Re: [xml] DTDs and null SystemID/ExternalID ?

[Stefan Jeglinski <jeglin 4pi com>]

I think you've already answered by saying that the ParseDTD functions 
are only for external subsets. There's where I perhaps went wrong. 
I'm creating the dtd content in memory in preparation for writing it 
out as part of an xml file that has an internal subset. I hope to be 
directed to that part of the API that should be used to do this. 
Please read further for my specific responses to your points:


>  Yes that's legal, that's called an internal subset, and no it's not
> the last third case as you can combine SYSTEM or PUBLIC identifiers and
> exend their definitions with an internal subset too.
>
>   Spec is there
>     http://www.w3.org/TR/REC-xml/#NT-doctypedecl
> please don't paraphrase especially when being wrong. take the time to read
> the spec it will help being understood and avoid this kind of errors.

A more-than-fair criticism. The rest of my answers are without the 
benefit of me having absorbed and understood the spec as well as I 
should. That reading is on my list of TDD.


>   the ParseDTD functions are there to load an external subset, not to
> try to tweak things like that. What you're attempting may work if you
> understand the internal representation of libxml2 and be careful though.

OK. See my opening comment. Hopefully I can also learn how to be 
careful. Please note that the reason my OP actually referred to the 
libxml2 source, is *precisely* because I'm trying to learn more about 
its internal representation.


>  > I've boxed myself into a corner. Snooping the libxml2 source
>  > (xmlsave.c, xmlDtdDumpOutput), I see that I can create the 3rd type
> >  only if SystemID and ExternalID are NULL.
>
>   Sorry, that premice sounds wrong to me. Please explain ! SystemID
>  and ExternalID where ?

The xmlDtd record structure, which contains an entry for both. Please 
refer to xmlsave.c, in xmlDtdDumpOutput (as quoted above in my OP). 
If you just read the code, you see that if SystemID and ExternalID 
are NULL, neither SYSTEM nor PUBLIC are inserted. At least that's the 
way it reads to me. Maybe I'm stupid.


>  > But this is directly at
>  > odds with xmlIOParseDTD, which ultimately allocates both and fills
> >  them in with "none".
>
>   I don't think I ever use "none", it may be NULL, and that's normal
> it's not part of the file being parsed.

Again I must be way off base, but in parser.c, in the xmlIOParseDTD 
routine, you call xmlNewDtd, and pass '"none"' not once but 3 times. 
If you refer to xmlNewDtd in tree.c, you will see that the last 2 
"nones" are for SystemID and ExternalID. Bottom line, neither 
SystemID nor ExternalID are initialized to NULL.

What can I say here? I mean, that's what the libxml2 code does. I 
can't explain why you say "none" is never used when it is, or that a 
premise about SystemID and ExternalID being NULL is wrong, when there 
are explicit conditionals testing for NULL. I can only conclude that 
my approach is *so* naive and off-base that it never occurred to you 
that anyone would ever try to look at it in so ridiculous a fashion 
:-)


>  > You can't naively can't set them to NULL after the fact, because you
> >  will die in xmlFreeDoc.
>
>   I'm not sure I understand, explain !

Hopefully my above explanation helps clear this up.


>   First read the spec it will help understanding the code and the
> terminology. Second I don't understand what you're trying to achieve.
>  "create the dtd in memory" how ? The DOM tree for the DTD ? The
> serialization of a DTD to a string ?
>
>   Very confusing ...

I am sorry to be so confusing. This has been discussed on the list 
before and you have participated in the threads - that's where I got 
the idea. I allocate a buffer in memory, and write the contents of 
the dtd (elements names etc) to that buffer. I then create the xml 
nodes and their content and write it all out to disk. I may be guilty 
of not understanding the spec as I should, but there is plenty of 
documentation out there about creating an xml file that has the dtd 
embedded in it.

My question boils down to that - if I want to create an xml file with 
embedded dtd according to my case #3, which is legal as you say, an 
internal subset, then both SYSTEM and PUBLIC are correctly missing. 
But they're not missing if they have been initialized as an xmlChar* 
to "none". Apparently I need to use a different part of the API. I 
may in fact stumble on that in the list archive or by googling if you 
decline to answer... but help is always appreciated.



Stefan Jeglinski