[fpc-devel] Read from memory stream with position before start of file

[C Western]

Michael Van Canneyt wrote:
> On Thu, 1 Jan 2009, Paul Ishenin wrote:
>
>   
>> Michael Van Canneyt wrote:
>>     
>>>> While finding the bug #12897 I found that a memory stream position could
>>>> be set to before the start of the file without giving an error, and
>>>> subsequent reads would appear to work (unless the position was so far
>>>> off an invalid region of memory was read). I found the precise problem
>>>> by patching TCustomMemoryStream to raise an exception if the position is
>>>> set before the start of the file. Should an exception be raised in this
>>>> circumstance, which would make finding bugs easier? Or should the
>>>> position just be set to 0, which would be more consistent with the
>>>> behaviour when going beyond the end of the stream?
>>>>     
>>>>         
>>> I think that an exception is better. If you set the position to a
>>> non-existing
>>> position, there is an error in your logic anyway.
>>>   
>>>       
>> Just tested with delphi
>>
>> procedure TForm1.Button1Click(Sender: TObject);
>> var
>>  S: TMemoryStream;
>> begin
>>  S := TMemoryStream.Create;
>>  S.Position := 10;
>>  ShowMessage(IntToStr(S.Position));
>>  S.Position := -1;
>>  ShowMessage(IntToStr(S.Position));
>>  S.Free;
>> end;
>>
>> It shows 10 and -1.
>>
>> If you do an exception or silent change of position it can cause
>> incompatibilities. Maybe read operation must be fixed to ignore positions out
>> of the range 0..Size -1?
>>     
>
> What does Delphi do in such a case ?
>
>   
Delphi reads and writes silently do nothing for position < 0, though I 
would prefer an exception.

Colin