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| <h1>The XML C library for Gnome</h1> |
| <h2>A real example</h2> |
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| <p>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 <a href="gjobs.xml">XML encoded jobs |
| base</a>:</p> |
| <pre><?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></pre> |
| <p>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 ata and |
| generate the internal structures is harder, and more error prone.</p> |
| <p>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:</p> |
| <pre>/* |
| * 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); |
| }</pre> |
| <p>Here are a couple of things to notice:</p> |
| <ul> |
| <li>Usually a recursive parsing style is the more convenient one: XML data |
| is by nature subject to repetitive constructs and usually exibits highly |
| stuctured patterns.</li> |
| <li>The two arguments of type <em>xmlDocPtr</em> and <em>xmlNsPtr</em>, |
| 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).</li> |
| <li>To retrieve text and attributes value, you can use the function |
| <em>xmlNodeListGetString</em> to gather all the text and entity reference |
| nodes generated by the DOM output and produce an single text string.</li> |
| </ul> |
| <p>Here is another piece of code used to parse another level of the |
| structure:</p> |
| <pre>#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); |
| }</pre> |
| <p>Once you are used to it, writing this kind of code is quite simple, but |
| boring. Ultimately, it could be possble 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 :-)</p> |
| <p>Feel free to use <a href="example/gjobread.c">the code for the full C |
| parsing example</a> as a template, it is also available with Makefile in the |
| Gnome CVS base under gnome-xml/example</p> |
| <p><a href="mailto:daniel@veillard.com">Daniel Veillard</a></p> |
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