[freetds] column_unicodedata considered harmful

ZIGLIO Frediano Frediano.Ziglio at vodafoneomnitel.it
Wed Apr 2 03:41:08 EST 2003


> > 
> > This is a problem. Assuming server is latin1 and we convert 
> > to utf8; we have
> > a varchar(3) with all accent letter (like 'ééé'). Converting 
> > to utf8 result
> > 6 bytes... If client ask for data length we return 6 
> > characters, but if
> > client ask for column size? IMHO we should return 3, cause 
> > server size is
> > 3.. but if client use column size to compute size of buffer 
> it fail...
> 
> That is exactly my point in talking about "fixing up" the 
> metadata.  The
> purpose of TDSCOLINFO is to convey the server's description 
> of the data to
> the client.  If we change the data (with iconv), then we must 
> change the
> metadata to match.  Using your example, we must make it look 
> as though the
> server had sent UTF-8.  
> 
> (A good way to think about all this, but much harder to 
> implement, is as a
> server gateway.  Pretend you're a proxy TDS server, doing 
> protocol and/or
> charset conversions by adjusting the TDS  packets flowing through you.
> Your representation to your client has to be consistent, else 
> it's useless.)
> 
> Just to be clear: the column size is a byte count, not a 
> character count.  I
> don't think the APIs ever use a character count.  
> 
> What should we say, for UTF-8?  UTF-8 uses 1-4 byte/char, so 
> 4 would be
> safe.  But, how many bytes it uses is a function of the 
> Unicode code point
> it's representing, and we know the charset we're converting from.  For
> Latin1, we know the max is 2 byte/char.  Therefore, if your 
> example client
> asks for the column size, we'd better say 3 * 2 = 6.  Everytime. 
> 

Well, so for every conversion you should know how many bytes it take...
I think any ms charset -> utf8 take 3 bytes maximun.

> Pretending for a moment that the server charset is Latin1 and 
> the client is
> UCS-2, 
> 
> > 
> > In db-lib and ct-lib you set charset only before connection, 
> > so this is the
> > correct way. In ODBC I can't see any method to set charset... 
> > very strange..
> 
> Actually, it's an option.  I should have known. :/
> 
> In ODBC, I think you want SQL_COPT_SS_TRANSLATE.  The 
> documentation says it
> "causes the driver to translate characters between the client 
> and server
> code pages".  
> 

Well, all *SS* macro are sql server macro, not odbc macro.
They are all extension to ODBC... In windows client ODBC is a per-tread
configuration (not in the registry but in the TLS..).
In *nix we haven't still this option (perhaps using glibc 2.3... called
thread-local locale, if anyone have more informations...)

> In db-lib, it's very poorly described as db-lib options 
> DBANSItoOEM and
> DBOEMtoANSI.  The documentation of dbsetopt() implies this 
> option is sent to
> the server, but I don't believe it.  Elsewhere, the docs 
> speak of using this
> "DB-Library option", and there's no DBCC or SET option for this
> functionality.  Conversion is performed by the driver.  
> 
> I'm still running 7.0, not 2000.  In SQL Server 2000, 
> individual columns in
> a result set may each have their own character set (it's no longer a
> server-wide setting).  It's not clear to me how db-lib is 
> supposed to cope
> with that; if anyone has clear information or is willing to 
> dig through the
> docs, I'd be interested.  For the present, I'd say we do any and all
> conversions if DBANSItoOEM is set (the default), else none.  
> 
...
> 
> > Perhaps problem with ODBC is that client can read characters using
> > single-byte or UCS2... (MS use very seldom utf8...)
> 
> Sybase can, though.  We should study Sybase's metadata while 
> it uses UTF-8.
> 

Currently my Sybase server (only a test server) is 11.9.2. So it do not
support utf8. I think someone can test the behavior of Sybase using utf8
data.
- if server is utf8 and return data in iso8859-1 (iso_1) how it return
column sizes?
- if server is iso_1 (reinstallation required?) how it return column sizes?
- test unichar and univarchar columns
- test longvarbinary TDS type with UTF16 inside (does server use UCS2 or
real UTF16?)

> 
> > I discovered how to skip a character using iconv !!
> 
> Maybe I'm missing something.  It doesn't look hard to me.  
> 
> For single- or double-byte encoded inputs, we know how many 
> bytes to skip.  
> 
> For UTF-8, see http://czyborra.com/utf/#UTF-8 especially this table:
> 
>  bytes | bits | representation
>      1 |    7 | 0vvvvvvv
>      2 |   11 | 110vvvvv 10vvvvvv
>      3 |   16 | 1110vvvv 10vvvvvv 10vvvvvv
>      4 |   21 | 11110vvv 10vvvvvv 10vvvvvv 10vvvvvv
> 
> Any UTF-8 value >127 is part of a multibyte character.  If 
> (byte & 64) is
> true, you have a lead byte that will be followed by some 
> "10vvvvvv" bytes; a
> simple shift-loop will get you to the end.  Insert a '?' in 
> the output, and
> continue from there.  
> 
> UTF-8 had self-synchronization as part of its design; they 
> wanted to make
> this easy.  
> 

Well, skipping a fixed-size character (single-byte, ucs2/4) is simple. utf8
is a bit harder. But all these charset haven't state!! big5 or shift-jis
charset have state. This mean that conversion of a series of bytes using
these charset depend on internal state of iconv_t structure...

So for every charset we should know:
- is fixed byte? if so, how many bytes take a character?
- how many bytes can take a character?
- does this charset have a state?
And for every conversion we should know the ratio...

I written a small program to test if a charset is single-byte or not... is
still a test but can be useful...

freddy77

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