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  • From: "Lowden, James K" <LowdenJK AT bernstein.com>
  • To: "'FreeTDS Development Group'" <freetds AT lists.ibiblio.org>
  • Subject: RE: [freetds] tdsstring.c ansi alias problem
  • Date: Mon, 5 May 2003 13:24:32 -0400

> From: ZIGLIO Frediano [mailto:Frediano.Ziglio AT vodafoneomnitel.it]
> Sent: May 5, 2003 11:55 AM
> >
> > Where is DSTR_CHAR defined? I find only a forward
> > declaration in tds.h.
>
> Yes, it's true. It's an undefined structure. I use a pointer
> to undefined structure to be more type safe using C...

Oh. That's an unusual technique; a comment would have saved us some time.
:-)

> Also in such way I can
> extend dstring in future (like adding length and buffer size)
> without breaking source compatibility.

Maybe, maybe not. You're trading some potential future flexibility for
permanent opacity.

If you add members to DSTR_CHAR, you're right, it will compile, because of
all the heavy duty casts. OTOH, you won't get what you want: the buffer
size and length won't be set correctly until you notice the problem -- at
runtime -- and then fix the code. And adjusting the code was what you're
trying to avoid, yes?

I think it's better to expose the structure, for clarity, assigning the
pointer obviously, viz:

typedef struct { char *s } DSTR_CHAR, *DSTR;
char src[] = "hi";
DSTR * s;
/* then ... */
s->s = src;

because that code can be understood by all compilers and many human beings.


To protect against outsiders peeking into DSTR_CHAR, it can be defined in
tdsstring.c.

If you decide to change DSTR_CHAR, it won't be hard. The logic and the
structure are both simple. Am I missing something?

> I did this to avoid free(dstring) or other wrong usage.
> If "*(char **) s = src;" is not OK perhaps it's a priority
> problem... *((char**)s) = src should work better.

You're probably right about that.

Regards,

--jkl



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