[Lazarus] Threads

[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)

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
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.
3. all variables described in the VAR part of the EXECUTE procedure are 
"Thread safe" (seems obvious)
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)
5. all useful code a thread needs should be encapsultated into a TObject 
descendant and instantiated within the thread's space.
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) )

This list is (certainly) not exhausted.

Antonio.


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