread>

#pragma comment (lib, "ws2_32.lib")
QString sqt;

server2::server2(QWidget *parent)
    : QMainWindow(parent)
{
    ui.setupUi(this);
}

DWORD WINAPI server2::receive_req(LPVOID lpParam) {
	std::string input;
	input = "<Trancode> msgcode";
	SOCKET clientsock = (SOCKET)lpParam;
	char buf[4096];
	
	while (true)
	{

		ZeroMemory(buf, 4096);
		
		
		int recbyte = recv(clientsock, buf, 4096, 0);
		std::string str = std::string(buf, 0, recbyte);
		QString qstr = QString::fromStdString(str);
		sqt += qstr;
		sqt += '\n';
		//ui.label->append(sqt);
		
		if (recbyte == 0 || std::string(buf, 0, recbyte) == "")
		{


			//cout << "Client disconnected " << endl;
			break;
		}

		if (recbyte == SOCKET_ERROR)
		{
			//cerr << "Error in recv(). Quitting" << endl;

			break;
		}



		send(clientsock, input.c_str(), input.size() + 1, 0);

		//

	}

	closesocket(clientsock);
	ExitThread(0);

}

void server2::handle_gui(SOCKET &listening) {
	int cnt = 0;
	DWORD threadd;
	SOCKET clientsock;

	sockaddr_in from;
	int fromlen = sizeof(from);
	while (cnt != 3)
	{

		clientsock = accept(listening, (struct sockaddr*)&from, &fromlen);
		/*char buf[4096];
		int recbyte = recv(clientsock, buf, 4096, 0);
		std::string str = std::string(buf, 0, recbyte);
		QString qstr = QString::fromStdString(str);*/
		ui.label->setText("hello world");
		cnt++;



		// created a recv_cmds thread
		CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)server2::receive_req, (LPVOID)clientsock, 0, &threadd);
		ui.label->setText(sqt);

	}
}

void server2::on_connect_clicked()
{
	//ui.label->setText("hello world");
	

	WSADATA data;
	WORD ver = MAKEWORD(2, 2);

	int wres = WSAStartup(ver, &data);
	if (wres != 0)
	{
		//cerr << "unable to initialize winsock" << endl;
		return;
	}

	SOCKET listening = socket(AF_INET, SOCK_STREAM, 0);
	
	if (listening == INVALID_SOCKET)
	{
		//cerr << "unable to create a socket" << endl;
		return;
	}

	sockaddr_in sdata;
	sdata.sin_family = AF_INET;
	sdata.sin_port = htons(54000);
	sdata.sin_addr.S_un.S_addr = INADDR_ANY;

	

	bind(listening, (sockaddr*)&sdata, sizeof(sdata));

	listen(listening, SOMAXCONN);
	server2 ser;
	std::thread th1(&server2::handle_gui,&ser,std::ref(listening));
	ui.label->append(sqt);
	th1.join();

	//function gui
	

	closesocket(listening);

	//closesocket(clientsock);

	WSACleanup();

	system("pause");


}

client.cpp

#include <iostream>
#include <string>
#include <WS2tcpip.h>
#pragma comment(lib, "ws2_32.lib")

using namespace std;

void main()
{
	string ipadd = "127.0.0.1";
	int port = 54000;

	WSAData wd;
	WORD ver = MAKEWORD(2, 2);
	int wres = WSAStartup(ver, &wd);
	if (wres != 0)
	{
		cerr << "Winsock Error" << wres << endl;
		return;
	}


	SOCKET sock = socket(AF_INET, SOCK_STREAM, 0);
	if (sock == INVALID_SOCKET)
	{
		cerr << "Socket not created" << WSAGetLastError() << endl;
		WSACleanup();
		return;
	}

	sockaddr_in data;
	data.sin_family = AF_INET;
	data.sin_port = htons(port);
	inet_pton(AF_INET, ipadd.c_str(), &data.sin_addr);


	int conn = connect(sock, (sockaddr*)&data, sizeof(data));
	if (conn == SOCKET_ERROR)
	{
		cerr << "Unable to connect to the server" << WSAGetLastError() << endl;
		closesocket(sock);
		WSACleanup();
		return;
	}


	char buf[4096];
	string Input;

	do
	{

		cout << "> ";
		getline(cin, Input);

		if (Input.size() > 0)
		{

			int sres = send(sock, Input.c_str(),Input.size() + 1, 0);
			if (sres != SOCKET_ERROR)
			{

				ZeroMemory(buf, 4096);
				int recbyte = recv(sock, buf, 4096, 0);
				if (recbyte > 0)
				{

					cout << "Server> " << string(buf, 0, recbyte) << endl;
				}
			}
		}

	} while (Input.size() > 0);

	closesocket(sock);
	WSACleanup();
}

[JonB]

@fari35
I don't know from all your code, other than what the message tells you.

But why in the world are you using Qt and then all these Windows and C++ library calls to do everything? You are asking for problems, and I'd rather use Qt threading than C++ threading in a Qt program. If you used QSockets properly you might not need any threads (Qt socket stuff is asynchronous) and then you wouldn't have the issue in the first place....

[fari35]

@JonB I'm not able to include <QTcpserver> class file in my project in visual studio 2019. How can I do that? it's not there in VS 2019 packages.

[jsulm]

@fari35 said in " cannot send events to objects owned by another thread". exception in Qt visual studio 2019:

QTcpserver

It's called QTcpServer

[fari35]

@jsulm Yeah I'm not able to include this header file in my visual studio 2019. I'm using Qt therre. So how can I include this file. I have searched in extensions online but it's not even there.

[jsulm]

@fari35 said in " cannot send events to objects owned by another thread". exception in Qt visual studio 2019:

So how can I include this file

Like any other Qt header file.
Did you install qt at all?

[fari35]

@jsulm I have this zip file downloaded "qt-everywhere-src-5.15.0" and qtcpserver directory is there in this zip file but I don't know how to include it as a header file?
And I've downloaded Qt from this link "https://marketplace.visualstudio.com/items?itemName=TheQtCompany.QtVisualStudioTools2019"

[jsulm]

@fari35 This is an archive containing source code!
You do not need it!
As already said many times: the header file is part of a Qt installation.
I repeat this simple question: did you install Qt?
The link you posted is to download Qt Tools for Visual Studio, it is not to download Qt!
Please go to https://www.qt.io/download download Open Source Qt installer and install Qt for Visual Studio using it...