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<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<!--
__ __ _
___\ \/ /_ __ __ _| |_
/ _ \\ /| '_ \ / _` | __|
| __// \| |_) | (_| | |_
\___/_/\_\ .__/ \__,_|\__|
|_| XML parser
Copyright (c) 2000 Clark Cooper <coopercc@users.sourceforge.net>
Copyright (c) 2000-2004 Fred L. Drake, Jr. <fdrake@users.sourceforge.net>
Copyright (c) 2002-2012 Karl Waclawek <karl@waclawek.net>
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Copyright (c) 2021 Tomas Korbar <tkorbar@redhat.com>
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Copyright (c) 2022 Thijs Schreijer <thijs@thijsschreijer.nl>
Copyright (c) 2023 Hanno Böck <hanno@gentoo.org>
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The above copyright notice and this permission notice shall be included
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
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-->
<title>Expat XML Parser</title>
<meta name="author" content="Clark Cooper, coopercc@netheaven.com" />
<meta http-equiv="Content-Style-Type" content="text/css" />
<link href="ok.min.css" rel="stylesheet" type="text/css" />
<link href="style.css" rel="stylesheet" type="text/css" />
</head>
<body>
<div>
<h1>
The Expat XML Parser
<small>Release 2.7.1</small>
</h1>
</div>
<div class="content">
<p>Expat is a library, written in C, for parsing XML documents. It's
the underlying XML parser for the open source Mozilla project, Perl's
<code>XML::Parser</code>, Python's <code>xml.parsers.expat</code>, and
other open-source XML parsers.</p>
<p>This library is the creation of James Clark, who's also given us
groff (an nroff look-alike), Jade (an implementation of ISO's DSSSL
stylesheet language for SGML), XP (a Java XML parser package), XT (a
Java XSL engine). James was also the technical lead on the XML
Working Group at W3C that produced the XML specification.</p>
<p>This is free software, licensed under the <a
href="../COPYING">MIT/X Consortium license</a>. You may download it
from <a href="https://libexpat.github.io/">the Expat home page</a>.
</p>
<p>The bulk of this document was originally commissioned as an article
by <a href="https://www.xml.com/">XML.com</a>. They graciously allowed
Clark Cooper to retain copyright and to distribute it with Expat.
This version has been substantially extended to include documentation
on features which have been added since the original article was
published, and additional information on using the original
interface.</p>
<hr />
<h2>Table of Contents</h2>
<ul>
<li><a href="#overview">Overview</a></li>
<li><a href="#building">Building and Installing</a></li>
<li><a href="#using">Using Expat</a></li>
<li><a href="#reference">Reference</a>
<ul>
<li><a href="#creation">Parser Creation Functions</a>
<ul>
<li><a href="#XML_ParserCreate">XML_ParserCreate</a></li>
<li><a href="#XML_ParserCreateNS">XML_ParserCreateNS</a></li>
<li><a href="#XML_ParserCreate_MM">XML_ParserCreate_MM</a></li>
<li><a href="#XML_ExternalEntityParserCreate">XML_ExternalEntityParserCreate</a></li>
<li><a href="#XML_ParserFree">XML_ParserFree</a></li>
<li><a href="#XML_ParserReset">XML_ParserReset</a></li>
</ul>
</li>
<li><a href="#parsing">Parsing Functions</a>
<ul>
<li><a href="#XML_Parse">XML_Parse</a></li>
<li><a href="#XML_ParseBuffer">XML_ParseBuffer</a></li>
<li><a href="#XML_GetBuffer">XML_GetBuffer</a></li>
<li><a href="#XML_StopParser">XML_StopParser</a></li>
<li><a href="#XML_ResumeParser">XML_ResumeParser</a></li>
<li><a href="#XML_GetParsingStatus">XML_GetParsingStatus</a></li>
</ul>
</li>
<li><a href="#setting">Handler Setting Functions</a>
<ul>
<li><a href="#XML_SetStartElementHandler">XML_SetStartElementHandler</a></li>
<li><a href="#XML_SetEndElementHandler">XML_SetEndElementHandler</a></li>
<li><a href="#XML_SetElementHandler">XML_SetElementHandler</a></li>
<li><a href="#XML_SetCharacterDataHandler">XML_SetCharacterDataHandler</a></li>
<li><a href="#XML_SetProcessingInstructionHandler">XML_SetProcessingInstructionHandler</a></li>
<li><a href="#XML_SetCommentHandler">XML_SetCommentHandler</a></li>
<li><a href="#XML_SetStartCdataSectionHandler">XML_SetStartCdataSectionHandler</a></li>
<li><a href="#XML_SetEndCdataSectionHandler">XML_SetEndCdataSectionHandler</a></li>
<li><a href="#XML_SetCdataSectionHandler">XML_SetCdataSectionHandler</a></li>
<li><a href="#XML_SetDefaultHandler">XML_SetDefaultHandler</a></li>
<li><a href="#XML_SetDefaultHandlerExpand">XML_SetDefaultHandlerExpand</a></li>
<li><a href="#XML_SetExternalEntityRefHandler">XML_SetExternalEntityRefHandler</a></li>
<li><a href="#XML_SetExternalEntityRefHandlerArg">XML_SetExternalEntityRefHandlerArg</a></li>
<li><a href="#XML_SetSkippedEntityHandler">XML_SetSkippedEntityHandler</a></li>
<li><a href="#XML_SetUnknownEncodingHandler">XML_SetUnknownEncodingHandler</a></li>
<li><a href="#XML_SetStartNamespaceDeclHandler">XML_SetStartNamespaceDeclHandler</a></li>
<li><a href="#XML_SetEndNamespaceDeclHandler">XML_SetEndNamespaceDeclHandler</a></li>
<li><a href="#XML_SetNamespaceDeclHandler">XML_SetNamespaceDeclHandler</a></li>
<li><a href="#XML_SetXmlDeclHandler">XML_SetXmlDeclHandler</a></li>
<li><a href="#XML_SetStartDoctypeDeclHandler">XML_SetStartDoctypeDeclHandler</a></li>
<li><a href="#XML_SetEndDoctypeDeclHandler">XML_SetEndDoctypeDeclHandler</a></li>
<li><a href="#XML_SetDoctypeDeclHandler">XML_SetDoctypeDeclHandler</a></li>
<li><a href="#XML_SetElementDeclHandler">XML_SetElementDeclHandler</a></li>
<li><a href="#XML_SetAttlistDeclHandler">XML_SetAttlistDeclHandler</a></li>
<li><a href="#XML_SetEntityDeclHandler">XML_SetEntityDeclHandler</a></li>
<li><a href="#XML_SetUnparsedEntityDeclHandler">XML_SetUnparsedEntityDeclHandler</a></li>
<li><a href="#XML_SetNotationDeclHandler">XML_SetNotationDeclHandler</a></li>
<li><a href="#XML_SetNotStandaloneHandler">XML_SetNotStandaloneHandler</a></li>
</ul>
</li>
<li><a href="#position">Parse Position and Error Reporting Functions</a>
<ul>
<li><a href="#XML_GetErrorCode">XML_GetErrorCode</a></li>
<li><a href="#XML_ErrorString">XML_ErrorString</a></li>
<li><a href="#XML_GetCurrentByteIndex">XML_GetCurrentByteIndex</a></li>
<li><a href="#XML_GetCurrentLineNumber">XML_GetCurrentLineNumber</a></li>
<li><a href="#XML_GetCurrentColumnNumber">XML_GetCurrentColumnNumber</a></li>
<li><a href="#XML_GetCurrentByteCount">XML_GetCurrentByteCount</a></li>
<li><a href="#XML_GetInputContext">XML_GetInputContext</a></li>
</ul>
</li>
<li>
<a href="#attack-protection">Attack Protection</a>
<ul>
<li><a href="#XML_SetBillionLaughsAttackProtectionMaximumAmplification">XML_SetBillionLaughsAttackProtectionMaximumAmplification</a></li>
<li><a href="#XML_SetBillionLaughsAttackProtectionActivationThreshold">XML_SetBillionLaughsAttackProtectionActivationThreshold</a></li>
<li><a href="#XML_SetReparseDeferralEnabled">XML_SetReparseDeferralEnabled</a></li>
</ul>
</li>
<li><a href="#miscellaneous">Miscellaneous Functions</a>
<ul>
<li><a href="#XML_SetUserData">XML_SetUserData</a></li>
<li><a href="#XML_GetUserData">XML_GetUserData</a></li>
<li><a href="#XML_UseParserAsHandlerArg">XML_UseParserAsHandlerArg</a></li>
<li><a href="#XML_SetBase">XML_SetBase</a></li>
<li><a href="#XML_GetBase">XML_GetBase</a></li>
<li><a href="#XML_GetSpecifiedAttributeCount">XML_GetSpecifiedAttributeCount</a></li>
<li><a href="#XML_GetIdAttributeIndex">XML_GetIdAttributeIndex</a></li>
<li><a href="#XML_GetAttributeInfo">XML_GetAttributeInfo</a></li>
<li><a href="#XML_SetEncoding">XML_SetEncoding</a></li>
<li><a href="#XML_SetParamEntityParsing">XML_SetParamEntityParsing</a></li>
<li><a href="#XML_SetHashSalt">XML_SetHashSalt</a></li>
<li><a href="#XML_UseForeignDTD">XML_UseForeignDTD</a></li>
<li><a href="#XML_SetReturnNSTriplet">XML_SetReturnNSTriplet</a></li>
<li><a href="#XML_DefaultCurrent">XML_DefaultCurrent</a></li>
<li><a href="#XML_ExpatVersion">XML_ExpatVersion</a></li>
<li><a href="#XML_ExpatVersionInfo">XML_ExpatVersionInfo</a></li>
<li><a href="#XML_GetFeatureList">XML_GetFeatureList</a></li>
<li><a href="#XML_FreeContentModel">XML_FreeContentModel</a></li>
<li><a href="#XML_MemMalloc">XML_MemMalloc</a></li>
<li><a href="#XML_MemRealloc">XML_MemRealloc</a></li>
<li><a href="#XML_MemFree">XML_MemFree</a></li>
</ul>
</li>
</ul>
</li>
</ul>
<hr />
<h2><a name="overview">Overview</a></h2>
<p>Expat is a stream-oriented parser. You register callback (or
handler) functions with the parser and then start feeding it the
document. As the parser recognizes parts of the document, it will
call the appropriate handler for that part (if you've registered one.)
The document is fed to the parser in pieces, so you can start parsing
before you have all the document. This also allows you to parse really
huge documents that won't fit into memory.</p>
<p>Expat can be intimidating due to the many kinds of handlers and
options you can set. But you only need to learn four functions in
order to do 90% of what you'll want to do with it:</p>
<dl>
<dt><code><a href= "#XML_ParserCreate"
>XML_ParserCreate</a></code></dt>
<dd>Create a new parser object.</dd>
<dt><code><a href= "#XML_SetElementHandler"
>XML_SetElementHandler</a></code></dt>
<dd>Set handlers for start and end tags.</dd>
<dt><code><a href= "#XML_SetCharacterDataHandler"
>XML_SetCharacterDataHandler</a></code></dt>
<dd>Set handler for text.</dd>
<dt><code><a href= "#XML_Parse"
>XML_Parse</a></code></dt>
<dd>Pass a buffer full of document to the parser</dd>
</dl>
<p>These functions and others are described in the <a
href="#reference">reference</a> part of this document. The reference
section also describes in detail the parameters passed to the
different types of handlers.</p>
<p>Let's look at a very simple example program that only uses 3 of the
above functions (it doesn't need to set a character handler.) The
program <a href="../examples/outline.c">outline.c</a> prints an
element outline, indenting child elements to distinguish them from the
parent element that contains them. The start handler does all the
work. It prints two indenting spaces for every level of ancestor
elements, then it prints the element and attribute
information. Finally it increments the global <code>Depth</code>
variable.</p>
<pre class="eg">
int Depth;
void XMLCALL
start(void *data, const char *el, const char **attr) {
int i;
for (i = 0; i < Depth; i++)
printf(" ");
printf("%s", el);
for (i = 0; attr[i]; i += 2) {
printf(" %s='%s'", attr[i], attr[i + 1]);
}
printf("\n");
Depth++;
} /* End of start handler */
</pre>
<p>The end tag simply does the bookkeeping work of decrementing
<code>Depth</code>.</p>
<pre class="eg">
void XMLCALL
end(void *data, const char *el) {
Depth--;
} /* End of end handler */
</pre>
<p>Note the <code>XMLCALL</code> annotation used for the callbacks.
This is used to ensure that the Expat and the callbacks are using the
same calling convention in case the compiler options used for Expat
itself and the client code are different. Expat tries not to care
what the default calling convention is, though it may require that it
be compiled with a default convention of "cdecl" on some platforms.
For code which uses Expat, however, the calling convention is
specified by the <code>XMLCALL</code> annotation on most platforms;
callbacks should be defined using this annotation.</p>
<p>The <code>XMLCALL</code> annotation was added in Expat 1.95.7, but
existing working Expat applications don't need to add it (since they
are already using the "cdecl" calling convention, or they wouldn't be
working). The annotation is only needed if the default calling
convention may be something other than "cdecl". To use the annotation
safely with older versions of Expat, you can conditionally define it
<em>after</em> including Expat's header file:</p>
<pre class="eg">
#include <expat.h>
#ifndef XMLCALL
#if defined(_MSC_EXTENSIONS) && !defined(__BEOS__) && !defined(__CYGWIN__)
#define XMLCALL __cdecl
#elif defined(__GNUC__)
#define XMLCALL __attribute__((cdecl))
#else
#define XMLCALL
#endif
#endif
</pre>
<p>After creating the parser, the main program just has the job of
shoveling the document to the parser so that it can do its work.</p>
<hr />
<h2><a name="building">Building and Installing Expat</a></h2>
<p>The Expat distribution comes as a compressed (with GNU gzip) tar
file. You may download the latest version from <a href=
"https://sourceforge.net/projects/expat/" >Source Forge</a>. After
unpacking this, cd into the directory. Then follow either the Win32
directions or Unix directions below.</p>
<h3>Building under Win32</h3>
<p>If you're using the GNU compiler under cygwin, follow the Unix
directions in the next section. Otherwise if you have Microsoft's
Developer Studio installed,
you can use CMake to generate a <code>.sln</code> file, e.g.
<code>
cmake -G"Visual Studio 16 2019" -DCMAKE_BUILD_TYPE=RelWithDebInfo .
</code>, and build Expat using <code>msbuild /m expat.sln</code> after.</p>
<p>Alternatively, you may download the Win32 binary package that
contains the "expat.h" include file and a pre-built DLL.</p>
<h3>Building under Unix (or GNU)</h3>
<p>First you'll need to run the configure shell script in order to
configure the Makefiles and headers for your system.</p>
<p>If you're happy with all the defaults that configure picks for you,
and you have permission on your system to install into /usr/local, you
can install Expat with this sequence of commands:</p>
<pre class="eg">
./configure
make
make install
</pre>
<p>There are some options that you can provide to this script, but the
only one we'll mention here is the <code>--prefix</code> option. You
can find out all the options available by running configure with just
the <code>--help</code> option.</p>
<p>By default, the configure script sets things up so that the library
gets installed in <code>/usr/local/lib</code> and the associated
header file in <code>/usr/local/include</code>. But if you were to
give the option, <code>--prefix=/home/me/mystuff</code>, then the
library and header would get installed in
<code>/home/me/mystuff/lib</code> and
<code>/home/me/mystuff/include</code> respectively.</p>
<h3>Configuring Expat Using the Pre-Processor</h3>
<p>Expat's feature set can be configured using a small number of
pre-processor definitions. The symbols are:</p>
<dl class="cpp-symbols">
<dt><a name="XML_GE">XML_GE</a></dt>
<dd>
Added in Expat 2.6.0.
Include support for
<a href="https://www.w3.org/TR/2006/REC-xml-20060816/#sec-physical-struct">general entities</a>
(syntax <code>&e1;</code> to reference and
syntax <code><!ENTITY e1 'value1'></code> (an internal general entity) or
<code><!ENTITY e2 SYSTEM 'file2'></code> (an external general entity) to declare).
With <code>XML_GE</code> enabled, general entities will be replaced by their declared replacement text;
for this to work for <em>external</em> general entities, in addition an
<code><a href="#XML_SetExternalEntityRefHandler">XML_ExternalEntityRefHandler</a></code> must be set using
<code><a href="#XML_SetExternalEntityRefHandler">XML_SetExternalEntityRefHandler</a></code>.
Also, enabling <code>XML_GE</code> makes
the functions <code><a href="#XML_SetBillionLaughsAttackProtectionMaximumAmplification">
XML_SetBillionLaughsAttackProtectionMaximumAmplification</a></code> and <code>
<a href="#XML_SetBillionLaughsAttackProtectionActivationThreshold">
XML_SetBillionLaughsAttackProtectionActivationThreshold</a></code> available.
<br/>
With <code>XML_GE</code> disabled, Expat has a smaller memory footprint and can be faster, but will
not load external general entities and will replace all general entities
(except the <a href="https://www.w3.org/TR/2006/REC-xml-20060816/#sec-predefined-ent">predefined five</a>:
<code>amp</code>, <code>apos</code>, <code>gt</code>, <code>lt</code>, <code>quot</code>)
with a self-reference:
for example, referencing an entity <code>e1</code> via <code>&e1;</code> will be replaced
by text <code>&e1;</code>.
</dd>
<dt><a name="XML_DTD">XML_DTD</a></dt>
<dd>Include support for using and reporting DTD-based content. If
this is defined, default attribute values from an external DTD subset
are reported and attribute value normalization occurs based on the
type of attributes defined in the external subset. Without
this, Expat has a smaller memory footprint and can be faster, but will
not load external parameter entities or process conditional sections. If defined, makes
the functions <code><a
href="#XML_SetBillionLaughsAttackProtectionMaximumAmplification">
XML_SetBillionLaughsAttackProtectionMaximumAmplification</a></code> and <code>
<a href="#XML_SetBillionLaughsAttackProtectionActivationThreshold">
XML_SetBillionLaughsAttackProtectionActivationThreshold</a></code> available.</dd>
<dt><a name="XML_NS">XML_NS</a></dt>
<dd>When defined, support for the <cite><a href=
"https://www.w3.org/TR/REC-xml-names/" >Namespaces in XML</a></cite>
specification is included.</dd>
<dt><a name="XML_UNICODE">XML_UNICODE</a></dt>
<dd>When defined, character data reported to the application is
encoded in UTF-16 using wide characters of the type
<code>XML_Char</code>. This is implied if
<code>XML_UNICODE_WCHAR_T</code> is defined.</dd>
<dt><a name="XML_UNICODE_WCHAR_T">XML_UNICODE_WCHAR_T</a></dt>
<dd>If defined, causes the <code>XML_Char</code> character type to be
defined using the <code>wchar_t</code> type; otherwise, <code>unsigned
short</code> is used. Defining this implies
<code>XML_UNICODE</code>.</dd>
<dt><a name="XML_LARGE_SIZE">XML_LARGE_SIZE</a></dt>
<dd>If defined, causes the <code>XML_Size</code> and <code>XML_Index</code>
integer types to be at least 64 bits in size. This is intended to support
processing of very large input streams, where the return values of
<code><a href="#XML_GetCurrentByteIndex" >XML_GetCurrentByteIndex</a></code>,
<code><a href="#XML_GetCurrentLineNumber" >XML_GetCurrentLineNumber</a></code> and
<code><a href="#XML_GetCurrentColumnNumber" >XML_GetCurrentColumnNumber</a></code>
could overflow. It may not be supported by all compilers, and is turned
off by default.</dd>
<dt><a name="XML_CONTEXT_BYTES">XML_CONTEXT_BYTES</a></dt>
<dd>The number of input bytes of markup context which the parser will
ensure are available for reporting via <code><a href=
"#XML_GetInputContext" >XML_GetInputContext</a></code>. This is
normally set to 1024, and must be set to a positive integer to enable.
If this is set to zero, the input context will not be available and <code><a
href= "#XML_GetInputContext" >XML_GetInputContext</a></code> will
always report <code>NULL</code>. Without this, Expat has a smaller memory
footprint and can be faster.</dd>
<dt><a name="XML_STATIC">XML_STATIC</a></dt>
<dd>On Windows, this should be set if Expat is going to be linked
statically with the code that calls it; this is required to get all
the right MSVC magic annotations correct. This is ignored on other
platforms.</dd>
<dt><a name="XML_ATTR_INFO">XML_ATTR_INFO</a></dt>
<dd>If defined, makes the additional function <code><a href=
"#XML_GetAttributeInfo" >XML_GetAttributeInfo</a></code> available
for reporting attribute byte offsets.</dd>
</dl>
<hr />
<h2><a name="using">Using Expat</a></h2>
<h3>Compiling and Linking Against Expat</h3>
<p>Unless you installed Expat in a location not expected by your
compiler and linker, all you have to do to use Expat in your programs
is to include the Expat header (<code>#include <expat.h></code>)
in your files that make calls to it and to tell the linker that it
needs to link against the Expat library. On Unix systems, this would
usually be done with the <code>-lexpat</code> argument. Otherwise,
you'll need to tell the compiler where to look for the Expat header
and the linker where to find the Expat library. You may also need to
take steps to tell the operating system where to find this library at
run time.</p>
<p>On a Unix-based system, here's what a Makefile might look like when
Expat is installed in a standard location:</p>
<pre class="eg">
CC=cc
LDFLAGS=
LIBS= -lexpat
xmlapp: xmlapp.o
$(CC) $(LDFLAGS) -o xmlapp xmlapp.o $(LIBS)
</pre>
<p>If you installed Expat in, say, <code>/home/me/mystuff</code>, then
the Makefile would look like this:</p>
<pre class="eg">
CC=cc
CFLAGS= -I/home/me/mystuff/include
LDFLAGS=
LIBS= -L/home/me/mystuff/lib -lexpat
xmlapp: xmlapp.o
$(CC) $(LDFLAGS) -o xmlapp xmlapp.o $(LIBS)
</pre>
<p>You'd also have to set the environment variable
<code>LD_LIBRARY_PATH</code> to <code>/home/me/mystuff/lib</code> (or
to <code>${LD_LIBRARY_PATH}:/home/me/mystuff/lib</code> if
LD_LIBRARY_PATH already has some directories in it) in order to run
your application.</p>
<h3>Expat Basics</h3>
<p>As we saw in the example in the overview, the first step in parsing
an XML document with Expat is to create a parser object. There are <a
href="#creation">three functions</a> in the Expat API for creating a
parser object. However, only two of these (<code><a href=
"#XML_ParserCreate" >XML_ParserCreate</a></code> and <code><a href=
"#XML_ParserCreateNS" >XML_ParserCreateNS</a></code>) can be used for
constructing a parser for a top-level document. The object returned
by these functions is an opaque pointer (i.e. "expat.h" declares it as
void *) to data with further internal structure. In order to free the
memory associated with this object you must call <code><a href=
"#XML_ParserFree" >XML_ParserFree</a></code>. Note that if you have
provided any <a href="#userdata">user data</a> that gets stored in the
parser, then your application is responsible for freeing it prior to
calling <code>XML_ParserFree</code>.</p>
<p>The objects returned by the parser creation functions are good for
parsing only one XML document or external parsed entity. If your
application needs to parse many XML documents, then it needs to create
a parser object for each one. The best way to deal with this is to
create a higher level object that contains all the default
initialization you want for your parser objects.</p>
<p>Walking through a document hierarchy with a stream oriented parser
will require a good stack mechanism in order to keep track of current
context. For instance, to answer the simple question, "What element
does this text belong to?" requires a stack, since the parser may have
descended into other elements that are children of the current one and
has encountered this text on the way out.</p>
<p>The things you're likely to want to keep on a stack are the
currently opened element and it's attributes. You push this
information onto the stack in the start handler and you pop it off in
the end handler.</p>
<p>For some tasks, it is sufficient to just keep information on what
the depth of the stack is (or would be if you had one.) The outline
program shown above presents one example. Another such task would be
skipping over a complete element. When you see the start tag for the
element you want to skip, you set a skip flag and record the depth at
which the element started. When the end tag handler encounters the
same depth, the skipped element has ended and the flag may be
cleared. If you follow the convention that the root element starts at
1, then you can use the same variable for skip flag and skip
depth.</p>
<pre class="eg">
void
init_info(Parseinfo *info) {
info->skip = 0;
info->depth = 1;
/* Other initializations here */
} /* End of init_info */
void XMLCALL
rawstart(void *data, const char *el, const char **attr) {
Parseinfo *inf = (Parseinfo *) data;
if (! inf->skip) {
if (should_skip(inf, el, attr)) {
inf->skip = inf->depth;
}
else
start(inf, el, attr); /* This does rest of start handling */
}
inf->depth++;
} /* End of rawstart */
void XMLCALL
rawend(void *data, const char *el) {
Parseinfo *inf = (Parseinfo *) data;
inf->depth--;
if (! inf->skip)
end(inf, el); /* This does rest of end handling */
if (inf->skip == inf->depth)
inf->skip = 0;
} /* End rawend */
</pre>
<p>Notice in the above example the difference in how depth is
manipulated in the start and end handlers. The end tag handler should
be the mirror image of the start tag handler. This is necessary to
properly model containment. Since, in the start tag handler, we
incremented depth <em>after</em> the main body of start tag code, then
in the end handler, we need to manipulate it <em>before</em> the main
body. If we'd decided to increment it first thing in the start
handler, then we'd have had to decrement it last thing in the end
handler.</p>
<h3 id="userdata">Communicating between handlers</h3>
<p>In order to be able to pass information between different handlers
without using globals, you'll need to define a data structure to hold
the shared variables. You can then tell Expat (with the <code><a href=
"#XML_SetUserData" >XML_SetUserData</a></code> function) to pass a
pointer to this structure to the handlers. This is the first
argument received by most handlers. In the <a href="#reference"
>reference section</a>, an argument to a callback function is named
<code>userData</code> and have type <code>void *</code> if the user
data is passed; it will have the type <code>XML_Parser</code> if the
parser itself is passed. When the parser is passed, the user data may
be retrieved using <code><a href="#XML_GetUserData"
>XML_GetUserData</a></code>.</p>
<p>One common case where multiple calls to a single handler may need
to communicate using an application data structure is the case when
content passed to the character data handler (set by <code><a href=
"#XML_SetCharacterDataHandler"
>XML_SetCharacterDataHandler</a></code>) needs to be accumulated. A
common first-time mistake with any of the event-oriented interfaces to
an XML parser is to expect all the text contained in an element to be
reported by a single call to the character data handler. Expat, like
many other XML parsers, reports such data as a sequence of calls;
there's no way to know when the end of the sequence is reached until a
different callback is made. A buffer referenced by the user data
structure proves both an effective and convenient place to accumulate
character data.</p>
<!-- XXX example needed here -->
<h3>XML Version</h3>
<p>Expat is an XML 1.0 parser, and as such never complains based on
the value of the <code>version</code> pseudo-attribute in the XML
declaration, if present.</p>
<p>If an application needs to check the version number (to support
alternate processing), it should use the <code><a href=
"#XML_SetXmlDeclHandler" >XML_SetXmlDeclHandler</a></code> function to
set a handler that uses the information in the XML declaration to
determine what to do. This example shows how to check that only a
version number of <code>"1.0"</code> is accepted:</p>
<pre class="eg">
static int wrong_version;
static XML_Parser parser;
static void XMLCALL
xmldecl_handler(void *userData,
const XML_Char *version,
const XML_Char *encoding,
int standalone)
{
static const XML_Char Version_1_0[] = {'1', '.', '0', 0};
int i;
for (i = 0; i < (sizeof(Version_1_0) / sizeof(Version_1_0[0])); ++i) {
if (version[i] != Version_1_0[i]) {
wrong_version = 1;
/* also clear all other handlers: */
XML_SetCharacterDataHandler(parser, NULL);
...
return;
}
}
...
}
</pre>
<h3>Namespace Processing</h3>
<p>When the parser is created using the <code><a href=
"#XML_ParserCreateNS" >XML_ParserCreateNS</a></code>, function, Expat
performs namespace processing. Under namespace processing, Expat
consumes <code>xmlns</code> and <code>xmlns:...</code> attributes,
which declare namespaces for the scope of the element in which they
occur. This means that your start handler will not see these
attributes. Your application can still be informed of these
declarations by setting namespace declaration handlers with <a href=
"#XML_SetNamespaceDeclHandler"
><code>XML_SetNamespaceDeclHandler</code></a>.</p>
<p>Element type and attribute names that belong to a given namespace
are passed to the appropriate handler in expanded form. By default
this expanded form is a concatenation of the namespace URI, the
separator character (which is the 2nd argument to <code><a href=
"#XML_ParserCreateNS" >XML_ParserCreateNS</a></code>), and the local
name (i.e. the part after the colon). Names with undeclared prefixes
are not well-formed when namespace processing is enabled, and will
trigger an error. Unprefixed attribute names are never expanded,
and unprefixed element names are only expanded when they are in the
scope of a default namespace.</p>
<p>However if <code><a href= "#XML_SetReturnNSTriplet"
>XML_SetReturnNSTriplet</a></code> has been called with a non-zero
<code>do_nst</code> parameter, then the expanded form for names with
an explicit prefix is a concatenation of: URI, separator, local name,
separator, prefix.</p>
<p>You can set handlers for the start of a namespace declaration and
for the end of a scope of a declaration with the <code><a href=
"#XML_SetNamespaceDeclHandler" >XML_SetNamespaceDeclHandler</a></code>
function. The StartNamespaceDeclHandler is called prior to the start
tag handler and the EndNamespaceDeclHandler is called after the
corresponding end tag that ends the namespace's scope. The namespace
start handler gets passed the prefix and URI for the namespace. For a
default namespace declaration (xmlns='...'), the prefix will be
<code>NULL</code>.
The URI will be <code>NULL</code> for the case where the default namespace is being
unset. The namespace end handler just gets the prefix for the closing
scope.</p>
<p>These handlers are called for each declaration. So if, for
instance, a start tag had three namespace declarations, then the
StartNamespaceDeclHandler would be called three times before the start
tag handler is called, once for each declaration.</p>
<h3>Character Encodings</h3>
<p>While XML is based on Unicode, and every XML processor is required
to recognized UTF-8 and UTF-16 (1 and 2 byte encodings of Unicode),
other encodings may be declared in XML documents or entities. For the
main document, an XML declaration may contain an encoding
declaration:</p>
<pre>
<?xml version="1.0" encoding="ISO-8859-2"?>
</pre>
<p>External parsed entities may begin with a text declaration, which
looks like an XML declaration with just an encoding declaration:</p>
<pre>
<?xml encoding="Big5"?>
</pre>
<p>With Expat, you may also specify an encoding at the time of
creating a parser. This is useful when the encoding information may
come from a source outside the document itself (like a higher level
protocol.)</p>
<p><a name="builtin_encodings"></a>There are four built-in encodings
in Expat:</p>
<ul>
<li>UTF-8</li>
<li>UTF-16</li>
<li>ISO-8859-1</li>
<li>US-ASCII</li>
</ul>
<p>Anything else discovered in an encoding declaration or in the
protocol encoding specified in the parser constructor, triggers a call
to the <code>UnknownEncodingHandler</code>. This handler gets passed
the encoding name and a pointer to an <code>XML_Encoding</code> data
structure. Your handler must fill in this structure and return
<code>XML_STATUS_OK</code> if it knows how to deal with the
encoding. Otherwise the handler should return
<code>XML_STATUS_ERROR</code>. The handler also gets passed a pointer
to an optional application data structure that you may indicate when
you set the handler.</p>
<p>Expat places restrictions on character encodings that it can
support by filling in the <code>XML_Encoding</code> structure.
include file:</p>
<ol>
<li>Every ASCII character that can appear in a well-formed XML document
must be represented by a single byte, and that byte must correspond to
it's ASCII encoding (except for the characters $@\^'{}~)</li>
<li>Characters must be encoded in 4 bytes or less.</li>
<li>All characters encoded must have Unicode scalar values less than or
equal to 65535 (0xFFFF)<em>This does not apply to the built-in support
for UTF-16 and UTF-8</em></li>
<li>No character may be encoded by more that one distinct sequence of
bytes</li>
</ol>
<p><code>XML_Encoding</code> contains an array of integers that
correspond to the 1st byte of an encoding sequence. If the value in
the array for a byte is zero or positive, then the byte is a single
byte encoding that encodes the Unicode scalar value contained in the
array. A -1 in this array indicates a malformed byte. If the value is
-2, -3, or -4, then the byte is the beginning of a 2, 3, or 4 byte
sequence respectively. Multi-byte sequences are sent to the convert
function pointed at in the <code>XML_Encoding</code> structure. This
function should return the Unicode scalar value for the sequence or -1
if the sequence is malformed.</p>
<p>One pitfall that novice Expat users are likely to fall into is that
although Expat may accept input in various encodings, the strings that
it passes to the handlers are always encoded in UTF-8 or UTF-16
(depending on how Expat was compiled). Your application is responsible
for any translation of these strings into other encodings.</p>
<h3>Handling External Entity References</h3>
<p>Expat does not read or parse external entities directly. Note that
any external DTD is a special case of an external entity. If you've
set no <code>ExternalEntityRefHandler</code>, then external entity
references are silently ignored. Otherwise, it calls your handler with
the information needed to read and parse the external entity.</p>
<p>Your handler isn't actually responsible for parsing the entity, but
it is responsible for creating a subsidiary parser with <code><a href=
"#XML_ExternalEntityParserCreate"
>XML_ExternalEntityParserCreate</a></code> that will do the job. This
returns an instance of <code>XML_Parser</code> that has handlers and
other data structures initialized from the parent parser. You may then
use <code><a href= "#XML_Parse" >XML_Parse</a></code> or <code><a
href= "#XML_ParseBuffer">XML_ParseBuffer</a></code> calls against this
parser. Since external entities my refer to other external entities,
your handler should be prepared to be called recursively.</p>
<h3>Parsing DTDs</h3>
<p>In order to parse parameter entities, before starting the parse,
you must call <code><a href= "#XML_SetParamEntityParsing"
>XML_SetParamEntityParsing</a></code> with one of the following
arguments:</p>
<dl>
<dt><code>XML_PARAM_ENTITY_PARSING_NEVER</code></dt>
<dd>Don't parse parameter entities or the external subset</dd>
<dt><code>XML_PARAM_ENTITY_PARSING_UNLESS_STANDALONE</code></dt>
<dd>Parse parameter entities and the external subset unless
<code>standalone</code> was set to "yes" in the XML declaration.</dd>
<dt><code>XML_PARAM_ENTITY_PARSING_ALWAYS</code></dt>
<dd>Always parse parameter entities and the external subset</dd>
</dl>
<p>In order to read an external DTD, you also have to set an external
entity reference handler as described above.</p>
<h3 id="stop-resume">Temporarily Stopping Parsing</h3>
<p>Expat 1.95.8 introduces a new feature: its now possible to stop
parsing temporarily from within a handler function, even if more data
has already been passed into the parser. Applications for this
include</p>
<ul>
<li>Supporting the <a href= "https://www.w3.org/TR/xinclude/"
>XInclude</a> specification.</li>
<li>Delaying further processing until additional information is
available from some other source.</li>
<li>Adjusting processor load as task priorities shift within an
application.</li>
<li>Stopping parsing completely (simply free or reset the parser
instead of resuming in the outer parsing loop). This can be useful
if an application-domain error is found in the XML being parsed or if
the result of the parse is determined not to be useful after
all.</li>
</ul>
<p>To take advantage of this feature, the main parsing loop of an
application needs to support this specifically. It cannot be
supported with a parsing loop compatible with Expat 1.95.7 or
earlier (though existing loops will continue to work without
supporting the stop/resume feature).</p>
<p>An application that uses this feature for a single parser will have
the rough structure (in pseudo-code):</p>
<pre class="pseudocode">
fd = open_input()
p = create_parser()
if parse_xml(p, fd) {
/* suspended */
int suspended = 1;
while (suspended) {
do_something_else()
if ready_to_resume() {
suspended = continue_parsing(p, fd);
}
}
}
</pre>
<p>An application that may resume any of several parsers based on
input (either from the XML being parsed or some other source) will
certainly have more interesting control structures.</p>
<p>This C function could be used for the <code>parse_xml</code>
function mentioned in the pseudo-code above:</p>
<pre class="eg">
#define BUFF_SIZE 10240
/* Parse a document from the open file descriptor 'fd' until the parse
is complete (the document has been completely parsed, or there's
been an error), or the parse is stopped. Return non-zero when
the parse is merely suspended.
*/
int
parse_xml(XML_Parser p, int fd)
{
for (;;) {
int last_chunk;
int bytes_read;
enum XML_Status status;
void *buff = XML_GetBuffer(p, BUFF_SIZE);
if (buff == NULL) {
/* handle error... */
return 0;
}
bytes_read = read(fd, buff, BUFF_SIZE);
if (bytes_read < 0) {
/* handle error... */
return 0;
}
status = XML_ParseBuffer(p, bytes_read, bytes_read == 0);
switch (status) {
case XML_STATUS_ERROR:
/* handle error... */
return 0;
case XML_STATUS_SUSPENDED:
return 1;
}
if (bytes_read == 0)
return 0;
}
}
</pre>
<p>The corresponding <code>continue_parsing</code> function is
somewhat simpler, since it only need deal with the return code from
<code><a href= "#XML_ResumeParser">XML_ResumeParser</a></code>; it can
delegate the input handling to the <code>parse_xml</code>
function:</p>
<pre class="eg">
/* Continue parsing a document which had been suspended. The 'p' and
'fd' arguments are the same as passed to parse_xml(). Return
non-zero when the parse is suspended.
*/
int
continue_parsing(XML_Parser p, int fd)
{
enum XML_Status status = XML_ResumeParser(p);
switch (status) {
case XML_STATUS_ERROR:
/* handle error... */
return 0;
case XML_ERROR_NOT_SUSPENDED:
/* handle error... */
return 0;.
case XML_STATUS_SUSPENDED:
return 1;
}
return parse_xml(p, fd);
}
</pre>
<p>Now that we've seen what a mess the top-level parsing loop can
become, what have we gained? Very simply, we can now use the <code><a
href= "#XML_StopParser" >XML_StopParser</a></code> function to stop
parsing, without having to go to great lengths to avoid additional
processing that we're expecting to ignore. As a bonus, we get to stop
parsing <em>temporarily</em>, and come back to it when we're
ready.</p>
<p>To stop parsing from a handler function, use the <code><a href=
"#XML_StopParser" >XML_StopParser</a></code> function. This function
takes two arguments; the parser being stopped and a flag indicating
whether the parse can be resumed in the future.</p>
<!-- XXX really need more here -->
<hr />
<!-- ================================================================ -->
<h2><a name="reference">Expat Reference</a></h2>
<h3><a name="creation">Parser Creation</a></h3>
<h4 id="XML_ParserCreate">XML_ParserCreate</h4>
<pre class="fcndec">
XML_Parser XMLCALL
XML_ParserCreate(const XML_Char *encoding);
</pre>
<div class="fcndef">
<p>
Construct a new parser. If encoding is non-<code>NULL</code>, it specifies a
character encoding to use for the document. This overrides the document
encoding declaration. There are four built-in encodings:
</p>
<ul>
<li>US-ASCII</li>
<li>UTF-8</li>
<li>UTF-16</li>
<li>ISO-8859-1</li>
</ul>
<p>
Any other value will invoke a call to the UnknownEncodingHandler.
</p>
</div>
<h4 id="XML_ParserCreateNS">XML_ParserCreateNS</h4>
<pre class="fcndec">
XML_Parser XMLCALL
XML_ParserCreateNS(const XML_Char *encoding,
XML_Char sep);
</pre>
<div class="fcndef">