Re: [xml] resolveEntity and SAX

[Daniel Veillard <veillard redhat com>]

On Fri, Nov 16, 2012 at 12:36:14AM +0800, Roland King wrote:
> I feel sure this is simple but after a day of reading code and googling I'm not getting it. 

  No it is not simple. SAX and entities handling is not simple in
  libxml2. See the warning in red at the bottom of
   http://xmlsoft.org/entities.html

> I want to implement a resolveEntity callback on a simple SAX parser in libxml2 so I can supply a DTD. I 
> have the following
>
>       xmlSAXHandlerPtr hdlrPtr = calloc( 1, sizeof( xmlSAXHandler ) );
>       
>       hdlrPtr->startDocument  = &startDocument;
>       hdlrPtr->endDocument    = &endDocument;
>       hdlrPtr->resolveEntity  = &resolveEntity;
>               
>       int result = xmlSAXUserParseMemory( hdlrPtr, NULL, buf, size );
>
> my resolveEntity code is never called even when fed a document which has a SYSTEM entity in the DOCTYPE. 
> The startDocument and endDocument callbacks are called. 
>
> <!DOCTYPE testxml SYSTEM "TestXML.dtd">
> ...
>
> Is this the wrong callback? I see the xmlParserOption enum which includes DTD loading and validation but 
> there's nothing in the simple SAX interface which seems to use that, it appears to be a level further up, 
> I'm just trying to use xmlSAXUserParseMemory() and calls at that level. Since the handler contains a 
> resolveEntity function I would have expected it to be called. 
>
> What basic misunderstanding do I have here?

  By default libxml2 XML parser does not fetch any of the external
  subset. You really need to give the DTD loading or validation option.
You can't do that with xmlSAXUserParseMemory()
Create a parser context, then set your SAX callback block and then
call xmlCtxtReadMemory() . There is no convenient APIs for doing this,
SAX processing and entities handling are difficult to handle properly.

  Unless you really have existing code relying on SAX, I really suggest
to instead look at using the Reader API,

Daniel

-- 
Daniel Veillard      | Open Source and Standards, Red Hat
veillard redhat com  | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
http://veillard.com/ | virtualization library  http://libvirt.org/