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
  ...
end;

The problem is that every time I try to execute the method Add of a TCollection or TObjectList I get a SIGSEGV.

I must point out that the method "runcode" in the shared library is not multithreaded. Also the shared library was linked against pthreads library, and the main program uses cthreads. I also tried to enclose the Callback inside a CriticalSection but the problem persists.

Thanks in advance,

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?.

Have you checked the calling convention. E.g. 'cdecl;'?

Mattias

Ah. In that case you can, yes.

Is the calling convention correct ?

Michael.

Yes, I'm using cdecl. The library is a C++ library running on Linux, defined as this:

typedef void (* CFindCallBack)(const char *, const void *);

So, correct me if I'm wrong, the calling convention should be cdecl.

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.

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,

06.04.2012 16:31, Leonardo M. Ram? ?????:

You should pass 'Self' here, not '@Self'. The latter would be correct for TP-style objects, but TThread descendant is a class.

Regards,
Sergei

Le 06/04/2012 15:04, Leonardo M. Ram? a ?crit :

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.

Yes!!!, you are right. That was the problem.

Regards,

Yes Antonio, Sergei noted the same error in my code, that fixed the issue.