break loop in C++

marek · 09-10-2003, 09:10 AM

Hi, Has anybody got an idea how interrupt loop in C++ program?. I'll show you situation:

I've created program with Qt which turn on and off the keyboard lights with some frequency. It is done with a loop :

start()
{
while(1)
{
....
}
}
which is a part of C++ class. I execute start() when i cilck on button. But how to stop this loopback?? (i mean by clicking different buton)
I tested solutions with using function fork() to creating new process, but it doesn't want to work well (i usually received the zombie process), i think about threads, but i am not sure if it works.
How is possibly to break loops in C++ class?????

ps. breaking loop in C is really simple - enough to send a signal from other process, but i have to use a Qt classes, so i have to work in C++

nephilim · 09-10-2003, 09:16 AM

the 'continue' and 'break' keywords break out of a loop.

Since you seem to know quite a bit about C, I'm not sure this is the answer you're looking for. If not, let me know and I'll try to help you further.

marek · 09-10-2003, 09:58 AM

You right but, but i want to break loop which exists in function1 from function2,
of course i may test - every time i make a looping - value of some global valuable (private for C++ class), and change this value in function2, but do you think it is a good idea?

wapcaplet · 09-10-2003, 10:15 AM

It's been a while since I've done anything with inter-process communication, but I seem to recall that there is a way for one process to send a signal to another. If you fork the process before entering the loop, then have a test for "signal received from other process?" as the while condition, it should do what you're after.

I am afraid I cannot offer any details on how to do this, though...

Is there a particular reason that the loop has to be interrupted by an outside process? If a click starts the keyboard lights blinking, then shouldn't you just have a click of some kind fo stop it also? If that's the case, perhaps what you want is a loop that says "has user clicked mouse? If not, go ahead and blink lights. Loop."

Dark_Helmet · 09-10-2003, 10:22 AM

I think you'll have to implement it in a thread. The problem you describe boils down to one thing. You want your program to do two things at once: wait for input from the user and blink lights in a continuous loop.

You could do it with forks, but I like threads much more.

Essentially, you will have a main thread that handles user input. Then, when you click a button, your main thread will call an interface task with your thread(s). That task will alter some internal variable for the thread; a flag to indicate some new behavior. Then your main thread will resume waiting for input.

Meanwhile, your thread's code would look something like this:

Code:

private:
  int enable_blinking; // internal thread variable
  int kill_thread;  // flag to terminate thread's execution

...

// Public interface function
void thread_class::change_blink_status(int new_status)
{
  enable_blinking = new_status;
}

// Function that is executed by the thread when it is
// created
void thread_class::start() 
{
  while( kill_thread == 0 )
  {
    if( enable_blinking == 1 )
    {
      toggle_lights();
      // put in some delay here to get your frequency
    }
  }
}

It's been a while since I've played with threads, but i think that will work. It would also be cleaner to have a timer signal when to toggle the lights on or off. Lots of different ways of doing it.

Hko · 09-10-2003, 11:26 AM

Quote:

Originally posted by marek
I've created program with Qt which turn on and off the keyboard lights with some frequency. It is done with a loop :

start()
{
while(1)
{
....
}
}
which is a part of C++ class. I execute start() when i cilck on button. But how to stop this loopback?? (i mean by clicking different buton)

The problem is that the program does not react to the "stop"-button while the loop is running, so that's why you are thinking about threads, or a second process to interrupt it. Did I get this right?

The simplest solution, if I understand your question right, is not to use threads or processes, and not even a loop. You are using Qt, so why not use a timer ?

You can make a timer call with a (Qt slot-) function periodically (with a minimum of about 20 ms, which can be too slow in some cases, but not in your case I guess.)

Using a timer, the Qt framework does handle button-clicks and other events, while your light switching stuff is done in intervals.

Example:

Code:

//
//       Header file (.h)
//

#include <qwidget.h>
#include <qtimer.h>

class yourMainWidget: public QWidget
{
     Q_OBJECT

public:
     yourMainWidget(QWidget *parent=0, const char *name=0);
     ~yourMainWidget();
     
     // more stuff, like the buttons

protected:
     void timerEvent(QTimerEvent *); 

private:
     int timerID;
     
};


//
//    C++ file (.cpp)
//

void MMGameboard::timerEvent(QTimerEvent *e)
{
     // do your light switching stuff here.
     // *without* looping.
}


// ...then in the "start"- button slot, do:

timerID = startTimer(1000);  // 1000 ms = 1 second. try 10 or 100.

// and in the "stop"-button slot, do:

killTimer(timerID);

See for details: http://doc.trolltech.com/3.2/qobject.html#killTimer

Hope this helps.

marek · 09-10-2003, 12:23 PM

It's me again. Hko, i am sure you have a right , i tested QTimer earlier and i created application which was working great, but i have (maybe) crazy idea to connect the old function from Linux with graphical interface (i chose Qt). Anyway i have one more question, what is it [defunct]

process -> zombie[defunct]???

Everybody knows what it is zombie, but Could you tell me what it is defunct???

Hko · 09-10-2003, 12:27 PM

Quote:

Originally posted by marek Anyway i have one more question, what is it [defunct]

process -> zombie[defunct]???

Everybody knows what it is zombie, but Could you tell me what it is defunct??? [/B]

"defunct" is the original (UNIX) term for the same thing.
In Linux a "defunct" process is called "zombie".

Hko · 09-10-2003, 12:33 PM

Quote:

Originally posted by marek but i have (maybe) crazy idea to connect the old function from Linux with graphical interface (i chose Qt). [/B]

Then why not start the old C program from your Qt app with system(), exec...() whatever. Only add a signalhandler that sets a global var, and have the loop watch that var.

Then from your Qt app, send it a signal. You can use all normal C-functions like kill(), and so in a C++/QT program.

marek · 09-11-2003, 07:38 AM

This is my program:

THIS IS FILE form1.ui.h created by QtDesignera:

#include <sys/perm.h> //niezbedne do wywolania funkcji
#include <asm/io.h>
#include <sys/types.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
#include <signal.h>
#include <sys/time.h>
#include <stdlib.h>

unsigned char lights_off = 0x0;
const unsigned char keyb_inp_port = 0x060;
pid_t parent_process = 0;
pid_t child_process = 0;
int zmienna = 0;
unsigned char num_lock_on = 0x02;
unsigned char caps_lock_on = 0x04;
unsigned char scroll_lock_on = 0x01;

struct itimerval my_itimerval;

void Form1::init()
{

}

void Form1::destroy()
{

}

void Form1::start_lampki()
{

printf("Numer PID dla start_lampki = %d\n", getpid());
child_process = fork();
if (child_process == 0)
{
ioperm(keyb_inp_port, 16, 1);
while(1)
{
outb(0xed, keyb_inp_port);
outb(num_lock_on, keyb_inp_port);
sleep(1);
outb(0xed, keyb_inp_port);
outb(caps_lock_on, keyb_inp_port);
sleep(1);
outb(0xed, keyb_inp_port);
outb(scroll_lock_on, keyb_inp_port);
}
exit(1);
}
else if (child_process > 0)
{
printf("weee\n");
wait(&status);
}

}

void Form1::stop_lampki()
{
pid_t p = getpid();
kill(p + 1, SIGTERM);
printf("Numer PID dla stop_lampki = %d\n", getpid());
printf("Numer PID parenta dla stop_lampki = %d\n", getppid());

outb(0xed, keyb_inp_port);
outb(lights_off, keyb_inp_port);
ioperm(keyb_inp_port, 16, 0);
}

THIS IS MY FILE main.cpp

#include <qapplication.h>
#include "zabawa.h"
#include <qpushbutton.h> //ten plik naglowkowy musi byc, zeby mozna bylo korzystac z PushButton'a dla funckji connect() !!

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Form1 z;// = new Form1(0,0,FALSE,0);
z.show();
a.connect(z.startPushButton, SIGNAL(clicked() ), &z, SLOT(start_lampki() ));
a.connect(z.stopPushButton, SIGNAL(clicked() ), &z, SLOT(stop_lampki() ));
a.connect(&a, SIGNAL(lastWindowClosed() ), &a, SLOT(quit() )); //po zamknieciu ostatniego okna formatka (QDialog) bedzie zamkniety a wraz z nim cala aplikacja
a.connect(&a, SIGNAL(aboutToQuit() ), &z, SLOT(stop_lampki() )); //w momencie, w ktorym aplikacja bedzie zamykana przerwe sterowanie swiatelkami
return a.exec();
}

except these two files written above i have two files form1.h and form1.cpp (declaration and implementation for Qt class)

What about trouble - when i click on one button, it execute slot "start_lampki", and that's all application is blocked and i can't click another buttons!!

marek · 09-11-2003, 10:59 AM

Ok, i made a lot of mistakes, which i have resolved now. Thank you everybody for help