Re: [Lazarus] Callback and Threads problem

[Antonio Fortuny]

Le 06/04/2012 15:04, Leonardo M. Ramé a écrit :
On 2012-04-06 14:53:13 +0200, michael.vancann...@wisa.be wrote:

On Fri, 6 Apr 2012, Leonardo M. Ramé wrote:

On 2012-04-06 14:39:10 +0200, michael.vancann...@wisa.be wrote:

On Fri, 6 Apr 2012, Leonardo M. Ramé wrote:

I have a similar problem to this:

http://www.lazarus.freepascal.org/index.php?topic=10571.0

My main program launches a thread, that loads a shared library, then
execute a function who receives as a parameter a pointer to a callback
function and an instance of the caller, as this:

procedure TMythread.execute;
begin
...
// external function
runcode(@MyCallBack, @Self);
...
end;

procedure MyCallback(const Result: PAnsiChar; const ACaller: pointer);
cdecl;
var
lCaller: TMythread;
begin
// the parameter ACaller is used because callbacks can't be of type
// procedure of object.
lCaller := TMyThread(ACaller);
lCaller.TestVar := Result; //<--- Ok.
lCaller.ListOfThings.Add;  //<--- SIGSEGV
IMHO the problem is not in threads.

You cannot pass objects to libraries or vice versa.
The VMTs will differ.

Michael.
But, I'm passing the object as a pointer that is received as (void *) and
returned back to the callback as a parameter, the library doesn't touch
it. Even in this case I can't pass an object?.
Ah. In that case you can, yes.

Is the calling convention correct ?

Michael.
Yes, I'm using cdecl.

BTW. I somewhat fixed the issue by defining a global variable of the
same type of my thread, the var points to "Self" just before calling the
external function and assigned to nil when the function ends.
Not exactly a fix but the good way. If I undertsand well, with this form:
runcode(@MyCallBack, @Self);
the second parameter is not a pointer pointing to some address but the 
address itself.

doing a previous
var
    PP: Pointer;
anf then
pp := @MyThread
runcode(@MyCallBack, pp);
passes a real pointer whose data effectively points to the desired 
object (the thread in the example).
I wonder if the same rule applies to the first paramaeter

This can explain why your second method works.

Antonio.



This way, the callback can use the thread by pointing to the global var.

I hate to use global variables, but, as the function is called only
once, I know it won't be overlapped by many callers.

Regards,



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