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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><link rel="SHORTCUT ICON" href="/favicon.ico" /><style type="text/css">
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TD {font-family: Verdana,Arial,Helvetica}
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BODY {font-family: Verdana,Arial,Helvetica; margin-top: 2em; margin-left: 0em; margin-right: 0em}
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H1 {font-family: Verdana,Arial,Helvetica}
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H2 {font-family: Verdana,Arial,Helvetica}
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H3 {font-family: Verdana,Arial,Helvetica}
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A:link, A:visited, A:active { text-decoration: underline }
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</style><title>A real example</title></head><body bgcolor="#8b7765" text="#000000" link="#a06060" vlink="#000000">
Action against software patentsGnome2 LogoW3C LogoRed Hat Logo
Made with Libxml2 Logo
The XML C parser and toolkit of Gnome
A real example
<center>Developer Menu</center>
<form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="20" value="" /><input name="submit" type="submit" value="Search ..." /></form>
<center>API Indexes</center>
<center>Related links</center>
Here is a real size example, where the actual content of the application
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data is not kept in the DOM tree but uses internal structures. It is based on
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a proposal to keep a database of jobs related to Gnome, with an XML based
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storage structure. Here is an XML encoded jobs
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base:
<?xml version="1.0"?>
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<gjob:Helping xmlns:gjob="http://www.gnome.org/some-location">
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  <gjob:Jobs>
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    <gjob:Job>
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      <gjob:Project ID="3"/>
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      <gjob:Application>GBackup</gjob:Application>
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      <gjob:Category>Development</gjob:Category>
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      <gjob:Update>
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        <gjob:Status>Open</gjob:Status>
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        <gjob:Modified>Mon, 07 Jun 1999 20:27:45 -0400 MET DST</gjob:Modified>
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        <gjob:Salary>USD 0.00</gjob:Salary>
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      </gjob:Update>
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      <gjob:Developers>
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        <gjob:Developer>
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        </gjob:Developer>
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      </gjob:Developers>
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      <gjob:Contact>
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        <gjob:Person>Nathan Clemons</gjob:Person>
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        <gjob:Email>nathan@windsofstorm.net</gjob:Email>
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        <gjob:Company>
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        </gjob:Company>
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        <gjob:Organisation>
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        </gjob:Organisation>
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        <gjob:Webpage>
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        </gjob:Webpage>
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        <gjob:Snailmail>
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        </gjob:Snailmail>
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        <gjob:Phone>
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        </gjob:Phone>
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      </gjob:Contact>
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      <gjob:Requirements>
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      The program should be released as free software, under the GPL.
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      </gjob:Requirements>
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      <gjob:Skills>
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      </gjob:Skills>
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      <gjob:Details>
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      A GNOME based system that will allow a superuser to configure
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      compressed and uncompressed files and/or file systems to be backed
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      up with a supported media in the system.  This should be able to
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      perform via find commands generating a list of files that are passed
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      to tar, dd, cpio, cp, gzip, etc., to be directed to the tape machine
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      or via operations performed on the filesystem itself. Email
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      notification and GUI status display very important.
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      </gjob:Details>
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    </gjob:Job>
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  </gjob:Jobs>
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</gjob:Helping>
While loading the XML file into an internal DOM tree is a matter of
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calling only a couple of functions, browsing the tree to gather the data and
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generate the internal structures is harder, and more error prone.
The suggested principle is to be tolerant with respect to the input
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structure. For example, the ordering of the attributes is not significant,
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the XML specification is clear about it. It's also usually a good idea not to
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depend on the order of the children of a given node, unless it really makes
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things harder. Here is some code to parse the information for a person:
/*
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 * A person record
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 */
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typedef struct person {
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    char *name;
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    char *email;
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    char *company;
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    char *organisation;
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    char *smail;
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    char *webPage;
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    char *phone;
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} person, *personPtr;
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/*
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 * And the code needed to parse it
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 */
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personPtr parsePerson(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
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    personPtr ret = NULL;
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DEBUG("parsePerson\n");
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    /*
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     * allocate the struct
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     */
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    ret = (personPtr) malloc(sizeof(person));
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    if (ret == NULL) {
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        fprintf(stderr,"out of memory\n");
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        return(NULL);
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    }
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    memset(ret, 0, sizeof(person));
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    /* We don't care what the top level element name is */
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    cur = cur->xmlChildrenNode;
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    while (cur != NULL) {
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        if ((!strcmp(cur->name, "Person")) && (cur->ns == ns))
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            ret->name = xmlNodeListGetString(doc, cur->xmlChildrenNode, 1);
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        if ((!strcmp(cur->name, "Email")) && (cur->ns == ns))
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            ret->email = xmlNodeListGetString(doc, cur->xmlChildrenNode, 1);
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        cur = cur->next;
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    }
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    return(ret);
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}
Here are a couple of things to notice:
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  • Usually a recursive parsing style is the more convenient one: XML data
    		•
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        is by nature subject to repetitive constructs and usually exhibits highly
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        structured patterns.
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  • The two arguments of type xmlDocPtr and xmlNsPtr,
    		•
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        i.e. the pointer to the global XML document and the namespace reserved to
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        the application. Document wide information are needed for example to
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        decode entities and it's a good coding practice to define a namespace for
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        your application set of data and test that the element and attributes
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        you're analyzing actually pertains to your application space. This is
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        done by a simple equality test (cur->ns == ns).
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  • To retrieve text and attributes value, you can use the function
    		•
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        xmlNodeListGetString to gather all the text and entity reference
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        nodes generated by the DOM output and produce an single text string.
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    Here is another piece of code used to parse another level of the
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    structure:
    #include <libxml/tree.h>
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    /*
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     * a Description for a Job
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     */
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    typedef struct job {
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        char *projectID;
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        char *application;
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        char *category;
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        personPtr contact;
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        int nbDevelopers;
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        personPtr developers[100]; /* using dynamic alloc is left as an exercise */
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    } job, *jobPtr;
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    /*
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     * And the code needed to parse it
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     */
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    jobPtr parseJob(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
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        jobPtr ret = NULL;
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    DEBUG("parseJob\n");
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        /*
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         * allocate the struct
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         */
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        ret = (jobPtr) malloc(sizeof(job));
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        if (ret == NULL) {
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            fprintf(stderr,"out of memory\n");
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            return(NULL);
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        }
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        memset(ret, 0, sizeof(job));
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        /* We don't care what the top level element name is */
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        cur = cur->xmlChildrenNode;
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        while (cur != NULL) {
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            if ((!strcmp(cur->name, "Project")) && (cur->ns == ns)) {
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                ret->projectID = xmlGetProp(cur, "ID");
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                if (ret->projectID == NULL) {
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                    fprintf(stderr, "Project has no ID\n");
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                }
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            }
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            if ((!strcmp(cur->name, "Application")) && (cur->ns == ns))
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                ret->application = xmlNodeListGetString(doc, cur->xmlChildrenNode, 1);
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            if ((!strcmp(cur->name, "Category")) && (cur->ns == ns))
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                ret->category = xmlNodeListGetString(doc, cur->xmlChildrenNode, 1);
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            if ((!strcmp(cur->name, "Contact")) && (cur->ns == ns))
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                ret->contact = parsePerson(doc, ns, cur);
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            cur = cur->next;
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        }
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        return(ret);
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    }
    Once you are used to it, writing this kind of code is quite simple, but
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    boring. Ultimately, it could be possible to write stubbers taking either C
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    data structure definitions, a set of XML examples or an XML DTD and produce
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    the code needed to import and export the content between C data and XML
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    storage. This is left as an exercise to the reader :-)
    Feel free to use the code for the full C
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    parsing example as a template, it is also available with Makefile in the
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    Gnome SVN base under libxml2/example
    Daniel Veillard
    </body></html>

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