Re: [Qemu-devel] KVM call minutes for Feb 15

[Avi Kivity]

On 02/17/2011 03:37 PM, Anthony Liguori wrote:
> On 02/17/2011 07:25 AM, Avi Kivity wrote:
> > On 02/17/2011 03:10 PM, Anthony Liguori wrote:
> > > On 02/17/2011 06:23 AM, Avi Kivity wrote:
> > > > On 02/17/2011 02:12 PM, Anthony Liguori wrote:
> > > > > > (btw what happens in a non-UTF-8 locale? I guess we should just 
> > > > > > reject unencodable strings).
> > > > >
> > > > >
> > > > > While QEMU is mostly ASCII internally, for the purposes of the 
> > > > > JSON parser, we always encode and decode UTF-8.  We reject invalid 
> > > > > UTF-8 sequences.  But since JSON is string-encoded unicode, we can 
> > > > > always decode a JSON string to valid UTF-8 as long as the string 
> > > > > is well formed.
> > > >
> > > > That is wrong.  If the user passes a Unicode filename it is 
> > > > expected to be translated to the current locale encoding for the 
> > > > purpose of, say, filename lookup.
> > >
> > > QEMU does not support anything but UTF-8.
> >
> > Since when?
> >
> > AFAICT, JSON string conversion is the only place where there is any 
> > dependency on UTF-8.  Anything else should just work.
> >
> > > That's pretty common with Unix software.  I don't think any modern 
> > > Unix platform actually uses UCS2 or UTF-16.  It's either ascii or 
> > > UTF-8.
> >
> > Most/all Linux distributions support UTF-8 as well as a zillion other 
> > encodings (single-byte ASCII + another charset, or multi-byte 
> > charsets for languages with many characters.
>
> Maybe there's some confusion here.  UTF-8 is an encoding, not a locale.
>
> The common encodings are ASCII, UTF-8, UCS2, UTF-16, and UTF-32.

ASCII is a character set and encoding.  The rest are encodings for 
Unicode.  There are lots of other encodings, say latin-1.

> An application has to explicitly support an encoding.  It is not 
> transparent.

It is fully transparent until you do wire conversions (like we do with 
qmp which is explicitly UTF-8).

>   UCS2/UTF-16 means that strings are not 'const char *'s but 'const 
> wchar_t *' where typedef unsigned short wchar_t;.
>
> QEMU assumes, in lots of places that strings are single-byte NUL 
> terminated.  Basically, any use of snprintf, printf, strcpy, strlen, 
> etc. pretty much tie you to ASCII/UTF-8.  You can have a single NUL 
> byte as part of a valid UCS2 string.

We're tied to single- or multiple- byte encodings, and can't do 
wchar_t.  But that's very different from ASCII/UTF-8 only.

> > > The only place it even matters is Windows and Windows has ASCII and 
> > > UTF-16 versions of their APIs.  So on Windows, non-ASCII characters 
> > > won't be handled correctly (yet another one of the many issues with 
> > > Windows support in QEMU).  UTF-8 is self-recovering though so it 
> > > degrades gracefully.
> >
> > It matters on Linux with el_GR.iso88597, for example.
>
> The whole series of iso8859 (8-bit encodings) are officially abandoned 
> in favor of UCS and encodings that support the full UCS code page 
> (UTF-8/UTF-16).
>
> I see no strong reason to try and support deprecated encodings when 
> there are perfectly valid replacements like el_GR.utf8.

All it takes is a call to iconv(3).  I agree it's unlikely to happen in 
practice.

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