[fpc-pascal] Re: FPC 2.6.2 throws SEGV in fpc_AnsiStr_Decr_Ref(). How is this possible?

Bruce Tulloch pascal at causal.com
Wed May 8 08:27:52 CEST 2013

I should clarify, foo is a virtual method of an object, not a regular
function. -b

On Wed, May 8, 2013 at 4:13 PM, Bruce Tulloch <pascal at causal.com> wrote:

>  After a random but very long period of time (i.e. very many successful
> calls) I get a SEGV in the built-in function fpc_AnsiStr_Decr_Ref.
> GDB reports the argument to fpc_AnsiStr_Decr_Ref (the string who's
> reference is to be decremented) is nil (i.e. 0x0).
> Prima facie, that's the reason for the SEGV, but how is it possible that
> the compiler would pass a nil pointer to this function the first place?
> To put this into context, I'm running FPC 2.6.2 on a 32 bit Linux system
> executing in a multi-threaded application (which uses python threads and
> fpc threads). I have not found obvious evidence of memory corruption from
> other execution contexts or shared memory handling problems.
> The SEGV occurs when called from a function, let's call it foo, that looks
> like this:
> function foo : AnsiString;
> begin
>   Result := '';
>  <other stuff>
> end;
> The AnsiString pointer that fpc_AnsiStr_Decr_Ref throws a SEGV is Result,
> at the first line of the function foo.
> It appears the compiler is passing Result to fpc_AnsiStr_Decr_Ref even
> though Result (at this point in the function) must be nil (having only just
> come into scope).
> How is is possible that fpc_AnsiStr_Decr_Ref is being called at all?
>  Any/all advice gratefully received.
> Cheers, Bruce.
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