FreeTDS Development Group

["James K. Lowden" <jklowden AT schemamania.org>]

Hi Freddy, 

On 30 Mar 2003 21:40:44 +0200, Frediano Ziglio <freddyz77 AT tin.it> wrote:
> Il dom, 2003-03-30 alle 21:06, James K. Lowden ha scritto:
> > typedef struct tdsiconvinfo
> > {
> >     char client_charset[64];
> >     char server_charset[64];
> >     iconv_t to_wire;   /* conversion from client charset to server's
> >     format
> > */
> >     iconv_t from_wire; /* conversion from server's format to client
> >     charset
> > */
> > } TDSICONVINFO;
> > 
> 
> Yes, but wire can be unicode and single-byte, this is the reason for 4
> iconv_t. 

It doesn't matter.  Don't think in terms of single/double byte, think in
terms of client/server.  For any session, there are only two character
sets: client and server.  We should not care which one needs more
bytes/char, except for buffer allocation.  

> Also bytes_per_char is not still used but required for
> allocating row...

I'll write a function that takes an iconv charset name and returns
bytes/char pessimisitically (i.e., 4 is the default).  

> > If either iconv_t is NULL, that's the equivalent of use_iconv == 0.  
> 
> iconv_t is not a pointer, use (iconv_t)-1 (like file handle...)

If !HAVE_ICONV or iconv_open fails, we initialize the iconv_t members to
NULL.  

> > Then, instead of 
> > 
> > char* tds7_ascii2unicode(TDSSOCKET * tds, const char *in_string, 
> >                      char *out_string, int maxlen);
> > 
> > and its complement, I propose:
> > 
> > size_t tds_iconv (iconv_t cd, const char *input, size_t * input_size,
> >                      char *out_string, size_t maxlen);
> > 
> 
> Yes and not. To skip a character (required to handle inconvertible
> characters without truncating) you should know how to skip it. 

tds_iconv requires a loop, just as tds7_ascii2unicode() & Co. do now.  Is
that what you mean?  

> > If we need to estimate how big a buffer to use in conversion, 4
> > bytes/char is always safe for the foreseeable future.  
> 
> Yes, but it's a waste of space. Every column in row occupy 4 times. 

OK, granted.  Here's my proposed function prototype:

        int bytes_per_char(const char charset_name[]);

That will require strcmp(3) on a lookup table, which isn't very efficient.
 We should probably cache the results in TDSICONVINFO for *both* server
and client charsets.  

A different approach would be to cache the client:server byte/char ratio. 
IOW, if client and server are both 1 byte/char, the ratio is 1:1 === 1.0. 
If the client in Latin1 and the server is UCS-2, the ratio is 1:2 ===
0.50.  Etc.  That way, given a buffer and knowing which direction you're
going, you know how much to multiply its size by to get the output buffer
size.  

Regards, 

--jkl