How to know thread is running from different button ?

jsulm

@a-normal-worker "You should make sure that you use a class member variable or they point to the same object" - he already does in his last post.

Hiloshi

Dear @jsulm , @a-normal-worker ,

Thank you all. First, the "redefine of class xxxxx" issue may cause by a silly mistake:

#ifndef MYTHREAD_H
#define MYTHREAD_H
#endif 
class MyThread   // <--I put my code here.

After fix above mistake and declare T3Thread pointer correctly then can compiler successfully.

But running the program show a strange message: "Process killed by signal"

In MyThread.h:

class MyThread : public QThread
{
    Q_OBJECT

private:
    void run()
     {       
        for (int i = 0; i <= 100; i++)
        {            
            delay(20);
        }       
     }
};

In mainwindow.h:

class MyThread;

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:
    void on_pushButton_pressed();

    void on_pushButton_2_clicked();

private:
    Ui::MainWindow *ui;
    MyThread *T3Thread;
};

In mainwindow.cpp:

#include "mythread.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);   
    MyThread  *T3Thread = new MyThread;
    T3Thread->start();
}
void MainWindow::on_pushButton_12_clicked()
{
    T3Thread->start();
}

When I run the program, the screen show as normal, but when I press button_2, the screen close suddently, and show "Process killed by signal" on QT terminal.

I suspect that problem is cause by T3Thread->start(), however, if I didn't press button_2, nothing happened.
T3Thread should also executed while creating main window?

Do I lack some declaration ?

SGaist

Your constructor is wrong. You are shadowing your T3Thread member variable. Thus when you call on_pushButton_12_clicked you are trying to access an uninitialised pointer hence the crash.

jsulm

@Hiloshi To add to @SGaist : why do you start the thread twice (once in the constructor and then in on_pushButton_12_clicked())?

Hiloshi

Dear @jsulm , @SGaist

Sorry to confuse you, I didn't explain clearly.

If I use "new" and "T3Thread->start" in the same area, then it will no error(process killed by signal),
as below:
https://www.dropbox.com/s/0488sqlhgn9u4wf/Screenshot from 2017-04-06 09-28-32.png?dl=0
alt text

However, if I put them in different place, application will crash and return "process killed by signal",
as below:
https://www.dropbox.com/s/3dfists06z3c5q9/Screenshot from 2017-04-06 09-33-10.png?dl=0
alt text

Moreover, if I put "new" just after the "ui->setupUi(this)" and "T3Thread in the on_pushButton_clicked,
it will show a warning message "unused variable ‘T3Thread’ [-Wunused-variable]".

@SGaist , I think maybe this is what you said : "uninitialised pointer " .

But, I am sorry, I don't know how to correct it.

SGaist

Remove MyThread * from your constructor.

jsulm

@Hiloshi You should really read more carefully what others are writing. What @SGaist said is:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);   
    T3Thread = new MyThread; // Remove MyThread* - else you are declaring a local variable which is completely unrelated to the one in your header!
    T3Thread->start();
}

And after this change think about where to start the thread: in the code you posted you're doing it twice.

Hiloshi

Very happy to learn from here. Thanks everyone.

Hiloshi

I want to study in depth in this question. In the original question, I have a running T3Thread, mainwindow cannot reach T3Thread, thanks for @a-normal-worker 's suggestion, we can make T3Thread as a class member of mainwindow then mainwindow have ability to access T3Thread.

Can we use signal/slot instead of using class member ?

SGaist

What exactly do you want to do with your thread ?

Hiloshi

Dear @SGaist ,

I am thinking to create a breathing LED in my thread, "run" function is keep turn on/off LED.
I want to put one button on mainwindow to start the thread.

I do not want this button to be pressed twice ( or thread will create twice), so in the button I put:

void MainWindow::on_pushButton_3_clicked()
{
    if(T3Thread->isRunning()==false)
    {
        MyThread  *T3Thread = new MyThread;  // local variable
        T3Thread->start;
    }
    else
        qDebug()<<"T3 thread is running";
}

If I do not want to use class member, can I use signal/slot to know T3Thread is running or not ?

Thanks.

J.Hilk

@Hiloshi
ok, I think we talk at cross here.

Here's a basic setup to check if your thread is running when you press a Button, if its not running the button will start the thread:

//in your header:
example.h

#include <QThread>
#include <qDebug>

private:
QThread *thread;

//in your source file

//Constructor
{
    ui->setupUi(this);   
    thread = new QThread;
....
}


void on_pushButton_clicked(){
   if(thread->isRunning())
      qDebug() << "Thread is running";
   else{
      qDebug() << "Thread started";
      thread->start(); 
   }
}

//Destructor
{
  if(thread->isRunning()){
   thread->quit();
   thread-> wait();
  }
...
  delete ui;
}

SGaist

What is the problem with using a member variable to handle that thread object properly ?

Hiloshi

@SGaist , There is no problem using member variable. I just thinking if it possible to use signal/slot to reach the same purpose.

SGaist

There could be ways however you are talking about creating an object somewhere without any reference to it and still want to interact with it. That's bad by design.

Hiloshi

@SGaist , Understood, thanks for the help.