<?xml version="1.0" standalone="no"?>
<!--
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
-->

<!DOCTYPE s1 SYSTEM "sbk:/style/dtd/document.dtd">

<s1 title="Programming Guide">
    <anchor name="Macro"/>
    <s2 title="Version Macro">
        <p>&XercesCName; defines a numeric preprocessor macro, _XERCES_VERSION, for users to
           introduce into their code to perform conditional compilation where the
           version of &XercesCName; is detected in order to enable or disable version
          specific capabilities. For example,
         </p>
<source>
#if _XERCES_VERSION >= 30102
  // Code specific to Xerces-C++ version 3.1.2 and later.
#else
  // Old code.
#endif
</source>
        <p>The minor and revision (patch level) numbers have two digits of resolution
           which means that '1' becomes '01' and '2' becomes '02' in this example.
        </p>
        <p>There are also other string macros or constants to represent the Xerces-C++ version.
           Please refer to the <code>xercesc/util/XercesVersion.hpp</code> header for details.
        </p>
    </s2>


    <anchor name="Schema"/>
    <s2 title="Schema Support">
        <p>&XercesCName; contains an implementation of the W3C XML Schema
           Language.  See the <jump href="schema-&XercesC3Series;.html">XML Schema Support</jump> page for details.
         </p>
    </s2>

    <anchor name="Progressive"/>
    <s2 title="Progressive Parsing">

        <p>In addition to using the <code>parse()</code> method to parse an XML File.
        You can use the other two parsing methods, <code>parseFirst()</code> and <code>parseNext()</code>
        to do the so called progressive parsing. This way you don't
        have to depend on throwing an exception to terminate the
        parsing operation.
         </p>
         <p>
        Calling <code>parseFirst()</code> will cause the DTD (both internal and
        external subsets), and any pre-content, i.e. everything up to
        but not including the root element, to be parsed. Subsequent calls to
        <code>parseNext()</code> will cause one more pieces of markup to be parsed,
        and propagated from the core scanning code to the parser (and
        hence either on to you if using SAX/SAX2 or into the DOM tree if
        using DOM).
         </p>
         <p>
        You can quit the parse any time by just not
        calling <code>parseNext()</code> anymore and breaking out of the loop. When
        you call <code>parseNext()</code> and the end of the root element is the
        next piece of markup, the parser will continue on to the end
        of the file and return false, to let you know that the parse
        is done. So a typical progressive parse loop will look like
        this:</p>

<source>// Create a progressive scan token
XMLPScanToken token;

if (!parser.parseFirst(xmlFile, token))
{
  cerr &lt;&lt; "scanFirst() failed\n" &lt;&lt; endl;
  return 1;
}

//
// We started ok, so lets call scanNext()
// until we find what we want or hit the end.
//
bool gotMore = true;
while (gotMore &amp;&amp; !handler.getDone())
  gotMore = parser.parseNext(token);</source>

        <p>In this case, our event handler object (named 'handler')
        is watching for some criteria and will
        return a status from its <code>getDone()</code> method. Since
        the handler
        sees the SAX events coming out of the SAXParser, it can tell
        when it finds what it wants. So we loop until we get no more
        data or our handler indicates that it saw what it wanted to
        see.</p>

        <p>When doing non-progressive parses, the parser can easily
        know when the parse is complete and insure that any used
        resources are cleaned up. Even in the case of a fatal parsing
        error, it can clean up all per-parse resources. However, when
        progressive parsing is done, the client code doing the parse
        loop might choose to stop the parse before the end of the
        primary file is reached. In such cases, the parser will not
        know that the parse has ended, so any resources will not be
        reclaimed until the parser is destroyed or another parse is started.</p>

        <p>This might not seem like such a bad thing; however, in this case,
        the files and sockets which were opened in order to parse the
        referenced XML entities will remain open. This could cause
        serious problems. Therefore, you should destroy the parser instance
        in such cases, or restart another parse immediately. In a future
        release, a reset method will be provided to do this more cleanly.</p>

        <p>Also note that you must create a scan token and pass it
        back in on each call. This insures that things don't get done
        out of sequence. When you call <code>parseFirst()</code> or
        <code>parse()</code>, any
        previous scan tokens are invalidated and will cause an error
        if used again. This prevents incorrect mixed use of the two
        different parsing schemes or incorrect calls to
        <code>parseNext()</code>.</p>

    </s2>

    <anchor name="GrammarCache"/>
    <s2 title="Pre-parsing Grammar and Grammar Caching">
        <p>&XercesCName; provides a function to pre-parse the grammar so that users
           can check for any syntax error before using the grammar.  Users can also optionally
           cache these pre-parsed grammars for later use during actual parsing.
        </p>
        <p>Here is an example:</p>
<source>
XercesDOMParser parser;

// Enable schema processing.
parser.setDoSchema(true);
parser.setDONamespaces(true);

// Let's preparse the schema grammar (.xsd) and cache it.
Grammar* grammar = parser.loadGrammar(xmlFile, Grammar::SchemaGrammarType, true);
</source>
        <p>Besides caching pre-parsed schema grammars, users can also cache any
           grammars encountered during an xml document parse.
        </p>
        <p>Here is an example:</p>
<source>
SAXParser parser;

// Enable grammar caching by setting cacheGrammarFromParse to true.
// The parser will cache any encountered grammars if it does not
// exist in the pool.
// If the grammar is DTD, no internal subset is allowed.
parser.cacheGrammarFromParse(true);

// Let's parse our xml file (DTD grammar)
parser.parse(xmlFile);

// We can get the grammar where the root element was declared
// by calling the parser's method getRootGrammar;
// Note: The parser owns the grammar, and the user should not delete it.
Grammar* grammar = parser.getRootGrammar();
</source>
        <p>We can use any previously cached grammars when parsing new xml
        documents. Here are some examples on how to use those cached grammars:
        </p>
<source>
/**
  * Caching and reusing XML Schema (.xsd) grammar
  * Parse an XML document and cache its grammar set. Then,  use the cached
  * grammar set in subsequent parses.
  */

XercesDOMParser parser;

// Enable schema processing
parser.setDoSchema(true);
parser.setDoNamespaces(true);

// Enable grammar caching
parser.cacheGrammarFromParse(true);

// Let's parse the XML document. The parser will cache any grammars encountered.
parser.parse(xmlFile);

// No need to enable re-use by setting useCachedGrammarInParse to true. It is
// automatically enabled with grammar caching.
for (int i=0; i&lt; 3; i++)
    parser.parse(xmlFile);

// This will flush the grammar pool
parser.resetCachedGrammarPool();
</source>

<source>
/**
  * Caching and reusing DTD grammar
  * Preparse a grammar and cache it in the pool. Then, we use the cached grammar
  * when parsing XML documents.
  */

SAX2XMLReader* parser = XMLReaderFactory::createXMLReader();

// Load grammar and cache it
parser->loadGrammar(dtdFile, Grammar::DTDGrammarType, true);

// enable grammar reuse
parser->setFeature(XMLUni::fgXercesUseCachedGrammarInParse, true);

// Parse xml files
parser->parse(xmlFile1);
parser->parse(xmlFile2);
</source>
        <p>There are some limitations about caching and using cached grammars:</p>
           <ul>
              <li>When caching/reusing DTD grammars, no internal subset is allowed.</li>
              <li>When preparsing grammars with caching option enabled, if a grammar, in the
              result set, already exists in the pool (same namespace for schema or same system
              id for DTD), the entire set will not be cached. This behavior is the default but can
              be overridden for XML Schema caching. See the SAX/SAX2/DOM parser features for details.</li>
              <li>When parsing an XML document with the grammar caching option enabled, the
              reuse option is also automatically enabled. We will only parse a grammar if it
              does not exist in the pool.</li>
           </ul>
    </s2>

    <anchor name="LoadableMessageText"/>
    <s2 title="Loadable Message Text">

        <p>The &XercesCName; supports loadable message text.   Although
        the current distribution only supports English, it is capable of
        supporting other
        languages. Anyone interested in contributing any translations
        should contact us. This would be an extremely useful
        service.</p>

        <p>In order to support the local message loading services, all the error messages
        are captured in an XML file in the src/xercesc/NLS/ directory.
        There is a simple program, in the tools/NLS/Xlat/ directory,
        which can translate that text in various formats. It currently
        supports a simple 'in memory' format (i.e. an array of
        strings), the Win32 resource format, and the message catalog
        format.  The 'in memory' format is intended for very simple
        installations or for use when porting to a new platform (since
        you can use it until you can get your own local message
        loading support done.)</p>

        <p>In the src/xercesc/util/ directory, there is an XMLMsgLoader
        class.  This is an abstraction from which any number of
        message loading services can be derived. Your platform driver
        file can create whichever type of message loader it wants to
        use on that platform.  &XercesCName; currently has versions for the in
        memory format, the Win32 resource format, the message
        catalog format, and ICU message loader.
        Some of the platforms can support multiple message
        loaders, in which case a #define token is used to control
        which one is used. You can set this in your build projects to
        control the message loader type used.</p>

    </s2>

    <anchor name="PluggableTranscoders"/>
    <s2 title="Pluggable Transcoders">

        <p>&XercesCName; also supports pluggable transcoding services. The
        XMLTransService class is an abstract API that can be derived
        from, to support any desired transcoding
        service. XMLTranscoder is the abstract API for a particular
        instance of a transcoder for a particular encoding. The
        platform driver file decides what specific type of transcoder
        to use, which allows each platform to use its native
        transcoding services, or the ICU service if desired.</p>

        <p>Implementations are provided for Win32 native services, ICU
        services, and the <ref>iconv</ref> services available on many
        Unix platforms. The Win32 version only provides native code
        page services, so it can only handle XML code in the intrinsic
        encodings ASCII, UTF-8, UTF-16 (Big/Small Endian), UCS4
        (Big/Small Endian), EBCDIC code pages IBM037, IBM1047 and
        IBM1140 encodings, ISO-8859-1 (aka Latin1) and Windows-1252. The ICU version
        provides all of the encodings that ICU supports. The
        <ref>iconv</ref> version will support the encodings supported
        by the local system. You can use transcoders we provide or
        create your own if you feel ours are insufficient in some way,
        or if your platform requires an implementation that &XercesCName; does not
        provide.</p>

    </s2>

    <anchor name="PortingGuidelines"/>
    <s2 title="Porting Guidelines">

      <p>All platform dependent code in &XercesCName; has been
      isolated to a couple of files, which should ease the porting
      effort. The <code>src/xercesc/util</code> directory
      contains all such files. In particular:</p>

      <ul>
        <li>The <code>src/xercesc/util/FileManagers</code> directory
            contains implementations of file managers for various
            platforms.</li>

        <li>The <code>src/xercesc/util/MutexManagers</code> directory
            contains implementations of mutex managers for various
            platforms.</li>

        <li>The <code>src/xercesc/util/Xerces_autoconf_const*</code> files
            provide base definitions for various platforms.</li>
      </ul>

      <p>Other concerns are:</p>

      <ul>
        <li>Does ICU compile on your platform? If not, then you'll need to
            create a transcoder implementation that uses your local transcoding
            services. The iconv transcoder should work for you, though perhaps
            with some modifications.</li>
        <li>What message loader will you use? To get started, you can use the
            "in memory" one, which is very simple and easy. Then, once you get
            going, you may want to adapt the message catalog message loader, or
            write one of your own that uses local services.</li>
        <li>What should I define XMLCh to be? Please refer to <jump
            href="build-misc-&XercesC3Series;.html#XMLChInfo">What should I define XMLCh to be?</jump> for
            further details.</li>
      </ul>

       <p>Finally, you need to decide about how to define XMLCh. Generally,
          XMLCh should be defined to be a type suitable for holding a
          utf-16 encoded (16 bit) value, usually an <code>unsigned short</code>. </p>

        <p>All XML data is handled within &XercesCName; as strings of
           XMLCh characters.  Regardless of the size of the
           type chosen, the data stored in variables of type XMLCh
           will always be utf-16 encoded values. </p>



        <p>Unlike XMLCh, the  encoding
               of wchar_t is platform dependent.  Sometimes it is utf-16
               (AIX, Windows), sometimes ucs-4 (Solaris,
               Linux), sometimes it is not based on Unicode at all
               (HP/UX, AS/400, system 390).  </p>

        <p>Some earlier releases of &XercesCName; defined XMLCh to be the
           same type as wchar_t on most platforms, with the goal of making
           it possible to pass XMLCh strings to library or system functions
           that were expecting wchar_t parameters.  This approach has
           been abandoned because of</p>

           <ul>
              <li>
                 Portability problems with any code that assumes that
                 the types of XMLCh and wchar_t are compatible
              </li>

              <li>Excessive memory usage, especially in the DOM, on
                  platforms with 32 bit wchar_t.
              </li>

              <li>utf-16 encoded XMLCh is not always compatible with
                  ucs-4 encoded wchar_t on Solaris and Linux.  The
                  problem occurs with Unicode characters with values
                  greater than 64k; in ucs-4 the value is stored as
                  a single 32 bit quantity.  With utf-16, the value
                  will be stored as a "surrogate pair" of two 16 bit
                  values.  Even with XMLCh equated to wchar_t, xerces will
                  still create the utf-16 encoded surrogate pairs, which
                  are illegal in ucs-4 encoded wchar_t strings.
               </li>
           </ul>



    </s2>

    <anchor name="CPPNamespace"/>
    <s2 title="Using C++ Namespace">

    <p>&XercesCName; makes use of C++ namespace to make sure its
       definitions do not conflict with other libraries and
       applications. As a result applications must
       namespace-qualify all &XercesCName; classes, data and
       variables using the <code>xercesc</code> name. Alternatively,
       applications can use <code>using xercesc::&lt;Name>;</code>
       declarations
       to make individual &XercesCName; names visible in the
       current scope
       or <code>using namespace xercesc;</code>
       definition to make all &XercesCName; names visible in the
       current scope.</p>

    <p>While the above information should be sufficient for the majority
       of applications, for cases where several versions of the &XercesCName;
       library must be used in the same application, namespace versioning is
       provided. The following convenience macros can be used to access the
       &XercesCName; namespace names with versions embedded
       (see <code>src/xercesc/util/XercesDefs.hpp</code>):</p>

<source>
    #define XERCES_CPP_NAMESPACE_BEGIN    namespace &XercesC3NSVersion; {
    #define XERCES_CPP_NAMESPACE_END    }
    #define XERCES_CPP_NAMESPACE_USE    using namespace &XercesC3NSVersion;;
    #define XERCES_CPP_NAMESPACE_QUALIFIER    &XercesC3NSVersion;::

    namespace &XercesC3NSVersion; { }
    namespace &XercesC3Namespace; = &XercesC3NSVersion;;
</source>
    </s2>


    <anchor name="SpecifyLocaleForMessageLoader"/>
    <s2 title="Specify Locale for Message Loader">

        <p>&XercesCName; provides mechanisms for Native Language Support (NLS).
        Even though
        the current distribution has only English message file, it is capable
        of supporting other languages once the translated version of the
        target language is available.</p>

        <p>An application can specify the locale for the message loader in their
        very first invocation to XMLPlatformUtils::Initialize() by supplying
        a parameter for the target locale intended. The default locale is "en_US".
        </p>
<source>
    // Initialize the parser system
    try
    {
         XMLPlatformUtils::Initialize("fr_FR");
    }
    catch ()
    {
    }
</source>
    </s2>


    <anchor name="SpecifyLocationForMessageLoader"/>
    <s2 title="Specify Location for Message Loader">

        <p>&XercesCName; searches for message files at the location
           specified in the <code>XERCESC_NLS_HOME</code> environment
           variable and, if that is not set, at the default
           message directory, <code>$XERCESCROOT/msg</code>.
        </p>

        <p>Application can specify an alternative location for the message files in their
        very first invocation to XMLPlatformUtils::Initialize() by supplying
        a parameter for the alternative location.
        </p>

<source>
    // Initialize the parser system
    try
    {
         XMLPlatformUtils::Initialize("en_US", "/usr/nls");
    }
    catch ()
    {
    }
</source>
    </s2>

    <anchor name="PluggablePanicHandler"/>
    <s2 title="Pluggable Panic Handler">

        <p>&XercesCName; reports panic conditions encountered to the panic
           handler installed. The panic handler can take whatever action
           appropriate to handle the panic condition.
        </p>
        <p>&XercesCName; allows application to provide a customized panic handler
           (class implementing the interface PanicHandler), in its very first invocation of
           XMLPlatformUtils::Initialize().
        </p>
        <p>In the absence of an application-specific panic handler, &XercesCName; default
           panic handler is installed and used, which aborts program whenever a panic
           condition is encountered.
        </p>

<source>
    // Initialize the parser system
    try
    {
         PanicHandler* ph = new MyPanicHandler();

         XMLPlatformUtils::Initialize("en_US",
                                      "/usr/nls",
                                      ph);
    }
    catch ()
    {
    }
</source>
    </s2>

    <anchor name="PluggableMemoryManager"/>
    <s2 title="Pluggable Memory Manager">
        <p>Certain applications wish to maintain precise control over
        memory allocation.  This enables them to recover more easily
        from crashes of individual components, as well as to allocate
        memory more efficiently than a general-purpose OS-level
        procedure with no knowledge of the characteristics of the
        program making the requests for memory.  In &XercesCName; this
        is supported via the Pluggable Memory Handler.
        </p>

        <p>Users who wish to implement their own MemoryManager,
        an interface found in <code>xercesc/framework/MemoryManager.hpp</code>,
        need to implement only two methods:</p>
<source>
// This method allocates requested memory.
// the parameter is the requested memory size
// A pointer to the allocated memory is returned.
virtual void* allocate(XMLSize_t size) = 0;

// This method deallocates memory
// The parameter is a pointer to the allocated memory to be deleted
virtual void deallocate(void* p) = 0;
</source>
        <p>To maximize the amount of flexibility that applications
        have in terms of controlling memory allocation, a
        MemoryManager instance may be set as part of the call to
        XMLPlatformUtils::Initialize() to allow for static
        initialization to be done with the given MemoryHandler; a
        (possibly different) MemoryManager may be passed in to the
        constructors of all Xerces parser objects as well, and all
        dynamic allocations within the parsers will make use of this
        object.  Assuming that MyMemoryHandler is a class that
        implements the MemoryManager interface, here is a bit of
        pseudocode which illustrates these ideas:
        </p>
<source>
MyMemoryHandler *mm_for_statics = new MyMemoryHandler();
MyMemoryHandler *mm_for_particular_parser = new MyMemoryManager();

// initialize the parser information; try/catch
// removed for brevity
XMLPlatformUtils::Initialize(XMLUni::fgXercescDefaultLocale, 0,0,
        mm_for_statics);

// create a parser object
XercesDOMParser *parser = new
        XercesDomParser(mm_for_particular_parser);

// ...
delete parser;
XMLPlatformUtils::Terminate();
</source>
      <p>
        If a user provides a MemoryManager object to the parser, then
        the user owns that object.  It is also important to note that
        &XercesCName; default implementation simply uses the global
        new and delete operators.
      </p>
    </s2>

    <anchor name="SecurityManager"/>
    <s2 title="Managing Security Vulnerabilities">
      <p>
        The purpose of the SecurityManager class is to permit applications a
        means to have the parser reject documents whose processing would
        otherwise consume large amounts of system resources.  Malicious
        use of such documents could be used to launch a denial-of-service
        attack against a system running the parser.  Initially, the
        SecurityManager only knows about attacks that can result from
        exponential entity expansion; this is the only known attack that
        involves processing a single XML document.  Other, similar attacks
        can be launched if arbitrary schemas may be parsed; there already
        exist means (via use of the EntityResolver interface) by which
        applications can deny processing of untrusted schemas.  In future,
        the SecurityManager will be expanded to take these other exploits
        into account.
      </p>
      <p>
        The SecurityManager class is very simple:  It will contain
        getters and setters corresponding to each known variety of
        exploit.  These will reflect limits that the application may
        impose on the parser with respect to the processing of various
        XML constructs.  When an instance of SecurityManager is
        instantiated, default values for these limits will be provided
        that should suit most applications.
      </p>
      <p>
        By default, &XercesCName; is a wholly conformant XML parser; that
        is, no security-related considerations will be observed by
        default. An application must provide an instance of the
        SecurityManager class to a parser in order to make that
        parser behave in a security-conscious manner.  For example:
      </p>
<source>
SAXParser *myParser = new SAXParser();
SecurityManager *myManager = new SecurityManager();
myManager->setEntityExpansionLimit(100000); // larger than default
myParser->setSecurityManager(myManager);
// ... use the parser
</source>
      <p>
        Note that SecurityManager instances may be set on all kinds of
        &XercesCName; parsers; please see the documentation for the
        individual parsers for details.
      </p>
      <p>
        Note also that the application always owns the SecurityManager
        instance.  The default SecurityManager that &XercesCName; provides
        is not thread-safe; although it only uses primitive operations at
        the moment, users may need to extend the class with a
        thread-safe implementation on some platforms.
      </p>
    </s2>
<anchor name="UseSpecificScanner"/>
    <s2 title="Use Specific Scanner">

        <p>For performance and modularity &XercesCName; provides a mechanism
        for specifying the scanner to be used when scanning an XML document.
        Such mechanism will enable the creation of special purpose scanners
        that can be easily plugged in.</p>

        <p>&XercesCName; supports the following scanners:</p>

        <s3 title="WFXMLScanner">

            <p>
            The WFXMLScanner is a non-validating scanner which performs well-formedness check only.
            It does not do any DTD/XMLSchema processing. If the XML document contains a DOCTYPE, it
            will be silently ignored (i.e. no warning message is issued). Similarly, any schema
            specific attributes (e.g. schemaLocation), will be treated as normal element attributes.
            Setting grammar specific features/properties will have no effect on its behavior
            (e.g. setLoadExternalDTD(true) is ignored).
            </p>

<source>
// Create a DOM parser
XercesDOMParser parser;

// Specify scanner name
parser.useScanner(XMLUni::fgWFXMLScanner);

// Specify other parser features, e.g.
parser.setDoNamespaces(true);
</source>


        </s3>

        <s3 title="DGXMLScanner">

            <p>
            The DGXMLScanner handles XML documents with DOCTYPE information. It does not do any
            XMLSchema processing, which means that any schema specific attributes (e.g. schemaLocation),
            will be treated as normal element attributes. Setting schema grammar specific features/properties
            will have no effect on its behavior (e.g. setDoSchema(true) and setLoadSchema(true) are ignored).
            </p>

<source>
// Create a SAX parser
SAXParser parser;

// Specify scanner name
parser.useScanner(XMLUni::fgDGXMLScanner);

// Specify other parser features, e.g.
parser.setLoadExternalDTD(true);
</source>

        </s3>

        <s3 title="SGXMLScanner">

            <p>
            The SGXMLScanner handles XML documents with XML schema grammar information.
            If the XML document contains a DOCTYPE, it will be ignored. Namespace and
            schema processing features are on by default, and setting them to off has
            not effect.
            </p>

<source>
// Create a SAX2 parser
SAX2XMLReader* parser = XMLReaderFactory::createXMLReader();

// Specify scanner name
parser->setProperty(XMLUni::fgXercesScannerName, (void *)XMLUni::fgSGXMLScanner);

// Specify other parser features, e.g.
parser->setFeature(XMLUni::fgXercesSchemaFullChecking, false);
</source>

        </s3>

        <s3 title="IGXMLScanner">

            <p>
            The IGXMLScanner is an integrated scanner and handles XML documents with DTD and/or
            XML schema grammar. This is the default scanner used by the various parsers if no
            scanner is specified.
            </p>

<source>
// Create a DOMLSParser parser
DOMLSParser *parser = ((DOMImplementationLS*)impl)->createLSParser(
  DOMImplementationLS::MODE_SYNCHRONOUS, 0);

// Specify scanner name - This is optional as IGXMLScanner is the default
parser->getDomConfig()->setParameter(
  XMLUni::fgXercesScannerName, (void *)XMLUni::fgIGXMLScanner);

// Specify other parser features, e.g.
parser->getDomConfig()->setParameter(XMLUni::fgDOMNamespaces, doNamespaces);
parser->getDomConfig()->setParameter(XMLUni::fgXercesSchema, doSchema);
</source>

        </s3>

    </s2>

    <anchor name="EntityResolution"/>
    <s2 title="Entity Resolution and NetAccessors">

        <p>All Xerces applications that wish to limit a wide range of security risks MUST install
        appropriate implementations of API-specific "entity/resource resolution" implementations to
        govern/police the resolution of external entities by the parser.
        </p>   

        <p>In the event that an entity resolver wants to delegate the job of resolving a particular resource
        to the parser, Xerces will automatically handle local file references but will delegate any non-local
        file URLs to a NetAccessor, a plugin controlled at build time supporting access to remote
        systems. Most of the built-in implementations are limited, with the principal exception of support
        for <jump href="https://curl.haxx.se">libcurl</jump> as an implementation, which supports a wide range
        of protocols, potentially including http over TLS, which is rarely supported in other cases.</p>

        <p>It is incredibly important to implement your own entity resolution code to ensure that if you
        allow the parser to perform its own resolution, it does so only under conditions and against URLs
        you trust. The library will NOT your application from the many potential threats
        arising from arbitrary resource resolution.</p>

        <p>Notably, entity/resource resolver implementations can be accompanied by changing the default
        parser features to disallow "default" resolution, preventing the parser from executing its own
        NetAccessors to perform these lookups.</p>
    </s2>
</s1>