Topic: expat -- API

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The expat xmlparse API is actually pretty easy to use. But let's face it, reading the .h just doesn't cut it for documentation (not for me, anyway.) So I decided to HTMLize it to a certain extent. During this process, I am realizing how much I don't yet understand about XML parsing; if you find any inaccuracies in what I write here, please don't hesitate to tell me about them. NOTE: This page isn't finished (yeah, you would have noticed on your own.) I'm trying to finish a couple of functions per day, but this API is a big one and it's boring, tedious work.

[ create ]
[ setup ]
[ handlers ]
[ parse ]
[ data structures ]
[ errors ]
Error table


XML_ParserCreate returns XML_Parser
const XML_Char *encoding
This function creates a new parser. If you know the encoding, then you can supply it now explicitly. If you don't, pass in a NULL, and the encoding will be NULL unless there is an encoding specified in the external protocol. You can also call XML_SetEncoding at any time before you actually start parsing.


XML_Parser parser
Frees up resources associated with a parser (including all the allocated names and values, so watch out!)

XML_ParserCreateNS returns XML_Parser

const XML_Char *encoding,
XML_Char namespaceSeparator
From the header:
/* Constructs a new parser and namespace processor. Element type names
and attribute names that belong to a namespace will be expanded;
unprefixed attribute names are never expanded; unprefixed element type
names are expanded only if there is a default namespace. The expanded
name is the concatenation of the namespace URI, the namespace separator character,
and the local part of the name. If the namespace separator is '\0' then
the namespace URI and the local part will be concatenated without any
separator. When a namespace is not declared, the name and prefix will be
passed through without expansion. */

So far none of my XML stuff has required namespaces, so I have no words of wisdom at this point.

XML_ExternalEntityParserCreate returns XML_Parser

XML_Parser parser,
const XML_Char *context,
const XML_Char *encoding
Used to parse an external entity in the context of an ongoing main parse.

From the header again:
/* Creates an XML_Parser object that can parse an external general entity;
context is a '\0'-terminated string specifying the parse context;
encoding is a '\0'-terminated string giving the name of the externally specified encoding,
or null if there is no externally specified encoding.
The context string consists of a sequence of tokens separated by formfeeds (\f);
a token consisting of a name specifies that the general entity of the name
is open; a token of the form prefix=uri specifies the namespace for a particular
prefix; a token of the form =uri specifies the default namespace.
This can be called at any point after the first call to an ExternalEntityRefHandler
so longer as the parser has not yet been freed.
The new parser is completely independent and may safely be used in a separate thread.
The handlers and userData are initialized from the parser argument.
Returns 0 if out of memory. Otherwise returns a new XML_Parser object. */


XML_Parser parser,
void *arg
If you specify a non-NULL value for arg, then the external entity reference handler will get this argument instead of the parser, which is its usual first argument.


XML_Parser parser,
void *userData
Sets a value to be passed to each handler during the parse. The void pointer is then cast to whatever data structure is appropriate.

/* 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. */
void XMLPARSEAPI XML_DefaultCurrent(XML_Parser parser);

/* This is equivalent to supplying an encoding argument
to XML_CreateParser. It must not be called after XML_Parse
or XML_ParseBuffer. */

XML_SetEncoding(XML_Parser parser, const XML_Char *encoding);

/* If this function is called, then the parser will be passed
as the first argument to callbacks instead of userData.
The userData will still be accessible using XML_GetUserData. */

XML_UseParserAsHandlerArg(XML_Parser parser);

/* Sets the base to be used for resolving relative URIs in system identifiers in
declarations.  Resolving relative identifiers is left to the application:
this value will be passed through as the base argument to the
XML_ExternalEntityRefHandler, XML_NotationDeclHandler
and XML_UnparsedEntityDeclHandler. The base argument will be copied.
Returns zero if out of memory, non-zero otherwise. */

XML_SetBase(XML_Parser parser, const XML_Char *base);


XML_Parser parser,
XML_StartElementHandler start,
XML_EndElementHandler end
/* atts is array of name/value pairs, terminated by 0; names and values are 0 terminated. */

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

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

XML_SetCharacterDataHandler(XML_Parser parser,
			    XML_CharacterDataHandler handler);

/* s is not 0 terminated. */
typedef void (*XML_CharacterDataHandler)(void *userData,
					 const XML_Char *s,
					 int len);

XML_SetProcessingInstructionHandler(XML_Parser parser, XML_ProcessingInstructionHandler handler); /* target and data are 0 terminated */ typedef void (*XML_ProcessingInstructionHandler)(void *userData, const XML_Char *target, const XML_Char *data);

XML_SetCommentHandler(XML_Parser parser, XML_CommentHandler handler); /* data is 0 terminated */ typedef void (*XML_CommentHandler)(void *userData, const XML_Char *data);

XML_SetCdataSectionHandler(XML_Parser parser, XML_StartCdataSectionHandler start, XML_EndCdataSectionHandler end); typedef void (*XML_StartCdataSectionHandler)(void *userData); typedef void (*XML_EndCdataSectionHandler)(void *userData);

XML_SetDefaultHandler(XML_Parser parser, XML_DefaultHandler handler); /* This sets the default handler and also inhibits expansion of internal entities. The entity reference will be passed to the default handler. */ XML_SetDefaultHandlerExpand(XML_Parser parser, XML_DefaultHandler handler); /* This sets the default handler but does not inhibit expansion of internal entities. The entity reference will not be passed to the default handler. */ /* This is called for any characters in the XML document for which there is no applicable handler. This includes both characters that are part of markup which is of a kind that is not reported (comments, markup declarations), or characters that are part of a construct which could be reported but for which no handler has been supplied. The characters are passed exactly as they were in the XML document except that they will be encoded in UTF-8. 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. */ typedef void (*XML_DefaultHandler)(void *userData, const XML_Char *s, int len);

XML_SetUnparsedEntityDeclHandler(XML_Parser parser, XML_UnparsedEntityDeclHandler handler); /* This is called for a declaration of an unparsed (NDATA) entity. The base argument is whatever was set by XML_SetBase. The entityName, systemId and notationName arguments will never be null. The other arguments may be. */ 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);

XML_SetNotationDeclHandler(XML_Parser parser, XML_NotationDeclHandler handler); /* This is called for a declaration of notation. The base argument is whatever was set by XML_SetBase. The notationName will never be null. The other arguments can be. */ typedef void (*XML_NotationDeclHandler)(void *userData, const XML_Char *notationName, const XML_Char *base, const XML_Char *systemId, const XML_Char *publicId);

XML_SetNamespaceDeclHandler(XML_Parser parser, XML_StartNamespaceDeclHandler start, XML_EndNamespaceDeclHandler end); /* When namespace processing is enabled, these are called once for each namespace declaration. The call to the start and end element handlers occur between the calls to the start and end namespace declaration handlers. For an xmlns attribute, prefix will be null. For an xmlns="" attribute, uri will be null. */ typedef void (*XML_StartNamespaceDeclHandler)(void *userData, const XML_Char *prefix, const XML_Char *uri); typedef void (*XML_EndNamespaceDeclHandler)(void *userData, const XML_Char *prefix);

XML_SetNotStandaloneHandler(XML_Parser parser, XML_NotStandaloneHandler handler); /* This is called if the document is not standalone (it has an external subset or a reference to a parameter entity, but does not have standalone="yes"). If this handler returns 0, then processing will not continue, and the parser will return a XML_ERROR_NOT_STANDALONE error. */ typedef int (*XML_NotStandaloneHandler)(void *userData);

XML_SetExternalEntityRefHandler(XML_Parser parser, XML_ExternalEntityRefHandler handler); /* This is called for a reference to an external parsed general entity. The referenced entity is not automatically parsed. The application can parse it immediately or later using XML_ExternalEntityParserCreate. The parser argument is the parser parsing the entity containing the reference; it can be passed as the parser argument to XML_ExternalEntityParserCreate. The systemId argument is the system identifier as specified in the entity declaration; it will not be null. The base argument is the system identifier that should be used as the base for resolving systemId if systemId was relative; this is set by XML_SetBase; it may be null. The publicId argument is the public identifier as specified in the entity declaration, or null if none was specified; the whitespace in the public identifier will have been normalized as required by the XML spec. The context argument specifies the parsing context in the format expected by the context argument to XML_ExternalEntityParserCreate; context is valid only until the handler returns, so if the referenced entity is to be parsed later, it must be copied. The handler should return 0 if processing should not continue because of a fatal error in the handling of the external entity. In this case the calling parser will return an XML_ERROR_EXTERNAL_ENTITY_HANDLING error. Note that unlike other handlers the first argument is the parser, not userData. */ typedef int (*XML_ExternalEntityRefHandler)(XML_Parser parser, const XML_Char *context, const XML_Char *base, const XML_Char *systemId, const XML_Char *publicId);

XML_SetUnknownEncodingHandler(XML_Parser parser, XML_UnknownEncodingHandler handler, void *encodingHandlerData); /* This is called for an encoding that is unknown to the parser. The encodingHandlerData argument is that which was passed as the second argument to XML_SetUnknownEncodingHandler. The name argument gives the name of the encoding as specified in the encoding declaration. If the callback can provide information about the encoding, it must fill in the XML_Encoding structure, and return 1. Otherwise it must return 0. If info does not describe a suitable encoding, then the parser will return an XML_UNKNOWN_ENCODING error. */ typedef int (*XML_UnknownEncodingHandler)(void *encodingHandlerData, const XML_Char *name, XML_Encoding *info);


/* Parses some input. Returns 0 if a fatal error is detected.
The last call to XML_Parse must have isFinal true;
len may be zero for this call (or any other). */
XML_Parse(XML_Parser parser, const char *s, int len, int isFinal);

XML_GetBuffer(XML_Parser parser, int len);

XML_ParseBuffer(XML_Parser parser, int len, int isFinal);

/* If XML_Parse or XML_ParseBuffer have returned 0, then XML_GetErrorCode
returns information about the error. */

enum XML_Error XMLPARSEAPI XML_GetErrorCode(XML_Parser parser);

/* Returns the last value set by XML_SetUserData or null. */
#define XML_GetUserData(parser) (*(void **)(parser))

XML_GetBase(XML_Parser parser);

/* Returns the number of the attributes passed in last call to the
XML_StartElementHandler that were specified in the start-tag rather
than defaulted. */

int XMLPARSEAPI XML_GetSpecifiedAttributeCount(XML_Parser parser);

/* These functions return information about the current parse location.
They may be called when XML_Parse or XML_ParseBuffer return 0;
in this case the location is the location of the character at which
the error was detected.
They may also be called from any other callback called to report
some parse event; in this the location is the location of the first
of the sequence of characters that generated the event. */

int XMLPARSEAPI XML_GetCurrentLineNumber(XML_Parser parser);
int XMLPARSEAPI XML_GetCurrentColumnNumber(XML_Parser parser);
long XMLPARSEAPI XML_GetCurrentByteIndex(XML_Parser parser);

/* Return the number of bytes in the current event.
Returns 0 if the event is in an internal entity. */

int XMLPARSEAPI XML_GetCurrentByteCount(XML_Parser parser);

/* For backwards compatibility with previous versions. */
#define XML_GetErrorLineNumber XML_GetCurrentLineNumber
#define XML_GetErrorColumnNumber XML_GetCurrentColumnNumber
#define XML_GetErrorByteIndex XML_GetCurrentByteIndex

/* Returns a string describing the error. */
const XML_LChar XMLPARSEAPI *XML_ErrorString(int code);


The central data structure of the xmlparse API, the XML_Parser provides a place to hang handlers, user data, and parsing state.

The usual enum of error returns from XML_Parser.
The list of errors is here.

Character types. The default is 8-byte (UTF-8), but if you define either XML_UNICODE or XML_UNICODE_WCHAR_T, then these will be 16-bit values. In the latter case, the type wchar_t will be used. I believe that wchar_t is Windows-specific, but I haven't worked much with 16-bit character types, so I'm probably wrong. So much to learn, so little time.

The #defines used in the above.

struct {
  int map[256];
  void *data;
  int (*convert)(void *data, const char *s);
  void (*release)(void *data);

From the header, but HTMLized a bit:
This structure is filled in by the
XML_UnknownEncodingHandler to provide information to the parser about encodings that are unknown to the parser.
The map[b] member gives information about byte sequences whose first byte is b.

  • If map[b] is c where c is >= 0, then b by itself encodes the Unicode scalar value c.
  • If map[b] is -1, then the byte sequence is malformed.
  • If map[b] is -n, where n >= 2, then b is the first byte of an n-byte sequence that encodes a single Unicode scalar value.
The data member will be passed as the first argument to the convert function. The convert function is used to convert multibyte sequences; s will point to a n-byte sequence where map[(unsigned char)*s] == -n. The convert function must return the Unicode scalar value represented by this byte sequence or -1 if the byte sequence is malformed. The convert function may be null if the encoding is a single-byte encoding, that is if map[b] >= -1 for all bytes b. When the parser is finished with the encoding, then if release is not null, it will call release passing it the data member; once release has been called, the convert function will not be called again.

Expat places certain restrictions on the encodings that are supported using this mechanism.

  • Every ASCII character that can appear in a well-formed XML document, other than the characters
    must be represented by a single byte, and that byte must be the same byte that represents that character in ASCII.
  • No character may require more than 4 bytes to encode.
  • All characters encoded must have Unicode scalar values <= 0xFFFF, (ie characters that would be encoded by surrogates in UTF-16 are not allowed). Note that this restriction doesn't apply to the built-in support for UTF-8 and UTF-16.
  • No Unicode character may be encoded by more than one distinct sequence of bytes.


These errors are of the enum type XML_Error. Use XML_ErrorString to get the descriptive string.
#define name
Message returned by XML_ErrorString
0 XML_ERROR_NONEno error
1 XML_ERROR_NO_MEMORYout of memory
2 XML_ERROR_SYNTAXsyntax error
3 XML_ERROR_NO_ELEMENTSno element found
4 XML_ERROR_INVALID_TOKENnot well-formed
6 XML_ERROR_PARTIAL_CHARunclosed token
7 XML_ERROR_TAG_MISMATCHmismatched tag
9 XML_ERROR_JUNK_AFTER_DOC_ELEMENTjunk after document element
10 XML_ERROR_PARAM_ENTITY_REFillegal parameter entity reference
12 XML_ERROR_RECURSIVE_ENTITY_REFrecursive entity reference
13 XML_ERROR_ASYNC_ENTITYasynchronous entity
14 XML_ERROR_BAD_CHAR_REFreference to invalid character number
15 XML_ERROR_BINARY_ENTITY_REFreference to binary entity
16 XML_ERROR_ATTRIBUTE_EXTERNAL_ENTITY_REFreference to external entity in attribute
17 XML_ERROR_MISPLACED_XML_PIxml processing instruction not at start of external entity
19 XML_ERROR_INCORRECT_ENCODINGencoding specified in XML declaration is incorrect
21 XML_ERROR_EXTERNAL_ENTITY_HANDLINGerror in processing external entity reference
22 XML_ERROR_NOT_STANDALONEdocument is not standalone

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