Re: [xml] libxml2 API crazy?

[Nikita Churaev <lamefun x0r gmail com>]

this gives out an error:

#include <libxml/parser.h>
#include <libxml/tree.h>
#include <stdio.h>

int
main (int argc, char** argv)
{
  xmlDocPtr doc = xmlReadMemory("<doc/>", 6, "egg.xml", NULL, 0);

                  /* XmlNode */            /* XmlDoc */
  printf("%s\n", (doc->children->parent == doc) ? "true" : "false");
  return 0;
}

and this crashes on my machine (as xmlDoc has different fields than
xmlName):

#include <libxml/parser.h>
#include <libxml/tree.h>
#include <stdio.h>

int
main (int argc, char** argv)
{
  xmlDocPtr doc = xmlReadMemory("<doc/>", 6, "egg.xml", NULL, 0);
         
  if (doc->children->parent->ns && doc->children->parent->ns->href)
    printf("%s\n", doc->children->parent->ns->href);

  return 0;
}

On Sat, 2013-05-04 at 10:08 +1000, Callum Gibson wrote:
> On 03May13 22:44, Nikita Churaev wrote:
> }> What do you want to achieve ? I think I understand but it is better if
> }> stated.
> }
> }Make so you can use libxml2 and be sure that you use it correctly rather
> }than just blindly guess.
> }
> }The main problem here is casting quirks, libxml2 tree seems to cast
> }xmlDoc*, xmlAttr* and even xmlNs* into xmlNode*, so not only the fields
> }after the common node parts are unsafe to access without checking, even
> }ones in the common part are, except xmlElementType type. The
> }documentation provides no answer whatsoever on what structures
> }correspond to each xmlElementType. Casting xmlNs* to xmlNode* is the
> }worst one.
>
> You seem to be talking about the internal implementation, rather than
> use of the API? Can you provide specific examples in the code of how one
> might be using the API in a valid fashion (ie. as documented and per the
> examples) and yet be potentially using the wrong struct type?
>
> If you are worried about "polymorphic" aspects of using certain structures
> this is perfectly standard behaviour and, as I mentioned earlier, has been
> well-known since K&R 2nd edition. It's also used extensively in the X11 API
> and associated widget toolkits.
>
> }I want to add:
> }
> }safe casting functions, for example:
> }
> }xmlNodePtr xmlDocToNode(xmlDocPtr node); 
> }
> }xmlDocPtr xmlNodeToDoc(xmlNodePtr node); // returns NULL if 
> }                                         // node is not a doc
>
> Why would an application being using docs as nodes and vice versa? Can you
> give examples where this happens?
>
> regards,
>     Callum