Programming Xerces-C++

| Questions |
| Does Xerces-C++ support XML Schema? Does Xerces-C++ support XPath? Why does my application crash when instantiating the parser? Is it OK to call the XMLPlatformUtils::Initialize/Terminate pair of routines multiple times in one program? Why does my application crash after calling XMLPlatformUtils::Terminate()? Is Xerces-C++ thread-safe? I am seeing memory leaks in Xerces-C++. Are they real? Can Xerces-C++ create an XML skeleton based on a DTD Can I use Xerces-C++ to perform write validation Can I validate the data contained in a DOM tree? How to write out a DOM tree into a string or an XML file? Why doesn't DOMNode::cloneNode() clone the pointer assigned to a DOMNode via DOMNode::setUserData()? How are entity reference nodes handled in DOM? Can I use Xerces-C++ to parse HTML? I keep getting an error: "invalid UTF-8 character". What's wrong? What encodings are supported by Xerces-C++? What character encoding should I use when creating XML documents? Why does deleting a transcoded string result in assertion on windows? How do I transcode to/from something besides the local code page? Why does the parser still try to locate the DTD even validation is turned off and how to ignore external DTD reference? Why does the XML data generated by the DOMLSSerializer does not match my original XML input? Why does my application crash when deleting the parser after releasing a document?
| Does Xerces-C++ support Schema? |
| Yes, Xerces-C++ 3.2.3 contains an implementation of the W3C XML Schema Language, a recommendation of the Worldwide Web Consortium available in three parts: XML Schema: Primer and XML Schema: Structures and XML Schema: Datatypes. We consider this implementation complete. See the XML Schema Support page for limitations.
| Does Xerces-C++ support XPath? |
| Xerces-C++ 3.2.3 provides partial XPath 1 implementation for the purposes of handling XML Schema identity constraints. The same engine is made available through the DOMDocument::evaluate API to let the user perform simple XPath queries involving DOMElement nodes only, with no predicate testing and allowing the "//" operator only as the initial step. For full XPath 1 and 2 support refer to the XQilla and Apache Xalan C++ open source projects.
| Why does my application crash when instantiating the parser? |
| In order to work with the Xerces-C++ parser, you have to first initialize the XML subsystem. The most common mistake is to forget this initialization. Before you make any calls to Xerces-C++ APIs, you must call XMLPlatformUtils::Initialize():
    try {
       XMLPlatformUtils::Initialize();
    }
    catch (const XMLException& toCatch) {
       // Do your failure processing here
    }
This initializes the Xerces system and sets its internal variables. Note that you must include the `xercesc/util/PlatformUtils.hpp` file for this to work.
| Is it OK to call the XMLPlatformUtils::Initialize/Terminate pair of routines multiple times in one program? |
| Yes. Note, however, that the application needs to guarantee that the XMLPlatformUtils::Initialize() and XMLPlatformUtils::Terminate() methods are called from the same thread (usually the initial thread executing main()) or proper synchronization is performed by the application if multiple threads call XMLPlatformUtils::Initialize() and XMLPlatformUtils::Terminate() concurrently. If you are calling XMLPlatformUtils::Initialize() a number of times, and then follow with XMLPlatformUtils::Terminate() the same number of times, only the first XMLPlatformUtils::Initialize() will do the initialization, and only the last XMLPlatformUtils::Terminate() will clean up the memory. The other calls are ignored.
| Why does my application crash after calling XMLPlatformUtils::Terminate()? |
| Please make sure the XMLPlatformUtils::Terminate() is the last Xerces-C++ function to be called in your program. NO explicit nor implicit Xerces-C++ destructor (those local data that are destructed when going out of scope) should be called after XMLPlatformUtils::Terminate(). For example consider the following code snippet which is incorrect:
    1: {
    2:    XMLPlatformUtils::Initialize();
    3:    XercesDOMParser parser;
    4:    XMLPlatformUtils::Terminate();
    5: }
The XercesDOMParser object "parser" is destructed when going out of scope at line 5 before the closing brace. As a result, XercesDOMParser destructor is called at line 5 after XMLPlatformUtils::Terminate() which is incorrect. Correct code should be:
    1: {
    2:    XMLPlatformUtils::Initialize();
    2a:    {
    3:           XercesDOMParser parser;
    3a:    }
    4:    XMLPlatformUtils::Terminate();
    5: }
The extra pair of braces (line 2a and 3a) ensures that all implicit destructors are called before terminating Xerces-C++. Note also that the application needs to guarantee that the XMLPlatformUtils::Initialize() and XMLPlatformUtils::Terminate() methods are called from the same thread (usually the initial thread executing main()) or proper synchronization is performed by the application if multiple threads call XMLPlatformUtils::Initialize() and XMLPlatformUtils::Terminate() concurrently.
| Is Xerces-C++ thread-safe? |
| The answer is yes if you observe the following rules for using Xerces-C++ in a multi-threaded environment: Within an address space, an instance of the parser may be used without restriction from a single thread, or an instance of the parser can be accessed from multiple threads, provided the application guarantees that only one thread has entered a method of the parser at any one time. When two or more parser instances exist in a process, the instances can be used concurrently, without external synchronization. That is, in an application containing two parsers and two threads, one parser can be running within the first thread concurrently with the second parser running within the second thread. The same rules apply to Xerces-C++ DOM documents. Multiple document instances may be concurrently accessed from different threads, but any given document instance can only be accessed by one thread at a time. The application also needs to guarantee that the XMLPlatformUtils::Initialize() and XMLPlatformUtils::Terminate() methods are called from the same thread (usually the initial thread executing main()) or proper synchronization is performed by the application if multiple threads call XMLPlatformUtils::Initialize() and XMLPlatformUtils::Terminate() concurrently.
| I am seeing memory leaks in Xerces-C++. Are they real? |
| The Xerces-C++ library allocates and caches some commonly reused items. The storage for these may be reported as memory leaks by some heap analysis tools; to avoid the problem, call the function `XMLPlatformUtils::Terminate()` before your application exits. This will free all memory that was being held by the library. For most applications, the use of `Terminate()` is optional. The system will recover all memory when the application process shuts down. The exception to this is the use of Xerces-C++ from DLLs that will be repeatedly loaded and unloaded from within the same process. To avoid memory leaks with this kind of use, `Terminate()` must be called before unloading the Xerces-C++ library To ensure all the memory held by the parser are freed, the number of XMLPlatformUtils::Terminate() calls should match the number of XMLPlatformUtils::Initialize() calls. If you have built Xerces-C++ with dependency on ICU then you may want to call the u_cleanup() ICU function to clean up ICU static data. Refer to the ICU documentation for details.
| Is there a function that creates an XML file from a DTD (obviously with the values missing, a skeleton)? |
| No, there is no such functionality.
| Can I use Xerces-C++ to perform "write validation"? That is, having an appropriate Grammar and being able to add elements to the DOM whilst validating against the grammar? |
| No, there is no such functionality. The best you can do for now is to create the DOM document, write it back as XML and re-parse it with validation turned on.
| Is there a facility in Xerces-C++ to validate the data contained in a DOM tree? That is, without saving and re-parsing the source document? |
| No, there is no such functionality. The best you can do for now is to create the DOM document, write it back as XML and re-parse it with validation turned on.
| How to write out a DOM tree into a string or an XML file? |
| You can use the DOMLSSerializer::writeToString, or DOMLSSerializer::writeNode to serialize a DOM tree. Please refer to the sample DOMPrint or the API documentation for more details of DOMLSSerializer.
| Why doesn't DOMNode::cloneNode() clone the pointer assigned to a DOMNode via DOMNode::setUserData()? |
| Xerces-C++ supports the DOMNode::userData specified in the DOM level 3 Node interface. As is made clear in the description of the behavior of `cloneNode()`, userData that has been set on the Node is not cloned. Thus, if the userData is to be copied to the new Node, this copy must be effected manually. Note further that the operation of `importNode()` is specified similarly.
| How are entity reference nodes handled in DOM? |
| If you are using the native DOM classes, the function `setCreateEntityReferenceNodes` controls how entities appear in the DOM tree. When setCreateEntityReferenceNodes is set to true (the default), an occurrence of an entity reference in the XML document will be represented by a subtree with an EntityReference node at the root whose children represent the entity expansion. Entity expansion will be a DOM tree representing the structure of the entity expansion, not a text node containing the entity expansion as text. If setCreateEntityReferenceNodes is false, an entity reference in the XML document is represented by only the nodes that represent the entity expansion. The DOM tree will not contain any entityReference nodes.
| Can I use Xerces-C++ to parse HTML? |
| Yes, but only if the HTML follows the rules given in the XML specification. Most HTML, however, does not follow the XML rules, and will generate XML well-formedness errors.
| I keep getting an error: "invalid UTF-8 character". What's wrong? |
| Most commonly, the XML `encoding =` declaration is either incorrect or missing. Without a declaration, XML defaults to the use utf-8 character encoding, which is not compatible with the default text file encoding on most systems. The XML declaration should look something like this: `<?xml version="1.0" encoding="iso-8859-1"?>` Make sure to specify the encoding that is actually used by file. The encoding for "plain" text files depends both on the operating system and the locale (country and language) in use. Another common source of problems is characters that are not allowed in XML documents, according to the XML spec. Typical disallowed characters are control characters, even if you escape them using the Character Reference form. See the XML specification, sections 2.2 and 4.1 for details. If the parser is generating an `Invalid character (Unicode: 0x???)` error, it is very likely that there's a character in there that you can't see. You can generally use a UNIX command like "od -hc" to find it.
| What encodings are supported by Xerces-C++? |
| Xerces-C++ has intrinsic support for 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. This means that it can always parse input XML files in these above mentioned encodings. Furthermore, if you build Xerces-C++ with the International Components for Unicode (ICU) as a transcoder then the list of supported encodings extends to over 100 different encodings that are supported by ICU. In particular, all the encodings registered with the Internet Assigned Numbers Authority (IANA) are supported in this configuration.
| What character encoding should I use when creating XML documents? |
| The best choice in most cases is either utf-8 or utf-16. Advantages of these encodings include: The best portability. These encodings are more widely supported by XML processors than any others, meaning that your documents will have the best possible chance of being read correctly, no matter where they end up. Full international character support. Both utf-8 and utf-16 cover the full Unicode character set, which includes all of the characters from all major national, international and industry character sets. Efficient. utf-8 has the smaller storage requirements for documents that are primarily composed of characters from the Latin alphabet. utf-16 is more efficient for encoding Asian languages. But both encodings cover all languages without loss. The only drawback of utf-8 or utf-16 is that they are not the native text file format for most systems, meaning that some text file editors and viewers can not be directly used. A second choice of encoding would be any of the others listed in the table above. This works best when the xml encoding is the same as the default system encoding on the machine where the XML document is being prepared, because the document will then display correctly as a plain text file. For UNIX systems in countries speaking Western European languages, the encoding will usually be iso-8859-1. A word of caution for Windows users: The default character set on Windows systems is windows-1252, not iso-8859-1. While Xerces-C++ does recognize this Windows encoding, it is a poor choice for portable XML data because it is not widely recognized by other XML processing tools. If you are using a Windows-based editing tool to generate XML, check which character set it generates, and make sure that the resulting XML specifies the correct name in the `encoding="..."` declaration.
| Why does deleting a transcoded string result in assertion on windows? |
| Both your application program and the Xerces-C++ DLL must use the same DLL version of the runtime library. If either statically links to the runtime library, this problem will still occur. For a Visual Studio build the runtime library setting MUST be "Multithreaded DLL" for release builds and "Debug Multithreaded DLL" for debug builds. To bypass such problem, instead of calling operator delete[] directly, you can use the provided function XMLString::release to delete any string that was allocated by the parser. This will ensure the string is allocated and deleted by the same DLL and such assertion problem should be resolved.
| How do I transcode to/from something besides the local code page? |
| XMLString::transcode() will transcode from XMLCh to the local code page, and other APIs which take a char* assume that the source text is in the local code page. If this is not true, you must transcode the text yourself. You can do this using local transcoding support on your OS, such as Iconv on Unix or IBM's ICU package. However, if your transcoding needs are simple, you can achieve better portability by using the Xerces-C++ parser's transcoder wrappers. You get a transcoder like this: Call XMLPlatformUtils::fgTransServer->MakeNewTranscoderFor() and provide the name of the encoding you wish to create a transcoder for. This will return a transcoder to you, which you own and must delete when you are through with it. NOTE: You must provide a maximum block size that you will pass to the transcoder at one time, and you must pass blocks of characters of this count or smaller when you do your transcoding. The reason for this is that this is really an internal API and is used by the parser itself to do transcoding. The parser always does transcoding in known block sizes, and this allows transcoders to be much more efficient for internal use since it knows the max size it will ever have to deal with and can set itself up for that internally. In general, you should stick to block sizes in the 4 to 64K range. The returned transcoder is something derived from XMLTranscoder, so they are all returned to you via that interface. This object is really just a wrapper around the underlying transcoding system actually in use by your version of Xerces-C++, and does whatever is necessary to handle differences between the XMLCh representation and the representation used by that underlying transcoding system. The transcoder object has two primary APIs, transcodeFrom() and transcodeTo(). These transcode between the XMLCh format and the encoding you indicated. These APIs will transcode as much of the source data as will fit into the outgoing buffer you provide. They will tell you how much of the source they ate and how much of the target they filled. You can use this information to continue the process until all source is consumed. char* data is always dealt with in terms of bytes, and XMLCh data is always dealt with in terms of characters. Don't mix up which you are dealing with or you will not get the correct results, since many encodings don't have a one to one relationship of characters to bytes. When transcoding from XMLCh to the target encoding, the transcodeTo() method provides an 'unrepresentable flag' parameter, which tells the transcoder how to deal with an XMLCh code point that cannot be converted legally to the target encoding, which can easily happen since XMLCh is Unicode and can represent thousands of code points. The options are to use a default replacement character (which the underlying transcoding service will choose, and which is guaranteed to be legal for the target encoding), or to throw an exception. Here is an example:
    // Create an XMLTranscoder that is able to transcode between
    // Unicode and UTF-8.
    //
    XMLTranscoder* t = XMLPlatformUtils::fgTransService->makeNewTranscoderFor(
      "UTF-8", failReason, 16*1024);

    // Source string is in Unicode, want to transcode to UTF-8
    t->transcodeTo(source_unicode,
                      length,
                      result_utf8,
                      length,
                      charsEaten,
                      XMLTranscoder::UnRep_Throw);

    // Source string in UTF-8, want to transcode to Unicode.
    t->transcodeFrom(source_utf8,
                        length,
                        result_unicode,
                        length,
                        bytesEaten,
                        (unsigned char*)charSz);
An even simpler way to transcode to a different encoding is to use the TranscodeToStr and TranscodeFromStr wrapper classes which represent a one-time transcoding and encapsulate all the memory management. Refer to the API Reference for more information.
| Why does the parser still try to locate the DTD even validation is turned off and how to ignore external DTD reference? |
| When DTD is referenced, the parser will try to read it, because DTDs can provide a lot more information than just validation. It defines entities and notations, external unparsed entities, default attributes, character entities, etc. Therefore the parser will always try to read it if present, even if validation is turned off. To ignore external DTDs completely you can call `setLoadExternalDTD(false)` (or `setFeature(XMLUni::fgXercesLoadExternalDTD, false)` to disable the loading of external DTD. The parser will then ignore any external DTD completely if the validationScheme is set to Val_Never. Note: This flag is ignored if the validationScheme is set to Val_Always or Val_Auto.
| Why does the XML data generated by the DOMLSSerializer does not match my original XML input? |
| If you parse an xml document using XercesDOMParser or DOMLSParser and pass such DOMNode to DOMLSSerializer for serialization, you may not get something that is exactly the same as the original XML data. The parser may have done normalization, end of line conversion, or has expanded the entity reference as per the XML 1.0 specification, 4.4 XML Processor Treatment of Entities and References. From DOMLSSerializer perspective, it does not know what the original string was, all it sees is a processed DOMNode generated by the parser. But since the DOMLSSerializer is supposed to generate something that is parsable if sent back to the parser, it will not print the DOMNode node value as is. The DOMLSSerializer may do some "touch up" to the output data for it to be parsable. See How does DOMLSSerializer handle built-in entity Reference in node value? to understand further how DOMLSSerializer touches up the entity reference.
| Why does my application crash when deleting the parser after releasing a document? |
| In most cases, the parser handles deleting documents when the parser gets deleted. However, if an application needs to release a document, it shall adopt the document before releasing it, so that the parser knows that the ownership of this particular document is transfered to the application and will not try to delete it once the parser gets deleted.
    XercesDOMParser *parser = new XercesDOMParser;
    ...
    try
    {
        parser->parse(xml_file);
    }
    catch ()
    {
    ...
    }
    DOMNode *doc = parser->getDocument();
    ...
    parser->adoptDocument();
    doc->release();
    ...
    delete parser;
The alternative to release document is to call parser's resetDocumentPool(), which releases all the documents parsed.

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