Difference between revisions of "Multithreaded Application Tutorial"

From Lazarus wiki
(See also)
(See also)
Line 515: Line 515:
 
* [[Streaming components]]
 
* [[Streaming components]]
 
* [[Manager Worker Threads System]]
 
* [[Manager Worker Threads System]]
* [[Array of threads]]
+
* [[Example of multi-threaded application: array of threads]]
 
[[Category:Tutorials]]
 
[[Category:Tutorials]]

Revision as of 22:17, 7 September 2010

Deutsch (de) English (en) español (es) français (fr) 日本語 (ja) polski (pl) português (pt) русский (ru) slovenčina (sk) 中文(中国大陆)‎ (zh_CN)

Overview

This page will try to explain how to write and debug a multi-threaded application with Free Pascal and Lazarus. A multi-threaded application is one that creates two or more threads of execution that work at the same time. If you are new to multi-threading, please read the paragraph "Do you need multi-threading?" to determine whether it is really required; this may save you many headaches.

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 interfaces with the user: otherwise, the application may hang.

The main idea is that the application can do some processing in background in a second thread while the user can continue working using the main thread.

Another use of threads is just to have a better responding application. If you create an application, and when the user presses a button the application starts processing a big job... and while processing, the screen stops responding, and gives the user the impression that the application is frozen, a poor or misleading impression will be created. If the big job runs in a second thread, the application keeps responding (almost) as if it were idle. In this case it is a good idea, before starting the thread, to disable the buttons of the form to avoid the user starting more than one thread for the job.

Another use of multi-threading may be a server application that is able to respond to many clients at the same time.

Do you need multi-threading?

If you are new to multi-threading and you only want to make your application more responsive while your application performs moderately long-running tasks, then multi-threading may be more than is required. Multi-threaded applications are always more difficult to debug and they are often much more complex; in many cases you don't need multi-threading. A single thread is enough. If you can split up the time-consuming task into several smaller chunks, then instead you should use Application.ProcessMessages. This method allows the LCL to handle all waiting messages and returns. The central idea is to call Application.ProcessMessages at regular intervals during the execution of a long-running task to determine whether the user has clicked on something, or a progress indicator must be repainted, and so on.

For example: Reading a big file and process it. See examples/multithreading/singlethreadingexample1.lpi.

Multi-threading is only needed for

  • blocking handles, like network communications
  • using multiple processors simultaneously (SMP)
  • algorithms and library calls that must be called through an API an as such cannot be split up into smaller parts.

The TThread Class

The following example can be found in the examples/multithreading/ directory.

To create a multi-threaded application, the easiest way is to use the TThread Class. This class permits the creation of an additional thread (alongside the main thread) in a simple way. Normally you are required to override only 2 methods: 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 you might expect, setting Suspended = True will prevent the thread starting 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. Deep procedure call recursions in a thread are a good example. If you don't specify the stack size parameter, a default OS stack size is used.

In the overridden 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 a loop (and this is typical), the loop should be exited when Terminated is true (it is false by default). Within each pass, the value of Terminated must be checked, and if it is true then the loop should be exited as quickly as is appropriate, after any necessary cleanup. Bear in mind that the Terminate method does not do anything by default: the .Execute method must explicitly implement support for it to quit its job.

As we explained earlier, the thread should not interact with the visible components. Updates to visible components must be made within the context of the main thread. To do this, a TThread method called Synchronize exists. Synchronize requires a method (that takes no parameters) as an argument. When you call that method through Synchronize(@MyMethod), the thread execution will be paused, the code of MyMethod will run in the main thread, and then the thread execution will be resumed. The exact working of Synchronize depends on the platform, but basically it does this: it posts a message onto the main message queue and goes to sleep. Eventually the main thread processes the message and calls MyMethod. This way MyMethod is called without context, that means not during a mouse down event or during paint event, but after. After the main thread executed MyMethod, it wakes the sleeping Thread and processes the next message. The Thread then continues.

There is another important property of TThread: FreeOnTerminate. If this property is true, the thread object is automatically freed when the thread execution (.Execute method) stops. Otherwise the application will need to free it manually.

Example:

<delphi> Type

   TMyThread = class(TThread)
   private
     fStatusText : string;
     procedure ShowStatus;
   protected
     procedure Execute; override;
   public
     Constructor Create(CreateSuspended : boolean);
   end;
 constructor TMyThread.Create(CreateSuspended : boolean);
 begin
   FreeOnTerminate := True;
   inherited Create(CreateSuspended);
 end;
 procedure TMyThread.ShowStatus;
 // this method is executed by the mainthread and can therefore access all GUI elements.
 begin
   Form1.Caption := fStatusText;
 end;

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

</delphi>

On the application,

<delphi>

 var
   MyThread : TMyThread;
 begin
   MyThread := TMyThread.Create(True); // This way it doesn't start automatically
   ...
   [Here the code initialises anything required before the threads starts executing]
   ...
   MyThread.Resume;
 end;</delphi>

If you want to make your application more flexible you can create an event for the thread; this way your synchronized method won't be tightly coupled with a specific form or class: you can attach listeners to the thread's event. Here is an example:

<delphi> Type

   TShowStatusEvent = procedure(Status: String) of Object;
   TMyThread = class(TThread)
   private
     fStatusText : string;
     FOnShowStatus: TShowStatusEvent;
     procedure ShowStatus;
   protected
     procedure Execute; override;
   public
     Constructor Create(CreateSuspended : boolean);
     property OnShowStatus: TShowStatusEvent read FOnShowStatus write FOnShowStatus;
   end;
 constructor TMyThread.Create(CreateSuspended : boolean);
 begin
   FreeOnTerminate := True;
   inherited Create(CreateSuspended);
 end;
 procedure TMyThread.ShowStatus;
 // this method is executed by the mainthread and can therefore access all GUI elements.
 begin
   if Assigned(FOnShowStatus) then
   begin
     FOnShowStatus(fStatusText);
   end;
 end;
 procedure TMyThread.Execute;
 var
   newStatus : string;
 begin
   fStatusText := 'TMyThread Starting...';
   Synchronize(@Showstatus);
   fStatusText := 'TMyThread Running...';
   while (not Terminated) and ([any condition required]) do
     begin
       ...
       [here goes the code of the main thread loop]
       ...
       if NewStatus <> fStatusText then
         begin
           fStatusText := newStatus;
           Synchronize(@Showstatus);
         end;
     end;
 end;

</delphi>

On the application,

<delphi> Type

   TForm1 = class(TForm)
     Button1: TButton;
     Label1: TLabel;
     procedure FormCreate(Sender: TObject);
     procedure FormDestroy(Sender: TObject);
   private
     { private declarations }
     MyThread: TMyThread; 
     procedure ShowStatus(Status: string);
   public
     { public declarations }
   end;
 procedure TForm1.FormCreate(Sender: TObject);
 begin
   inherited;
   MyThread := TMyThread.Create(true);
   MyThread.OnShowStatus := @ShowStatus;
 end;
 procedure TForm1.FormDestroy(Sender: TObject);
 begin
   MyThread.Terminate;
   MyThread.Free;
   inherited;
 end;
 procedure TForm1.Button1Click(Sender: TObject);
 begin
  MyThread.Resume;
 end;
 procedure TForm1.ShowStatus(Status: string);
 begin
   Label1.Caption := Status;
 end;</delphi>

Special things to take care of

Stack checking under Windows

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:

<delphi>MyThread := TThread.Create(False); if Assigned(MyThread.FatalException) then

 raise MyThread.FatalException;</delphi>

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

Units needed for a multi-threaded application

You don´t need any special unit for this to work with Windows. However with Linux, Mac OS X and FreeBSD, you need the cthreads unit and it must be the first used unit of the project (the program unit, .lpr)!

So, your Lazarus application code should look like:

<Delphi> program MyMultiThreadedProgram; {$mode objfpc}{$H+} uses {$ifdef unix}

 cthreads,
 cmem, // the c memory manager is on some systems much faster for multi-threading

{$endif}

 Interfaces, // this includes the LCL widgetset
 Forms
 { you can add units here },

</Delphi>

If you forget this and you use TThread you will get this error on startup:

 This binary has no thread support compiled in.
 Recompile the application with a thread-driver in the program uses clause before other units using thread.

Multithreading in packages

Packages which uses multi-threading should add the -dUseCThreads flag to the custom usage options. Open the package editor of the package, then Options > Usage > Custom and add -dUseCThreads. This will define this flag to all projects and packages using this package, including the IDE. The IDE and all new applications created by the IDE have already the following code in their .lpr file: <Delphi> uses

 {$IFDEF UNIX}{$IFDEF UseCThreads}
 cthreads,
 cmem, // the c memory manager is on some systems much faster for multi-threading
 {$ENDIF}{$ENDIF}

</DELPHI>

Heaptrc

You can not use the -gh switch with the cmem unit. The -gh switch uses the heaptrc unit, which extends the heap manager. Therefore the heaptrc unit must be used after the cmem unit.

<Delphi>uses

 {$IFDEF UNIX}{$IFDEF UseCThreads}
 cthreads,
 cmem, // the c memory manager is on some systems much faster for multi-threading
 {$ENDIF}{$ENDIF}
 heaptrc,</DELPHI>

SMP Support

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

Debugging Multi-threaded Applications with Lazarus

The debugging on Lazarus requires GDB and is rapidly becoming more and more fully featured and stable. However, there still exists a few Linux distributions with some problems.

Debugging output

In a single threaded application, you can simply write to console/terminal/whatever and the order of the lines is the same as they were written. In multi-threaded application things are more complicated. If two threads are writing, say a line is written by thread A before a line by thread B, then the lines are not necessarily written in that order. It can even happen, that a thread writes its output, while the other thread is writing a line.

The LCLProc unit contains several functions, to let each thread write to its own log file: <delphi> procedure DbgOutThreadLog(const Msg: string); overload;

 procedure DebuglnThreadLog(const Msg: string); overload;
 procedure DebuglnThreadLog(Args: array of const); overload;
 procedure DebuglnThreadLog; overload;</delphi>

For example: Instead of writeln('Some text ',123); use

 DebuglnThreadLog(['Some text ',123]);

This will append a line 'Some text 123' to Log<PID>.txt, where <PID> is the process ID of the current thread.

It is a good idea to remove the log files before each run:

 rm -f Log* && ./project1

Linux

If you try to debug a multi-threaded application on Linux, you will have one big problem: the Desktop Manager on X server will hang. When that happens you can simply exit out of that session and create a new session by pressing CTRL+ALT+F3. That will give you a new session prompt. Once there enter sudo /etc/init.d/gdm restart. This will restart the desktop manager and get you back into your desktop.

One method that solved this problem for Ubuntu x64 is to set the Project options for debugging required extra information file...

 Project Options -> Compiler Options -> Linking -> Debugging: Check Use external gdb debug symbols file (-Xg).

If the above solution does not work a workaround is:

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 second 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.



Instead of creating a new X session, one can use Xnest. Xnest is a X session on a window. Using it X server didn't lock while debugging threads, and it's much easier to debug without keeping changing terminals.

The command line to run Xnest is

 Xnest :1 -ac

to create a X session on :1, and disabling access control.

Widgetsets

The win32, the gtk and the carbon interfaces fully support multi-threading. This means, TThread, critical sections and Synchronize work.

Critical sections

A critical section is an object used to make sure, that some part of the code is executed only by one thread at a time. A critical section needs to be created/initialized before it can be used and be freed when it is not needed anymore.

Critical sections are normally used this way:

Add the unit SyncObjs.

Declare the section (globally for all threads which should access the section):

 MyCriticalSection: TRTLCriticalSection;

Create the section:

 InitializeCriticalSection(MyCriticalSection);

Run some threads. Doing something exclusively <delphi> EnterCriticalSection(MyCriticalSection); try

 // access some variables, write files, send some network packets, etc

finally

 LeaveCriticalSection(MyCriticalSection);

end;</delphi>

After all threads terminated, free it:

 DeleteCriticalSection(MyCriticalSection);

As an alternative, you can use a TCriticalSection object. The creation does the initialization, the Enter method does the EnterCriticalSection, the Leave method does the LeaveCriticalSection and the destruction of the object does the deletion.

For example: 5 threads incrementing a counter. See lazarus/examples/multithreading/criticalsectionexample1.lpi

BEWARE: There are two sets of the above 4 functions. The RTL and the LCL ones. The LCL ones are defined in the unit LCLIntf and LCLType. Both work pretty much the same. You can use both at the same time in your application, but you should not use a RTL function with an LCL Critical Section and vice versus.


Sharing Variables

If some threads share a variable, that is read only, then there is nothing to worry about. Just read it. But if one or several threads changes the variable, then you must make sure, that only one thread accesses the variables at a time.

For example: 5 threads incrementing a counter. See lazarus/examples/multithreading/criticalsectionexample1.lpi

Waiting for another thread

If a thread A needs a result of another thread B, it must wait, till B has finished.

Important: The main thread should never wait for another thread. Instead use Synchronize (see above).

See for an example: lazarus/examples/multithreading/waitforexample1.lpi

<delphi>{ TThreadA }

procedure TThreadA.Execute; begin

 Form1.ThreadB:=TThreadB.Create(false);
 // create event
 WaitForB:=RTLEventCreate;
 while not Application.Terminated do begin
   // wait infinitely (until B wakes A)
   RtlEventWaitFor(WaitForB);
   writeln('A: ThreadB.Counter='+IntToStr(Form1.ThreadB.Counter));
 end;

end;

{ TThreadB }

procedure TThreadB.Execute; var

 i: Integer;

begin

 Counter:=0;
 while not Application.Terminated do begin
   // B: Working ...
   Sleep(1500);
   inc(Counter);
   // wake A
   RtlEventSetEvent(Form1.ThreadA.WaitForB);
 end;

end;</delphi>

Note: RtlEventSetEvent can be called before RtlEventWaitFor. Then RtlEventWaitFor will return immediately. Use RTLeventResetEvent to clear a flag.

Fork

When forking in a multi-threaded application, be aware that any threads created and running BEFORE the fork (or fpFork) call, will NOT be running in the child process. As stated on the fork() man page, any threads that were running before the fork call, their state will be undefined.

So be aware of any threads initializing before the call (including on the initialization section). They will NOT work.

Parallel procedures/loops

A special case of multi threading is running a single procedure in parallel. See Parallel procedures.

Distributed computing

The next higher steps after multi threading is running the threads on multiple machines.

  • You can use one of the TCP suites like synapse, lnet or indy for communications. This gives you maximum flexibility and is mostly used for loosely connected Client / Server applications.
  • You can use message passing libraries like MPICH, which are used for HPC (High Performance Computing) on clusters.


External threads

To make Free Pascal's threading system to work properly, each newly created FPC thread needs to be initialized (more exactly, the exception, I/O system and threadvar system per thread needs to be initialized so threadvars and heap are working). That is fully automatically done for you if you use BeginThread (or indirectly by using the TThread class). However, if you use threads that were created without BeginThread (i.e. external threads), additional work (currently) might be required. External threads also include those that were created in external C libraries (.DLL/.so).


Things to consider when using external threads (might not be needed in all or future compiler versions):

  • Do not use external threads at all - use FPC threads. If can you can get control over how the thread is created, create the thread by yourself by using BeginThread.

If the calling convention doesn't fit (e.g. if your original thread function needs cdecl calling convention but BeginThread needs pascal convention, create a record, store the original required thread function in it, and call that function in your pascal thread function:

<Delphi>type

TCdeclThreadFunc = function (user_data:Pointer):Pointer;cdecl; 
PCdeclThreadFuncData = ^TCdeclThreadFunc; 
TCdeclThreadFuncData = record
  Func: TCdeclThreadFunc;  //cdecl function
  Data: Pointer;           //original data
end;     

// The Pascal thread calls the cdecl function function C2P_Translator(FuncData: pointer) : ptrint; var

 ThreadData: TCdeclThreadFuncData;

begin

 ThreadData := PCdeclThreadFuncData(FuncData)^;
 Result := ptrint(ThreadData.Func(ThreadData.Data));

end;

procedure CreatePascalThread; var

 ThreadData: PCdeclThreadFunc; 

begin

 New(ThreadData);
 // this is the desired cdecl thread function
 ThreadData^.Func := func; 
 ThreadData^.Data := user_data;    
 // this creates the Pascal thread
 BeginThread(@C2P_Translator, ThreadData );

end;</Delphi>


  • Initialize the FPC's threading system by creating a dummy thread. If you don't create any Pascal thread in your app, the thread system won't be initialized (and thus threadvars won't work and thus heap will not work correctly).

<Delphi>type

  tc = class(tthread)
    procedure execute;override;
  end;
  procedure tc.execute;
  begin
  end;

{ main program } begin

 { initialise threading system }
  with tc.create(false) do
  begin
    waitfor;
    free;
  end;
  { ... your code follows } 

end.</Delphi>

(After the threading system is initialized, the runtime may set the system variable "IsMultiThread" to true which is used by FPC routines to perform locks here and there. You should not set this variable manually.)


  • If for some reason this doesn't work for you, try this code in your external thread function:

<Delphi>function ExternalThread(param: Pointer): LongInt; stdcall; var

 tm: TThreadManager;

begin

 GetThreadManager(tm);
 tm.AllocateThreadVars;
 InitThread(1000000); // adjust inital stack size here
 
 { do something threaded here ... }
   
 Result:=0;

end;</Delphi>


Identifying external threads

Sometimes you even don't know if you have to deal with external threads (e.g. if some C library makes a callback). This can help to analyse this:

1. Ask the OS for the ID of the current thread at your application's start

<Delphi>Win32: GetCurrentThreadID(); Darwin: GetThreadID(); Linux: TThreadID(pthread_self);</Delphi>

2. Ask again for the ID of the current thread inside the thread function and compare this by the result of step 1.

See also