Re: [xml] xmlUTF8Strpos()'s use of xmlUTF8Strlen()

["William M. Brack" <wbrack mmm com hk>]

Bill Moseley said:
> On Mon, Dec 20, 2004 at 02:37:16AM +0800, William M. Brack wrote:
> > Bill Moseley said:
> > > I'm wondering why xmlUTF8Strpos() needs to call xmlUTF8Strlen().
> > >
> > >     const xmlChar *
> > >     xmlUTF8Strpos(const xmlChar *utf, int pos) {
> > >         xmlChar ch;
> > >
> > >         if (utf == NULL) return(NULL);
> > >         if ( (pos < 0) || (pos >= xmlUTF8Strlen(utf)) )
> > >             return(NULL);
> > >         while (pos--) {
> > >             if ((ch=*utf++) == 0) return(NULL);
> > >
> > > xmlUTF8Strpos() is already checking for a \0 byte so doesn't seem like it
> > > does any good to go over the string twice: once in xmlUTF8Strlen() and
> > then
> > > again in xmlUTF8Strpos().
> >
> > The problem which this is meant to guard against is a caller passing in a
> > large value for pos which potentially causes a crash (analogous to the check
> > for a NULL pointer).  Since the 'while' loop is starting from the end of the
> > string and working backward, the check within the loop for termination
> > doesn't
> > help.
>
> Hi Bill,
>
> We had a related discussion back in 2001[1]
>
> It's not starting from the end of the string.  xmlUTF8Strpos() only
> calls xmlUTF8Strlen() to see if pos (the number of chars to skip) is
> more than the utf-8 length of the string.
>
> xmlUTF8Strlen() walks the utf-8 string counting characters
> until a null is found and returns the count or -1 if an invalid utf-8
> char is found.
>
> xmlUTF8Strpos() also walks the utf-8 string.  It returns the ending
> position after stepping "pos" chars or NULL if it hits \0 before "pos"
> chars have be found or if an invalid utf-8 sequence is found.  So it's
> making the same checks as xmlUTF8Strlen() does.
>
> At least that's my reading of the code.
>
> So, I don't think calling xmlUTF8Strlen() does anything that isn't
> done in xmlUTF8Strpos().

I *love* it when other people understand my code better than I do :-).  Yes,
you are completely correct, and I have no valid excuse for my previous post
(maybe it's the onset of senility?).  I have modified the CVS code - thanks
for pointing this out!

> Makes me wonder if it would be useful to have different type for
> null-terminated strings.  xmlSAX2Characters() returns a *non* null-
> terminated string of type xmlChar and so does xmlUTF8Strlen() which
> requires a null-terminated string.  If they were different
> types then the compiler would have caught this.

This might be a nice feature, but I don't see how we could do it without
breaking source compatibility with the existing API.

> Thanks for the help,
>
>
>
>
>
> [1] http://mail.gnome.org/archives/xml/2001-October/msg00063.html
>
> --
> Bill Moseley
> moseley hank org