qt multi threads

[weixj2003ld]

Hello,everry one.
I want do the following things with qt multi threads.But I do not know how to realize it?
main ()
{
.....
create thread1// do thing A
create thread 2// do thing B
do thing C.
......
}
I want to the thing C does after the A and B both have finished.When the program run,C did before A and B finish(because thread1 and thread2 have not finished), how Could I realize what I want to do?

[wysota]

What's the reason for using threads then? If you insist on using threads, use wait conditions or mutexes to synchronize them. Threads A and B will release respective mutexes and only then thread C will do its job. If you don't want to block thread C (if it's your GUI thread) then you have to use another approach. In that case read this article:
http://doc.trolltech.com/qq/qq27-responsive-guis.html

[weixj2003ld]

Thank you for your answer.

1.The reason for using thread.

main()
{
         ...
          for(i=0;i<max;i++)
         {
              //  there are a lot things will be done in this circle,and it will consume much time. 
         }
         ...
}

main()
{
         ...
          for(i=0;i<max;i++)
         {
              //  there are a lot things will be done in this circle,and it will consume much time. 
         }
         ...
}

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In order to decrease the time,I want to do it with multi -threads.

for(i=0;i<max/2;i++)
   //threads A does
for(i=max/2+1;i<max;i++)
  //threads B does.

for(i=0;i<max/2;i++)
   //threads A does
for(i=max/2+1;i<max;i++)
  //threads B does.

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2.When I create a class like this.

class A: public QThread
{
          void run()
         {
              QMessageBox::warning(0,"dddd","ddddd",0,0,1);
         }
}

class A: public QThread
{
          void run()
         {
              QMessageBox::warning(0,"dddd","ddddd",0,0,1);
         }
}

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run the progam,the following error will occur:
"Widgets must be created in the GUI thread.", file kernel/qwidget.cpp,
I know I do not inherit from gui widget,but if I write like this

class A: public QWidget,public QThread
{
          void run()
         {
              QMessageBox::warning(0,"dddd","ddddd",0,0,1);
         }
}

class A: public QWidget,public QThread
{
          void run()
         {
              QMessageBox::warning(0,"dddd","ddddd",0,0,1);
         }
}

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error still exists.
How could I do with it?

[faldzip]

You have to create Widgets in main (GUI) thread, so instead of

QMessageBox::warning(...);

QMessageBox::warning(...);

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use qDebug() to print to console or, for example, notify GUI thread via signal/slot about what you want to display as a QMessageBox and the GUI thread will do that in slot.

[talk2amulya]

are you sure u'd need 3 different threads to accomplish your tasks..using too many threads can be overkill and as explained in the article by wysota, will most of the times bring the performance of ur application down..try to use just one extra thread and see how ur app performs..to make sure u dont start task C before A and B..start an event loop in main()..let A and B be done in one thread..when they are done, emit a signal, which is caught in the main thread upon which the eventloop is quitted..then perform task C..

Class A&B : public QThread
{
protected run()
{
    TaskA();
    TaskB();
    emit done();
}
signals:
    done();
}
 
main thread:
int main(...)
{
    //start thread A&B
    QEventLoop loop;
    connect(threadObject, SIGNAL(done()), &loop, SLOT(quit()));
    loop.exec();
    taskC(); 
    ....
}

Class A&B : public QThread
{
protected run()
{
    TaskA();
    TaskB();
    emit done();
}
signals:
    done();
}

main thread:
int main(...)
{
    //start thread A&B
    QEventLoop loop;
    connect(threadObject, SIGNAL(done()), &loop, SLOT(quit()));
    loop.exec();
    taskC(); 
    ....
}

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if u read wysota's article carefully, u might have already figured that out by now.

[wysota]

In order to decrease the time,I want to do it with multi -threads.

for(i=0;i<max/2;i++)
   //threads A does
for(i=max/2+1;i<max;i++)
  //threads B does.

for(i=0;i<max/2;i++)
   //threads A does
for(i=max/2+1;i<max;i++)
  //threads B does.

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This is wrong. Either the for loops need to be inside the threads or you have to change your architecture. Otherwise you're losing computing power. I suggest you use QtConcurrent to manage multithreading for you - the runnables solution is something you want to have a look at.

[weixj2003ld]

Thanks for answer.
1. Now,I create 2 threads,the thread class like this:
mythread.h

class Mythread:public QThread
{
   Mythread::Mythread(int start,int end,MyclassA *a)
     void MyThread::run()
{
 
}
  public:
        MyclassA *a1;
       int start1,end1;     
}
mythread.cpp
Mythread::Mythread(int start,int end,MyclassA *a)
{
start1=start;
end1=end;
a1=a;
}
Mythread::run()
{
    for(i=start1;i<end1;i++)
{
//do task.Myclass  will be used here.
}
}
main.cpp
{
...
  MyclassA myclassa is created.
.....
  Mythread *thread1=new Mythread(0,10,myclassa);
  Mythread *thread2=new Mythread(11,30,myclassa);
  thread1.start();
thread2.start();
...
}

class Mythread:public QThread
{
   Mythread::Mythread(int start,int end,MyclassA *a)
     void MyThread::run()
{
       
}
  public:
        MyclassA *a1;
       int start1,end1;     
}
mythread.cpp
Mythread::Mythread(int start,int end,MyclassA *a)
{
start1=start;
end1=end;
a1=a;
}
Mythread::run()
{
    for(i=start1;i<end1;i++)
{
//do task.Myclass  will be used here.
}
}
main.cpp
{
...
  MyclassA myclassa is created.
.....
  Mythread *thread1=new Mythread(0,10,myclassa);
  Mythread *thread2=new Mythread(11,30,myclassa);
  thread1.start();
thread2.start();
...
}

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2.If I only create one thread(thread1,for example) and the program is ok,but if I create 2 threads,when I run the program,it will crash.Why?

[talk2amulya]

you shouldnt ignore what different replies you get in the posts..your case is a classic example for which Qt Concurrent is applicable..read about it and try to implement it..u wont have to use those multiple threads..Qt will take care of everything without letting anything crash..

[weixj2003ld]

Thank you for answering my question patiently.
My program built with qt4.3.1,and a lot of things have been done by other people,I am afraid I can not change it to upper version.but Q4.3.1 does not support Qt Concurrent.
I think I will manage the threads by myself ,but not Qt Concurrent.
My questions:
1.What should I do if I want to let Q4.3.1 support Qt Concurrent?
2. Can data type( class defined by user) be shared by threads?
Thank you.

[wysota]

Thank you for answering my question patiently.
My program built with qt4.3.1,and a lot of things have been done by other people,I am afraid I can not change it to upper version.but Q4.3.1 does not support Qt Concurrent.

Why can't you upgrade Qt?

2. Can data type( class defined by user) be shared by threads?

Not without proper synchronization. If you do synchronize using a mutex, then it can be shared. Some of the classes will be safe without synchronization but it depends on a case by case basis. And in general widgets can't be shared between threads, regardless if you do the synchronization or not.

[weixj2003ld]

A class(defined by myself) shared by the threads must be lock and unlock?

[talk2amulya]

yes, using mutex

[weixj2003ld]

when I used two threads like what I have said in my program and compile and run it,it will be crashed at radom position,Is it because of my not synchronizing my threads?Must I synchronize them?

[talk2amulya]

if there is a shared object, u have to make it thread-safe..and yes, synchronize..and again, why cant u upgrade Qt? u have a solution waiting for u in there, so i cant understand ur reluctance