The XML C parser and toolkit of Gnome

This section is directly intended to help programmers getting bootstrapped using the XML tollkit from the C language. It is not intended to be extensive. I hope the automatically generated documents will provide the completeness required, but as a separate set of documents. The interfaces of the XML parser are by principle low level, Those interested in a higher level API should look at DOM.

The parser interfaces for XML are separated from the HTML parser interfaces. Let's have a look at how the XML parser can be called:

Invoking the parser : the pull method

Usually, the first thing to do is to read an XML input. The parser accepts documents either from in-memory strings or from files. The functions are defined in "parser.h":

xmlDocPtr xmlParseMemory(char *buffer, int size);

Parse a null-terminated string containing the document.

xmlDocPtr xmlParseFile(const char *filename);

Parse an XML document contained in a (possibly compressed) file.

The parser returns a pointer to the document structure (or NULL in case of failure).

Invoking the parser: the push method

In order for the application to keep the control when the document is being fetched (which is common for GUI based programs) libxml2 provides a push interface, too, as of version 1.8.3. Here are the interface functions:

xmlParserCtxtPtr xmlCreatePushParserCtxt(xmlSAXHandlerPtr sax,
                                         void *user_data,
                                         const char *chunk,
                                         int size,
                                         const char *filename);
int              xmlParseChunk          (xmlParserCtxtPtr ctxt,
                                         const char *chunk,
                                         int size,
                                         int terminate);

and here is a simple example showing how to use the interface:

            FILE *f;

            f = fopen(filename, "r");
            if (f != NULL) {
                int res, size = 1024;
                char chars[1024];
                xmlParserCtxtPtr ctxt;

                res = fread(chars, 1, 4, f);
                if (res > 0) {
                    ctxt = xmlCreatePushParserCtxt(NULL, NULL,
                                chars, res, filename);
                    while ((res = fread(chars, 1, size, f)) > 0) {
                        xmlParseChunk(ctxt, chars, res, 0);
                    }
                    xmlParseChunk(ctxt, chars, 0, 1);
                    doc = ctxt->myDoc;
                    xmlFreeParserCtxt(ctxt);
                }
            }

The HTML parser embedded into libxml2 also has a push interface; the functions are just prefixed by "html" rather than "xml".

Invoking the parser: the SAX interface

The tree-building interface makes the parser memory-hungry, first loading the document in memory and then building the tree itself. Reading a document without building the tree is possible using the SAX interfaces (see SAX.h and James Henstridge's documentation). Note also that the push interface can be limited to SAX: just use the two first arguments of xmlCreatePushParserCtxt().

Building a tree from scratch

The other way to get an XML tree in memory is by building it. Basically there is a set of functions dedicated to building new elements. (These are also described in <libxml/tree.h>.) For example, here is a piece of code that produces the XML document used in the previous examples:

    #include <libxml/tree.h>
    xmlDocPtr doc;
    xmlNodePtr tree, subtree;

    doc = xmlNewDoc("1.0");
    doc->children = xmlNewDocNode(doc, NULL, "EXAMPLE", NULL);
    xmlSetProp(doc->children, "prop1", "gnome is great");
    xmlSetProp(doc->children, "prop2", "& linux too");
    tree = xmlNewChild(doc->children, NULL, "head", NULL);
    subtree = xmlNewChild(tree, NULL, "title", "Welcome to Gnome");
    tree = xmlNewChild(doc->children, NULL, "chapter", NULL);
    subtree = xmlNewChild(tree, NULL, "title", "The Linux adventure");
    subtree = xmlNewChild(tree, NULL, "p", "bla bla bla ...");
    subtree = xmlNewChild(tree, NULL, "image", NULL);
    xmlSetProp(subtree, "href", "linus.gif");

Not really rocket science ...

Traversing the tree

Basically by including "tree.h" your code has access to the internal structure of all the elements of the tree. The names should be somewhat simple like parent, children, next, prev, properties, etc... For example, still with the previous example:

doc->children->children->children

points to the title element,

doc->children->children->next->children->children

points to the text node containing the chapter title "The Linux adventure".

NOTE: XML allows PIs and comments to be present before the document root, so doc->children may point to an element which is not the document Root Element; a function xmlDocGetRootElement() was added for this purpose.

Modifying the tree

Functions are provided for reading and writing the document content. Here is an excerpt from the tree API:

xmlAttrPtr xmlSetProp(xmlNodePtr node, const xmlChar *name, const xmlChar *value);

This sets (or changes) an attribute carried by an ELEMENT node. The value can be NULL.

const xmlChar *xmlGetProp(xmlNodePtr node, const xmlChar *name);

This function returns a pointer to new copy of the property content. Note that the user must deallocate the result.

Two functions are provided for reading and writing the text associated with elements:

xmlNodePtr xmlStringGetNodeList(xmlDocPtr doc, const xmlChar *value);

This function takes an "external" string and converts it to one text node or possibly to a list of entity and text nodes. All non-predefined entity references like &Gnome; will be stored internally as entity nodes, hence the result of the function may not be a single node.

xmlChar *xmlNodeListGetString(xmlDocPtr doc, xmlNodePtr list, int inLine);

This function is the inverse of xmlStringGetNodeList(). It generates a new string containing the content of the text and entity nodes. Note the extra argument inLine. If this argument is set to 1, the function will expand entity references. For example, instead of returning the &Gnome; XML encoding in the string, it will substitute it with its value (say, "GNU Network Object Model Environment").

Saving a tree

Basically 3 options are possible:

void xmlDocDumpMemory(xmlDocPtr cur, xmlChar**mem, int *size);

Returns a buffer into which the document has been saved.

extern void xmlDocDump(FILE *f, xmlDocPtr doc);

Dumps a document to an open file descriptor.

int xmlSaveFile(const char *filename, xmlDocPtr cur);

Saves the document to a file. In this case, the compression interface is triggered if it has been turned on.

Compression

The library transparently handles compression when doing file-based accesses. The level of compression on saves can be turned on either globally or individually for one file:

int xmlGetDocCompressMode (xmlDocPtr doc);

Gets the document compression ratio (0-9).

void xmlSetDocCompressMode (xmlDocPtr doc, int mode);

Sets the document compression ratio.

int xmlGetCompressMode(void);

Gets the default compression ratio.

void xmlSetCompressMode(int mode);

Sets the default compression ratio.

Daniel Veillard