# phpman > man > XML::LibXML::InputCallback

## NAME
    [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown) - [XML::LibXML](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML/markdown) Class for Input Callbacks

## SYNOPSIS
      use [XML::LibXML](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML/markdown);

## DESCRIPTION
    You may get unexpected results if you are trying to load external documents during libxml2
    parsing if the location of the resource is not a HTTP, FTP or relative location but a absolute
    path for example. To get around this limitation, you may add your own input handler to open,
    read and close particular types of locations or URI classes. Using this input callback handlers,
    you can handle your own custom URI schemes for example.

    The input callbacks are used whenever [XML::LibXML](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML/markdown) has to get something other than externally
    parsed entities from somewhere. They are implemented using a callback stack on the Perl layer in
    analogy to libxml2's native callback stack.

    The [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown) class transparently registers the input callbacks for the
    libxml2's parser processes.

  How does [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown) work?
    The libxml2 library offers a callback implementation as global functions only. To work-around
    the troubles resulting in having only global callbacks - for example, if the same global
    callback stack is manipulated by different applications running together in a single Apache
    Web-server environment -, [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown) comes with a object-oriented and a
    function-oriented part.

    Using the function-oriented part the global callback stack of libxml2 can be manipulated. Those
    functions can be used as interface to the callbacks on the C- and XS Layer. At the
    object-oriented part, operations for working with the "pseudo-localized" callback stack are
    implemented. Currently, you can register and de-register callbacks on the Perl layer and
    initialize them on a per parser basis.

   Callback Groups
    The libxml2 input callbacks come in groups. One group contains a URI matcher (*match*), a data
    stream constructor (*open*), a data stream reader (*read*), and a data stream destructor
    (*close*). The callbacks can be manipulated on a per group basis only.

   The Parser Process
    The parser process works on an XML data stream, along which, links to other resources can be
    embedded. This can be links to external DTDs or XIncludes for example. Those resources are
    identified by URIs. The callback implementation of libxml2 assumes that one callback group can
    handle a certain amount of URIs and a certain URI scheme. Per default, callback handlers for
    *file://**, *file:://*.gz*, *http://** and *ftp://** are registered.

    Callback groups in the callback stack are processed from top to bottom, meaning that callback
    groups registered later will be processed before the earlier registered ones.

    While parsing the data stream, the libxml2 parser checks if a registered callback group will
    handle a URI - if they will not, the URI will be interpreted as *file://URI*. To handle a URI,
    the *match* callback will have to return '1'. If that happens, the handling of the URI will be
    passed to that callback group. Next, the URI will be passed to the *open* callback, which should
    return a *reference* to the data stream if it successfully opened the file, '0' otherwise. If
    opening the stream was successful, the *read* callback will be called repeatedly until it
    returns an empty string. After the read callback, the *close* callback will be called to close
    the stream.

   Organisation of callback groups in [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown)
    Callback groups are implemented as a stack (Array), each entry holds a reference to an array of
    the callbacks. For the libxml2 library, the [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown) callback implementation
    appears as one single callback group. The Perl implementation however allows one to manage
    different callback stacks on a per libxml2-parser basis.

### Using [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown)
    After object instantiation using the parameter-less constructor, you can register callback
    groups.

      my $input_callbacks = [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown)->new();
      $input_callbacks->register_callbacks([ $match_cb1, $open_cb1,
                                             $read_cb1, $close_cb1 ] );
      $input_callbacks->register_callbacks([ $match_cb2, $open_cb2,
                                             $read_cb2, $close_cb2 ] );
      $input_callbacks->register_callbacks( [ $match_cb3, $open_cb3,
                                              $read_cb3, $close_cb3 ] );

      $parser->input_callbacks( $input_callbacks );
      $parser->parse_file( $some_xml_file );

  What about the old callback system prior to [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown)?
    In [XML::LibXML](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML/markdown) versions prior to 1.59 - i.e. without the [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown) module - you
    could define your callbacks either using globally or locally. You still can do that using
    [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown), and in addition to that you can define the callbacks on a per parser
    basis!

    If you use the old callback interface through global callbacks, [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown) will
    treat them with a lower priority as the ones registered using the new interface. The global
    callbacks will not override the callback groups registered using the new interface. Local
    callbacks are attached to a specific parser instance, therefore they are treated with highest
    priority. If the *match* callback of the callback group registered as local variable is
    identical to one of the callback groups registered using the new interface, that callback group
    will be replaced.

    Users of the old callback implementation whose *open* callback returned a plain string, will
    have to adapt their code to return a reference to that string after upgrading to version >=
    1.59. The new callback system can only deal with the *open* callback returning a reference!

## INTERFACE DESCRIPTION
### Global Variables
    $_CUR_CB
        Stores the current callback and can be used as shortcut to access the callback stack.

    @_GLOBAL_CALLBACKS
        Stores all callback groups for the current parser process.

    @_CB_STACK
        Stores the currently used callback group. Used to prevent parser errors when dealing with
        nested XML data.

### Global Callbacks
    _callback_match
        Implements the interface for the *match* callback at C-level and for the selection of the
        callback group from the callbacks defined at the Perl-level.

    _callback_open
        Forwards the *open* callback from libxml2 to the corresponding callback function at the
        Perl-level.

    _callback_read
        Forwards the read request to the corresponding callback function at the Perl-level and
        returns the result to libxml2.

    _callback_close
        Forwards the *close* callback from libxml2 to the corresponding callback function at the
        Perl-level..

### Class methods
### new
        A simple constructor.

### register_callbacks
        The four callbacks *have* to be given as array reference in the above order *match*, *open*,
        *read*, *close*!

### unregister_callbacks
        With no arguments given, "unregister_callbacks()" will delete the last registered callback
        group from the stack. If four callbacks are passed as array reference, the callback group to
        unregister will be identified by the *match* callback and deleted from the callback stack.
        Note that if several identical *match* callbacks are defined in different callback groups,
        ALL of them will be deleted from the stack.

### init_callbacks
        Initializes the callback system for the provided parser before starting a parsing process.

### cleanup_callbacks
        Resets global variables and the libxml2 callback stack.

### lib_init_callbacks
        Used internally for callback registration at C-level.

### lib_cleanup_callbacks
        Used internally for callback resetting at the C-level.

## EXAMPLE CALLBACKS
    The following example is a purely fictitious example that uses a [MyScheme::Handler](https://www.chedong.com/phpMan.php/perldoc/MyScheme%3A%3AHandler/markdown) object that
    responds to methods similar to an [IO::Handle](https://www.chedong.com/phpMan.php/perldoc/IO%3A%3AHandle/markdown).

      # Define the four callback functions
      sub match_uri {
          my $uri = shift;
          return $uri =~ /^myscheme:/; # trigger our callback group at a 'myscheme' URIs
      }

      sub open_uri {
          my $uri = shift;
          my $handler = [MyScheme::Handler](https://www.chedong.com/phpMan.php/perldoc/MyScheme%3A%3AHandler/markdown)->new($uri);
          return $handler;
      }

      # The returned $buffer will be parsed by the libxml2 parser
      sub read_uri {
          my $handler = shift;
          my $length = shift;
          my $buffer;
          read($handler, $buffer, $length);
          return $buffer; # $buffer will be an empty string '' if read() is done
      }

      # Close the handle associated with the resource.
      sub close_uri {
          my $handler = shift;
          close($handler);
      }

      # Register them with a instance of [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown)
      my $input_callbacks = [XML::LibXML::InputCallback](https://www.chedong.com/phpMan.php/perldoc/XML%3A%3ALibXML%3A%3AInputCallback/markdown)->new();
      $input_callbacks->register_callbacks([ \&match_uri, \&open_uri,
                                             \&read_uri, \&close_uri ] );

      # Register the callback group at a parser instance
      $parser->input_callbacks( $input_callbacks );

      # $some_xml_file will be parsed using our callbacks
      $parser->parse_file( $some_xml_file );

## AUTHORS
    Matt Sergeant, Christian Glahn, Petr Pajas

## VERSION
    2.0134

## COPYRIGHT
    2001-2007, AxKit.com Ltd.

    2002-2006, Christian Glahn.

    2006-2009, Petr Pajas.

## LICENSE
    This program is free software; you can redistribute it and/or modify it under the same terms as
    Perl itself.