# 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 implemention 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.

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

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, then from Windows Explorer double-click on "expat.dsp" in the lib directory and build and install in the usual manner.

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.

## 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 libary 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
    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
    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 typically the first argument received by most handlers.

### 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
    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 passed through to the handlers unchanged, with the prefix and colon still attached. 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 before 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 entites 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.

## Expat Reference

### Parser Creation

    XML_Parser
    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
    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 a legal URI.

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

    typedef struct {
      void *(*malloc_fcn)(size_t size);
      void *(*realloc_fcn)(void *ptr, size_t size);
      void (*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
    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
    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
    XML_ParserReset(XML_Parser p);

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. 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`.

    XML_Status
    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.

    XML_Status
    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 *
    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;
    }

### 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 defined in expat.h as `char *` and contains bytes encoding UTF-8. Note that you'll receive them in this form independent of the original encoding of the document.

    XML_SetStartElementHandler(XML_Parser p,
                               XML_StartElementHandler start);

    typedef void
    (*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(XML_Parser p,
                             XML_EndElementHandler);

    typedef void
    (*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(XML_Parser p,
                          XML_StartElementHandler start,
                          XML_EndElementHandler end);

Set handlers for start and end tags with one call.

    XML_SetCharacterDataHandler(XML_Parser p,
                                XML_CharacterDataHandler charhndl)

    typedef void
    (*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.

    XML_SetProcessingInstructionHandler(XML_Parser p,
                                        XML_ProcessingInstructionHandler proc)

    typedef void
    (*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(XML_Parser p,
                          XML_CommentHandler cmnt)

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

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

    XML_SetStartCdataSectionHandler(XML_Parser p,
                                    XML_StartCdataSectionHandler start);

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

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

    XML_SetEndCdataSectionHandler(XML_Parser p,
                                  XML_EndCdataSectionHandler end);

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

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

    XML_SetCdataSectionHandler(XML_Parser p,
                               XML_StartCdataSectionHandler start,
                               XML_EndCdataSectionHandler end)

Sets both CDATA section handlers with one call.

    XML_SetDefaultHandler(XML_Parser p,
                          XML_DefaultHandler hndl)

    typedef void
    (*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`.

    XML_SetDefaultHandlerExpand(XML_Parser p,
                                XML_DefaultHandler hndl)

    typedef void
    (*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`.

    XML_SetExternalEntityRefHandler(XML_Parser p,
                                    XML_ExternalEntityRefHandler hndl)

    typedef int
    (*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(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(XML_Parser p,
                                XML_SkippedEntityHandler handler)

    typedef void
    (*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

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

    typedef int
    (*XML_UnknownEncodingHandler)(void *encodingHandlerData,
                                  const XML_Char *name,
                                  XML_Encoding *info);

    typedef struct {
      int map[256];
      void *data;
      int (*convert)(void *data, const char *s);
      void (*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_ERROR`. Otherwise it should return `XML_STATUS_OK`. 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 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.

    XML_SetStartNamespaceDeclHandler(XML_Parser p,
    			         XML_StartNamespaceDeclHandler start);

    typedef void
    (*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.

Note: Due to limitations of the implementation, the StartNamespaceDeclHandler is not called unless the StartElementHandler is also set. The specific value of the StartElementHandler is allowed to change freely, so long as it is not NULL.

    XML_SetEndNamespaceDeclHandler(XML_Parser p,
    			       XML_EndNamespaceDeclHandler end);

    typedef void
    (*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.

Note: Due to limitations of the implementation, the EndNamespaceDeclHandler is not called unless the StartElementHandler is also set. The specific value of the StartElementHandler is allowed to change freely, so long as it is not NULL.

    XML_SetNamespaceDeclHandler(XML_Parser p,
                                XML_StartNamespaceDeclHandler start,
                                XML_EndNamespaceDeclHandler end)

Sets both namespace declaration handlers with a single call.

Note: Due to limitations of the implementation, the StartNamespaceDeclHandler and EndNamespaceDeclHandler are not called unless the StartElementHandler is also set. The specific value of the StartElementHandler is allowed to change freely, so long as it is not NULL.

    XML_SetXmlDeclHandler(XML_Parser p,
    		      XML_XmlDeclHandler xmldecl);

    typedef void
    (*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(XML_Parser p,
    			       XML_StartDoctypeDeclHandler start);

    typedef void
    (*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(XML_Parser p,
    			     XML_EndDoctypeDeclHandler end);

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

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

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

Set both doctype handlers with one call.

    XML_SetElementDeclHandler(XML_Parser p,
    			  XML_ElementDeclHandler eldecl);

    typedef void
    (*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.

    XML_SetAttlistDeclHandler(XML_Parser p,
                              XML_AttlistDeclHandler attdecl);

    typedef void
    (*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(XML_Parser p,
    			 XML_EntityDeclHandler handler);

    typedef void
    (*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(XML_Parser p,
                                     XML_UnparsedEntityDeclHandler h)

    typedef void
    (*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(XML_Parser p,
                               XML_NotationDeclHandler h)

    typedef void
    (*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(XML_Parser p,
                                XML_NotStandaloneHandler h)

    typedef int
    (*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 ocurred), 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 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
    XML_GetErrorCode(XML_Parser p);

Return what type of error has occurred.

    const XML_LChar *
    XML_ErrorString(int 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`.

    long
    XML_GetCurrentByteIndex(XML_Parser p);

Return the byte offset of the position.

    int
    XML_GetCurrentLineNumber(XML_Parser p);

Return the line number of the position.

    int
    XML_GetCurrentColumnNumber(XML_Parser p);

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

    int
    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 *
    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.

### Miscellaneous functions

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

    void
    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 *
    XML_GetUserData(XML_Parser p);

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

    void
    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
    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 *
    XML_GetBase(XML_Parser p);

Return the base for resolving relative URIs.

    int
    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
    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.

    enum XML_Status
    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
    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`

    enum XML_Error
    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`.

    void
    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
    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 *
    XML_ExpatVersion();

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

    struct XML_Expat_Version
    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 *
    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
    };

    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
    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 *
    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 *
    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
    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.