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<a name="Missing-Components-1"></a>
<h4 class="subsection">38.4.4 Missing Components</h4>
<p>If a package relies on a component, such as another Octave package, that may
not be present it may be useful to install a function which informs users what
to do when a particular component is missing. The function must be written by
the package maintainer and registered with Octave using
<code>missing_component_hook</code>.
</p>
<a name="XREFmissing_005fcomponent_005fhook"></a><dl>
<dt><a name="index-missing_005fcomponent_005fhook"></a>Built-in Function: <em><var>val</var> =</em> <strong>missing_component_hook</strong> <em>()</em></dt>
<dt><a name="index-missing_005fcomponent_005fhook-1"></a>Built-in Function: <em><var>old_val</var> =</em> <strong>missing_component_hook</strong> <em>(<var>new_val</var>)</em></dt>
<dt><a name="index-missing_005fcomponent_005fhook-2"></a>Built-in Function: <em></em> <strong>missing_component_hook</strong> <em>(<var>new_val</var>, "local")</em></dt>
<dd><p>Query or set the internal variable that specifies the function to call when
a component of Octave is missing. This can be useful for packagers that
may split the Octave installation into multiple sub-packages, for example,
to provide a hint to users for how to install the missing components.
</p>
<p>When called from inside a function with the <code>"local"</code> option, the
variable is changed locally for the function and any subroutines it calls.
The original variable value is restored when exiting the function.
</p>
<p>The hook function is expected to be of the form
</p>
<div class="example">
<pre class="example"><var>fcn</var> (<var>component</var>)
</pre></div>
<p>Octave will call <var>fcn</var> with the name of the function that requires the
component and a string describing the missing component. The hook function
should return an error message to be displayed.
</p>
<p><strong>See also:</strong> <a href="Parser.html#XREFmissing_005ffunction_005fhook">missing_function_hook</a>.
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