Multithreaded Application Tutorial

From Lazarus wiki
Revision as of 10:03, 7 November 2005 by Almindor (talk | contribs) (Special things to take care of)


In this page I will try to explain how to write and debug a mutithreaded application with Free Pascal and Lazarus.

A multithreaded application is one that creates two or more threads of execution that works at the same time.

One of the threads is called the Main Thread. The Main Thread is the one that is created by the Operating System, once our application starts. The Main Thread MUST BE the only thread that updates the components that interfases the application with the uses (else, the application may hang).

The main idea is that the application can do some processing in background (seccond thread) while the user could keep working (on the main thread).

Another use of threads is just to have a better responding application. If you create an application, and when the user press a button, the application start processing (a big process)... and while processing, the screen stop responding, and gives the user the sensation that the application is dead... if the big process runs on a seccond thread, the application keeps responding as good as it were idle. In this case is a good idea, before starting the thread, disable the buttons of the form to avoid the user to start more than one thread for the process.

Other use, is to create a server application that is able to respond to many clients at the same time.

The TThread Class

To create a multithreaded application, the easiest way is to use the TThread Class.

This class permits the creation of aditional thread (over the mail thread) in a simple way.

Normally you only have to override 2 methods (or may be just one): the Create constructor, and the Execute method.

In the constructor, you will prepare the thread to run. You will set the initial values of the variables or properties you need. The original constructor of TThread requires a parameter called Suspended. As guessed, being Suspended = True will prevent the thread to start automatically after the creation. If Suspended = False, the thread will start running just after the creation. If the thread is created suspended, then it will run only after the Resume method is called.

As of FPC version 2.0.1 and later TThread.Create also has an implicit parameter for Stack Size. You can now change the default stack size of each thread you create if you need it. Huge recursions in a thread are good example. If you don't specify the stack size parameter default OS stack size is used.

In the overrided Execute method you will write the code that will run on the thread.

The TThread class has one important property: Terminated : boolean;

If the thread has cicles (and this is usual), every cicle shoud be exited when Terminated is true (is false by default). So in the cicle evaluation it must check if Terminated is True, and if it is, must exit the cicle.

As we explained earlier, the thread should not interact with the visible components. To show something to the user it must acomplish that in the main thread. To do that exists a TThread method called Synchronize. Synchronize requires a parameter less method as an argument. When you call that method throw Synchronize(mymethod), the thread execution will be paused, the code of mymethod will run on the main thread, and then the thread execution will be resumed.

There is another important property of TThread: FreeOnTerminate. If this property is true, the thread object is automatically fried. Else the application will need to free it manually.


   TMyThread = class(TThread)
     fStatusText : string;
     procedure ShowStatus;
     Constructor Create(Suspended : boolean); override;
     procedure Execute; override;
 constructor TMyThread.Create(Suspended : boolean);
   inherited Create(Suspended);
   FreeOnTerminate := True;
 procedure TMyThread.ShowStatus;
   Form1.Caption := fStatusText;

 procedure TMyThread.Execute;
   newStatus : string;
   fStatusText := 'Starting...';
   fStatusText := 'Running...';
   while (not Terminated) and ([any condition required]) do
       [here goes the code of the main thread loop]
       if NewStatus <> fStatus then
           fStatusText = newStatus;

On the application,

   MyThread : TMyThread;
   MyThread := TMyThread.Create(True); // This way it starts   automatically
   [Here the code initialices anythig required before the threads starts   executing]

Special things to take care of

There is a potential headache in Windows with Threads if you use the -Ct (stack check) switch. For reasons not so clear the stack check will "trigger" on any TThread.Create if you use the default stack size. The only work-around for the moment is to simply not use -Ct switch. Note that it does NOT cause an exception in the main thread, but in the newly created one. This "looks" like if the thread was never started.

A good code to check for this and other exceptions which can occur in thread creation is:

    if Assigned(MyThread.FatalException) then
      raise MyThread.FatalException;

This code will asure that any exception which occured during thread creation will be raised in your main thread.

Units needed for multithreaded application

On Windows, you don´t need any special unit for this to work. On Linux, you need the cthreads unit. But, not only you need it. It must be the first used unit on the project!

So, your Lazarus application code should look like:

 program MyMultithreadedServer;
 {$ifdef linux}
   Interfaces, // this includes the LCL widgetset
   { add your units here },

SMP Support

The good news is that if your application works properly multithreaded this way, it is already SMP enabled!

Debuging Multithreaded Applications with Lazarus

The debuging on Lazarus is not fully fuctional yet. But, if you try to debug a multithreaded application on Linux, you will have one big problem: the X server will hang.

You will ask: How to solve it? I will answer: I DON´T KNOW... but I will give you a work around:

You could create a new instance of X with:

 X :1 &

It will open, and when you switch to another desktop (the one you are working with pressing CTRL+ALT+F7), you will be able to go back to the new graphical desktop with CTRL+ALT+F8 (if this combination does not work, try with CTRL+ALT+F2... this one worked on Slackware).

Then you could, if you want, create a desktop session on the X started with:

 gnome-session --display=:1 &

Then, in Lazarus, on the run parameters dialog for the project, check "Use display" and enter :1.

Now the application will run on the seccond X server and you will be able to debug it on the first one.

This was tested with Free Pascal 2.0 and Lazarus 0.9.10 on Windows and Linux.