[Lazarus] Threads

Antonio Fortuny a.fortuny at sitasoftware.lu
Fri Mar 23 11:24:36 CET 2012

Thanks Sven, I really appreciate.

Le 23/03/2012 10:59, Sven Barth a écrit :
> Am 23.03.2012 10:25, schrieb Antonio Fortuny:
>> Le 22/03/2012 16:20, Mattias Gaertner a écrit :
>>> Each thread has its own stack.
>>> All threads share the same address space and the same heap.
>>> Objects (here: class instances) are created on the heap. The heap is
>>> thread safe.
>>> The data segment is for constants. They are shared.
>> Le 22/03/2012 17:10, Michael Schnell a écrit :
>>> Besides what the other said a very basic comment.
>>> The location where an object is defined (i.e. within a TThread enabled
>>> unit) or who created it (the main line code or the thread code) does
>>> not matter. The Concept of classes, objects and instances is a matter
>>> of memory allocation and pointers and not a concept of program flow.
>>> same is absolutely independent. You can use one instance of a class in
>>> one thread and another one in another thread. You can create an
>>> instance in one thread and call its procedures and properties by
>>> another one. (BTW this results in the fact that its very hard to
>>> define something like "Thread-safe" for a class).
>> Thanks to you all .
>> Trying to be as much clear as possible, there are some sentences:
>> Assuming this few statements (aka my own rules when writing thread's 
>> code):
>> a. all thread code+data are encapsulated into a TThread object
>> b. "Thread safe" means that there is no overlap of data references from
>> thread to thread, the main thread inclusive, and that no references are
>> passed outside the thread's control from thread to thread (by the means
>> of the owner or global vars for instance)
>> c. the thread creator does not interfere in any way with the thread's
>> data and methods as soon as the thread has been started (Terminated
>> execpted of course)
>> d. no function neither procedure calls are made to any function or
>> procedure defined outside of the thread control (aka self defined 
>> methods)
> Ok, I hereby assume the above as given.
>> Do you all agree on the following asserts:
>> 1. All variables in the thread definition (TThread's private, protected
>> and public variables) are "Thread safe" BUT are accessible to the thread
>> creator
> Yes (though the private vars aren't available to the thread creator if 
> the creator and the definition of your TThread descendant reside in 
> different units ;) )
>> 2. an instance of an object (aka TObject descendant) created into the
>> thread's EXECUTE procedure is invisible to all other instances of the
>> same object whichever the creator could be (the same thread or other
>> threads created with the same thread definition object) and to other
>> thread object instances, even when the reference variable of the created
>> object is defined into the thread vars (see 1.) provided that all object
>> methods do not call any function or procedure outside of the object 
>> methods.
> If I've understood that correctly: yes
>> 3. all variables described in the VAR part of the EXECUTE procedure are
>> "Thread safe" (seems obvious)
> Yes
>> 4. all local variables and constants defined into local Thread object
>> methods are "Thread safe" (they are defined into each thread stack for
>> the vars and the heap for constants)
> It's true for variables. Local constants are defined in a section of 
> the executable, so if you have writable constants enabled (only then 
> it's a problem) and you write to these constants then the change will 
> be reflected in other constants as well. If you don't write to the 
> constants than they are safe.
Well, when I define a *const* anywhere in a program it is a *constant* 
actually otherwise I use a var. In this case I guess than there is no 
problem for any thread to read a constant whatever the context could be
>> 5. all useful code a thread needs should be encapsultated into a TObject
>> descendant and instantiated within the thread's space.
> Note necessarily. You can also call global procedures/functions that 
> don't rely on global state (e.g. IntToStr, etc.). If you want to call 
> functions/procedures that rely on the state (e.g. some registration 
> systems for classes) you'll need to synchronize the access.
Is it more accurate to state that any global procedure or function could 
be called by any thread as far as the called code does not reference 
anything outside itself apart from other global code considered as 
"Tread safe" too or constants. For example StrToIntDef.
>> 6. all methods defined in the thread's definition, aprat from the
>> EXECUTE procedure (obvious !), run into the thread creator space (the
>> one which instantiates the thread, aka does something like wThread :=
>> TMyThread.Create (False) )
> I don't know whether I understood you correctly, but if you have this:
> === example begin ===
> type
>   TTestThread = class(TThread)
>   protected
>     procedure Execute; override;
>   public
>     procedure DoSomething;
>   end;
> procedure TTestThread.Execute;
> begin
>   DoSomething;
> end;
> procedure TTestThread.DoSomething;
> begin
>   Writeln('Something');
> end;
> begin
>   with TTestThread.Create(True) do begin
>     FreeOnTerminate := True;
>     Start;
>   end;
> end.
> === example end ===
> Then (to my understandment of your assertion) your assertion is wrong, 
> because DoSomething is (although it is public) only called in context 
> of Execute (Note: Not that you should make such a method public if you 
> don't need to, but this is merely an example).
Very nice indeed. You did understand, really.
> Regards,
> Sven
> -- 
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