# The XML C parser and toolkit of Gnome ## The parser interfaces

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This section is directly intended to help programmers gettingbootstrappedusing the XML tollkit from the C language. It is not intended tobeextensive. I hope the automatically generated documents will providethecompleteness required, but as a separate set of documents. The interfacesofthe XML parser are by principle low level, Those interested in a higherlevelAPI should look at DOM. The parser interfaces forXMLareseparated from the HTMLparserinterfaces. Let's have a look at how the XML parser can becalled: ### Invoking the parser : the pull method Usually, the first thing to do is to read an XML input. The parseracceptsdocuments either from in-memory strings or from files. The functionsaredefined 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 caseoffailure). ### Invoking the parser: the push method In order for the application to keep the control when the document isbeingfetched (which is common for GUI based programs) libxml2 provides apushinterface, too, as of version 1.8.3. Here are the interfacefunctions:
    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;thefunctions are just prefixed by "html" rather than "xml". ### Invoking the parser: the SAX interface The tree-building interface makes the parser memory-hungry, firstloadingthe document in memory and then building the tree itself. Reading adocumentwithout building the tree is possible using the SAX interfaces (seeSAX.h andJamesHenstridge'sdocumentation). Note also that the push interface can belimited 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.Basicallythere is a set of functions dedicated to building new elements.(These arealso described in <libxml/tree.h>.) For example, here is apiece ofcode 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"yourcode has access to the internal structure of all the elementsof the tree.The names should be somewhat simple likeparent,children, next,prev,properties, etc... For example, stillwith the previousexample:
    doc->children->children->children
points to the title element,
    doc->children->children->next->children->children
points to the text node containing the chapter title "TheLinuxadventure". NOTE: XML allows PIs and commentstobepresent before the document root, so `doc->children`maypointto an element which is not the document Root Element; afunction`xmlDocGetRootElement()`was added for this purpose. ### Modifying the tree Functions are provided for reading and writing the document content.Hereis an excerpt from the tree API: `xmlAttrPtr xmlSetProp(xmlNodePtr node, const xmlChar *name,constxmlChar *value);` This sets (or changes) an attribute carried by an ELEMENT node.Thevalue can be NULL. `const xmlChar *xmlGetProp(xmlNodePtr node, constxmlChar*name);` This function returns a pointer to new copy of thepropertycontent. Note that the user must deallocate the result. Two functions are provided for reading and writing the text associatedwithelements: `xmlNodePtr xmlStringGetNodeList(xmlDocPtr doc, constxmlChar*value);` This function takes an "external" string and converts it toonetext node or possibly to a list of entity and text nodes.Allnon-predefined entity references like &Gnome; will bestoredinternally as entity nodes, hence the result of the function maynot bea single node. `xmlChar *xmlNodeListGetString(xmlDocPtr doc, xmlNodePtr list,intinLine);` This function is the inverseof`xmlStringGetNodeList()`. It generates a newstringcontaining the content of the text and entity nodes. Note theextraargument inLine. If this argument is set to 1, the function willexpandentity references. For example, instead of returning the&Gnome;XML encoding in the string, it will substitute it with itsvalue (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, thecompressioninterface is triggered if it has been turned on. ### Compression The library transparently handles compression when doingfile-basedaccesses. The level of compression on saves can be turned on eithergloballyor 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