The XML C parser and toolkit of Gnome

Here is a real size example, where the actual content of the application data is not kept in the DOM tree but uses internal structures. It is based on a proposal to keep a database of jobs related to Gnome, with an XML based storage structure. Here is an XML encoded jobs base:

<?xml version="1.0"?>
<gjob:Helping xmlns:gjob="http://www.gnome.org/some-location">
  <gjob:Jobs>

    <gjob:Job>
      <gjob:Project ID="3"/>
      <gjob:Application>GBackup</gjob:Application>
      <gjob:Category>Development</gjob:Category>

      <gjob:Update>
        <gjob:Status>Open</gjob:Status>
        <gjob:Modified>Mon, 07 Jun 1999 20:27:45 -0400 MET DST</gjob:Modified>
        <gjob:Salary>USD 0.00</gjob:Salary>
      </gjob:Update>

      <gjob:Developers>
        <gjob:Developer>
        </gjob:Developer>
      </gjob:Developers>

      <gjob:Contact>
        <gjob:Person>Nathan Clemons</gjob:Person>
        <gjob:Email>nathan@windsofstorm.net</gjob:Email>
        <gjob:Company>
        </gjob:Company>
        <gjob:Organisation>
        </gjob:Organisation>
        <gjob:Webpage>
        </gjob:Webpage>
        <gjob:Snailmail>
        </gjob:Snailmail>
        <gjob:Phone>
        </gjob:Phone>
      </gjob:Contact>

      <gjob:Requirements>
      The program should be released as free software, under the GPL.
      </gjob:Requirements>

      <gjob:Skills>
      </gjob:Skills>

      <gjob:Details>
      A GNOME based system that will allow a superuser to configure 
      compressed and uncompressed files and/or file systems to be backed 
      up with a supported media in the system.  This should be able to 
      perform via find commands generating a list of files that are passed 
      to tar, dd, cpio, cp, gzip, etc., to be directed to the tape machine 
      or via operations performed on the filesystem itself. Email 
      notification and GUI status display very important.
      </gjob:Details>

    </gjob:Job>

  </gjob:Jobs>
</gjob:Helping>

While loading the XML file into an internal DOM tree is a matter of calling only a couple of functions, browsing the tree to gather the data and generate the internal structures is harder, and more error prone.

The suggested principle is to be tolerant with respect to the input structure. For example, the ordering of the attributes is not significant, the XML specification is clear about it. It's also usually a good idea not to depend on the order of the children of a given node, unless it really makes things harder. Here is some code to parse the information for a person:

/*
 * A person record
 */
typedef struct person {
    char *name;
    char *email;
    char *company;
    char *organisation;
    char *smail;
    char *webPage;
    char *phone;
} person, *personPtr;

/*
 * And the code needed to parse it
 */
personPtr parsePerson(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
    personPtr ret = NULL;

DEBUG("parsePerson\n");
    /*
     * allocate the struct
     */
    ret = (personPtr) malloc(sizeof(person));
    if (ret == NULL) {
        fprintf(stderr,"out of memory\n");
        return(NULL);
    }
    memset(ret, 0, sizeof(person));

    /* We don't care what the top level element name is */
    cur = cur->xmlChildrenNode;
    while (cur != NULL) {
        if ((!strcmp(cur->name, "Person")) && (cur->ns == ns))
            ret->name = xmlNodeListGetString(doc, cur->xmlChildrenNode, 1);
        if ((!strcmp(cur->name, "Email")) && (cur->ns == ns))
            ret->email = xmlNodeListGetString(doc, cur->xmlChildrenNode, 1);
        cur = cur->next;
    }

    return(ret);
}

Here are a couple of things to notice:

• Usually a recursive parsing style is the more convenient one: XML data is by nature subject to repetitive constructs and usually exhibits highly structured patterns.
• The two arguments of type xmlDocPtr and xmlNsPtr, i.e. the pointer to the global XML document and the namespace reserved to the application. Document wide information are needed for example to decode entities and it's a good coding practice to define a namespace for your application set of data and test that the element and attributes you're analyzing actually pertains to your application space. This is done by a simple equality test (cur->ns == ns).
• To retrieve text and attributes value, you can use the function xmlNodeListGetString to gather all the text and entity reference nodes generated by the DOM output and produce an single text string.

Here is another piece of code used to parse another level of the structure:

#include <libxml/tree.h>
/*
 * a Description for a Job
 */
typedef struct job {
    char *projectID;
    char *application;
    char *category;
    personPtr contact;
    int nbDevelopers;
    personPtr developers[100]; /* using dynamic alloc is left as an exercise */
} job, *jobPtr;

/*
 * And the code needed to parse it
 */
jobPtr parseJob(xmlDocPtr doc, xmlNsPtr ns, xmlNodePtr cur) {
    jobPtr ret = NULL;

DEBUG("parseJob\n");
    /*
     * allocate the struct
     */
    ret = (jobPtr) malloc(sizeof(job));
    if (ret == NULL) {
        fprintf(stderr,"out of memory\n");
        return(NULL);
    }
    memset(ret, 0, sizeof(job));

    /* We don't care what the top level element name is */
    cur = cur->xmlChildrenNode;
    while (cur != NULL) {
        
        if ((!strcmp(cur->name, "Project")) && (cur->ns == ns)) {
            ret->projectID = xmlGetProp(cur, "ID");
            if (ret->projectID == NULL) {
                fprintf(stderr, "Project has no ID\n");
            }
        }
        if ((!strcmp(cur->name, "Application")) && (cur->ns == ns))
            ret->application = xmlNodeListGetString(doc, cur->xmlChildrenNode, 1);
        if ((!strcmp(cur->name, "Category")) && (cur->ns == ns))
            ret->category = xmlNodeListGetString(doc, cur->xmlChildrenNode, 1);
        if ((!strcmp(cur->name, "Contact")) && (cur->ns == ns))
            ret->contact = parsePerson(doc, ns, cur);
        cur = cur->next;
    }

    return(ret);
}

Once you are used to it, writing this kind of code is quite simple, but boring. Ultimately, it could be possible to write stubbers taking either C data structure definitions, a set of XML examples or an XML DTD and produce the code needed to import and export the content between C data and XML storage. This is left as an exercise to the reader :-)

Feel free to use the code for the full C parsing example as a template, it is also available with Makefile in the Gnome SVN base under libxml2/example

Daniel Veillard