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<div class="section" id="general-plugin-concepts">
<h1>General plugin concepts<a class="headerlink" href="#general-plugin-concepts" title="Permalink to this headline">¶</a></h1>
<p>A krb5 dynamic plugin module is a Unix shared object or Windows DLL.
Typically, the source code for a dynamic plugin module should live in
its own project with a build system using <a class="reference external" href="http://www.gnu.org/software/automake/">automake</a> and <a class="reference external" href="http://www.gnu.org/software/libtool/">libtool</a>, or
tools with similar functionality.</p>
<p>A plugin module must define a specific symbol name, which depends on
the pluggable interface and module name. For most pluggable
interfaces, the exported symbol is a function named
<tt class="docutils literal"><span class="pre">INTERFACE_MODULE_initvt</span></tt>, where <em>INTERFACE</em> is the name of the
pluggable interface and <em>MODULE</em> is the name of the module. For these
interfaces, it is possible for one shared object or DLL to implement
multiple plugin modules, either for the same pluggable interface or
for different ones. For example, a shared object could implement both
KDC and client preauthentication mechanisms, by exporting functions
named <tt class="docutils literal"><span class="pre">kdcpreauth_mymech_initvt</span></tt> and <tt class="docutils literal"><span class="pre">clpreauth_mymech_initvt</span></tt>.</p>
<p>A plugin module implementation should include the header file
<tt class="docutils literal"><span class="pre"><krb5/INTERFACE_plugin.h></span></tt>, where <em>INTERFACE</em> is the name of the
pluggable interface. For instance, a ccselect plugin module
implementation should use <tt class="docutils literal"><span class="pre">#include</span> <span class="pre"><krb5/ccselect_plugin.h></span></tt>.</p>
<p>initvt functions have the following prototype:</p>
<div class="highlight-python"><div class="highlight"><pre>krb5_error_code interface_modname_initvt(krb5_context context,
int maj_ver, int min_ver,
krb5_plugin_vtable vtable);
</pre></div>
</div>
<p>and should do the following:</p>
<ol class="arabic simple">
<li>Check that the supplied maj_ver argument is supported by the
module. If it is not supported, the function should return
KRB5_PLUGIN_VER_NOTSUPP.</li>
<li>Cast the supplied vtable pointer to the structure type
corresponding to the major version, as documented in the pluggable
interface header file.</li>
<li>Fill in the structure fields with pointers to method functions and
static data, stopping at the field indicated by the supplied minor
version. Fields for unimplemented optional methods can be left
alone; it is not necessary to initialize them to NULL.</li>
</ol>
<p>In most cases, the context argument will not be used. The initvt
function should not allocate memory; think of it as a glorified
structure initializer. Each pluggable interface defines methods for
allocating and freeing module state if doing so is necessary for the
interface.</p>
<p>Pluggable interfaces typically include a <strong>name</strong> field in the vtable
structure, which should be filled in with a pointer to a string
literal containing the module name.</p>
<p>Here is an example of what an initvt function might look like for a
fictional pluggable interface named fences, for a module named
“wicker”:</p>
<div class="highlight-python"><div class="highlight"><pre>krb5_error_code
fences_wicker_initvt(krb5_context context, int maj_ver,
int min_ver, krb5_plugin_vtable vtable)
{
krb5_ccselect_vtable vt;
if (maj_ver == 1) {
krb5_fences_vtable vt = (krb5_fences_vtable)vtable;
vt->name = "wicker";
vt->slats = wicker_slats;
vt->braces = wicker_braces;
} else if (maj_ver == 2) {
krb5_fences_vtable_v2 vt = (krb5_fences_vtable_v2)vtable;
vt->name = "wicker";
vt->material = wicker_material;
vt->construction = wicker_construction;
if (min_ver < 2)
return 0;
vt->footing = wicker_footing;
if (min_ver < 3)
return 0;
vt->appearance = wicker_appearance;
} else {
return KRB5_PLUGIN_VER_NOTSUPP;
}
return 0;
}
</pre></div>
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<li class="toctree-l1 current"><a class="reference internal" href="index.html">For plugin module developers</a><ul class="current">
<li class="toctree-l2 current"><a class="current reference internal" href="">General plugin concepts</a></li>
<li class="toctree-l2"><a class="reference internal" href="clpreauth.html">Client preauthentication interface (clpreauth)</a></li>
<li class="toctree-l2"><a class="reference internal" href="kdcpreauth.html">KDC preauthentication interface (kdcpreauth)</a></li>
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<li class="toctree-l2"><a class="reference internal" href="profile.html">Configuration interface (profile)</a></li>
<li class="toctree-l2"><a class="reference internal" href="gssapi.html">GSSAPI mechanism interface</a></li>
<li class="toctree-l2"><a class="reference internal" href="internal.html">Internal pluggable interfaces</a></li>
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<li class="toctree-l1"><a class="reference internal" href="../build/index.html">Building Kerberos V5</a></li>
<li class="toctree-l1"><a class="reference internal" href="../basic/index.html">Kerberos V5 concepts</a></li>
<li class="toctree-l1"><a class="reference internal" href="../formats/index.html">Protocols and file formats</a></li>
<li class="toctree-l1"><a class="reference internal" href="../mitK5features.html">MIT Kerberos features</a></li>
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