[Lazarus] LongString does not work?

[Sven Barth]

On 03.03.2013 14:26, Xiangrong Fang wrote:
> A (Ansi)string in {$H+} mode is just a pointer (to the characters of the
> string). So sizeof(DataFolder)=4 (or 8) in all cases. There is no room
> for the string unless
>
> I guess this is the cause, but then there is no need to specify "packed"
> if I just use array of char.

The "packed" should always be used if you want to read/write data. Only 
then you can be sure that the memory layout of the record is correct. 
Otherwise padding bytes will be added. Consider the following record:

=== example begin ===

type
   TMyRecord = record
     b: Byte;
     i: LongInt;
     w: Word;
   end;

=== example end ===

The size of this record will be 12, because between b and i and after w 
padding bytes are added. While if you add "packed" the size will be 7 
and the fields will directly follow after each other.

The memory layout of non-packed records is not guaranteed to stay the 
same between FPC versions! It could be for example that we find a bug in 
the code that generates the memory layout of a record and then suddenly 
you can no longer read in files that were written by a version of your 
program compiled with the older compiler. For "packed" there is the 
guarantee though that the layout will stay the same.

Regards,
Sven