2 - Getting Started with Mini-XML

This chapter describes how to write programs that use Mini-XML to access data in an XML file.

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.

New nodes can be created using the mxmlNewElement(), mxmlNewInteger(), mxmlNewOpaque(), mxmlNewReal(), and mxmlNewText() functions. Only elements can have child nodes, and the top node must be an element, usually "?xml".

Each node has pointers for the node above (parent), below (child), to the left (prev), and to the 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>
        <node>val9</node>
    </data>

the node tree returned by mxmlLoadFile() would look like the following in memory:

    ?xml
      |
    data
      |
    node - node - node - group - node - node - node
      |      |      |      |       |      |      |
    val1   val2   val3     |     val7   val8   val9
                           |
                         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);

Loading and Saving XML Files

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_NO_CALLBACK);
    fclose(fp);

The third argument specifies a callback function which returns the value type of the immediate children for a new element node: MXML_INTEGER, MXML_OPAQUE, MXML_REAL, or MXML_TEXT. This function is called after 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. The default value type is MXML_TEXT if no callback is used.

Similarly, 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);

Callback functions for saving are used to optionally insert whitespace before and after elements in the node tree. Your function will be called up to four times for each element node with a pointer to the node and a "where" value of MXML_WS_BEFORE_OPEN, MXML_WS_AFTER_OPEN, MXML_WS_BEFORE_CLOSE, or MXML_WS_AFTER_CLOSE. The callback function should return NULL if no whitespace should be added and the string to insert (spaces, tabs, carriage returns, and newlines) otherwise.

The mxmlLoadString(), mxmlSaveAllocString(), and mxmlSaveString() functions load XML node trees from and save XML node trees to strings:

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

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

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

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

Finding and Iterating Nodes

The mxmlWalkPrev() and mxmlWalkNext()functions can be used to iterate through the XML node tree:

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

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

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

    mxml_node_t *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/~mike/mxml/", 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: