Trying to understand semaphore

[ManiRon]

@dheerendra ok Sir

[VRonin]

I think you are confusing semaphores with mutexes. When you have 1 resource to protect there's no need to use a semaphore.

Can you tell us where do wou declare a, addSem and disSem?

@jsulm said in Trying to understand semaphore:

you're emitting signals in a loop that means slots will be executed after the loop is finished.

Not totally correct. If the connected slot is in a thread with an active event loop it will execute "immediately".

[ManiRon]

@VRonin

declared 'a' locally and addsem and disSem globally at MainWindow.cpp

[ManiRon]

@VRonin

Actually my aim was to check how the thread runs when we use a semaphore thats why thought of having a sample which displays each value after performing the addition but it just displays the first data after that the for loop is not running

[dheerendra]

This indicates that one thread has acquired & did not release the semaphore. Show us the code with your loop for both the threads.

[ManiRon]

@dheerendra actually i got both the values from both the thread but the value was only the first value then the next value were not updated donno whether my for loop is running?

[ManiRon]

@dheerendra
My thread Code:

void add_thread::run()
{

    int b = 0;
    int c = 0;
qDebug("ADD_");
    for (int a = 30; a < 40;++a)
    {
        addSem.acquire();
        c = a + b;
        qDebug("ADD_DATA %d",c);
        disSem.release();
        sleep(1000);
        b = c;
        addSem.available();
        emit ADD_Data(c);

        sleep(1000);
    }
    qDebug("ADD_DATA %d",c);
}


My Other thread code:

void display_thread::run()
{

    int b = 0;
    int c = 0;

    for (int a = 10; a < 20; ++a)
    {
       addSem.acquire();
        c = a + b;
        addSem.release();
        b = c;
        addSem.available();
        emit ADD_Data1(c);
        sleep(1000);

    }


}

[dheerendra]

@ManiRon said in Trying to understand semaphore:

disSem.release();

My guess is right. Still you are releasing someother semaphore. Replace above line with addSem.release. Both the threads should work with same semaphore.

addSem.aquire()
// do something
addSem.release()

[ManiRon]

@dheerendra
Sorry sir i changed and tried but the same result. But that too the second thread data i am able to display in line edit but the first thread data i am able to print using Qdebug but couldnt see the data in line edit

[dheerendra]

Can you show how did you declare semaphore ?

[ManiRon]

@dheerendra

QSemaphore addSem;

[dheerendra]

1. addSem is not initialised with initial semaphore count
2. sleep(..) it is seconds. So to complete the loop 10 times, it takes the 10*1000 seconds. So approximately you have to wait for 3 hrs to complete this loop:). So change your sleep to sleep(1).

[ManiRon]

@dheerendra
done sir its working

[dheerendra]

cool man. If you want the detailed example look at git

[ManiRon]

@dheerendra
Ok Sir

[dheerendra]

Now issue is resolved, you can move the issue to SOLVED state. Enjoy Qt Programming.