doc/advanced.html

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<h1 align='right'><a name='ADVANCED'><img src="3.gif" align="right"
hspace="10" width="100" height="100" alt="3"></a>More Mini-XML
Programming Techniques</h1>

<p>This chapter shows additional ways to use the Mini-XML
library in your programs.</p>

<h2><a name='LOAD_CALLBACKS'>Load Callbacks</a></h2>

<p><a href='#LOAD_XML'>Chapter 2</a> introduced the <a
href='#mxmlLoadFile'><tt>mxmlLoadFile()</tt></a> and <a
href='#mxmlLoadString'><tt>mxmlLoadString()</tt></a> functions.
The last argument to these functions is a callback function
which is used to determine the value type of each data node in
an XML document.</p>

<p>Mini-XML defines several standard callbacks for simple
XML data files:</p>

<ul>

        <li><tt>MXML_INTEGER_CALLBACK</tt> - All data nodes
        contain whitespace-separated integers.</li>

        <li><tt>MXML_OPAQUE_CALLBACK</tt> - All data nodes
        contain opaque strings ("CDATA").</li>

	<li><tt>MXML_REAL_CALLBACK</tt> - All data nodes contain
	whitespace-separated floating-point numbers.</li>

	<li><tt>MXML_TEXT_CALLBACK</tt> - All data nodes contain
	whitespace-separated strings.</li>

</ul>

<p>You can provide your own callback functions for more complex
XML documents. Your callback function will receive a pointer to
the current element node and must return the value type of the
immediate children for that element node: <tt>MXML_INTEGER</tt>,
<tt>MXML_OPAQUE</tt>, <tt>MXML_REAL</tt>, or <tt>MXML_TEXT</tt>.
The function is called <i>after</i> the element and its
attributes have been read, so you can look at the element name,
attributes, and attribute values to determine the proper value
type to return.</p>

<!-- NEED 2in -->
<p>The following callback function looks for an attribute named
"type" or the element name to determine the value type for its
child nodes:</p>

<pre>
    mxml_type_t
    type_cb(mxml_node_t *node)
    {
      const char *type;

     /*
      * You can lookup attributes and/or use the
      * element name, hierarchy, etc...
      */

      type = mxmlElementGetAttr(node, "type");
      if (type == NULL)
	type = mxmlGetElement(node);

      if (!strcmp(type, "integer"))
	return (MXML_INTEGER);
      else if (!strcmp(type, "opaque"))
	return (MXML_OPAQUE);
      else if (!strcmp(type, "real"))
	return (MXML_REAL);
      else
	return (MXML_TEXT);
    }
</pre>

<p>To use this callback function, simply use the name when you
call any of the load functions:</p>

<pre>
    FILE *fp;
    mxml_node_t *tree;

    fp = fopen("filename.xml", "r");
    tree = mxmlLoadFile(NULL, fp, <b>type_cb</b>);
    fclose(fp);
</pre>


<h2><a name='SAVE_CALLBACKS'>Save Callbacks</a></h2>

<p><a href='#LOAD_XML'>Chapter 2</a> also introduced the <a
href='#mxmlSaveFile'><tt>mxmlSaveFile()</tt></a>, <a
href='#mxmlSaveString'><tt>mxmlSaveString()</tt></a>, and <a
href='#mxmlSaveAllocString'><tt>mxmlSaveAllocString()</tt></a>
functions. The last argument to these functions is a callback
function which is used to automatically insert whitespace in an
XML document.</p>

<p>Your callback function will be called up to four times for
each element node with a pointer to the node and a "where" value
of <tt>MXML_WS_BEFORE_OPEN</tt>, <tt>MXML_WS_AFTER_OPEN</tt>,
<tt>MXML_WS_BEFORE_CLOSE</tt>, or <tt>MXML_WS_AFTER_CLOSE</tt>.
The callback function should return <tt>NULL</tt> if no
whitespace should be added and the string to insert (spaces,
tabs, carriage returns, and newlines) otherwise.</p>

<p>The following whitespace callback can be used to add
whitespace to XHTML output to make it more readable in a standard
text editor:</p>

<pre>
    const char *
    whitespace_cb(mxml_node_t *node,
                  int where)
    {
      const char *name;

     /*
      * We can conditionally break to a new line
      * before or after any element. These are
      * just common HTML elements...
      */

      name = mxmlGetElement(node);

      if (!strcmp(name, "html") ||
          !strcmp(name, "head") ||
          !strcmp(name, "body") ||
	  !strcmp(name, "pre") ||
          !strcmp(name, "p") ||
	  !strcmp(name, "h1") ||
          !strcmp(name, "h2") ||
          !strcmp(name, "h3") ||
	  !strcmp(name, "h4") ||
          !strcmp(name, "h5") ||
          !strcmp(name, "h6"))
      {
       /*
	* Newlines before open and after
        * close...
	*/

	if (where == MXML_WS_BEFORE_OPEN ||
            where == MXML_WS_AFTER_CLOSE)
	  return ("\n");
      }
      else if (!strcmp(name, "dl") ||
               !strcmp(name, "ol") ||
               !strcmp(name, "ul"))
      {
       /*
	* Put a newline before and after list
        * elements...
	*/

	return ("\n");
      }
      else if (!strcmp(name, "dd") ||
               !strcmp(name, "dt") ||
               !strcmp(name, "li"))
      {
       /*
	* Put a tab before &lt;li>'s, * &lt;dd>'s,
        * and &lt;dt>'s, and a newline after them...
	*/

	if (where == MXML_WS_BEFORE_OPEN)
	  return ("\t");
	else if (where == MXML_WS_AFTER_CLOSE)
	  return ("\n");
      }

     /*
      * Return NULL for no added whitespace...
      */

      return (NULL);
    }
</pre>

<p>To use this callback function, simply use the name when you
call any of the save functions:</p>

<pre>
    FILE *fp;
    mxml_node_t *tree;

    fp = fopen("filename.xml", "w");
    mxmlSaveFile(tree, fp, <b>whitespace_cb</b>);
    fclose(fp);
</pre>


<!-- NEED 10 -->
<h2>Custom Data Types</h2>

<p>Mini-XML supports custom data types via global load and save
callbacks. Only a single set of callbacks can be active at any
time, however your callbacks can store additional information in
order to support multiple custom data types as needed. The
<tt>MXML_CUSTOM</tt> node type identifies custom data nodes.</p>

<p>The load callback receives a pointer to the current data node
and a string of opaque character data from the XML source with
character entities converted to the corresponding UTF-8
characters. For example, if we wanted to support a custom
date/time type whose value is encoded as "yyyy-mm-ddThh:mm:ssZ"
(ISO format), the load callback would look like the
following:</p>

<pre>
    typedef struct
    {
      unsigned      year,    /* Year */
                    month,   /* Month */
                    day,     /* Day */
                    hour,    /* Hour */
                    minute,  /* Minute */
                    second;  /* Second */
      time_t        unix;    /* UNIX time */
    } iso_date_time_t;

    int
    load_custom(mxml_node_t *node,
                const char *data)
    {
      iso_date_time_t *dt;
      struct tm tmdata;

     /*
      * Allocate data structure...
      */

      dt = calloc(1, sizeof(iso_date_time_t));

     /*
      * Try reading 6 unsigned integers from the
      * data string...
      */

      if (sscanf(data, "%u-%u-%uT%u:%u:%uZ",
                 &amp;(dt->year), &amp;(dt->month),
                 &amp;(dt->day), &amp;(dt->hour),
                 &amp;(dt->minute),
                 &amp;(dt->second)) != 6)
      {
       /*
        * Unable to read numbers, free the data
        * structure and return an error...
        */

        free(dt);

        return (-1);
      }

     /*
      * Range check values...
      */

      if (dt->month < 1 || dt->month > 12 ||
          dt->day < 1 || dt->day > 31 ||
          dt->hour < 0 || dt->hour > 23 ||
          dt->minute < 0 || dt->minute > 59 ||
          dt->second < 0 || dt->second > 59)
      {
       /*
        * Date information is out of range...
        */

        free(dt);

        return (-1);
      }

     /*
      * Convert ISO time to UNIX time in
      * seconds...
      */

      tmdata.tm_year = dt->year - 1900;
      tmdata.tm_mon  = dt->month - 1;
      tmdata.tm_day  = dt->day;
      tmdata.tm_hour = dt->hour;
      tmdata.tm_min  = dt->minute;
      tmdata.tm_sec  = dt->second;

      dt->unix = gmtime(&amp;tmdata);

     /*
      * Assign custom node data and destroy
      * function pointers...
      */

      mxmlSetCustom(node, data, destroy);

     /*
      * Return with no errors...
      */

      return (0);
    }
</pre>

<p>The function itself can return 0 on success or -1 if it is
unable to decode the custom data or the data contains an error.
Custom data nodes contain a <tt>void</tt> pointer to the
allocated custom data for the node and a pointer to a destructor
function which will free the custom data when the node is
deleted.</p>

<!-- NEED 15 -->
<p>The save callback receives the node pointer and returns an
allocated string containing the custom data value. The following
save callback could be used for our ISO date/time type:</p>

<pre>
    char *
    save_custom(mxml_node_t *node)
    {
      char data[255];
      iso_date_time_t *dt;


      dt = (iso_date_time_t *)mxmlGetCustom(node);

      snprintf(data, sizeof(data),
               "%04u-%02u-%02uT%02u:%02u:%02uZ",
               dt->year, dt->month, dt->day,
               dt->hour, dt->minute, dt->second);

      return (strdup(data));
    }
</pre>

<p>You register the callback functions using the <a
href='#mxmlSetCustomHandlers'><tt>mxmlSetCustomHandlers()</tt></a>
function:</p>

<pre>
    mxmlSetCustomHandlers(<b>load_custom</b>,
                          <b>save_custom</b>);
</pre>


<!-- NEED 20 -->
<h2>Changing Node Values</h2>

<p>All of the examples so far have concentrated on creating and
loading new XML data nodes. Many applications, however, need to
manipulate or change the nodes during their operation, so
Mini-XML provides functions to change node values safely and
without leaking memory.</p>

<p>Existing nodes can be changed using the <a
href='#mxmlSetElement'><tt>mxmlSetElement()</tt></a>, <a
href='#mxmlSetInteger'><tt>mxmlSetInteger()</tt></a>, <a
href='#mxmlSetOpaque'><tt>mxmlSetOpaque()</tt></a>, <a
href='#mxmlSetReal'><tt>mxmlSetReal()</tt></a>, <a
href='#mxmlSetText'><tt>mxmlSetText()</tt></a>, and <a
href='#mxmlSetTextf'><tt>mxmlSetTextf()</tt></a> functions. For
example, use the following function call to change a text node
to contain the text "new" with leading whitespace:</p>

<pre>
    mxml_node_t *node;

    mxmlSetText(node, 1, "new");
</pre>


<h2>Formatted Text</h2>

<p>The <a href='#mxmlNewTextf'><tt>mxmlNewTextf()</tt></a> and <a
href='#mxmlSetTextf'><tt>mxmlSetTextf()</tt></a> functions create
and change text nodes, respectively, using <tt>printf</tt>-style
format strings and arguments. For example, use the following
function call to create a new text node containing a constructed
filename:</p>

<pre>
    mxml_node_t</a> *node;

    node = mxmlNewTextf(node, 1, "%s/%s",
                        path, filename);
</pre>


<h2>Indexing</h2>

<p>Mini-XML provides functions for managing indices of nodes.
The current implementation provides the same functionality as
<a href='#mxmlFindElement'><tt>mxmlFindElement()</tt></a>.
The advantage of using an index is that searching and
enumeration of elements is significantly faster. The only
disadvantage is that each index is a static snapshot of the XML
document, so indices are not well suited to XML data that is
updated more often than it is searched. The overhead of creating
an index is approximately equal to walking the XML document
tree. Nodes in the index are sorted by element name and
attribute value.</p>

<p>Indices are stored in <a href='#mxml_index_t'><tt>mxml_index_t</tt></a>
structures. The <a href='#mxmlIndexNew'><tt>mxmlIndexNew()</tt></a> function
creates a new index:</p>

<pre>
    mxml_node_t *tree;
    mxml_index_t *ind;

    ind = mxmlIndexNew(tree, "element",
                       "attribute");
</pre>

<p>The first argument is the XML node tree to index. Normally this
will be a pointer to the <tt>?xml</tt> element.</p>

<p>The second argument contains the element to index; passing
<tt>NULL</tt> indexes all element nodes alphabetically.</p>

<p>The third argument contains the attribute to index; passing
<tt>NULL</tt> causes only the element name to be indexed.</p>

<p>Once the index is created, the <a
href='#mxmlIndexEnum'><tt>mxmlIndexEnum()</tt></a>,  <a
href='#mxmlIndexFind'><tt>mxmlIndexFind()</tt></a>, and  <a
href='#mxmlIndexReset'><tt>mxmlIndexReset()</tt></a> functions
are used to access the nodes in the index. The <a
href='#mxmlIndexReset'><tt>mxmlIndexReset()</tt></a> function
resets the "current" node pointer in the index, allowing you to
do new searches and enumerations on the same index. Typically
you will call this function prior to your calls to <a
href='#mxmlIndexEnum'><tt>mxmlIndexEnum()</tt></a> and <a
href='#mxmlIndexFind'><tt>mxmlIndexFind()</tt></a>.</p>

<p>The <a href='#mxmlIndexEnum'><tt>mxmlIndexEnum()</tt></a>
function enumerates each of the nodes in the index and can be
used in a loop as follows:</p>

<pre>
    mxml_node_t *node;

    mxmlIndexReset(ind);

    while ((node = mxmlIndexEnum(ind)) != NULL)
    {
      // do something with node
    }
</pre>

<p>The <a href='#mxmlIndexFind'><tt>mxmlIndexFind()</tt></a>
function locates the next occurrence of the named element and
attribute value in the index. It can be used to find all
matching elements in an index, as follows:</p>

<pre>
    mxml_node_t *node;

    mxmlIndexReset(ind);

    while ((node = mxmlIndexFind(ind, "element",
                                 "attr-value"))
                != NULL)
    {
      // do something with node
    }
</pre>

<p>The second and third arguments represent the element name and
attribute value, respectively. A <tt>NULL</tt> pointer is used
to return all elements or attributes in the index. Passing
<tt>NULL</tt> for both the element name and attribute value
is equivalent to calling <tt>mxmlIndexEnum</tt>.</p>

<p>When you are done using the index, delete it using the
<a href='#mxmlIndexDelete()'><tt>mxmlIndexDelete()</tt></a>
function:</p>

<pre>
    mxmlIndexDelete(ind);
</pre>

<h2>SAX (Stream) Loading of Documents</h2>

<p>Mini-XML supports an implementation of the Simple API for XML
(SAX) which allows you to load and process an XML document as a
stream of nodes. Aside from allowing you to process XML documents of
any size, the Mini-XML implementation also allows you to retain
portions of the document in memory for later processing.</p>

<p>The <a href='#mxmlSAXLoad'><tt>mxmlSAXLoadFd</tt></a>, <a
href='#mxmlSAXLoadFile'><tt>mxmlSAXLoadFile</tt></a>, and <a
href='#mxmlSAXLoadString'><tt>mxmlSAXLoadString</tt></a> functions
provide the SAX loading APIs. Each function works like the
corresponding <tt>mxmlLoad</tt> function but uses a callback to
process each node as it is read.</p>

<p>The callback function receives the node, an event code, and
a user data pointer you supply:</p>

<pre>
    void
    sax_cb(mxml_node_t *node,
           mxml_sax_event_t event,
           void *data)
    {
      ... do something ...
    }
</pre>

<p>The event will be one of the following:</p>

<ul>

	<li><tt>MXML_SAX_CDATA</tt> - CDATA was just read</li>

	<li><tt>MXML_SAX_COMMENT</tt> - A comment was just read</li>

	<li><tt>MXML_SAX_DATA</tt> - Data (custom, integer, opaque, real, or text) was just read</li>

	<li><tt>MXML_SAX_DIRECTIVE</tt> - A processing directive was just read</li>

	<li><tt>MXML_SAX_ELEMENT_CLOSE</tt> - A close element was just read (<tt>&lt;/element&gt;</tt>)</li>

	<li><tt>MXML_SAX_ELEMENT_OPEN</tt> - An open element was just read (<tt>&lt;element&gt;</tt>)</li>

</ul>

<p>Elements are <em>released</em> after the close element is
processed. All other nodes are released after they are processed.
The SAX callback can <em>retain</em> the node using the <a
href='#mxmlRetain'><tt>mxmlRetain</tt></a> function. For example,
the following SAX callback will retain all nodes, effectively
simulating a normal in-memory load:</p>

<pre>
    void
    sax_cb(mxml_node_t *node,
           mxml_sax_event_t event,
           void *data)
    {
      if (event != MXML_SAX_ELEMENT_CLOSE)
        mxmlRetain(node);
    }
</pre>

<p>More typically the SAX callback will only retain a small portion
of the document that is needed for post-processing. For example, the
following SAX callback will retain the title and headings in an
XHTML file. It also retains the (parent) elements like <tt>&lt;html&gt;</tt>, <tt>&lt;head&gt;</tt>, and <tt>&lt;body&gt;</tt>, and processing
directives like  <tt>&lt;?xml ... ?&gt;</tt> and  <tt>&lt;!DOCTYPE ... &gt;</tt>:</p>

<!-- NEED 10 -->
<pre>
    void
    sax_cb(mxml_node_t *node,
           mxml_sax_event_t event,
           void *data)
    {
      if (event == MXML_SAX_ELEMENT_OPEN)
      {
       /*
        * Retain headings and titles...
        */

        char *name = mxmlGetElement(node);

        if (!strcmp(name, "html") ||
            !strcmp(name, "head") ||
            !strcmp(name, "title") ||
            !strcmp(name, "body") ||
            !strcmp(name, "h1") ||
            !strcmp(name, "h2") ||
            !strcmp(name, "h3") ||
            !strcmp(name, "h4") ||
            !strcmp(name, "h5") ||
            !strcmp(name, "h6"))
          mxmlRetain(node);
      }
      else if (event == MXML_SAX_DIRECTIVE)
        mxmlRetain(node);
      else if (event == MXML_SAX_DATA)
      {
        if (mxmlGetRefCount(mxmlGetParent(node)) > 1)
        {
         /*
          * If the parent was retained, then retain
          * this data node as well.
          */

          mxmlRetain(node);
        }
      }
    }
</pre>

<p>The resulting skeleton document tree can then be searched just
like one loaded using the <tt>mxmlLoad</tt> functions. For example,
a filter that reads an XHTML document from stdin and then shows the
title and headings in the document would look like:</p>

<pre>
    mxml_node_t *doc, *title, *body, *heading;

    doc = mxmlSAXLoadFd(NULL, 0,
                        MXML_TEXT_CALLBACK,
                        <b>sax_cb</b>, NULL);

    title = mxmlFindElement(doc, doc, "title",
                            NULL, NULL,
                            MXML_DESCEND);

    if (title)
      print_children(title);

    body = mxmlFindElement(doc, doc, "body",
                           NULL, NULL,
                           MXML_DESCEND);

    if (body)
    {
      for (heading = mxmlGetFirstChild(body);
           heading;
           heading = mxmlGetNextSibling(heading))
        print_children(heading);
    }
</pre>

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