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<html><head><title>Apache Axis2/C - AXOM Tutorial</title><h2>Apache Axis2/C AXIOM Tutorial</h2><div class="subsection"><a name="Contents"></a><h3>Contents</h3><ul>
<li><a href="#Introduction">Introduction</a>
<ul>
<li><a href="#What_is_OM">What is AXIOM?</a></li>
<li><a href="#For_Whom_is_This_Tutorial">For whom is this
tutorial?</a></li>
<li><a href="#What_is_Pull_Parsing">What is Pull Parsing?</a></li>
<li><a href="#Features_of_OM">Features of AXIOM</a></li>
<li><a href="#Where_Does_SOAP_Come_into_Play">Where does SOAP come into
play?</a></li>
</ul>
</li>
<li><a href="#Working_with_OM">Working with AXIOM</a>
<ul>
<li><a href="#Env">Axis2/C Environment</a></li>
<li><a href="#Creation">Building AXIOM</a></li>
<li><a href="#Addition_and_Detaching_of_Nodes">Adding and Detaching
Nodes</a></li>
<li><a href="#Traversing">Traversing</a></li>
<li><a href="#Serialization">Serialization</a></li>
<li><a href="#Reader_and_Writer">Using axiom_xml_reader and
axiom_xml_writer</a></li>
<li><a href="#Mem_Leaks">How to avoid memory leaks and double frees
when using AXIOM</a></li>
<li><a href="#Complete_Sample">Complete Sample</a></li>
</ul>
</li>
</ul><p><a id="Introduction"></a></p></div><div class="subsection"><a name="Introduction"></a><h3>Introduction</h3><p><a id="What_is_OM"></a></p></div><div class="subsection"><a name="What_is_AXIOM_"></a><h3>What is AXIOM?</h3><p>AXIOM stands for AXis Object Model and refers to the XML infoset model
that is developed for Apache Axis2. XML infoset refers to the information
included inside the XML. For programmatical manipulation, it is convenient to
have a representation of this XML infoset in a language specific manner. DOM
and JDOM are two such XML models. AXIOM is conceptually similar to such an
XML model in its external behavior but deep down it is very different.</p><p>The objective of this tutorial is to introduce the basics of AXIOM/C and
explain the best practices while using AXIOM.</p><p>AXIOM/C is a C equivalant of AXIOM/Java. We have done our best to get
almost the same kind of API in C.</p><p><a id="For_Whom_is_This_Tutorial"></a></p></div><div class="subsection"><a name="For_whom_is_this_tutorial_"></a><h3>For whom is this tutorial?</h3><p>This tutorial can be used by anybody who is interested and wants to go
deeper in to AXIOM/C. Knowledge in similar object models such as DOM will be
helpful in understanding AXIOM, but such knowledge has not been assumed.
Several links are listed in the links section that will help you understand
the basics of XML.</p><p><a id="What_is_Pull_Parsing"></a></p></div><div class="subsection"><a name="What_is_Pull_Parsing__"></a><h3>What is Pull Parsing ?</h3><p>
Pull parsing is a new trend in XML processing. The previously popular XML
processing frameworks such as DOM were "push-based", which means that the
control of parsing was with the parser itself. This approach is fine and easy
to use, but it is not efficient in handling large XML documents since a
complete memory model will be generated in the memory. Pull parsing inverts
the control and hence the parser only proceeds at the user's command. The
user can decide to store or discard events generated from the parser. AXIOM
is based on pull parsing. To learn more about XML pull parsing, see the <a href="http://www.bearcave.com/software/java/xml/xmlpull.html" class="externalLink" title="External Link">XML pull
parsing introduction</a>. <a id="Features_of_OM"></a></p></div><div class="subsection"><a name="Features_of_AXIOM"></a><h3>Features of AXIOM</h3><p>AXIOM is a lightweight, differed built XML infoset representation based on
StAX API derived from <a href="http://www.jcp.org/aboutJava/communityprocess/first/jsr173/" class="externalLink" title="External Link">JSR
173</a>, which is the standard streaming pull parser API. AXIOM can be
manipulated as flexibly as any other object model such as <a href="http://www.jdom.org/" class="externalLink" title="External Link">JDOM</a>, but underneath, the objects will be
created only when they are absolutely required. This leads to much less
memory-intensive programming.</p><p>The following is a short feature overview of AXIOM.</p><ul>
<li>Lightweight: AXIOM is specifically targeted to be lightweight. This is
achieved by reducing the depth of the hierarchy, the number of methods,
and the attributes enclosed in the objects. This makes the objects less
memory intensive.</li>
<li>Differed building: By far, this is the most important feature of AXIOM.
The objects are not made unless a need arises for them. This passes the
control of building to the object model itself, rather than an external
builder.</li>
<li>Pull based: For a differed building mechanism, a pull-based parser is
required. AXIOM is based on StAX, which is the standard pull parser API.
<p>Since different XML parsers offer different kinds of pull parser APIs,
we define an API derived from StAX. That API is defined in
<code>axiom_xml_reader.h</code>. Similarly, we define an XML writer API
in <code>axiom_xml_writer.h</code>. These two APIs work as an abstarction
layer between any XML parser and AXIOM. So any parser that is going to be
used for AXIOM should implement the <code>axiom_xml_reader</code> API and
the <code>axiom_xml_writer</code> API using a wrapper layer.</p>
<p></p>
<p>Currenly we use <a href="http://xmlsoft.org/downloads.html" class="externalLink" title="External Link">Libxml2</a> as our default XML
parser.</p>
<p></p>
</li>
</ul><p class="img"><img alt="" src="images/archi006.jpg" class="img" width="490" height="282"></img></p><p>The AXIOM Builder wraps the raw XML character stream through the
<code>axiom_xml_reader</code> API. Hence the complexities of the pull event
stream are hidden from the user.</p><p><a id="Where_Does_SOAP_Come_into_Play"></a></p></div><div class="subsection"><a name="Where_does_SOAP_come_into_play_"></a><h3>Where does SOAP come into play?</h3><p>In a nutshell, SOAP is an information exchange protocol based on XML. SOAP
has a defined set of XML elements that should be used in messages. Since
Axis2 is a "SOAP Engine" and AXIOM is designed for Axis2, a SOAP specific API
was implemented on top of AXIOM. We have defined a number of structs to
represent SOAP constructs, which wrap general AXIOM structures. Learn more
about <a href="http://www.w3schools.com/SOAP/soap_intro.asp" class="externalLink" title="External Link">SOAP</a>.</p><p>
<a id="Working_with_OM"></a></p></div><div class="subsection"><a name="Working_with_AXIOM"></a><h3>Working with AXIOM</h3><p><a id="Env"></a></p></div><div class="subsection"><a name="Axis2_C_Environment"></a><h3>Axis2/C Environment</h3><p>Before starting the discussion on AXIOM, it is necessary to get a good
understanding of the basics of Axis2/C. Axis2/C is designed to be pluggable
to any system written in C or C++. Therefore, Axis2/C has abstracted the
functionalities that differ from system to system into a structure
<code>axutil_env_t</code>, which we refer to as the Axis2 environment. The
environment holds <code>axutil_allocator_t</code>, which is used for memory
allocation and deallocation, <code>axutil_error_t</code>, which is used for
error reporting, <code>axutil_log_t</code>, which is used for logging
mechanisms, and <code>axutil_thread_t</code> which is used for threading
mechanisms.</p><p>When creating the Axis2 environment, the first thing is to create the
allocator.</p><p><code>axutil_allocator_t *allocator = NULL;</code></p><p><code>allocator = axutil_allocator_init(NULL);</code></p><p>We pass <code>NULL</code> to the above function in order to use the
default allocator functions. Then the allocator functions will use the
<code>malloc</code>, and <code>free</code> functions for memory management.
If you have your own allocator structure, with custom malloc and free
functions, you can pass them instead.</p><p>Convenient macros <code>AXIS2_MALLOC</code> and <code>AXIS2_FREE</code>
are defined to use allocator functions (please have a look at
<code>axutil_allocator.h</code> for more information).</p><p>In a similar fashion, you can create the error and log structures.</p><p><code>axutil_log_t *log = NULL;</code></p><p><code>axutil_error_t *error = NULL;</code></p><p><code>log = axutil_log_create(allocator, NULL, NULL);</code></p><p><code>log = axutil_log_create(allocator, NULL, "mylog.log");</code></p><p></p><p>Now we can create the environment by parsing the allocator, error and log
to <code>axutil_env_create_with_error_log()</code> function.</p><p><code>axutil_env_t *env = NULL;</code></p><p><code>env = axutil_env_create_with_error_log(allocator, error,
log);</code></p><p>Apart from the above abstraction, all the other library functions used are
ANSI C compliant. Further, platform dependent functions are also
abstracted.</p><p>As a rule of thumb, all <code>create</code> functions take a pointer to
the environment as its first argument, and all the other functions take
pointer to this particular struct as the first argument, and a pointer to the
environment as the second argument. (Please refer to our <a href="../coding_conventions.html">coding convention page</a> to learn more
about this.)</p><p>Example,</p><p><code>axiom_node_t *node = NULL;</code></p><p><code>axiom_node_t *child = NULL;</code></p><p><code>node = axiom_node_create(env);</code></p><p><code>child = axiom_node_get_first_child(node, env);</code></p><p>Note that we are passing the node (pointer to <code>axiom_node_t</code> )
as the first argument and the pointer to the environment as the second.</p><p><a id="Creation"></a></p></div><div class="subsection"><a name="Building_AXIOM"></a><h3>Building AXIOM</h3><p>This section explains how AXIOM can be built either from an existing
document or programmatically. AXIOM provides a notion of a builder to create
objects. Since AXIOM is tightly bound to StAX, a StAX compliant reader should
be created first with the desired input stream.</p><p>In our AXIOM implementation, we define a struct <code>axiom_node_t</code>
which acts as the container of the other structs. <code>axiom_node_t</code>
maintains the links that form the linked list used to hold the AXIOM
structure.</p><p>To traverse this structure, the functions defined in
<code>axiom_node.h</code> must be used. To access XML information, the 'data
element' struct stored in <code>axiom_node_t</code> must be obtained using
the <code>axiom_node_get_data_element</code> function. The type of the struct
stored in the <code>axiom_node_t</code> struct can be obtained by the
<code>axiom_node_get_node_type</code> function. When we create
<code>axiom_element_t</code>, <code>axiom_text_t</code> etc., it is required
to parse a double pointer to the node struct as the last parameter of the
<code>create</code> function, so that the corresponding node struct can be
referenced using that pointer.</p><p>Example</p><p><code>axiom_node_t *my_node = NULL;</code></p><p><code>axiom_element_t *my_ele = NULL;</code></p><p><code>my_ele = axiom_element_create(env, NULL, "MY_ELEMENT", NULL,
&amp;my_node);</code></p><p></p><p>Now if we call the <code>axiom_node_get_node_type</code> function on the
<code>my_node</code> pointer, it will return <code>AXIOM_ELEMENT</code>.</p><p><b>Code Listing 1</b></p>
<div class="source"><pre>axiom_xml_reader_t *xml_reader = NULL;
axiom_stax_builder_t *om_builder = NULL;
axiom_soap_builder_t *soap_builder = NULL;
axiom_soap_envelope_t *soap_envelope = NULL;
xml_reader = axiom_xml_reader_create_for_file(env, "test_soap.xml", NULL);
om_builder = axiom_stax_builder_create(env, xml_reader);
soap_builder = axiom_soap_builder_create(env, om_builder , AXIOM_SOAP11_SOAP_ENVELOPE_NAMESPACE_URI);
soap_envelope = axiom_soap_builder_get_soap_envelope(soap_builder, env);
</pre></div>
<p>As the example shows, creating an AXIOM from <code>xml_reader</code> is
pretty straight forward. Elements and nodes can be created programmatically
to modify the structure as well. Currently AXIOM has two builders, namely the
<code>axiom_stax_builder_t</code> and the <code>axiom_soap_builder_t</code>.
These builders provide the necessary information to the XML infoset model to
build the AXIOM tree.</p><p><b>Code Listing 2</b></p>
<div class="source"><pre>axiom_namespace_t *ns1 = NULL;
axiom_namespace_t *ns2 = NULL;
axiom_element_t* root_ele = NULL;
axiom_node_t* root_ele_node = NULL;
axiom_element_t *ele1 = NULL;
axiom_node_t *ele1_node = NULL;
ns1 = axiom_namespace_create(env, "bar", "x");
ns2 = axiom_namespace_create(env, "bar1", "y");
root_ele = axiom_element_create(env, NULL, "root", ns1, &amp;root_ele_node);
ele1 = axiom_element_create(env, root_node, "foo1", ns2, &amp;ele1_node);
</pre></div>
<p>Several differences exist between a programmatically created
<code>axiom_node_t</code> and a conventionally built
<code>axiom_node_t</code>. The most important difference is that the latter
will have a pointer to its builder, where as the former does not have a
builder.</p><p>The SOAP struct hierarchy is made in the most natural way for a
programmer. It acts as a wrapper layer on top of the AXIOM implementation.
The SOAP structs wrap the corresponding <code>axiom_node_t</code> structs to
store XML information.</p><p><a id="Addition_and_Detaching_of_Nodes"></a></p></div><div class="subsection"><a name="Adding_and_Detaching_Nodes"></a><h3>Adding and Detaching Nodes</h3><p>Addition and removal methods are defined in the <code>axiom_node.h</code>
header file.</p><p><b>Code Listing 3</b></p><p>Add child operation</p>
<div class="source"><pre>axis2_status_t
axiom_node_add_child(axiom_node_t *om_node,
const axutil_env_t *env,
axiom_node_t *child_node);
</pre></div>
<p>Detach operation</p>
<div class="source"><pre>axiom_node_t *
axiom_node_detach(axiom_node_t *om_node,
const axutil_env_t *env);
</pre></div>
<p>The detach operation resets the links and removes a node from the AXIOM
tree.</p><p>This code segment shows how child addition can be done.</p><p><b>Code Listing 4</b></p>
<div class="source"><pre>axiom_node_t *foo_node = NULL;
axiom_element_t *foo_ele = NULL;
axiom_node_t *bar_node = NULL;
axiom_element_t *bar_ele = NULL;
foo_ele = axiom_element_create(env, NULL, "FOO", NULL, &amp;foo_node);
bar_ele = axiom_element_create(env, NULL, "BAR", NULL. &amp;bar_node);
axiom_node_add_child(foo_node, env, bar_node);
</pre></div>
<p>Alternatively, we can pass the <code>foo_node</code> as the parent node at
the time of creating the <code>bar_ele</code> as follows.</p>
<div class="source"><pre> bar_ele = axiom_element_create(env, foo_node, "BAR", NULL, &amp;bar_node);
</pre></div>
<p>The following shows important methods available in
<code>axiom_element</code> to be used to deal with namespaces.</p><p><b>Code Listing 5</b></p>
<div class="source"><pre>axiom_namespace_t *
axiom_element_declare_namespace(axiom_element_t *om_ele,
const axutil_env_t *env,
axiom_node_t *om_node,
axiom_namespace_t *om_ns);
axiom_namespace_t *
axiom_element_find_namespace(axiom_element_t *om_ele,
const axutil_env_t *env,
axiom_node_t *om_node,
axis2_char_t *uri,
axis2_char_t *prefix);
axiom_namespace_t *
axiom_element_find_declared_namespace(axiom_element_t *om_element,
const axutil_env_t *env,
axis2_char_t *uri,
axis2_char_t *prefix);
axis2_status_t
axiom_element_set_namespace(axiom_element_t *om_element,
const axutil_env_t *env,
axiom_namespace_t *ns,
axiom_node_t *element_node);
</pre></div>
<p>An <code>axiom_element</code> has a namespace list, the declared
namespaces, and a pointer to its own namespace if one exists.</p><p>The <code>axiom_element_declare_namespace</code> function is straight
forward. It adds a namespace to the declared namespace list. Note that a
namespace that is already declared will not be declared again.</p><p><code>axiom_element_find_namespace</code> is a very handy method to locate
a namespace in the AXIOM tree. It searches for a matching namespace in its
own declared namespace list and jumps to the parent if it's not found. The
search progresses up the tree until a matching namespace is found or the root
has been reached.</p><p><code>axiom_element_find_declared_namespace</code> can be used to search
for a namespace in the current element's declared namespace list.</p><p><code>axiom_element_set_namespace</code> sets <code>axiom_element</code>'s
own namespace. Note that an element's own namespace should be declared in its
own namespace declaration list or in one of its parent elements. This method
first searches for a matching namespace using
<code>axiom_element_find_namespace</code> and if a matching namespace is not
found, a namespace is declared to this <code>axiom_element</code>'s namespace
declarations list before setting the own namespace reference.</p><p>The following sample code segment shows how the namespaces are dealt with
in AXIOM.</p><p><b>Code Listing 6</b></p>
<div class="source"><pre>axiom_namespace_t *ns1 = NULL;
axiom_namespace_t *ns2 = NULL;
axiom_namespace_t *ns3 = NULL;
axiom_node_t *root_node = NULL;
axiom_element_t *root_ele = NULL;
axiom_node_t *ele1_node = NULL;
axiom_element_t *ele1 = NULL;
axiom_node_t *text_node = NULL;
axiom_text_t *om_text = NULL;
ns1 = axiom_namespace_create(env, "bar", "x");
ns2 = axiom_namespace_create(env, "bar1", "y");
root_ele = axiom_element_create(env, NULL , "root", ns1, &amp;root_node);
ele1 = axiom_element_create(env, root_node, "foo", ns2, &amp;ele1_node);
om_text = axiom_text_create(env, ele1_node, "blah", &amp;text_node);
</pre></div>
<p></p><p>Serialization of the root element produces the following XML:</p>
<div class="source"><pre>&lt;x:root xmlns:x="bar"&gt;
&lt;y:foo xmlns:y="bar1"&gt;blah&lt;/y:foo&gt;
&lt;/x:root&gt;
</pre></div>
<p>If we want to produce</p>
<div class="source"><pre>&lt;x:foo xmlns:x="bar" xmlns:y="bar1"&gt;Test&lt;/x:foo&gt;
</pre></div>
<p>we can use set_namespace and declare namespace functions as follows.</p>
<div class="source"><pre>axiom_node_t *foo_node = NULL;
axiom_element_t *foo_ele = NULL;
axiom_namespace_t *ns1 = NULL;
axiom_namespace_t *ns2 = NULL;
foo_ele = axiom_element_create(env, NULL,"foo" ,NULL, &amp;foo_node);
ns1 = axiom_namespace_create(env, "bar", "x");
ns2 = axiom_namespace_create(env, "bar1","y");
axiom_element_set_namespace(foo_ele, env, ns1, foo_node);
axiom_element_declare_namespace(foo_ele, env, ns2, foo_node);
axiom_element_set_text(foo_ele, env, "Test", &amp;foo_node);
</pre></div>
<p><a id="Traversing"></a></p></div><div class="subsection"><a name="Traversing"></a><h3>Traversing</h3><p>Traversing the AXIOM structure can be done by obtaining an iterator
struct. You can either call the appropriate function on an AXIOM element or
create the iterator manually. AXIOM/C offers three iterators to traverse the
AXIOM structure. They are:</p><ul>
<li>axiom_children_iterator_t</li>
<li>axiom_child_element_iterator_t</li>
<li>axiom_children_qname_iterator_t</li>
</ul><p>The iterator supports the 'AXIOM way' of accessing elements and is more
convenient than a list for sequential access. The following code sample shows
how the children can be accessed. The children can be of type
<code>AXIOM_TEXT</code> or <code>AXIOM_ELEMENT</code>.</p><p><b>Code Listing 7</b></p>
<div class="source"><pre>axiom_children_iterator_t *children_iter = NULL;
children_iter = axiom_element_get_children(om_ele, env, om_node);
if(NULL != children_iter )
{
while(axiom_children_iterator_has_next(children_iter, env))
{
axiom_node_t *node = NULL;
node = axiom_children_iterator_next(children_iter, env);
if(NULL != node)
{
if(axiom_node_get_node_type(node, env) == AXIOM_ELEMENT)
{
/* processing logic goes here */
}
}
}
}
</pre></div>
<p>Apart from this, every <code>axiom_node_t</code> struct has links to its
siblings. If a thorough navigation is needed, the
<code>axiom_node_get_next_sibling()</code> and
<code>axiom_node_get_previous_sibling()</code> functions can be used. A
restrictive set can be chosen by using
<code>axiom_element_xxx_with_qname()</code> methods. The
<code>axiom_element_get_first_child_with_qname()</code> method returns the
first child that matches the given <code>axutil_qname_t</code> and
<code>axiom_element_get_children_with_qname()</code> returns
<code>axiom_children_qname_iterator_t</code> which can be used to traverse
all the matching children. The advantage of these iterators is that they
won't build the whole object structure at once; it builds only what is
required.</p><table class="bodyTable"><tbody>
<tr class="a"><td><img src="images/OM005.gif" alt="" width="35" height="57"></img></td><td class="special-td">Internally, all iterator implementations stay
one step ahead of their apparent location to provide the correct
value for the <code>has_next()</code> function . This hidden
advancement can build elements that are not intended to be built at
all.</td><td></td></tr>
</tbody></table><p>
</p><p></p><p><a id="Serialization"></a></p></div><div class="subsection"><a name="Serialization"></a><h3>Serialization</h3><p>AXIOM can be serialized using the <code>axiom_node_serialize</code>
function. The serialization uses <code>axiom_xml_writer.h</code> and
<code>axiom_output.h</code> APIs.</p><p>Here is an example that shows how to write the output to the console (we
have serialized the SOAP envelope created in code listing 1).</p><p><b>Code Listing 8</b></p>
<div class="source"><pre>axiom_xml_writer_t *xml_writer = NULL;
axiom_output_t *om_output = NULL;
axis2_char_t *buffer = NULL;
..............
xml_writer = axiom_xml_writer_create(env, NULL, 0, 0);
om_output = axiom_output_create(env, xml_writer);
axiom_soap_envelope_serialize(envelope, env, om_output);
buffer = (axis2_char_t*)axis2_xml_writer_get_xml(xml_writer, env);
printf("%s ", buffer);
</pre></div>
<p>An easy way to serialize is to use the <code>to_string</code> function in
<code>om_element</code></p><p><b>Code Listing 9</b></p>
<div class="source"><pre>axis2_char_t *xml_output = NULL;
axiom_node_t *foo_node = NULL;
axiom_element_t *foo_ele = NULL;
axiom_namespace_t* ns = NULL;
ns = axiom_namespace_create(env, "bar", "x");
foo_ele = axiom_element_create(env, NULL, "foo", ns, &amp;foo_node);
axiom_element_set_text(foo_ele, env, "EASY SERAILIZATION", foo_node);
xml_output = axiom_element_to_string(foo_ele, env, foo_node);
printf("%s", xml_output);
AXIS2_FREE(env-&gt;allocator, xml_output);
</pre></div>
<p>Note that freeing the returned buffer is the user's responsibility.</p><p><a id="Reader_and_Writer"></a></p></div><div class="subsection"><a name="Using_axiom_xml_reader_and_axiom_xml_writer"></a><h3>Using axiom_xml_reader and axiom_xml_writer</h3><p><code>axiom_xml_reader</code> provides three create functions that can be
used for different XML input sources.</p><ul>
<li><code>axiom_xml_reader_create_for_file</code> can be used to read from
a file</li>
<li><code>axiom_xml_reader_create_for_io</code> uses a user defined
callback function to pull XML</li>
<li><code>axiom_xml_reader_create_for_memory</code> can be used to read
from an XML string that is in a character buffer</li>
</ul><p>
ls of the latest version can be found on the Apache Axis2/C
<ul>
<li><code>axiom_xml_writer_create_for_file</code> can be used to write to a
file</li>
<li><code>axiom_xml_writer_create_for_memory</code> can be used to write to
an internal memory buffer and obtain the XML string as a character
buffer</li>
</ul></p><p></p><p>Please refer to <code>axiom_xml_reader.h</code> and
<code>axiom_xml_writer.h</code> for more information.</p><p></p><p><a id="Mem_Leaks"></a></p></div><div class="subsection"><a name="How_to_Avoid_Memory_Leaks_and_Double_Frees_When_Using_AXIOM"></a><h3>How to Avoid Memory Leaks and Double Frees When Using AXIOM</h3><p>You have to be extremely careful when using AXIOM, in order to avoid
memory leaks and double free errors. The following guidelines will be
extremely useful:</p><p>1. The <code>axiom_element</code> struct keeps a list of attributes and a
list of namespaces, when an <code>axiom_namespace</code> pointer or an
<code>axiom_attribute</code> pointer is added to these lists, which will be
freed when the <code>axiom_element</code> is freed. Therefore a pointer to a
namespace or an attribute should not be freed, once it is used with an
<code>axiom_element</code>.</p><p>To avoid any inconvenience, clone functions have been implemented for both
the <code>axiom_namespace</code> and <code>axiom_attribute</code>
structures.</p><p>2. AXIOM returns shallow references to its string values. Therefore, when
you want deep copies of returned values, the <code>axutil_strdup()</code>
function should be used to avoid double free errors.</p><p>Example</p><p><code>axiom_namespace_t *ns = NULL;</code></p><p><code>axis2_char_t *uri = NULL;</code></p><p><code>ns = axiom_namespace_create(env, "http://ws.apache.org",
"AXIOM");</code></p><p><code>uri = axiom_namespace_get_uri(ns, env);</code></p><p><code>/* now uri points to the same place where namespace struct's uri <br></br>
pointer is pointing. Therefore following will cause a double free
*/</code></p><p><code>AXIS2_FREE(env-&gt;allocator, uri);</code></p><p><code>axiom_namespace_free(ns, env);</code></p><p>3. When creating AXIOM programatically, if you are declaring a namespace
with an <code>axiom_element</code>, it is advisable to find whether the
namespace is already available in the elements scope using the
<code>axiom_element_find_namespace</code> function. If available, that
pointer can be used instead of creating another namespace struct instance to
minimize memory usage.</p><p></p><p><a id="Complete_Sample"></a></p></div><div class="subsection"><a name="Complete_Code_for_the_AXIOM_Based_Document_Building_and_Serialization"></a><h3>Complete Code for the AXIOM Based Document Building and Serialization</h3><p>The following code segment shows how to use AXIOM for building a document
completely and then serializing it into text, pushing the output to the
console.</p><p></p><p><b>Code Listing 10</b></p>
<div class="source"><pre>#include &lt;axiom.h&gt;
#include &lt;axis2_util.h&gt;
#include &lt;axutil_env.h&gt;
#include &lt;axutil_log_default.h&gt;
#include &lt;axutil_error_default.h&gt;
#include &lt;stdio.h&gt;
FILE *f = NULL;
int read_input_callback(char *buffer, int size, void* ctx)
{
     fread(buffer, (char), size, f);
}
int close_input_callback(void *ctx)
{
     fclose(f);
}
axutil_env_t * create_environment()
{
    axutil_allocator_t *allocator = NULL;
    axutil_env_t *env = NULL;
    axutil_log_t *log = NULL;
    axutil_error_t *error = NULL;
    allocator = axutil_allocator_init(NULL);
    log = axutil_log_create(allocator, NULL, NULL);
    error = axutil_error_create(allocator);
    env = axutil_env_create_with_error_log(allocator, error, log);
     env;
}
build_and_serialize_om(axutil_env_t *env)
{
    axiom_node_t *root_node = NULL;
    axiom_element_t *root_ele = NULL;
    axiom_document_t *document = NULL;
    axiom_stax_builder_t *om_builder = NULL;
    axiom_xml_reader_t *xml_reader = NULL;
    axiom_xml_writer_t *xml_writer = NULL;
    axiom_output_t *om_output = NULL;
    axis2_char_t *buffer = NULL;
    
    f = fopen("test.xml","r");
    xml_reader = axiom_xml_reader_create_for_io(env, read_input_callback,
                                                    close_input_callback, NULL, NULL);
    (!xml_reader)
         -1;
    om_builder = axiom_stax_builder_create(env, xml_reader);
    (!om_builder)
    {
        axiom_xml_reader_free(xml_reader, env);
         AXIS2_FAILURE;
    }
    document = axiom_stax_builder_get_document(om_builder, env);
    (!document)
    {
         axiom_stax_builder_free(om_builder, env);
         AXIS2_FAILURE;
    }
    
    root_node = axiom_document_get_root_element(document, env);
    (!root_node)
    {
        axiom_stax_builder_free(om_builder, env);
         AXIS2_FAILURE;
    }        
    (root_node)
    {
        (axiom_node_get_node_type(root_node, env) == AXIOM_ELEMENT)
        {
            root_ele = (axiom_element_t*)axiom_node_get_data_element(root_node, env);
            (root_ele)
            {
printf(" %s" ,axiom_element_get_localname(root_ele, env));
}
}
}
axiom_document_build_all(document, env);
axiom_document_build_all(document, env);
xml_writer = axiom_xml_writer_create_for_memory(env, NULL, AXIS2_TRUE, 0, AXIS2_XML_PARSER_TYPE_BUFFER);
om_output = axiom_output_create(env, xml_writer);
axiom_node_serialize(root_node, env, om_output);
buffer = (axis2_char_t*)axiom_xml_writer_get_xml(xml_writer, env);
printf("The output XML is -&gt;&gt;&gt;&gt;\n %s ", buffer);
  
    
    
    axiom_output_free(om_output, env);
    
    
    axiom_stax_builder_free(om_builder, env);
    
     AXIS2_SUCCESS;
    
}
int main()
{
    int status = AXIS2_SUCCESS;
    
    axutil_env_t *env = NULL;
    axutil_allocator_t *allocator = NULL;
    env = create_environment();
    status = build_and_serialize_om(env);
    (status == AXIS2_FAILURE)
    {
        printf(" build AXIOM failed");
    }
    
    axutil_env_free(env);
    
     0;
}
</pre></div>
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