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

[Bruce Tulloch]

Thanks José, I can see that might cause a problem given bar passes result
by reference to foo without initializing result first. My question to Jonas
or others more knowledgeable than me about what the compiler does, is
whether result (in your example and my own case) is guaranteed to be
initialized to nil when it first appears in scope (i.e. before it's been
assigned any value in our code). If it is initialized to nil, then foo
would receive a reference to bar's result variable (via para) and the value
of that variable would be nil (and all would be okay). If it isn't
initialized to nil, the same rule applies but the value of result (as seen
by foo via para) would likely be invalid and would probably blow up in foo
when dereferenced (as a string).

My problem is similar except that I know it's not nil when passed in
(because the initial test in fpc_AnsiStr_Decr_Ref looking for nil passes)
but that it becomes nil very soon afterward (because the SEGV arises as an
indirect result of it being nil, as I explained in my reply to Ludo just
now).

I'm pretty sure I have a shared memory problem somewhere between threads in
my code but I can't understand how this could be given the "erroneously
shared" variable appears to be an automatic variable (i.e. Result) that has
just been created on the stack in the function foo that calls
fpc_AnsiStr_Decr_Ref where the SEGV occurs.

I'll keep looking :-) Bruce.


On Thu, May 9, 2013 at 9:48 PM, José Mejuto <joshyfun at gmail.com> wrote:

> El 09/05/2013 5:19, Bruce Tulloch escribió:
>
>  If there is no other explanation, then it means I need to find out how
>> the string variable referred to by (%eax) could have been been accessed
>> (or even known to exist) by any other thread in the same address space.--
>>
>
> Hello,
>
> In the past I had suffered a problem like yours and the culprit was
> another different function that passes result (string) as a parameter when
> calling a function without initialization, something like this:
>
> function foo(var para: string): string;
> begin
>   //Something with para
> end;
>
> function bar(): string;
> begin
>   result:=foo(result);
> end;
>
> I hope this helps...
>
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