Re: Exclusive file access

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

On Mon, 31 Aug 2015 23:12:28 +0200, Florian Weimer wrote:

> * Dmitry A. Kazakov:
> 
>>>> Semantically no. Wide_String according to RM 3.5.2 (3/3) represents a
>>>> narrower set of Unicode than UTF-16.
>>> 
>>> I don't have a current Windows system to try this, but I think Windows
>>> allows you to use lone surrogates in file names.  Such names are not
>>> valid UTF-16, but valid UCS-2.
>>
>> The system may have integrated AI that accepts names in English: "a file
>> with the name of Swahili dhadi". Would it make ASCII same as Unicode?
> 
> Sorry, there is no need for being silly.

It is not silly. It is the difference between semantics of the type and a
possibility to misuse bit patterns of type values representation for
anything else. You could put a whole system kernel into a string. That
won't make characters machine instructions.

> Non-encodable file names are
> definite problems and happen in practice (see Java programs on
> non-Windows platforms in a multi-byte locale).

Anything you can encode in Unicode you can encode in Unicode.

> The user may select a
> file, but the application cannot open it.  That's a poor user
> experience.

That is not a problem at all. You cannot create a 999TB large file either.
System-specific constraints put on an implementation do not effect the
interface, which has Name_Error in it already. 

>>> You can end up with non-expressible names, depending on how the
>>> conversion to the external representation is performed.  (I.e., the
>>> system may have file names which cannot be encoded as
>>> Wide_Wide_String.)
>>
>> Wasn't the purpose of Unicode to represent all possible characters?
> 
> This discussion isn't about characters, it's about conversions for
> sequences of code units which do not quite match the (current) Unicode
> specifications.

String is an array of characters. Anything that is not a sequence of
characters is not a sequence of characters.

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