Strange AV?

[chronozphere]

Hey everyone,

I'm getting a strange AV. Don't really know why it's popping up.

type
  PMyRec = ^TMyRec;
  TMyRec = record
    Str: String;
    Int: Integer;
  end;

procedure TForm1.Button5Click(Sender: TObject);
var
  P: PMyRec;
begin
  GetMem(P, sizeof(TMyRec));
  P^.Str := 'test';   //<< AV HERE!
  P^.Int := 5;

  FreeMem(P);
end;

This should go fine. I just set the Str which is actually a special kind of pointer, to point to 'test'.

Can somebody enlighten me?

Thanks

[Andru]

Hmm, strange, because only one line of code is wrong:

FreeMem( P );

You must free String in record first, like this:

P^.Str := '';

And only then call FreeMem.

But... try to call FillChar( P^, SizeOf( P^ ), 0 ) after GetMem

[chronozphere]

Hmm.. this is really odd. It does work if I replace getMem() and FreeMem() by New() and Dispose().

Does anyone know the differences between these functions?

I'll do some more testing ASAP. Thanks for your reply.

Edit:

  GetMem(P, sizeof(TMyRec));
  FillChar( P^, SizeOf( P^ ), 0 );
  P^.Str := 'test';
  P^.Int := 5;

This works too. How is that possible?

[JSoftware]

GetMem(P, sizeof(TMyRec));

results in P, a chunk of allocated memory filled with crap. When you assign a string to this pool of crap you first have to free the old string. If it tries to free Nil then it won't try, but if there's any value but nil, then you are freeing something that isn't allocated

Basically, you need to use either allocmem(it'll zero out the memory), or use fillchar after getmem. They are basically the same

Edit: and I think new uses the same method as allocmem. Eg. zeroes before returning

[de_jean_7777]

new and dispose are used with pointers to records or other structures that contain automated types, so they are initialized/finalized properly. Since the string in your example is probably an ansistring, and therefore an automated type, you need to use new and dispose, which I suggest you use instead of getmem/freemem. I only use getmem/freemem for memory which has no type (e.g. plain pointers). As JSoftware said, the compiler will try to free the old string when it assigns the new one, but there is no old string, only garbage which was on the heap where your record got allocated.

[User137]

Str: String;

is merely a pointer.

You should define strings with length if you want to use them in data blocks:

Str: String[64];

I think this is will take 65 bytes but usable like normal string. Just that it can have at max 64 characters if defined like that. You can alternatively use array[1..64] of char;