[xml] Re: [xslt] Few questions

On Mon, Sep 08, 2003 at 11:41:54AM +0200, Carlo Contavalli wrote:
  I'm almost done writing a module using libxml/libxslt (mainly 
2.5.7 and 1.0.30), and I have a few question marks still to answer:  

  - (libxslt) the module provides some extension tags (registered 
    with xsltRegisterExtElement). The function handling one of the 
    extension tags needs to retrieve all the text that would
    be expanded by the xslt processor in the corresponding ``subtree''. 
    To do this, I've used:


    is this correct?

   Maybe maybe not, I don't undertsnad fully what you're looking for
and I don't remember that function in particular.
If my tag contains a <fallback handler, however,
    althought working correctly, I keep getting the non-fatal error:

      xsltApplyOneTemplate: fallback was not compiled

    is there another function I should use? What can I do to avoid
    the error (something smarter than removing the fallback node 
    from the tree)?

   No idea. Use the source, put a breakpoint in your routine and see what's
happen !

  - (libxml) Some xml documents that get parsed use external entityes 
    fetched from http urls. I get an uncoherent behavior due to the fact 
    that if the dtd does not exist on the local file system, I get a sort
    of warning telling me the dtd does not exist, but parsing goes on. 

   Right the presence of a DTd is not critical for processing.

    However, when it is fetched from a remote url, the http status of 
    the request is not checked and if the document does not exist 
    libxml tryes to parse the error page as if it was the DTD, leading 
    to a fatal error (it is quite uncommon to have a 404 page
    containing a valid DTD).

   Well you served some markup. Libxml2 can parse HTML is asked for,
and could parse that URL 
       xmllint --html your_404_url

    So, if the dtd does not exist on the local file system -> ignorable
    warning, if it does not exist on a remote url -> parsing stops with
    an error not always easy to understand (Extra content at the end
    of the document... but where did it get the document?). Is this
    an expected behavior? Is there something I can do to avoid the 

   Define you own entity handler, see the documentation of libxml2 in
the "I/O Interfaces" page.

  - (libxslt) The application changes the error handlers, calling 
    xsltSetGenericErrorFunc with its own handler and its own
    context. However, I sometimes get segfault due to transform.c
    calling xsltGenericError(xsltGenericDebugContext, where
    xsltGenericDebugContext is a different (unknown) data 
    type than the one expected by my handler as GenericErrorContext. 
    Is this behavior expected? How can I make sure 
    xsltGenericError gets called only with its own context
    (without enabling debugging - still there in 1.0.32)?

  No idea how you got there. Sorry you will have to debug !

  - (libxml) The application I'm talking about is _not_ multithreaded
    (at least by default). However, it is made
    of a list of modules being called in a big loop, where 
    more than a module may make use of the libxml library in 
    the same thread of execution. libxml makes use of many global 
    variables, some of which may be changed to customize its 
    behavior. Actually, to avoid one module influencing the
    parsing of another module, I compile libxml with multithreading
    support and save the ``state'' of the library by copying
    the memory area returned by xmlGetGlobalState with memcpy
    and by restoring it after the module has executed, leading
    to something like:
      At module initialization
        - create new global state (malloc and InitializeGlobalState)
      - save global state (GetGlobalState and memcpy) 
      - overwrite the global state with the one created above
      - (change libxml defaults)
      - save my own global state
      - restore previous global state

  That sounds very messy. If you compile with thread you already have
per thread global data so I don't understand what you're doing with this

      At module execution (in the big loop)
        - save global state
      - overwrite the global state with my own
      - (work with libxml)
      - restore global state

    Is there a better way to do this? Is there any other

  I don't see why you think you needed it. Get a debugger and see
what's the default behaviour actually is.

    solution than copying every single global variable
    when multithreading is disabled? I've looked at the

  Oh it's without multithreading compiled in, well your code
is yours, you can try this kind of things but you're on your own.

    source code, and I believe it would be nice to let 
    the applications access the statics NewGlobalState
    and FreeGlobalState, and to have a SetGlobalState
    that acts directly on the tsd variables being used
    (if you believe it worth, I could easily provide the
    needed code).

   Well I don't understand why you want to do that. if you have
specific setup and you want to switch save the variable and restore
them I really don't see why it's not appropriate !

  - (libxslt) What's the status for libxslt? I haven't seen 
    any threading support and the only globals I've seen 
    are those for error handling.
    Am I on the safe side (with regard to the problem
    highlited above) if I save and restore just them
    before and after running the module? 

   Pleople are using libxslt in multithreaded apps without troubles
nearly since the beginning. For the errors yes they are shared, 
so far nobody provided code for something like libxml2 multithread
support so it must be that nobody felt an urgent need for it.

Sorry for the long mail and sorry for the OT about libxml, but 
I really didn't know how to best split the mail.  Feel free to
answer me on any of the two lists.

  I'm not sure I understand every aspects of your problems nor
why you selected some solutions which looks rather complex.


Daniel Veillard      | Red Hat Network https://rhn.redhat.com/
veillard redhat com  | libxml GNOME XML XSLT toolkit  http://xmlsoft.org/
http://veillard.com/ | Rpmfind RPM search engine http://rpmfind.net/

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