[freetds] charset, mssql

Lowden, James K LowdenJK at bernstein.com
Tue Apr 29 17:56:35 EDT 2003


Hi Freddy, 

> From: Frediano Ziglio [mailto:freddyz77 at tin.it]
> Sent: April 29, 2003 2:37 PM
> 
> Current CVS do not work with TDS8 (mssql2k protocol).

I don't have access to a 2K server, so this will be a challenge.  :-(

> If you want to try FreeTDS snapshot use TDS7 even for mssql2k.
> Problem is due to conversion stuff. During login TDS8 server send
> default collation for database using env type 7. This collation is
> readed and used to setup iconv according (using collation charset)

To do this right, we need a map from Microsoft's charset names to iconv
names.  

> however all next conversation use still ucs2...
> TDS7 do not have this problem cause return collation information in a
> different way (not still supported by FreeTDS, env types 5 and 6).

UTF-8 clients really need (at least) two TDSICONVINFO structures: 

1.  client <-> UCS-2 [nchar]
2.  client <-> server charset [char]

TDS 8.0 clients need more, potentially, one for each distinct client-server
charset pair.  

My solution so far:

TDSSOCKET::iconv_info becomes an array (not done yet).  

TDSCOLINFO gets a new member, TDSICONVINFO *iconv_info, that points to one
of the elements in TDSSOCKET::iconv_info.  (Done already, but only for UCS-2
columns, and not committed).  If the server column doesn't need iconv
intervention, its iconv_info is NULL.  

I set up the TDSICONVINFO* while adjusting the column_length, when I know
the server's and client's character set.  ATM is relies on the one-and-only
TDSICONVINFO structure, in the TDSSOCKET.  To make it general, we will need
utility functions to search the socket's array based on a pair of charset
names.  

I think an array for TDSSOCKET::iconv_info is better than a linked list,
because it will be reallocated infrequently.  Tell me if you think
different.  

> TDS7 and TDS8 handle collation in slightly different ways:
> 
> TDS7
> - every database have it's collation
          ^^^^^^^^ every *server*
...

> I think we should have some fixed iconv structures (like database one
> and ucs2) and cache others conversions.

Right.  That's my array in TDSSOCKET.  :-)

> Also tds_get_string is not sufficient 

Cf. tds_get_char_data().  Takes a TDSCOLINFO structure.  Soon, that
structure will have a pointer to a valid (cached) TDSICONVINFO structure.
Then it will be easy to decide what kind of conversion is needed.  

If tds_get_char_data() encounters a charset pair not already present in the
TDSSOCKET::iconv_info array, it will have to ask for one to be added.  That
function doesn't exist yet, either.  

tds_get_string() is only useful if you *know* the inbound string is UCS-2
and you don't have a TDSCOLINFO structure.  I.e., at login and when reading
metadata.  

We had a long discussion about handling conversion errors.  Per my earlier
message, we cannot silently insert a '?' whenever the conversion fails.  We
must emit an error message.  For reads, we can proceed, but for writes we
must refuse to process the data.  

I think it's OK to abort reading BIG5 data on failed conversions, too.  

Regards, 

--jkl


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