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

Bruce Tulloch pascal at causal.com
Thu May 9 01:33:47 CEST 2013

Thanks Jonas, that confirms what I suspected. Next time I trap an instance
of this (rare) fault I will inspect exactly which CPU instruction raised
the SEGV inside fpc_AnsiStr_Decr_Ref in search of a source of memory


On Wed, May 8, 2013 at 11:49 PM, Jonas Maebe <jonas.maebe at elis.ugent.be>wrote:

> On 08 May 2013, at 08:13, Bruce Tulloch 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?
> The first thing fpc_AnsiStr_Decr_Ref does is check whether its parameter
> is nil, and if so it immediately exists. It can be nil in case the
> ansistring contains an empty string.
> That routine itself also sets its argument to nil in case this was not the
> case initially (it's a var-parameter), and I assume your crash happens
> after this has been done.
>  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.
> It's nevertheless most likely memory corruption. You can try compiling
> with -gv and running your program under valgrind to see whether it finds
> anything (you will probably get some false positives about certain RTL
> pchar routines such as strscan and strlen, but you can ignore those).
> Jonas
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