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<h4 class="subsection">25.3.5 Argp Parser Functions</h4>

<p>The function pointed to by the <code>parser</code> field in a <code>struct
argp</code> (see <a href="Argp-Parsers.html#Argp-Parsers">Argp Parsers</a>) defines what actions take place in response
to each option or argument parsed.  It is also used as a hook, allowing a
parser to perform tasks at certain other points during parsing.

   <p>Argp parser functions have the following type signature:

   <p><a name="index-argp-parser-functions-3048"></a>
<pre class="smallexample">     error_t <var>parser</var> (int <var>key</var>, char *<var>arg</var>, struct argp_state *<var>state</var>)
</pre>
   <p class="noindent">where the arguments are as follows:

     <dl>
<dt><var>key</var><dd>For each option that is parsed, <var>parser</var> is called with a value of
<var>key</var> from that option's <code>key</code> field in the option
vector.  See <a href="Argp-Option-Vectors.html#Argp-Option-Vectors">Argp Option Vectors</a>.  <var>parser</var> is also called at
other times with special reserved keys, such as <code>ARGP_KEY_ARG</code> for
non-option arguments.  See <a href="Argp-Special-Keys.html#Argp-Special-Keys">Argp Special Keys</a>.

     <br><dt><var>arg</var><dd>If <var>key</var> is an option, <var>arg</var> is its given value.  This defaults
to zero if no value is specified.  Only options that have a non-zero
<code>arg</code> field can ever have a value.  These must <em>always</em> have a
value unless the <code>OPTION_ARG_OPTIONAL</code> flag is specified.  If the
input being parsed specifies a value for an option that doesn't allow
one, an error results before <var>parser</var> ever gets called.

     <p>If <var>key</var> is <code>ARGP_KEY_ARG</code>, <var>arg</var> is a non-option
argument.  Other special keys always have a zero <var>arg</var>.

     <br><dt><var>state</var><dd><var>state</var> points to a <code>struct argp_state</code>, containing useful
information about the current parsing state for use by
<var>parser</var>.  See <a href="Argp-Parsing-State.html#Argp-Parsing-State">Argp Parsing State</a>. 
</dl>

   <p>When <var>parser</var> is called, it should perform whatever action is
appropriate for <var>key</var>, and return <code>0</code> for success,
<code>ARGP_ERR_UNKNOWN</code> if the value of <var>key</var> is not handled by this
parser function, or a unix error code if a real error
occurred.  See <a href="Error-Codes.html#Error-Codes">Error Codes</a>.

<!-- argp.h -->
<!-- GNU -->
<div class="defun">
&mdash; Macro: int <b>ARGP_ERR_UNKNOWN</b><var><a name="index-ARGP_005fERR_005fUNKNOWN-3049"></a></var><br>
<blockquote><p>Argp parser functions should return <code>ARGP_ERR_UNKNOWN</code> for any
<var>key</var> value they do not recognize, or for non-option arguments
(<var>key</var><code> == ARGP_KEY_ARG</code>) that they are not equipped to handle. 
</p></blockquote></div>

   <p>A typical parser function uses a switch statement on <var>key</var>:

<pre class="smallexample">     error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       switch (key)
         {
         case <var>option_key</var>:
           <var>action</var>
           break;
         ...
         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }
</pre>
   <ul class="menu">
<li><a accesskey="1" href="Argp-Special-Keys.html#Argp-Special-Keys">Keys</a>:            Special values for the <var>key</var> argument. 
<li><a accesskey="2" href="Argp-Parsing-State.html#Argp-Parsing-State">State</a>:          What the <var>state</var> argument refers to. 
<li><a accesskey="3" href="Argp-Helper-Functions.html#Argp-Helper-Functions">Functions</a>:   Functions to help during argp parsing. 
</ul>

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