Re: Strings with discriminated records

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2018-05-27 19:11, NiGHTS wrote:
> I've expanded my example to include a strange run-time crash I am getting.
> 
> Type Message (Length : Positive) is new Ada.Finalization.Controlled with record
>      Text  :  String (1 .. Length);
>      Cstr  :  Interfaces.C.Strings.chars_ptr;
> end record;
>         
> function Create (Value : String) return Message is
> begin
>      return (
>          Ada.Finalization.Controlled with Length => Value'Length,
>          Text => Value,
>          Cstr => Interfaces.C.Strings.New_String (Value)
>      );
> end Create;
> 
> procedure Finalize (
>      M : in out Message
> ) is
> begin
>      Interfaces.C.Strings.Free ( M.Cstr ); -- Crashes here
> end;
> 
> declare
>      M : Message := Create ("Hello World");
> begin
>      null;
> end;
> 
> Why does my program crash on Finalization?

Because it is wrong.

You have a controlled object which gets copied all the time. It means 
that the pointer Cstr is shared by all copies and is freed multiple 
times. Either

1. You must override Adjust and make a new string for the result.

or

2. You could deploy some reference counting schema cloning the content 
when the string at the pointer is updated. This is how dynamic strings 
are usually implemented.

P.S. If you want C-compatible strings use char_array, it is exactly the 
thing you need.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de