# The Expat XML Parser

Release 2.0.1 |

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.

## Table of Contents

- Overview
- Building and Installing
- Using Expat
- Reference
  - Parser Creation Functions
    - XML_ParserCreate
    - XML_ParserCreateNS
    - XML_ParserCreate_MM
    - XML_ExternalEntityParserCreate
    - XML_ParserFree
    - XML_ParserReset
  - Parsing Functions
    - XML_Parse
    - XML_ParseBuffer
    - XML_GetBuffer
    - XML_StopParser
    - XML_ResumeParser
    - XML_GetParsingStatus
  - Handler Setting Functions
    - 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
  - Parse Position and Error Reporting Functions
    - XML_GetErrorCode
    - XML_ErrorString
    - XML_GetCurrentByteIndex
    - XML_GetCurrentLineNumber
    - XML_GetCurrentColumnNumber
    - XML_GetCurrentByteCount
    - XML_GetInputContext
  - Miscellaneous Functions
    - 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
    - XML_ExpatVersion
    - XML_ExpatVersionInfo
    - XML_GetFeatureList
    - XML_FreeContentModel
    - XML_MemMalloc
    - XML_MemRealloc
    - XML_MemFree

## 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 15 2017" -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 definition of this symbols does not affect the set of entry points for Expat, only the behavior of the API and the definition of character types in the case of `XML_UNICODE_WCHAR_T`. The symbols are:

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 entities or process conditional sections. This does not affect the set of functions available in the API.
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. If this is not defined, 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:

1. 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 $@\^'{}~)
2. Characters must be encoded in 4 bytes or less.
3. 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
4. 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_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_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.

    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_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).

    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_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.

    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. 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.

    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.

    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. 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;
    }

    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_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.

    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.

    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.

    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).

    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.

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

Set handlers for start and end tags with one call.

    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 nul-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.

    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.

    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.

    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.

    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.

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

Sets both CDATA section handlers with one call.

    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.

See also `XML_DefaultCurrent`.

    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.

See also `XML_DefaultCurrent`.

    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.

    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.

    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:

1. An entity reference is encountered for which no declaration has been read and this is not an error.
2. 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

    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 nul-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.

    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.

    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.

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

Sets both namespace declaration handlers with a single call.

    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.

    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.

    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.

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

Set both doctype handlers with one call.

    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 is the application's responsibility to free this data structure using `XML_FreeContentModel`.

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.

    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.

    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.

    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.

    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.

    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 trigged 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.

    enum XML_Error XMLCALL
    XML_GetErrorCode(XML_Parser p);

Return what type of error has occurred.

    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_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_Size XMLCALL
    XML_GetCurrentLineNumber(XML_Parser p);

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

    XML_Size XMLCALL
    XML_GetCurrentColumnNumber(XML_Parser p);

Return the offset, from the beginning of the current line, of the position.

    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).

    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 not defined, this will always return NULL.

### Miscellaneous functions

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

    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.

    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.

    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.

    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`.

    const XML_Char * XMLCALL
    XML_GetBase(XML_Parser p);

Return the base for resolving relative URIs.

    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.

    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.

    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`.

    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.

    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.

    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.

    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.

    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.

    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_LChar * XMLCALL
    XML_ExpatVersion();

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

    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.

    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`.

    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).

    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`.

    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`.

    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.