Expat XML Parser

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 XML::Parser, Python's xml.parsers.expat, and other open-source XML parsers.

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.

This is free software, licensed under the MIT/X Consortium license. You may download it from the Expat home page.

The bulk of this document was originally commissioned as an article by XML.com. 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.

Overview
Building and Installing
Using Expat
Reference

Overview

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.

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:

XML_ParserCreate: Create a new parser object.
XML_SetElementHandler: Set handlers for start and end tags.
XML_SetCharacterDataHandler: Set handler for text.
XML_Parse: Pass a buffer full of document to the parser

These functions and others are described in the reference part of this document. The reference section also describes in detail the parameters passed to the different types of handlers.

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 outline.c 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 Depth variable.

 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 */

The end tag simply does the bookkeeping work of decrementing Depth.

 void XMLCALL
 end(void *data, const char *el) {
   Depth--;
 }  /* End of end handler */

Note the XMLCALL 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 XMLCALL annotation on most platforms; callbacks should be defined using this annotation.

The XMLCALL 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 after including Expat's header file:

 #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

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.

Building and Installing Expat

The Expat distribution comes as a compressed (with GNU gzip) tar file. You may download the latest version from Source Forge. After unpacking this, cd into the directory. Then follow either the Win32 directions or Unix directions below.

Building under Win32

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 .sln file, e.g. cmake -G"Visual Studio 17 2022" -DCMAKE_BUILD_TYPE=RelWithDebInfo ., and build Expat using msbuild /m expat.sln after.

Alternatively, you may download the Win32 binary package that contains the "expat.h" include file and a pre-built DLL.

Building under Unix (or GNU)

First you'll need to run the configure shell script in order to configure the Makefiles and headers for your system.

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:

 ./configure
 make
 make install

There are some options that you can provide to this script, but the only one we'll mention here is the --prefix option. You can find out all the options available by running configure with just the --help option.

By default, the configure script sets things up so that the library gets installed in /usr/local/lib and the associated header file in /usr/local/include. But if you were to give the option, --prefix=/home/me/mystuff, then the library and header would get installed in /home/me/mystuff/lib and /home/me/mystuff/include respectively.

Configuring Expat Using the Pre-Processor

Expat's feature set can be configured using a small number of pre-processor definitions. The symbols are:

XML_GE: Added in Expat 2.6.0. Include support for general entities (syntax &e1; to reference and syntax <!ENTITY e1 'value1'> (an internal general entity) or <!ENTITY e2 SYSTEM 'file2'> (an external general entity) to declare). With XML_GE enabled, general entities will be replaced by their declared replacement text; for this to work for external general entities, in addition an XML_ExternalEntityRefHandler must be set using XML_SetExternalEntityRefHandler. Also, enabling XML_GE makes the functions XML_SetBillionLaughsAttackProtectionMaximumAmplification and XML_SetBillionLaughsAttackProtectionActivationThreshold available.
With XML_GE 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 predefined five: amp, apos, gt, lt, quot) with a self-reference: for example, referencing an entity e1 via &e1; will be replaced by text &e1;.
XML_DTD: 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 XML_SetBillionLaughsAttackProtectionMaximumAmplification and XML_SetBillionLaughsAttackProtectionActivationThreshold available.
XML_NS: When defined, support for the Namespaces in XML specification is included.
XML_UNICODE: When defined, character data reported to the application is encoded in UTF-16 using wide characters of the type XML_Char. This is implied if XML_UNICODE_WCHAR_T is defined.
XML_UNICODE_WCHAR_T: If defined, causes the XML_Char character type to be defined using the wchar_t type; otherwise, unsigned short is used. Defining this implies XML_UNICODE.
XML_LARGE_SIZE: If defined, causes the XML_Size and XML_Index 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 XML_GetCurrentByteIndex, XML_GetCurrentLineNumber and XML_GetCurrentColumnNumber could overflow. It may not be supported by all compilers, and is turned off by default.
XML_CONTEXT_BYTES: The number of input bytes of markup context which the parser will ensure are available for reporting via XML_GetInputContext. 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 XML_GetInputContext will always report NULL. Without this, Expat has a smaller memory footprint and can be faster.
XML_STATIC: 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.
XML_ATTR_INFO: If defined, makes the additional function XML_GetAttributeInfo available for reporting attribute byte offsets.

Using Expat

Compiling and Linking Against Expat

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 (#include <expat.h>) 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 -lexpat 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.

On a Unix-based system, here's what a Makefile might look like when Expat is installed in a standard location:

 CC=cc
 LDFLAGS=
 LIBS= -lexpat
 xmlapp: xmlapp.o
         $(CC) $(LDFLAGS) -o xmlapp xmlapp.o $(LIBS)

If you installed Expat in, say, /home/me/mystuff, then the Makefile would look like this:

 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)

You'd also have to set the environment variable LD_LIBRARY_PATH to /home/me/mystuff/lib (or to ${LD_LIBRARY_PATH}:/home/me/mystuff/lib if LD_LIBRARY_PATH already has some directories in it) in order to run your application.

Expat Basics

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 three functions in the Expat API for creating a parser object. However, only two of these (XML_ParserCreate and XML_ParserCreateNS) 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 XML_ParserFree. Note that if you have provided any user data that gets stored in the parser, then your application is responsible for freeing it prior to calling XML_ParserFree.

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.

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.

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.

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.

 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 */

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 after the main body of start tag code, then in the end handler, we need to manipulate it before 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.

Communicating between handlers

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 XML_SetUserData function) to pass a pointer to this structure to the handlers. This is the first argument received by most handlers. In the reference section, an argument to a callback function is named userData and have type void * if the user data is passed; it will have the type XML_Parser if the parser itself is passed. When the parser is passed, the user data may be retrieved using XML_GetUserData.

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 XML_SetCharacterDataHandler) 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.

XML Version

Expat is an XML 1.0 parser, and as such never complains based on the value of the version pseudo-attribute in the XML declaration, if present.

If an application needs to check the version number (to support alternate processing), it should use the XML_SetXmlDeclHandler 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 "1.0" is accepted:

 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;
     }
   }
   ...
 }

Namespace Processing

When the parser is created using the XML_ParserCreateNS, function, Expat performs namespace processing. Under namespace processing, Expat consumes xmlns and xmlns:... 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 XML_SetNamespaceDeclHandler.

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 XML_ParserCreateNS), 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.

However if XML_SetReturnNSTriplet has been called with a non-zero do_nst parameter, then the expanded form for names with an explicit prefix is a concatenation of: URI, separator, local name, separator, prefix.

You can set handlers for the start of a namespace declaration and for the end of a scope of a declaration with the XML_SetNamespaceDeclHandler 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 NULL. The URI will be NULL for the case where the default namespace is being unset. The namespace end handler just gets the prefix for the closing scope.

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.

Character Encodings

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:

 <?xml version="1.0" encoding="ISO-8859-2"?>

External parsed entities may begin with a text declaration, which looks like an XML declaration with just an encoding declaration:

 <?xml encoding="Big5"?>

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.)

There are four built-in encodings in Expat:

UTF-8
UTF-16
ISO-8859-1
US-ASCII

Anything else discovered in an encoding declaration or in the protocol encoding specified in the parser constructor, triggers a call to the UnknownEncodingHandler. This handler gets passed the encoding name and a pointer to an XML_Encoding data structure. Your handler must fill in this structure and return XML_STATUS_OK if it knows how to deal with the encoding. Otherwise the handler should return XML_STATUS_ERROR. The handler also gets passed a pointer to an optional application data structure that you may indicate when you set the handler.

Expat places restrictions on character encodings that it can support by filling in the XML_Encoding structure. include file:

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 $@\^'{}~)
Characters must be encoded in 4 bytes or less.
All characters encoded must have Unicode scalar values less than or equal to 65535 (0xFFFF)This does not apply to the built-in support for UTF-16 and UTF-8
No character may be encoded by more that one distinct sequence of bytes

XML_Encoding 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 XML_Encoding structure. This function should return the Unicode scalar value for the sequence or -1 if the sequence is malformed.

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.

Handling External Entity References

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 ExternalEntityRefHandler, then external entity references are silently ignored. Otherwise, it calls your handler with the information needed to read and parse the external entity.

Your handler isn't actually responsible for parsing the entity, but it is responsible for creating a subsidiary parser with XML_ExternalEntityParserCreate that will do the job. This returns an instance of XML_Parser that has handlers and other data structures initialized from the parent parser. You may then use XML_Parse or XML_ParseBuffer calls against this parser. Since external entities my refer to other external entities, your handler should be prepared to be called recursively.

Parsing DTDs

In order to parse parameter entities, before starting the parse, you must call XML_SetParamEntityParsing with one of the following arguments:

XML_PARAM_ENTITY_PARSING_NEVER: Don't parse parameter entities or the external subset
XML_PARAM_ENTITY_PARSING_UNLESS_STANDALONE: Parse parameter entities and the external subset unless standalone was set to "yes" in the XML declaration.
XML_PARAM_ENTITY_PARSING_ALWAYS: Always parse parameter entities and the external subset

In order to read an external DTD, you also have to set an external entity reference handler as described above.

Temporarily Stopping Parsing

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

Supporting the XInclude specification.
Delaying further processing until additional information is available from some other source.
Adjusting processor load as task priorities shift within an application.
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.

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).

An application that uses this feature for a single parser will have the rough structure (in pseudo-code):

 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);
     }
   }
 }

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.

This C function could be used for the parse_xml function mentioned in the pseudo-code above:

 #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;
   }
 }

The corresponding continue_parsing function is somewhat simpler, since it only need deal with the return code from XML_ResumeParser; it can delegate the input handling to the parse_xml function:

 /* 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);
 }

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 XML_StopParser 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 temporarily, and come back to it when we're ready.

To stop parsing from a handler function, use the XML_StopParser function. This function takes two arguments; the parser being stopped and a flag indicating whether the parse can be resumed in the future.

Expat Reference

Parser Creation

XML_ParserCreate

 XML_Parser XMLCALL
 XML_ParserCreate(const XML_Char *encoding);

Construct a new parser. If encoding is non-NULL, it specifies a character encoding to use for the document. This overrides the document encoding declaration. There are four built-in encodings:

US-ASCII
UTF-8
UTF-16
ISO-8859-1

Any other value will invoke a call to the UnknownEncodingHandler.

XML_ParserCreateNS

 XML_Parser XMLCALL
 XML_ParserCreateNS(const XML_Char *encoding,
                    XML_Char sep);

Constructs a new parser that has namespace processing in effect. Namespace expanded element names and attribute names are returned as a concatenation of the namespace URI, sep, and the local part of the name. This means that you should pick a character for sep that can't be part of an URI. Since Expat does not check namespace URIs for conformance, the only safe choice for a namespace separator is a character that is illegal in XML. For instance, '\xFF' is not legal in UTF-8, and '\xFFFF' is not legal in UTF-16. There is a special case when sep is the null character '\0': the namespace URI and the local part will be concatenated without any separator - this is intended to support RDF processors. It is a programming error to use the null separator with namespace triplets.

Note: Expat does not validate namespace URIs (beyond encoding) against RFC 3986 today (and is not required to do so with regard to the XML 1.0 namespaces specification) but it may start doing that in future releases. Before that, an application using Expat must be ready to receive namespace URIs containing non-URI characters.

XML_ParserCreate_MM

 XML_Parser XMLCALL
 XML_ParserCreate_MM(const XML_Char *encoding,
                     const XML_Memory_Handling_Suite *ms,
 		    const XML_Char *sep);

 typedef struct {
   void *(XMLCALL *malloc_fcn)(size_t size);
   void *(XMLCALL *realloc_fcn)(void *ptr, size_t size);
   void (XMLCALL *free_fcn)(void *ptr);
 } XML_Memory_Handling_Suite;

Construct a new parser using the suite of memory handling functions specified in ms. If ms is NULL, then use the standard set of memory management functions. If sep is non-NULL, then namespace processing is enabled in the created parser and the character pointed at by sep is used as the separator between the namespace URI and the local part of the name.

XML_ExternalEntityParserCreate

 XML_Parser XMLCALL
 XML_ExternalEntityParserCreate(XML_Parser p,
                                const XML_Char *context,
                                const XML_Char *encoding);

Construct a new XML_Parser object for parsing an external general entity. Context is the context argument passed in a call to a ExternalEntityRefHandler. Other state information such as handlers, user data, namespace processing is inherited from the parser passed as the 1st argument. So you shouldn't need to call any of the behavior changing functions on this parser (unless you want it to act differently than the parent parser).

XML_ParserFree

 void XMLCALL
 XML_ParserFree(XML_Parser p);

Free memory used by the parser. Your application is responsible for freeing any memory associated with user data.

XML_ParserReset

 XML_Bool XMLCALL
 XML_ParserReset(XML_Parser p,
                 const XML_Char *encoding);

Clean up the memory structures maintained by the parser so that it may be used again. After this has been called, parser is ready to start parsing a new document. All handlers are cleared from the parser, except for the unknownEncodingHandler. The parser's external state is re-initialized except for the values of ns and ns_triplets. This function may not be used on a parser created using XML_ExternalEntityParserCreate; it will return XML_FALSE in that case. Returns XML_TRUE on success. Your application is responsible for dealing with any memory associated with user data.

Parsing

To state the obvious: the three parsing functions XML_Parse, XML_ParseBuffer and XML_GetBuffer must not be called from within a handler unless they operate on a separate parser instance, that is, one that did not call the handler. For example, it is OK to call the parsing functions from within an XML_ExternalEntityRefHandler, if they apply to the parser created by XML_ExternalEntityParserCreate.

Note: The len argument passed to these functions should be considerably less than the maximum value for an integer, as it could create an integer overflow situation if the added lengths of a buffer and the unprocessed portion of the previous buffer exceed the maximum integer value. Input data at the end of a buffer will remain unprocessed if it is part of an XML token for which the end is not part of that buffer.

The application must make a concluding XML_Parse or XML_ParseBuffer call with isFinal set to XML_TRUE.

XML_Parse

 enum XML_Status XMLCALL
 XML_Parse(XML_Parser p,
           const char *s,
           int len,
           int isFinal);

 enum XML_Status {
   XML_STATUS_ERROR = 0,
   XML_STATUS_OK = 1
 };

Parse some more of the document. The string s is a buffer containing part (or perhaps all) of the document. The number of bytes of s that are part of the document is indicated by len. This means that s doesn't have to be null-terminated. It also means that if len is larger than the number of bytes in the block of memory that s points at, then a memory fault is likely. Negative values for len are rejected since Expat 2.2.1. The isFinal parameter informs the parser that this is the last piece of the document. Frequently, the last piece is empty (i.e. len is zero.)

If a parse error occurred, it returns XML_STATUS_ERROR. Otherwise it returns XML_STATUS_OK value. Note that regardless of the return value, there is no guarantee that all provided input has been parsed; only after the concluding call will all handler callbacks and parsing errors have happened.

Simplified, XML_Parse can be considered a convenience wrapper that is pairing calls to XML_GetBuffer and XML_ParseBuffer (when Expat is built with macro XML_CONTEXT_BYTES defined to a positive value, which is both common and default). XML_Parse is then functionally equivalent to calling XML_GetBuffer, memcpy, and XML_ParseBuffer.

To avoid double copying of the input, direct use of functions XML_GetBuffer and XML_ParseBuffer is advised for most production use, e.g. if you're using read or similar functionality to fill your buffers, fill directly into the buffer from XML_GetBuffer, then parse with XML_ParseBuffer.

XML_ParseBuffer

 enum XML_Status XMLCALL
 XML_ParseBuffer(XML_Parser p,
                 int len,
                 int isFinal);

This is just like XML_Parse, except in this case Expat provides the buffer. By obtaining the buffer from Expat with the XML_GetBuffer function, the application can avoid double copying of the input.

Negative values for len are rejected since Expat 2.6.3.

XML_GetBuffer

 void * XMLCALL
 XML_GetBuffer(XML_Parser p,
               int len);

Obtain a buffer of size len to read a piece of the document into. A NULL value is returned if Expat can't allocate enough memory for this buffer. A NULL value may also be returned if len is zero. This has to be called prior to every call to XML_ParseBuffer. A typical use would look like this:

 for (;;) {
   int bytes_read;
   void *buff = XML_GetBuffer(p, BUFF_SIZE);
   if (buff == NULL) {
     /* handle error */
   }
 
   bytes_read = read(docfd, buff, BUFF_SIZE);
   if (bytes_read < 0) {
     /* handle error */
   }
 
   if (! XML_ParseBuffer(p, bytes_read, bytes_read == 0)) {
     /* handle parse error */
   }
 
   if (bytes_read == 0)
     break;
 }

XML_StopParser

 enum XML_Status XMLCALL
 XML_StopParser(XML_Parser p,
                XML_Bool resumable);

Stops parsing, causing XML_Parse or XML_ParseBuffer to return. Must be called from within a call-back handler, except when aborting (when resumable is XML_FALSE) an already suspended parser. Some call-backs may still follow because they would otherwise get lost, including

the end element handler for empty elements when stopped in the start element handler,
the end namespace declaration handler when stopped in the end element handler,
the character data handler when stopped in the character data handler while making multiple call-backs on a contiguous chunk of characters,

and possibly others.

This can be called from most handlers, including DTD related call-backs, except when parsing an external parameter entity and resumable is XML_TRUE. Returns XML_STATUS_OK when successful, XML_STATUS_ERROR otherwise. The possible error codes are:

XML_ERROR_NOT_STARTED: when stopping or suspending a parser before it has started, added in Expat 2.6.4.
XML_ERROR_SUSPENDED: when suspending an already suspended parser.
XML_ERROR_FINISHED: when the parser has already finished.
XML_ERROR_SUSPEND_PE: when suspending while parsing an external PE.

Since the stop/resume feature requires application support in the outer parsing loop, it is an error to call this function for a parser not being handled appropriately; see Temporarily Stopping Parsing for more information.

When resumable is XML_TRUE then parsing is suspended, that is, XML_Parse and XML_ParseBuffer return XML_STATUS_SUSPENDED. Otherwise, parsing is aborted, that is, XML_Parse and XML_ParseBuffer return XML_STATUS_ERROR with error code XML_ERROR_ABORTED.

Note: This will be applied to the current parser instance only, that is, if there is a parent parser then it will continue parsing when the external entity reference handler returns. It is up to the implementation of that handler to call XML_StopParser on the parent parser (recursively), if one wants to stop parsing altogether.

When suspended, parsing can be resumed by calling XML_ResumeParser.

New in Expat 1.95.8.

XML_ResumeParser

 enum XML_Status XMLCALL
 XML_ResumeParser(XML_Parser p);

Resumes parsing after it has been suspended with XML_StopParser. Must not be called from within a handler call-back. Returns same status codes as XML_Parse or XML_ParseBuffer. An additional error code, XML_ERROR_NOT_SUSPENDED, will be returned if the parser was not currently suspended.

Note: This must be called on the most deeply nested child parser instance first, and on its parent parser only after the child parser has finished, to be applied recursively until the document entity's parser is restarted. That is, the parent parser will not resume by itself and it is up to the application to call XML_ResumeParser on it at the appropriate moment.

New in Expat 1.95.8.

XML_GetParsingStatus

 void XMLCALL
 XML_GetParsingStatus(XML_Parser p,
                      XML_ParsingStatus *status);

 enum XML_Parsing {
   XML_INITIALIZED,
   XML_PARSING,
   XML_FINISHED,
   XML_SUSPENDED
 };
 
 typedef struct {
   enum XML_Parsing parsing;
   XML_Bool finalBuffer;
 } XML_ParsingStatus;

Returns status of parser with respect to being initialized, parsing, finished, or suspended, and whether the final buffer is being processed. The status parameter must not be NULL.

New in Expat 1.95.8.

Handler Setting

Although handlers are typically set prior to parsing and left alone, an application may choose to set or change the handler for a parsing event while the parse is in progress. For instance, your application may choose to ignore all text not descended from a para element. One way it could do this is to set the character handler when a para start tag is seen, and unset it for the corresponding end tag.

A handler may be unset by providing a NULL pointer to the appropriate handler setter. None of the handler setting functions have a return value.

Your handlers will be receiving strings in arrays of type XML_Char. This type is conditionally defined in expat.h as either char, wchar_t or unsigned short. The former implies UTF-8 encoding, the latter two imply UTF-16 encoding. Note that you'll receive them in this form independent of the original encoding of the document.

XML_SetStartElementHandler

 void XMLCALL
 XML_SetStartElementHandler(XML_Parser p,
                            XML_StartElementHandler start);

 typedef void
 (XMLCALL *XML_StartElementHandler)(void *userData,
                                    const XML_Char *name,
                                    const XML_Char **atts);

Set handler for start (and empty) tags. Attributes are passed to the start handler as a pointer to a vector of char pointers. Each attribute seen in a start (or empty) tag occupies 2 consecutive places in this vector: the attribute name followed by the attribute value. These pairs are terminated by a NULL pointer.

Note that an empty tag generates a call to both start and end handlers (in that order).

XML_SetEndElementHandler

 void XMLCALL
 XML_SetEndElementHandler(XML_Parser p,
                          XML_EndElementHandler);

 typedef void
 (XMLCALL *XML_EndElementHandler)(void *userData,
                                  const XML_Char *name);

Set handler for end (and empty) tags. As noted above, an empty tag generates a call to both start and end handlers.

XML_SetElementHandler

 void XMLCALL
 XML_SetElementHandler(XML_Parser p,
                       XML_StartElementHandler start,
                       XML_EndElementHandler end);

Set handlers for start and end tags with one call.

XML_SetCharacterDataHandler

 void XMLCALL
 XML_SetCharacterDataHandler(XML_Parser p,
                             XML_CharacterDataHandler charhndl)

 typedef void
 (XMLCALL *XML_CharacterDataHandler)(void *userData,
                                     const XML_Char *s,
                                     int len);

Set a text handler. The string your handler receives is NOT null-terminated. You have to use the length argument to deal with the end of the string. A single block of contiguous text free of markup may still result in a sequence of calls to this handler. In other words, if you're searching for a pattern in the text, it may be split across calls to this handler. Note: Setting this handler to NULL may NOT immediately terminate call-backs if the parser is currently processing such a single block of contiguous markup-free text, as the parser will continue calling back until the end of the block is reached.

XML_SetProcessingInstructionHandler

 void XMLCALL
 XML_SetProcessingInstructionHandler(XML_Parser p,
                                     XML_ProcessingInstructionHandler proc)

 typedef void
 (XMLCALL *XML_ProcessingInstructionHandler)(void *userData,
                                             const XML_Char *target,
                                             const XML_Char *data);

Set a handler for processing instructions. The target is the first word in the processing instruction. The data is the rest of the characters in it after skipping all whitespace after the initial word.

XML_SetCommentHandler

 void XMLCALL
 XML_SetCommentHandler(XML_Parser p,
                       XML_CommentHandler cmnt)

 typedef void
 (XMLCALL *XML_CommentHandler)(void *userData,
                               const XML_Char *data);

Set a handler for comments. The data is all text inside the comment delimiters.

XML_SetStartCdataSectionHandler

 void XMLCALL
 XML_SetStartCdataSectionHandler(XML_Parser p,
                                 XML_StartCdataSectionHandler start);

 typedef void
 (XMLCALL *XML_StartCdataSectionHandler)(void *userData);

Set a handler that gets called at the beginning of a CDATA section.

XML_SetEndCdataSectionHandler

 void XMLCALL
 XML_SetEndCdataSectionHandler(XML_Parser p,
                               XML_EndCdataSectionHandler end);

 typedef void
 (XMLCALL *XML_EndCdataSectionHandler)(void *userData);

Set a handler that gets called at the end of a CDATA section.

XML_SetCdataSectionHandler

 void XMLCALL
 XML_SetCdataSectionHandler(XML_Parser p,
                            XML_StartCdataSectionHandler start,
                            XML_EndCdataSectionHandler end)

Sets both CDATA section handlers with one call.

XML_SetDefaultHandler

 void XMLCALL
 XML_SetDefaultHandler(XML_Parser p,
                       XML_DefaultHandler hndl)

 typedef void
 (XMLCALL *XML_DefaultHandler)(void *userData,
                               const XML_Char *s,
                               int len);

Sets a handler for any characters in the document which wouldn't otherwise be handled. This includes both data for which no handlers can be set (like some kinds of DTD declarations) and data which could be reported but which currently has no handler set. The characters are passed exactly as they were present in the XML document except that they will be encoded in UTF-8 or UTF-16. Line boundaries are not normalized. Note that a byte order mark character is not passed to the default handler. There are no guarantees about how characters are divided between calls to the default handler: for example, a comment might be split between multiple calls. Setting the handler with this call has the side effect of turning off expansion of references to internally defined general entities. Instead these references are passed to the default handler.

XML_SetDefaultHandlerExpand

 void XMLCALL
 XML_SetDefaultHandlerExpand(XML_Parser p,
                             XML_DefaultHandler hndl)

 typedef void
 (XMLCALL *XML_DefaultHandler)(void *userData,
                               const XML_Char *s,
                               int len);

This sets a default handler, but doesn't inhibit the expansion of internal entity references. The entity reference will not be passed to the default handler.

XML_SetExternalEntityRefHandler

 void XMLCALL
 XML_SetExternalEntityRefHandler(XML_Parser p,
                                 XML_ExternalEntityRefHandler hndl)

 typedef int
 (XMLCALL *XML_ExternalEntityRefHandler)(XML_Parser p,
                                         const XML_Char *context,
                                         const XML_Char *base,
                                         const XML_Char *systemId,
                                         const XML_Char *publicId);

Set an external entity reference handler. This handler is also called for processing an external DTD subset if parameter entity parsing is in effect. (See XML_SetParamEntityParsing.)

The context parameter specifies the parsing context in the format expected by the context argument to XML_ExternalEntityParserCreate. code is valid only until the handler returns, so if the referenced entity is to be parsed later, it must be copied. context is NULL only when the entity is a parameter entity, which is how one can differentiate between general and parameter entities.

The base parameter is the base to use for relative system identifiers. It is set by XML_SetBase and may be NULL. The publicId parameter is the public id given in the entity declaration and may be NULL. systemId is the system identifier specified in the entity declaration and is never NULL.

There are a couple of ways in which this handler differs from others. First, this handler returns a status indicator (an integer). XML_STATUS_OK should be returned for successful handling of the external entity reference. Returning XML_STATUS_ERROR indicates failure, and causes the calling parser to return an XML_ERROR_EXTERNAL_ENTITY_HANDLING error.

Second, instead of having the user data as its first argument, it receives the parser that encountered the entity reference. This, along with the context parameter, may be used as arguments to a call to XML_ExternalEntityParserCreate. Using the returned parser, the body of the external entity can be recursively parsed.

Since this handler may be called recursively, it should not be saving information into global or static variables.

XML_SetExternalEntityRefHandlerArg

 void XMLCALL
 XML_SetExternalEntityRefHandlerArg(XML_Parser p,
                                    void *arg)

Set the argument passed to the ExternalEntityRefHandler. If arg is not NULL, it is the new value passed to the handler set using XML_SetExternalEntityRefHandler; if arg is NULL, the argument passed to the handler function will be the parser object itself.

Note: The type of arg and the type of the first argument to the ExternalEntityRefHandler do not match. This function takes a void * to be passed to the handler, while the handler accepts an XML_Parser. This is a historical accident, but will not be corrected before Expat 2.0 (at the earliest) to avoid causing compiler warnings for code that's known to work with this API. It is the responsibility of the application code to know the actual type of the argument passed to the handler and to manage it properly.

XML_SetSkippedEntityHandler

 void XMLCALL
 XML_SetSkippedEntityHandler(XML_Parser p,
                             XML_SkippedEntityHandler handler)

 typedef void
 (XMLCALL *XML_SkippedEntityHandler)(void *userData,
                                     const XML_Char *entityName,
                                     int is_parameter_entity);

Set a skipped entity handler. This is called in two situations:

An entity reference is encountered for which no declaration has been read and this is not an error.
An internal entity reference is read, but not expanded, because XML_SetDefaultHandler has been called.

The is_parameter_entity argument will be non-zero for a parameter entity and zero for a general entity.

Note: Skipped parameter entities in declarations and skipped general entities in attribute values cannot be reported, because the event would be out of sync with the reporting of the declarations or attribute values

XML_SetUnknownEncodingHandler

 void XMLCALL
 XML_SetUnknownEncodingHandler(XML_Parser p,
                               XML_UnknownEncodingHandler enchandler,
 			      void *encodingHandlerData)

 typedef int
 (XMLCALL *XML_UnknownEncodingHandler)(void *encodingHandlerData,
                                       const XML_Char *name,
                                       XML_Encoding *info);
 
 typedef struct {
   int map[256];
   void *data;
   int (XMLCALL *convert)(void *data, const char *s);
   void (XMLCALL *release)(void *data);
 } XML_Encoding;

Set a handler to deal with encodings other than the built in set. This should be done before XML_Parse or XML_ParseBuffer have been called on the given parser.

If the handler knows how to deal with an encoding with the given name, it should fill in the info data structure and return XML_STATUS_OK. Otherwise it should return XML_STATUS_ERROR. The handler will be called at most once per parsed (external) entity. The optional application data pointer encodingHandlerData will be passed back to the handler.

The map array contains information for every possible leading byte in a byte sequence. If the corresponding value is >= 0, then it's a single byte sequence and the byte encodes that Unicode value. If the value is -1, then that byte is invalid as the initial byte in a sequence. If the value is -n, where n is an integer > 1, then n is the number of bytes in the sequence and the actual conversion is accomplished by a call to the function pointed at by convert. This function may return -1 if the sequence itself is invalid. The convert pointer may be NULL if there are only single byte codes. The data parameter passed to the convert function is the data pointer from XML_Encoding. The string s is NOT null-terminated and points at the sequence of bytes to be converted.

The function pointed at by release is called by the parser when it is finished with the encoding. It may be NULL.

XML_SetStartNamespaceDeclHandler

 void XMLCALL
 XML_SetStartNamespaceDeclHandler(XML_Parser p,
 			         XML_StartNamespaceDeclHandler start);

 typedef void
 (XMLCALL *XML_StartNamespaceDeclHandler)(void *userData,
                                          const XML_Char *prefix,
                                          const XML_Char *uri);

Set a handler to be called when a namespace is declared. Namespace declarations occur inside start tags. But the namespace declaration start handler is called before the start tag handler for each namespace declared in that start tag.

XML_SetEndNamespaceDeclHandler

 void XMLCALL
 XML_SetEndNamespaceDeclHandler(XML_Parser p,
 			       XML_EndNamespaceDeclHandler end);

 typedef void
 (XMLCALL *XML_EndNamespaceDeclHandler)(void *userData,
                                        const XML_Char *prefix);

Set a handler to be called when leaving the scope of a namespace declaration. This will be called, for each namespace declaration, after the handler for the end tag of the element in which the namespace was declared.

XML_SetNamespaceDeclHandler

 void XMLCALL
 XML_SetNamespaceDeclHandler(XML_Parser p,
                             XML_StartNamespaceDeclHandler start,
                             XML_EndNamespaceDeclHandler end)

Sets both namespace declaration handlers with a single call.

XML_SetXmlDeclHandler

 void XMLCALL
 XML_SetXmlDeclHandler(XML_Parser p,
 		      XML_XmlDeclHandler xmldecl);

 typedef void
 (XMLCALL *XML_XmlDeclHandler)(void            *userData,
                               const XML_Char  *version,
                               const XML_Char  *encoding,
                               int             standalone);

Sets a handler that is called for XML declarations and also for text declarations discovered in external entities. The way to distinguish is that the version parameter will be NULL for text declarations. The encoding parameter may be NULL for an XML declaration. The standalone argument will contain -1, 0, or 1 indicating respectively that there was no standalone parameter in the declaration, that it was given as no, or that it was given as yes.

XML_SetStartDoctypeDeclHandler

 void XMLCALL
 XML_SetStartDoctypeDeclHandler(XML_Parser p,
 			       XML_StartDoctypeDeclHandler start);

 typedef void
 (XMLCALL *XML_StartDoctypeDeclHandler)(void           *userData,
                                        const XML_Char *doctypeName,
                                        const XML_Char *sysid,
                                        const XML_Char *pubid,
                                        int            has_internal_subset);

Set a handler that is called at the start of a DOCTYPE declaration, before any external or internal subset is parsed. Both sysid and pubid may be NULL. The has_internal_subset will be non-zero if the DOCTYPE declaration has an internal subset.

XML_SetEndDoctypeDeclHandler

 void XMLCALL
 XML_SetEndDoctypeDeclHandler(XML_Parser p,
 			     XML_EndDoctypeDeclHandler end);

 typedef void
 (XMLCALL *XML_EndDoctypeDeclHandler)(void *userData);

Set a handler that is called at the end of a DOCTYPE declaration, after parsing any external subset.

XML_SetDoctypeDeclHandler

 void XMLCALL
 XML_SetDoctypeDeclHandler(XML_Parser p,
 			  XML_StartDoctypeDeclHandler start,
 			  XML_EndDoctypeDeclHandler end);

Set both doctype handlers with one call.

XML_SetElementDeclHandler

 void XMLCALL
 XML_SetElementDeclHandler(XML_Parser p,
 			  XML_ElementDeclHandler eldecl);

 typedef void
 (XMLCALL *XML_ElementDeclHandler)(void *userData,
                                   const XML_Char *name,
                                   XML_Content *model);

 enum XML_Content_Type {
   XML_CTYPE_EMPTY = 1,
   XML_CTYPE_ANY,
   XML_CTYPE_MIXED,
   XML_CTYPE_NAME,
   XML_CTYPE_CHOICE,
   XML_CTYPE_SEQ
 };
 
 enum XML_Content_Quant {
   XML_CQUANT_NONE,
   XML_CQUANT_OPT,
   XML_CQUANT_REP,
   XML_CQUANT_PLUS
 };
 
 typedef struct XML_cp XML_Content;
 
 struct XML_cp {
   enum XML_Content_Type		type;
   enum XML_Content_Quant	quant;
   const XML_Char *		name;
   unsigned int			numchildren;
   XML_Content *			children;
 };

Sets a handler for element declarations in a DTD. The handler gets called with the name of the element in the declaration and a pointer to a structure that contains the element model. It's the user code's responsibility to free model when finished with via a call to XML_FreeContentModel. There is no need to free the model from the handler, it can be kept around and freed at a later stage.

The model argument is the root of a tree of XML_Content nodes. If type equals XML_CTYPE_EMPTY or XML_CTYPE_ANY, then quant will be XML_CQUANT_NONE, and the other fields will be zero or NULL. If type is XML_CTYPE_MIXED, then quant will be XML_CQUANT_NONE or XML_CQUANT_REP and numchildren will contain the number of elements that are allowed to be mixed in and children points to an array of XML_Content structures that will all have type XML_CTYPE_NAME with no quantification. Only the root node can be type XML_CTYPE_EMPTY, XML_CTYPE_ANY, or XML_CTYPE_MIXED.

For type XML_CTYPE_NAME, the name field points to the name and the numchildren and children fields will be zero and NULL. The quant field will indicate any quantifiers placed on the name.

Types XML_CTYPE_CHOICE and XML_CTYPE_SEQ indicate a choice or sequence respectively. The numchildren field indicates how many nodes in the choice or sequence and children points to the nodes.

XML_SetAttlistDeclHandler

 void XMLCALL
 XML_SetAttlistDeclHandler(XML_Parser p,
                           XML_AttlistDeclHandler attdecl);

 typedef void
 (XMLCALL *XML_AttlistDeclHandler)(void           *userData,
                                   const XML_Char *elname,
                                   const XML_Char *attname,
                                   const XML_Char *att_type,
                                   const XML_Char *dflt,
                                   int            isrequired);

Set a handler for attlist declarations in the DTD. This handler is called for each attribute. So a single attlist declaration with multiple attributes declared will generate multiple calls to this handler. The elname parameter returns the name of the element for which the attribute is being declared. The attribute name is in the attname parameter. The attribute type is in the att_type parameter. It is the string representing the type in the declaration with whitespace removed.

The dflt parameter holds the default value. It will be NULL in the case of "#IMPLIED" or "#REQUIRED" attributes. You can distinguish these two cases by checking the isrequired parameter, which will be true in the case of "#REQUIRED" attributes. Attributes which are "#FIXED" will have also have a true isrequired, but they will have the non-NULL fixed value in the dflt parameter.

XML_SetEntityDeclHandler

 void XMLCALL
 XML_SetEntityDeclHandler(XML_Parser p,
 			 XML_EntityDeclHandler handler);

 typedef void
 (XMLCALL *XML_EntityDeclHandler)(void           *userData,
                                  const XML_Char *entityName,
                                  int            is_parameter_entity,
                                  const XML_Char *value,
                                  int            value_length, 
                                  const XML_Char *base,
                                  const XML_Char *systemId,
                                  const XML_Char *publicId,
                                  const XML_Char *notationName);

Sets a handler that will be called for all entity declarations. The is_parameter_entity argument will be non-zero in the case of parameter entities and zero otherwise.

For internal entities (<!ENTITY foo "bar">), value will be non-NULL and systemId, publicId, and notationName will all be NULL. The value string is not null-terminated; the length is provided in the value_length parameter. Do not use value_length to test for internal entities, since it is legal to have zero-length values. Instead check for whether or not value is NULL.

The notationName argument will have a non-NULL value only for unparsed entity declarations.

XML_SetUnparsedEntityDeclHandler

 void XMLCALL
 XML_SetUnparsedEntityDeclHandler(XML_Parser p,
                                  XML_UnparsedEntityDeclHandler h)

 typedef void
 (XMLCALL *XML_UnparsedEntityDeclHandler)(void *userData,
                                          const XML_Char *entityName, 
                                          const XML_Char *base,
                                          const XML_Char *systemId,
                                          const XML_Char *publicId,
                                          const XML_Char *notationName);

Set a handler that receives declarations of unparsed entities. These are entity declarations that have a notation (NDATA) field:

 <!ENTITY logo SYSTEM "images/logo.gif" NDATA gif>

This handler is obsolete and is provided for backwards compatibility. Use instead XML_SetEntityDeclHandler.

XML_SetNotationDeclHandler

 void XMLCALL
 XML_SetNotationDeclHandler(XML_Parser p,
                            XML_NotationDeclHandler h)

 typedef void
 (XMLCALL *XML_NotationDeclHandler)(void *userData, 
                                    const XML_Char *notationName,
                                    const XML_Char *base,
                                    const XML_Char *systemId,
                                    const XML_Char *publicId);

Set a handler that receives notation declarations.

XML_SetNotStandaloneHandler

 void XMLCALL
 XML_SetNotStandaloneHandler(XML_Parser p,
                             XML_NotStandaloneHandler h)

 typedef int 
 (XMLCALL *XML_NotStandaloneHandler)(void *userData);

Set a handler that is called if the document is not "standalone". This happens when there is an external subset or a reference to a parameter entity, but does not have standalone set to "yes" in an XML declaration. If this handler returns XML_STATUS_ERROR, then the parser will throw an XML_ERROR_NOT_STANDALONE error.

Parse position and error reporting functions

These are the functions you'll want to call when the parse functions return XML_STATUS_ERROR (a parse error has occurred), although the position reporting functions are useful outside of errors. The position reported is the byte position (in the original document or entity encoding) of the first of the sequence of characters that generated the current event (or the error that caused the parse functions to return XML_STATUS_ERROR.) The exceptions are callbacks triggered by declarations in the document prologue, in which case they exact position reported is somewhere in the relevant markup, but not necessarily as meaningful as for other events.

The position reporting functions are accurate only outside of the DTD. In other words, they usually return bogus information when called from within a DTD declaration handler.

XML_GetErrorCode

 enum XML_Error XMLCALL
 XML_GetErrorCode(XML_Parser p);

Return what type of error has occurred.

XML_ErrorString

 const XML_LChar * XMLCALL
 XML_ErrorString(enum XML_Error code);

Return a string describing the error corresponding to code. The code should be one of the enums that can be returned from XML_GetErrorCode.

XML_GetCurrentByteIndex

 XML_Index XMLCALL
 XML_GetCurrentByteIndex(XML_Parser p);

Return the byte offset of the position. This always corresponds to the values returned by XML_GetCurrentLineNumber and XML_GetCurrentColumnNumber.

XML_GetCurrentLineNumber

 XML_Size XMLCALL
 XML_GetCurrentLineNumber(XML_Parser p);

Return the line number of the position. The first line is reported as 1.

XML_GetCurrentColumnNumber

 XML_Size XMLCALL
 XML_GetCurrentColumnNumber(XML_Parser p);

-Return the offset, from the beginning of the current line, of -the position. +Return the offset, from the beginning of the current line, of +the position. The first column is reported as 0.

XML_GetCurrentByteCount

 int XMLCALL
 XML_GetCurrentByteCount(XML_Parser p);

Return the number of bytes in the current event. Returns 0 if the event is inside a reference to an internal entity and for the end-tag event for empty element tags (the later can be used to distinguish empty-element tags from empty elements using separate start and end tags).

XML_GetInputContext

 const char * XMLCALL
 XML_GetInputContext(XML_Parser p,
                     int *offset,
                     int *size);

Returns the parser's input buffer, sets the integer pointed at by offset to the offset within this buffer of the current parse position, and set the integer pointed at by size to the size of the returned buffer.

This should only be called from within a handler during an active parse and the returned buffer should only be referred to from within the handler that made the call. This input buffer contains the untranslated bytes of the input.

Only a limited amount of context is kept, so if the event triggering a call spans over a very large amount of input, the actual parse position may be before the beginning of the buffer.

If XML_CONTEXT_BYTES is zero, this will always return NULL.

Attack Protection

XML_SetBillionLaughsAttackProtectionMaximumAmplification

 /* Added in Expat 2.4.0. */
 XML_Bool XMLCALL
 XML_SetBillionLaughsAttackProtectionMaximumAmplification(XML_Parser p,
                                                          float maximumAmplificationFactor);

Sets the maximum tolerated amplification factor for protection against billion laughs attacks (default: 100.0) of parser p to maximumAmplificationFactor, and returns XML_TRUE upon success and XML_FALSE upon error.

The amplification factor is calculated as ..

+ Once the threshold for activation is reached, + the amplification factor is calculated as .. +

amplification := (direct + indirect) / direct

.. while parsing, whereas direct is the number of bytes read from the primary document in parsing and indirect is the number of bytes added by expanding entities and reading of external DTD files, combined.

For a call to XML_SetBillionLaughsAttackProtectionMaximumAmplification to succeed:

parser p must be a non-NULL root parser (without any parent parsers) and
maximumAmplificationFactor must be non-NaN and greater than or equal to 1.0.

Note: If you ever need to increase this value for non-attack payload, please file a bug report.

Note: Peak amplifications of factor 15,000 for the entire payload and of factor 30,000 in the middle of parsing have been observed with small benign files in practice. So if you do reduce the maximum allowed amplification, please make sure that the activation threshold is still big enough to not end up with undesired false positives (i.e. benign files being rejected).

XML_SetBillionLaughsAttackProtectionActivationThreshold

 /* Added in Expat 2.4.0. */
 XML_Bool XMLCALL
 XML_SetBillionLaughsAttackProtectionActivationThreshold(XML_Parser p,
                                                         unsigned long long activationThresholdBytes);

Sets number of output bytes (including amplification from entity expansion and reading DTD files) needed to activate protection against billion laughs attacks (default: 8 MiB) of parser p to activationThresholdBytes, and returns XML_TRUE upon success and XML_FALSE upon error.

For a call to XML_SetBillionLaughsAttackProtectionActivationThreshold to succeed:

parser p must be a non-NULL root parser (without any parent parsers).

Note: If you ever need to increase this value for non-attack payload, please file a bug report.

Note: Activation thresholds below 4 MiB are known to break support for DITA 1.3 payload and are hence not recommended.

XML_SetAllocTrackerMaximumAmplification

 /* Added in Expat 2.7.2. */
 XML_Bool
 XML_SetAllocTrackerMaximumAmplification(XML_Parser p,
                                         float maximumAmplificationFactor);

Sets the maximum tolerated amplification factor between direct input and bytes of dynamic memory allocated (default: 100.0) of parser p to maximumAmplificationFactor, and returns XML_TRUE upon success and XML_FALSE upon error.

Note: There are three types of allocations that intentionally bypass tracking and limiting:

application calls to functions XML_MemMalloc and XML_MemRealloc — healthy use of these two functions continues to be a responsibility of the application using Expat —,
the main character buffer used by functions XML_GetBuffer and XML_ParseBuffer (and thus also by plain XML_Parse), and
the content model memory (that is passed to the element declaration handler and freed by a call to XML_FreeContentModel).

The amplification factor is calculated as ..

+ Once the threshold for activation is reached, + the amplification factor is calculated as .. +

amplification := allocated / direct

.. while parsing, whereas direct is the number of bytes read from the primary document in parsing and allocated is the number of bytes of dynamic memory allocated in the parser hierarchy.

For a call to XML_SetAllocTrackerMaximumAmplification to succeed:

parser p must be a non-NULL root parser (without any parent parsers) and
maximumAmplificationFactor must be non-NaN and greater than or equal to 1.0.

Note: If you ever need to increase this value for non-attack payload, please file a bug report.

Note: - Amplifications factors greater than 100 can been observed near the start of parsing + Amplifications factors greater than 100.0 can been observed near the start of parsing even with benign files in practice. So if you do reduce the maximum allowed amplification, please make sure that the activation threshold is still big enough to not end up with undesired false positives (i.e. benign files being rejected).

XML_SetAllocTrackerActivationThreshold

 /* Added in Expat 2.7.2. */
 XML_Bool
 XML_SetAllocTrackerActivationThreshold(XML_Parser p,
                                        unsigned long long activationThresholdBytes);

Sets number of allocated bytes of dynamic memory needed to activate protection against disproportionate use of RAM (default: 64 MiB) of parser p to activationThresholdBytes, and returns XML_TRUE upon success and XML_FALSE upon error.

Note: For types of allocations that intentionally bypass tracking and limiting, please see XML_SetAllocTrackerMaximumAmplification above.

For a call to XML_SetAllocTrackerActivationThreshold to succeed:

parser p must be a non-NULL root parser (without any parent parsers).

Note: If you ever need to increase this value for non-attack payload, please file a bug report.

XML_SetReparseDeferralEnabled

 /* Added in Expat 2.6.0. */
 XML_Bool XMLCALL
 XML_SetReparseDeferralEnabled(XML_Parser parser, XML_Bool enabled);

Large tokens may require many parse calls before enough data is available for Expat to parse it in full. If Expat retried parsing the token on every parse call, parsing could take quadratic time. To avoid this, Expat only retries once a significant amount of new data is available. This function allows disabling this behavior.

The enabled argument should be XML_TRUE or XML_FALSE.

Returns XML_TRUE on success, and XML_FALSE on error.

Miscellaneous functions

The functions in this section either obtain state information from the parser or can be used to dynamically set parser options.

XML_SetUserData

 void XMLCALL
 XML_SetUserData(XML_Parser p,
                 void *userData);

This sets the user data pointer that gets passed to handlers. It overwrites any previous value for this pointer. Note that the application is responsible for freeing the memory associated with userData when it is finished with the parser. So if you call this when there's already a pointer there, and you haven't freed the memory associated with it, then you've probably just leaked memory.

XML_GetUserData

 void * XMLCALL
 XML_GetUserData(XML_Parser p);

This returns the user data pointer that gets passed to handlers. It is actually implemented as a macro.

XML_UseParserAsHandlerArg

 void XMLCALL
 XML_UseParserAsHandlerArg(XML_Parser p);

After this is called, handlers receive the parser in their userData arguments. The user data can still be obtained using the XML_GetUserData function.

XML_SetBase

 enum XML_Status XMLCALL
 XML_SetBase(XML_Parser p,
             const XML_Char *base);

Set the base to be used for resolving relative URIs in system identifiers. The return value is XML_STATUS_ERROR if there's no memory to store base, otherwise it's XML_STATUS_OK.

XML_GetBase

 const XML_Char * XMLCALL
 XML_GetBase(XML_Parser p);

Return the base for resolving relative URIs.

XML_GetSpecifiedAttributeCount

 int XMLCALL
 XML_GetSpecifiedAttributeCount(XML_Parser p);

When attributes are reported to the start handler in the atts vector, attributes that were explicitly set in the element occur before any attributes that receive their value from default information in an ATTLIST declaration. This function returns the number of attributes that were explicitly set times two, thus giving the offset in the atts array passed to the start tag handler of the first attribute set due to defaults. It supplies information for the last call to a start handler. If called inside a start handler, then that means the current call.

XML_GetIdAttributeIndex

 int XMLCALL
 XML_GetIdAttributeIndex(XML_Parser p);

Returns the index of the ID attribute passed in the atts array in the last call to XML_StartElementHandler, or -1 if there is no ID attribute. If called inside a start handler, then that means the current call.

XML_GetAttributeInfo

 const XML_AttrInfo * XMLCALL
 XML_GetAttributeInfo(XML_Parser parser);

 typedef struct {
   XML_Index  nameStart;  /* Offset to beginning of the attribute name. */
   XML_Index  nameEnd;    /* Offset after the attribute name's last byte. */
   XML_Index  valueStart; /* Offset to beginning of the attribute value. */
   XML_Index  valueEnd;   /* Offset after the attribute value's last byte. */
 } XML_AttrInfo;

Returns an array of XML_AttrInfo structures for the attribute/value pairs passed in the last call to the XML_StartElementHandler that were specified in the start-tag rather than defaulted. Each attribute/value pair counts as 1; thus the number of entries in the array is XML_GetSpecifiedAttributeCount(parser) / 2.

XML_SetEncoding

 enum XML_Status XMLCALL
 XML_SetEncoding(XML_Parser p,
                 const XML_Char *encoding);

Set the encoding to be used by the parser. It is equivalent to passing a non-NULL encoding argument to the parser creation functions. It must not be called after XML_Parse or XML_ParseBuffer have been called on the given parser. Returns XML_STATUS_OK on success or XML_STATUS_ERROR on error.

XML_SetParamEntityParsing

 int XMLCALL
 XML_SetParamEntityParsing(XML_Parser p,
                           enum XML_ParamEntityParsing code);

This enables parsing of parameter entities, including the external parameter entity that is the external DTD subset, according to code. The choices for code are:

XML_PARAM_ENTITY_PARSING_NEVER
XML_PARAM_ENTITY_PARSING_UNLESS_STANDALONE
XML_PARAM_ENTITY_PARSING_ALWAYS

Note: If XML_SetParamEntityParsing is called after XML_Parse or XML_ParseBuffer, then it has no effect and will always return 0.

XML_SetHashSalt

 int XMLCALL
 XML_SetHashSalt(XML_Parser p,
                 unsigned long hash_salt);

Sets the hash salt to use for internal hash calculations. Helps in preventing DoS attacks based on predicting hash function behavior. In order to have an effect this must be called before parsing has started. Returns 1 if successful, 0 when called after XML_Parse or XML_ParseBuffer.

Note: This call is optional, as the parser will auto-generate a new random salt value if no value has been set at the start of parsing.

Note: One should not call XML_SetHashSalt with a hash salt value of 0, as this value is used as sentinel value to indicate that XML_SetHashSalt has not been called. Consequently such a call will have no effect, even if it returns 1.

XML_UseForeignDTD

 enum XML_Error XMLCALL
 XML_UseForeignDTD(XML_Parser parser, XML_Bool useDTD);

This function allows an application to provide an external subset for the document type declaration for documents which do not specify an external subset of their own. For documents which specify an external subset in their DOCTYPE declaration, the application-provided subset will be ignored. If the document does not contain a DOCTYPE declaration at all and useDTD is true, the application-provided subset will be parsed, but the startDoctypeDeclHandler and endDoctypeDeclHandler functions, if set, will not be called. The setting of parameter entity parsing, controlled using XML_SetParamEntityParsing, will be honored.

The application-provided external subset is read by calling the external entity reference handler set via XML_SetExternalEntityRefHandler with both publicId and systemId set to NULL.

If this function is called after parsing has begun, it returns XML_ERROR_CANT_CHANGE_FEATURE_ONCE_PARSING and ignores useDTD. If called when Expat has been compiled without DTD support, it returns XML_ERROR_FEATURE_REQUIRES_XML_DTD. Otherwise, it returns XML_ERROR_NONE.

Note: For the purpose of checking WFC: Entity Declared, passing useDTD == XML_TRUE will make the parser behave as if the document had a DTD with an external subset. This holds true even if the external entity reference handler returns without action.

XML_SetReturnNSTriplet

 void XMLCALL
 XML_SetReturnNSTriplet(XML_Parser parser,
                        int        do_nst);

This function only has an effect when using a parser created with XML_ParserCreateNS, i.e. when namespace processing is in effect. The do_nst sets whether or not prefixes are returned with names qualified with a namespace prefix. If this function is called with do_nst non-zero, then afterwards namespace qualified names (that is qualified with a prefix as opposed to belonging to a default namespace) are returned as a triplet with the three parts separated by the namespace separator specified when the parser was created. The order of returned parts is URI, local name, and prefix.

If do_nst is zero, then namespaces are reported in the default manner, URI then local_name separated by the namespace separator.

XML_DefaultCurrent

 void XMLCALL
 XML_DefaultCurrent(XML_Parser parser);

This can be called within a handler for a start element, end element, processing instruction or character data. It causes the corresponding markup to be passed to the default handler set by XML_SetDefaultHandler or XML_SetDefaultHandlerExpand. It does nothing if there is not a default handler.

XML_ExpatVersion

 XML_LChar * XMLCALL
 XML_ExpatVersion();

Return the library version as a string (e.g. "expat_1.95.1").

XML_ExpatVersionInfo

 struct XML_Expat_Version XMLCALL
 XML_ExpatVersionInfo();

 typedef struct {
   int major;
   int minor;
   int micro;
 } XML_Expat_Version;

Return the library version information as a structure. Some macros are also defined that support compile-time tests of the library version:

XML_MAJOR_VERSION
XML_MINOR_VERSION
XML_MICRO_VERSION

Testing these constants is currently the best way to determine if particular parts of the Expat API are available.

XML_GetFeatureList

 const XML_Feature * XMLCALL
 XML_GetFeatureList();

 enum XML_FeatureEnum {
   XML_FEATURE_END = 0,
   XML_FEATURE_UNICODE,
   XML_FEATURE_UNICODE_WCHAR_T,
   XML_FEATURE_DTD,
   XML_FEATURE_CONTEXT_BYTES,
   XML_FEATURE_MIN_SIZE,
   XML_FEATURE_SIZEOF_XML_CHAR,
   XML_FEATURE_SIZEOF_XML_LCHAR,
   XML_FEATURE_NS,
   XML_FEATURE_LARGE_SIZE
 };
 
 typedef struct {
   enum XML_FeatureEnum  feature;
   XML_LChar            *name;
   long int              value;
 } XML_Feature;

Returns a list of "feature" records, providing details on how Expat was configured at compile time. Most applications should not need to worry about this, but this information is otherwise not available from Expat. This function allows code that does need to check these features to do so at runtime.

The return value is an array of XML_Feature, terminated by a record with a feature of XML_FEATURE_END and name of NULL, identifying the feature-test macros Expat was compiled with. Since an application that requires this kind of information needs to determine the type of character the name points to, records for the XML_FEATURE_SIZEOF_XML_CHAR and XML_FEATURE_SIZEOF_XML_LCHAR will be located at the beginning of the list, followed by XML_FEATURE_UNICODE and XML_FEATURE_UNICODE_WCHAR_T, if they are present at all.

Some features have an associated value. If there isn't an associated value, the value field is set to 0. At this time, the following features have been defined to have values:

XML_FEATURE_SIZEOF_XML_CHAR: The number of bytes occupied by one XML_Char character.
XML_FEATURE_SIZEOF_XML_LCHAR: The number of bytes occupied by one XML_LChar character.
XML_FEATURE_CONTEXT_BYTES: The maximum number of characters of context which can be reported by XML_GetInputContext.

XML_FreeContentModel

 void XMLCALL
 XML_FreeContentModel(XML_Parser parser, XML_Content *model);

Function to deallocate the model argument passed to the XML_ElementDeclHandler callback set using XML_ElementDeclHandler. This function should not be used for any other purpose.

The following functions allow external code to share the memory allocator an XML_Parser has been configured to use. This is especially useful for third-party libraries that interact with a parser object created by application code, or heavily layered applications. This can be essential when using dynamically loaded libraries which use different C standard libraries (this can happen on Windows, at least).

XML_MemMalloc

 void * XMLCALL
 XML_MemMalloc(XML_Parser parser, size_t size);

Allocate size bytes of memory using the allocator the parser object has been configured to use. Returns a pointer to the memory or NULL on failure. Memory allocated in this way must be freed using XML_MemFree.

XML_MemRealloc

 void * XMLCALL
 XML_MemRealloc(XML_Parser parser, void *ptr, size_t size);

Allocate size bytes of memory using the allocator the parser object has been configured to use. ptr must point to a block of memory allocated by XML_MemMalloc or XML_MemRealloc, or be NULL. This function tries to expand the block pointed to by ptr if possible. Returns a pointer to the memory or NULL on failure. On success, the original block has either been expanded or freed. On failure, the original block has not been freed; the caller is responsible for freeing the original block. Memory allocated in this way must be freed using XML_MemFree.

XML_MemFree

 void XMLCALL
 XML_MemFree(XML_Parser parser, void *ptr);

Free a block of memory pointed to by ptr. The block must have been allocated by XML_MemMalloc or XML_MemRealloc, or be NULL.

The Expat XML Parser - Release 2.7.2 + Release 2.7.3

Table of Contents

Building under Win32

Building under Unix (or GNU)

Configuring Expat Using the Pre-Processor

Compiling and Linking Against Expat

Expat Basics

Communicating between handlers

XML Version

Namespace Processing

Character Encodings

Handling External Entity References

Parsing DTDs

Temporarily Stopping Parsing

XML_ParserCreate

XML_ParserCreateNS

XML_ParserCreate_MM

XML_ExternalEntityParserCreate

XML_ParserFree

XML_ParserReset

XML_Parse

XML_ParseBuffer

XML_GetBuffer

XML_StopParser

XML_ResumeParser

XML_GetParsingStatus

XML_SetStartElementHandler

XML_SetEndElementHandler

XML_SetElementHandler

XML_SetCharacterDataHandler

XML_SetProcessingInstructionHandler

XML_SetCommentHandler

XML_SetStartCdataSectionHandler

XML_SetEndCdataSectionHandler

XML_SetCdataSectionHandler

XML_SetDefaultHandler

XML_SetDefaultHandlerExpand

XML_SetExternalEntityRefHandler

XML_SetExternalEntityRefHandlerArg

XML_SetSkippedEntityHandler

XML_SetUnknownEncodingHandler

XML_SetStartNamespaceDeclHandler

XML_SetEndNamespaceDeclHandler

XML_SetNamespaceDeclHandler

XML_SetXmlDeclHandler

XML_SetStartDoctypeDeclHandler

XML_SetEndDoctypeDeclHandler

XML_SetDoctypeDeclHandler

XML_SetElementDeclHandler

XML_SetAttlistDeclHandler

XML_SetEntityDeclHandler

XML_SetUnparsedEntityDeclHandler

XML_SetNotationDeclHandler

XML_SetNotStandaloneHandler

XML_GetErrorCode

XML_ErrorString

XML_GetCurrentByteIndex

XML_GetCurrentLineNumber

XML_GetCurrentColumnNumber

XML_GetCurrentByteCount

XML_GetInputContext

XML_SetBillionLaughsAttackProtectionMaximumAmplification

XML_SetBillionLaughsAttackProtectionActivationThreshold

XML_SetAllocTrackerMaximumAmplification

XML_SetAllocTrackerActivationThreshold

XML_SetReparseDeferralEnabled

XML_SetUserData

XML_GetUserData

XML_UseParserAsHandlerArg

XML_SetBase

XML_GetBase

XML_GetSpecifiedAttributeCount

XML_GetIdAttributeIndex

XML_GetAttributeInfo

XML_SetEncoding

XML_SetParamEntityParsing

XML_SetHashSalt

XML_UseForeignDTD

XML_SetReturnNSTriplet

XML_DefaultCurrent