How to know thread is running from different button ?

Hiloshi

Dear All,

I create a button and make it run a infinity loop thread:

void MainWindow::on_pushButton_2_clicked()
{
    MyThread  *T3Thread = new MyThread;
    T3Thread->start(); // <-- in run is a infinity loop with delay(20)
}

Now I want to check if T3 thread is running by

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

I believe everyone can see the mistake, T3Thread in button_2 is different from button_3 so T3Thread->isRunning() will never return true.

What should I do ?

Thanks for any suggestions.

a normal worker

t3 just a local variable?Why not use member variables

Hiloshi

@a-normal-worker said in How to know thread is running from different button ?:

t3 just a local variable?Why not use member variables

Do you mean a global variable ?

Hiloshi

@a-normal-worker said in How to know thread is running from different button ?:

t3 just a local variable?Why not use member variables

If I change T3Thread to global variable, then I met a different situation:
I wrote a myThread.h, inside this header file, I create a "my thread class":

class MyThread : public QThread
{
    Q_OBJECT

signals:
    

private:
    void run()
     {
        while(1)
        {
            for (int i = 0; i <= 100; i++)           
                delay(20);  
        }
     }
};

In order to declare a global variable, I add this line to mainwindow.h :

private:
     MyThread *T3Thread;

However, because mainwindow.h do not know "MyThread", so I include "myThread.h" to mainwindow.h.
Then compiler said "redefine of class xxxxx"

So....I cannot include another header file ? How to declare a variable like "MyThread *T3Thread" correctly.

Thanks everyone.

jsulm

@Hiloshi Does your myThread.h header file has an "include guard"? See https://en.wikipedia.org/wiki/Include_guard
They usually look like:

#ifndef MYTHREAD_H
#define MYTHREAD_H
class MyThread...
#endif

Another note: since T3Thread is a pointer you do not need to include myThread.h in mainwindow.h. Instead use forward declaration:

// In mainwindow.h before MainWindow class
class MyThread;

and include myThread.h in mainwindow.cpp.

One more note: T3Thread is NOT a global variable (and this is good), it is an class member variable.

a normal worker

@Hiloshi Maybe my idea is not clear. Your code is created with two different objects, so their states are different. You should make sure that you use a class member variable or they point to the same object

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.