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<h4 class="subsection">25.3.8 Customizing Argp Help Output</h4>

<p>The <code>help_filter</code> field in a <code>struct argp</code> is a pointer to a
function that filters the text of help messages before displaying
them.  They have a function signature like:

<pre class="smallexample">     char *<var>help-filter</var> (int <var>key</var>, const char *<var>text</var>, void *<var>input</var>)
</pre>
   <p class="noindent">Where <var>key</var> is either a key from an option, in which case <var>text</var>
is that option's help text.  See <a href="Argp-Option-Vectors.html#Argp-Option-Vectors">Argp Option Vectors</a>.  Alternately, one
of the special keys with names beginning with &lsquo;<samp><span class="samp">ARGP_KEY_HELP_</span></samp>&rsquo;
might be used, describing which other help text <var>text</var> will contain. 
See <a href="Argp-Help-Filter-Keys.html#Argp-Help-Filter-Keys">Argp Help Filter Keys</a>.

   <p>The function should return either <var>text</var> if it remains as-is, or a
replacement string allocated using <code>malloc</code>.  This will be either be
freed by argp or zero, which prints nothing.  The value of <var>text</var> is
supplied <em>after</em> any translation has been done, so if any of the
replacement text needs translation, it will be done by the filter
function.  <var>input</var> is either the input supplied to <code>argp_parse</code>
or it is zero, if <code>argp_help</code> was called directly by the user.

<ul class="menu">
<li><a accesskey="1" href="Argp-Help-Filter-Keys.html#Argp-Help-Filter-Keys">Keys</a>:   Special <var>key</var> values for help filter functions. 
</ul>

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