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

Bruce Tulloch pascal at causal.com
Wed May 8 08:13:29 CEST 2013

 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;
  Result := '';
 <other stuff>

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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