Getting Started with Mini-XML

This chapter describes how to write programs that use Mini-XML to access data in an XML file. Mini-XML provides the following functionality:

Functions for creating and managing XML documents in memory.
Reading of UTF-8 and UTF-16 encoded XML files and strings.
Writing of UTF-8 encoded XML files and strings.
Support for arbitrary element names, attributes, and attribute values with no preset limits, just available memory.
Support for integer, real, opaque ("cdata"), and text data types in "leaf" nodes.
"Find", "index", and "walk" functions for easily accessing data in an XML document.

Mini-XML doesn't do validation or other types of processing on the data based upon schema files or other sources of definition information, nor does it support character entities other than those required by the XML specification.

The Basics

Mini-XML provides a single header file which you include:

    #include <mxml.h>

The Mini-XML library is included with your program using the -lmxml option:

    gcc -o myprogram myprogram.c -lmxml ENTER

If you have the pkg-config(1) software installed, you can use it to determine the proper compiler and linker options for your installation:

    pkg-config --cflags mxml ENTER
    pkg-config --libs mxml ENTER

Nodes

Every piece of information in an XML file (elements, text, numbers) is stored in memory in "nodes". Nodes are defined by the mxml_node_t structure. The type member defines the node type (element, integer, opaque, real, or text) which determines which value you want to look at in the value union.

*Table 2-1: Mini-XML Node Value Members*
Value	Type	Node member
Custom	`void *`	`node->value.custom.data`
Element	`char *`	`node->value.element.name`
Integer	`int`	`node->value.integer`
Opaque (string)	`char *`	`node->value.opaque`
Real	`double`	`node->value.real`
Text	`char *`	`node->value.text.string`

Each node also has a user_data member which allows you to associate application-specific data with each node as needed.

New nodes are created using the mxmlNewElement, mxmlNewInteger, mxmlNewOpaque, mxmlNewReal, mxmlNewText mxmlNewTextf mxmlNewXML functions. Only elements can have child nodes, and the top node must be an element, usually the <?xml version="1.0"?> node created by mxmlNewXML().

Nodes have pointers to the node above (parent), below (child), left (prev), and right (next) of the current node. If you have an XML file like the following:

    <?xml version="1.0"?>
    <data>
        <node>val1</node>
        <node>val2</node>
        <node>val3</node>
        <group>
            <node>val4</node>
            <node>val5</node>
            <node>val6</node>
        </group>
        <node>val7</node>
        <node>val8</node>
    </data>

the node tree for the file would look like the following in memory:

    ?xml
      |
    data
      |
    node - node - node - group - node - node
      |      |      |      |       |      |
    val1   val2   val3     |     val7   val8
                           |
                         node - node - node
                           |      |      |
                         val4   val5   val6

where "-" is a pointer to the next node and "|" is a pointer to the first child node.

Once you are done with the XML data, use the mxmlDelete function to recursively free the memory that is used for a particular node or the entire tree:

    mxmlDelete(tree);

Creating XML Documents

You can create and update XML documents in memory using the various mxmlNew functions. The following code will create the XML document described in the previous section:

    mxml_node_t *xml;    /* <?xml ... ?> */
    mxml_node_t *data;   /* <data> */
    mxml_node_t *node;   /* <node> */
    mxml_node_t *group;  /* <group> */

    xml = mxmlNewXML("1.0");

    data = mxmlNewElement(xml, "data");

        node = mxmlNewElement(data, "node");
        mxmlNewText(node, 0, "val1");
        node = mxmlNewElement(data, "node");
        mxmlNewText(node, 0, "val2");
        node = mxmlNewElement(data, "node");
        mxmlNewText(node, 0, "val3");

        group = mxmlNewElement(data, "group");

            node = mxmlNewElement(group, "node");
            mxmlNewText(node, 0, "val4");
            node = mxmlNewElement(group, "node");
            mxmlNewText(node, 0, "val5");
            node = mxmlNewElement(group, "node");
            mxmlNewText(node, 0, "val6");

        node = mxmlNewElement(data, "node");
        mxmlNewText(node, 0, "val7");
        node = mxmlNewElement(data, "node");
        mxmlNewText(node, 0, "val8");

We start by creating the <?xml version="1.0"?> node common to all XML files using the mxmlNewXML function:

    xml = mxmlNewXML("1.0");

We then create the <data> node used for this document using the mxmlNewElement function. The first argument specifies the parent node (xml) while the second specifies the element name (data):

    data = mxmlNewElement(xml, "data");

Each <node>...</node> in the file is created using the mxmlNewElement and mxmlNewText functions. The first argument of mxmlNewText specifies the parent node (node). The second argument specifies whether whitespace appears before the text - 0 or false in this case. The last argument specifies the actual text to add:

    node = mxmlNewElement(data, "node");
    mxmlNewText(node, 0, "val1");

The resulting in-memory XML document can then be saved or processed just like one loaded from disk or a string.

Loading XML

You load an XML file using the mxmlLoadFile function:

    FILE *fp;
    mxml_node_t *tree;

    fp = fopen("filename.xml", "r");
    tree = mxmlLoadFile(NULL, fp,
                        MXML_TEXT_CALLBACK);
    fclose(fp);

The first argument specifies an existing XML parent node, if any. Normally you will pass NULL for this argument unless you are combining multiple XML sources. The XML file must contain a complete XML document including the ?xml element if the parent node is NULL.

The second argument specifies the stdio file to read from, as opened by fopen() or popen(). You can also use stdin if you are implementing an XML filter program.

The third argument specifies a callback function which returns the value type of the immediate children for a new element node: MXML_CUSTOM, MXML_IGNORE, MXML_INTEGER, MXML_OPAQUE, MXML_REAL, or MXML_TEXT. Load callbacks are described in detail in Chapter 3. The example code uses the MXML_TEXT_CALLBACK constant which specifies that all data nodes in the document contain whitespace-separated text values. Other standard callbacks include MXML_IGNORE_CALLBACK, MXML_INTEGER_CALLBACK, MXML_OPAQUE_CALLBACK, and MXML_REAL_CALLBACK.

The mxmlLoadString function loads XML node trees from a string:

    char buffer[8192];
    mxml_node_t *tree;

    ...
    tree = mxmlLoadString(NULL, buffer,
                          MXML_TEXT_CALLBACK);

The first and third arguments are the same as used for mxmlLoadFile(). The second argument specifies the string or character buffer to load and must be a complete XML document including the ?xml element if the parent node is NULL.

Saving XML

You save an XML file using the mxmlSaveFile function:

    FILE *fp;
    mxml_node_t *tree;

    fp = fopen("filename.xml", "w");
    mxmlSaveFile(tree, fp, MXML_NO_CALLBACK);
    fclose(fp);

The first argument is the XML node tree to save. It should normally be a pointer to the top-level ?xml node in your XML document.

The second argument is the stdio file to write to, as opened by fopen() or popen(). You can also use stdout if you are implementing an XML filter program.

The third argument is the whitespace callback to use when saving the file. Whitespace callbacks are covered in detail in Chapter 3. The previous example code uses the MXML_NO_CALLBACK constant to specify that no special whitespace handling is required.

The mxmlSaveAllocString, and mxmlSaveString functions save XML node trees to strings:

    char buffer[8192];
    char *ptr;
    mxml_node_t *tree;

    ...
    mxmlSaveString(tree, buffer, sizeof(buffer),
                   MXML_NO_CALLBACK);

    ...
    ptr = mxmlSaveAllocString(tree, MXML_NO_CALLBACK);

The first and last arguments are the same as used for mxmlSaveFile(). The mxmlSaveString function takes pointer and size arguments for saving the XML document to a fixed-size buffer, while mxmlSaveAllocString() returns a string buffer that was allocated using malloc().

Controlling Line Wrapping

When saving XML documents, Mini-XML normally wraps output lines at column 75 so that the text is readable in terminal windows. The mxmlSetWrapMargin function overrides the default wrap margin:

    /* Set the margin to 132 columns */
    mxmlSetWrapMargin(132);

    /* Disable wrapping */
    mxmlSetWrapMargin(0);

Finding and Iterating Nodes

The mxmlWalkPrev and mxmlWalkNextfunctions can be used to iterate through the XML node tree:

    mxml_node_t *node;
    
    node = mxmlWalkPrev(current, tree,
                        MXML_DESCEND);

    node = mxmlWalkNext(current, tree,
                        MXML_DESCEND);

In addition, you can find a named element/node using the mxmlFindElement function:

    mxml_node_t *node;
    
    node = mxmlFindElement(tree, tree, "name",
                           "attr", "value",
                           MXML_DESCEND);

The name, attr, and value arguments can be passed as NULL to act as wildcards, e.g.:

    /* Find the first "a" element */
    node = mxmlFindElement(tree, tree, "a",
                           NULL, NULL,
                           MXML_DESCEND);

    /* Find the first "a" element with "href"
       attribute */
    node = mxmlFindElement(tree, tree, "a",
                           "href", NULL,
                           MXML_DESCEND);

    /* Find the first "a" element with "href"
       to a URL */
    node = mxmlFindElement(tree, tree, "a",
                           "href",
                           "http://www.easysw.com/",
                           MXML_DESCEND);

    /* Find the first element with a "src"
       attribute */
    node = mxmlFindElement(tree, tree, NULL,
                           "src", NULL,
                           MXML_DESCEND);

    /* Find the first element with a "src"
       = "foo.jpg" */
    node = mxmlFindElement(tree, tree, NULL,
                           "src", "foo.jpg",
                           MXML_DESCEND);

You can also iterate with the same function:

    mxml_node_t *node;

    for (node = mxmlFindElement(tree, tree,
                                "name",
                                NULL, NULL,
                                MXML_DESCEND);
         node != NULL;
         node = mxmlFindElement(node, tree,
                                "name",
                                NULL, NULL,
                                MXML_DESCEND))
    {
      ... do something ...
    }

The MXML_DESCEND argument can actually be one of three constants:

MXML_NO_DESCEND means to not to look at any child nodes in the element hierarchy, just look at siblings at the same level or parent nodes until the top node or top-of-tree is reached.
The previous node from "group" would be the "node" element to the left, while the next node from "group" would be the "node" element to the right.
MXML_DESCEND_FIRST means that it is OK to descend to the first child of a node, but not to descend further when searching. You'll normally use this when iterating through direct children of a parent node, e.g. all of the "node" and "group" elements under the "?xml" parent node in the example above.
This mode is only applicable to the search function; the walk functions treat this as MXML_DESCEND since every call is a first time.
MXML_DESCEND means to keep descending until you hit the bottom of the tree. The previous node from "group" would be the "val3" node and the next node would be the first node element under "group".
If you were to walk from the root node "?xml" to the end of the tree with mxmlWalkNext(), the order would be:

?xml data node val1 node val2 node val3 group node val4 node val5 node val6 node val7 node val8

If you started at "val8" and walked using mxmlWalkPrev(), the order would be reversed, ending at "?xml".