[Lazarus] Mutlithreaded Dll Callback crashes my Application

[Sven Barth]

Am 25.08.2010 15:46, schrieb Alexander Grau:
> Sven Barth schrieb:
>> You can read this thread on the fpc-pascal list:
>> http://www.hu.freepascal.org/lists/fpc-pascal/2010-July/025939.html
>>
>> Jonas has implemented a possibilty for external C threads to work, but
>> you need to use a SVN FPC and you need to create at least one thread
>> in FPC (it can terminate immediatly, it's just needed to initialise
>> the threading subsystem). (And this fix is *nix only)
> If you read this thread carefully, you can notice that I already posted
> this here in my first answer to Maik ;-)
>

That was on 3rd August... How am I supposed to remember this? :P

> I also showed in a test program in a later post that this problem
> doesn't seem to exist in the Win32 target, it was just the Darwin
> target. Here's again the test program I wrote . I creates a non-FPC
> thread and inside that thread it calls back a FPC funtion and object.
> This fully works without crashes.
>

I looked at your example program and I also looked into FPC's Thread 
implementation for Windows. You need to play around with the Heap a bit 
more to make things interesting:

Modify someFunc like in the following and someObj.someMethod won't be 
executed any more, because an exception is raised somewhere after the 
second Writeln.

Regards,
Sven


procedure someFunc(param: pchar); cdecl;
var
   t: TObject;
begin
   WriteLn('External is called - ThreadID=', GetCurrentThreadId);
   if assigned(param) then writeln('blub');
   writeln('param=', param);
   t := TObject.Create;
   try
     Writeln(t.ClassName);
   finally
     t.Free;
   end;
   // this point will not be reached
   someObj.someMethod();
end;