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<?xml version="1.0" encoding = "iso-8859-1" standalone="no"?>
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<!DOCTYPE faqs SYSTEM "sbk:/style/dtd/faqs.dtd">
<faqs title="Programming &XercesCName;">
<faq title="Does &XercesCName; support XML Schema?">
<q> Does &XercesCName; support Schema?</q>
<a>
<p>Yes, &XercesCName; &XercesC3Version; contains an implementation
of the W3C XML Schema Language, a recommendation of the Worldwide Web Consortium
available in three parts:
<jump href="http://www.w3.org/TR/xmlschema-0/">XML Schema: Primer</jump> and
<jump href="http://www.w3.org/TR/xmlschema-1/">XML Schema: Structures</jump> and
<jump href="http://www.w3.org/TR/xmlschema-2/">XML Schema: Datatypes</jump>.
We consider this implementation complete. See the
<jump href="schema-&XercesC3Series;.html#limitation">XML Schema Support</jump> page for limitations.</p>
</a>
</faq>
<faq title="Does &XercesCName; support XPath?">
<q> Does &XercesCName; support XPath?</q>
<a>
<p>&XercesCName; &XercesC3Version; 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
<jump href="http://xqilla.sourceforge.net">XQilla</jump> and
<jump href="http://xml.apache.org/xalan-c/overview.html">Apache Xalan C++</jump>
open source projects.
</p>
</a>
</faq>
<faq title="Why does my application crash when instantiating the parser?">
<q>Why does my application crash when instantiating the parser?</q>
<a>
<p>In order to work with the &XercesCName; parser, you have to first
initialize the XML subsystem. The most common mistake is to forget this
initialization. Before you make any calls to &XercesCName; APIs, you must
call XMLPlatformUtils::Initialize(): </p>
<source>
try {
XMLPlatformUtils::Initialize();
}
catch (const XMLException&amp; toCatch) {
// Do your failure processing here
}</source>
<p>This initializes the &XercesCProjectName; system and sets its internal
variables. Note that you must include the <code>xercesc/util/PlatformUtils.hpp</code> file for this to work.</p>
</a>
</faq>
<faq title="Is it OK to call the XMLPlatformUtils::Initialize/Terminate pair of routines multiple times in one program?">
<q>Is it OK to call the XMLPlatformUtils::Initialize/Terminate pair of routines multiple times in one program?</q>
<a>
<p>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.</p>
<p>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.
</p>
</a>
</faq>
<faq title="Why does my application crash after calling XMLPlatformUtils::Terminate()?">
<q>Why does my application crash after calling XMLPlatformUtils::Terminate()?</q>
<a>
<p>Please make sure the XMLPlatformUtils::Terminate() is the last &XercesCName; function to be called
in your program. NO explicit nor implicit &XercesCName; destructor (those local data that are
destructed when going out of scope) should be called after XMLPlatformUtils::Terminate().
</p>
<p>
For example consider the following code snippet which is incorrect:
</p>
<source>
1: {
2: XMLPlatformUtils::Initialize();
3: XercesDOMParser parser;
4: XMLPlatformUtils::Terminate();
5: }
</source>
<p>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:
</p>
<source>
1: {
2: XMLPlatformUtils::Initialize();
2a: {
3: XercesDOMParser parser;
3a: }
4: XMLPlatformUtils::Terminate();
5: }
</source>
<p>The extra pair of braces (line 2a and 3a) ensures that all implicit destructors are called
before terminating &XercesCName;.</p>
<p>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.</p>
</a>
</faq>
<faq title="Is &XercesCName; thread-safe?">
<q>Is &XercesCName; thread-safe?</q>
<a>
<p>The answer is yes if you observe the following rules for using
&XercesCName; in a multi-threaded environment:</p>
<p>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.</p>
<p>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.</p>
<p>The same rules apply to &XercesCName; 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.</p>
<p>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.</p>
</a>
</faq>
<faq title="I am seeing memory leaks in &XercesCName;. Are they real?">
<q>I am seeing memory leaks in &XercesCName;. Are they real?</q>
<a>
<p>The &XercesCName; 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 <code>XMLPlatformUtils::Terminate()</code> before your application exits. This will free all memory that was being
held by the library.</p>
<p>For most applications, the use of <code>Terminate()</code> is optional. The system will recover all memory when the application
process shuts down. The exception to this is the use of &XercesCName; from DLLs
that will be repeatedly loaded and unloaded from within the same process. To
avoid memory leaks with this kind of use, <code>Terminate()</code> must be called before unloading the &XercesCName; library</p>
<p>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.
</p>
<p>If you have built &XercesCName; 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.
</p>
</a>
</faq>
<faq title="Can &XercesCName; create an XML skeleton based on a DTD">
<q>Is there a function that creates an XML file from a DTD (obviously
with the values missing, a skeleton)?</q>
<a>
<p>No, there is no such functionality.</p>
</a>
</faq>
<faq title="Can I use &XercesCName; to perform write validation">
<q>Can I use &XercesCName; to perform "write validation"? That is, having an
appropriate Grammar and being able to add elements to the DOM whilst validating
against the grammar?</q>
<a>
<p>No, there is no such functionality.</p>
<p>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.</p>
</a>
</faq>
<faq title="Can I validate the data contained in a DOM tree?">
<q>Is there a facility in &XercesCName; to validate the data contained in a
DOM tree? That is, without saving and re-parsing the source document?</q>
<a>
<p>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.</p>
</a>
</faq>
<faq title="How to write out a DOM tree into a string or an XML file?">
<q>How to write out a DOM tree into a string or an XML file?</q>
<a>
<p>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.</p>
</a>
</faq>
<faq title="Why doesn't DOMNode::cloneNode() clone the pointer assigned to a DOMNode via DOMNode::setUserData()?">
<q>Why doesn't DOMNode::cloneNode() clone the pointer assigned to a DOMNode via DOMNode::setUserData()?</q>
<a>
<p>&XercesCName; supports the DOMNode::userData specified
in <jump href="http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407/core.html#ID-3A0ED0A4">
the DOM level 3 Node interface</jump>. As
is made clear in the description of the behavior of
<code>cloneNode()</code>, 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 <code>importNode()</code>
is specified similarly.
</p>
</a>
</faq>
<faq title="How are entity reference nodes handled in DOM?">
<q>How are entity reference nodes handled in DOM?</q>
<a>
<p>If you are using the native DOM classes, the function <code>setCreateEntityReferenceNodes</code>
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.</p>
<p>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.</p>
</a>
</faq>
<faq title="Can I use &XercesCName; to parse HTML?">
<q>Can I use &XercesCName; to parse HTML?</q>
<a>
<p>Yes, but only if the HTML follows the rules given in the
<jump href="http://www.w3.org/TR/REC-xml">XML specification</jump>. Most HTML,
however, does not follow the XML rules, and will generate XML well-formedness
errors.</p>
</a>
</faq>
<faq title="I keep getting an error: &quot;invalid UTF-8 character&quot;. What's wrong?">
<q>I keep getting an error: "invalid UTF-8 character". What's wrong?</q>
<a>
<p>Most commonly, the XML <code>encoding =</code> 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.</p>
<p>The XML declaration should look something like this:</p>
<p><code>&lt;?xml version="1.0" encoding="iso-8859-1"?&gt;</code></p>
<p>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.</p>
<p>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 <code>Invalid character (Unicode: 0x???)</code> 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.</p>
</a>
</faq>
<faq title="What encodings are supported by &XercesCName;?">
<q>What encodings are supported by &XercesCName;?</q>
<a>
<p>&XercesCName; 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.</p>
<p>Furthermore, if you build &XercesCName; 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.</p>
</a>
</faq>
<faq title="What character encoding should I use when creating XML documents?">
<q>What character encoding should I use when creating XML documents?</q>
<a>
<p>The best choice in most cases is either utf-8 or utf-16. Advantages of
these encodings include:</p>
<ul>
<li>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.</li>
<li>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.</li>
<li>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.</li>
</ul>
<p>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.</p>
<p>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.</p>
<p>A word of caution for Windows users: The default character set on
Windows systems is windows-1252, not iso-8859-1. While &XercesCName; 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 <code>encoding="..."</code> declaration.</p>
</a>
</faq>
<faq title="Why does deleting a transcoded string result in assertion on windows?">
<q>Why does deleting a transcoded string result in assertion on windows?</q>
<a>
<p>Both your application program and the &XercesCName; DLL must use the same DLL version of the
runtime library. If either statically links to the runtime library, this
problem will still occur.</p>
<p>For a Visual Studio build the runtime library setting MUST
be "Multithreaded DLL" for release builds and "Debug Multithreaded DLL" for
debug builds.</p>
<p>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.</p>
</a>
</faq>
<faq title="How do I transcode to/from something besides the local code page?">
<q>How do I transcode to/from something besides the local code page?</q>
<a>
<p>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 &XercesCName; parser's
transcoder wrappers. You get a transcoder like this:
</p>
<ul>
<li>
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.
</li>
<li>
The returned transcoder is something derived from XMLTranscoder, so they
are all returned to you via that interface.
</li>
<li>
This object is really just a wrapper around the underlying transcoding
system actually in use by your version of &XercesCName;, and does whatever is
necessary to handle differences between the XMLCh representation and the
representation used by that underlying transcoding system.
</li>
<li>
The transcoder object has two primary APIs, transcodeFrom() and
transcodeTo(). These transcode between the XMLCh format and the encoding you
indicated.
</li>
<li>
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.
</li>
<li>
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.
</li>
<li>
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.
</li>
</ul>
<p>Here is an example:</p>
<source>
// 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-&gt;transcodeTo(source_unicode,
length,
result_utf8,
length,
charsEaten,
XMLTranscoder::UnRep_Throw);
// Source string in UTF-8, want to transcode to Unicode.
t-&gt;transcodeFrom(source_utf8,
length,
result_unicode,
length,
bytesEaten,
(unsigned char*)charSz);
</source>
<p>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.</p>
</a>
</faq>
<faq title="Why does the parser still try to locate the DTD even validation is turned off
and how to ignore external DTD reference?">
<q>Why does the parser still try to locate the DTD even validation is turned off
and how to ignore external DTD reference?</q>
<a>
<p>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.
</p>
<p>To ignore external DTDs completely you can call
<code>setLoadExternalDTD(false)</code> (or
<code>setFeature(XMLUni::fgXercesLoadExternalDTD, false)</code>
to disable the loading of external DTD. The parser will then ignore
any external DTD completely if the validationScheme is set to Val_Never.
</p>
<p>Note: This flag is ignored if the validationScheme is set to Val_Always or Val_Auto.
</p>
</a>
</faq>
<faq title="Why does the XML data generated by the DOMLSSerializer does not match my original XML input?">
<q>Why does the XML data generated by the DOMLSSerializer does not match my original XML input?</q>
<a>
<p>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.</p>
<p>See <jump href="program-dom-&XercesC3Series;.html#DOMLSSerializerEntityRef">How does DOMLSSerializer handle built-in entity
Reference in node value?</jump> to understand further how DOMLSSerializer touches up the entity reference.
</p>
</a>
</faq>
<faq title="Why does my application crash when deleting the parser after releasing a document?">
<q>Why does my application crash when deleting the parser after releasing a document?</q>
<a>
<p>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.
</p>
<source>
XercesDOMParser *parser = new XercesDOMParser;
...
try
{
parser->parse(xml_file);
}
catch ()
{
...
}
DOMNode *doc = parser->getDocument();
...
parser->adoptDocument();
doc->release();
...
delete parser;
</source>
<p>The alternative to release document is to call parser's resetDocumentPool(), which releases
all the documents parsed.
</p>
</a>
</faq>
</faqs>