[fpc-pascal] Stack problem in externally called callback function

Wimpie Nortje wimpienortje at gmail.com
Sun Aug 2 20:02:34 CEST 2009

Thanks for the response.

The library uses a centralised function to start all its threads using 
pthread_create. If I can make this central function inform FPC about all 
the threads created, would this work?

Assuming it would work:  what information would FPC require? When would 
this info be required, before or after thread creation? What needs to be 
done on the FPC side with this info?


Jonas Maebe wrote:
> On 31 Jul 2009, at 19:05, Wimpie Nortje wrote:
>> I am trying to use a shared lib written in C in my FPC program. The 
>> lib creates a number of threads and requires callbacks into the main 
>> app. It can call the callback from any of the threads.
> Calling FPC code from threads not created by FPC does not work at this 
> time on any unix-like platform.
>> When I compile without stack checking everything (seems) to work fine 
>> but when stack checking is on I get stack error (error 202) on entry 
>> to the callback function. I can break on the callback's 'begin' and 
>> see valid values for all the function parameters but when I step into 
>> the function body it crashes.
> That's because the stack checking has to be initialised per thread and 
> this is normally done when you create a thread via tthread.create() or 
> beginthread(). Disable stack checking.
>> I say 'seems' when not using stack checking because when the app 
>> becomes large I get a lot of strange problems which would probably be 
>> explained by stack problems.
> No, the problems are probably due to the FPC rtl not being initialised 
> for multi-threading. You will probably get slightly less problems if 
> you include the "cthreads" unit in your uses-clause and create a dummy 
> thread as soon as your program starts (after which you can destroy it 
> again), but even then you will get memory leaks and sporadic random 
> errors due to threadvars containing wrong values. As said above: it 
> does not work at this time (under unix-like platforms; in 2.3.1, it 
> should work on Windows).
> The reason that it does not work on Unix-like platforms, is that there 
> is no way to make libthread inform FPC whenever a thread is created 
> (while this is possible on Windows), so the FPC rtl cannot initialise 
> the per-thread information properly. There is no known way to fix this 
> problem (although some people have guesses, no one really knows for 
> sure).
> Jonas
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