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<h5 class="subsubsection">25.3.5.2 Functions For Use in Argp Parsers</h5>

<p>Argp provides a number of functions available to the user of argp
(see <a href="Argp-Parser-Functions.html#Argp-Parser-Functions">Argp Parser Functions</a>), mostly for producing error messages. 
These take as their first argument the <var>state</var> argument to the
parser function.  See <a href="Argp-Parsing-State.html#Argp-Parsing-State">Argp Parsing State</a>.

   <p><a name="index-usage-messages_002c-in-argp-3058"></a><!-- argp.h -->
<!-- GNU -->

<div class="defun">
&mdash; Function: void <b>argp_usage</b> (<var>const struct argp_state *state</var>)<var><a name="index-argp_005fusage-3059"></a></var><br>
<blockquote><p>Outputs the standard usage message for the argp parser referred to by
<var>state</var> to <var>state</var><code>-&gt;err_stream</code> and terminate the program
with <code>exit (argp_err_exit_status)</code>.  See <a href="Argp-Global-Variables.html#Argp-Global-Variables">Argp Global Variables</a>. 
</p></blockquote></div>

   <p><a name="index-syntax-error-messages_002c-in-argp-3060"></a><!-- argp.h -->
<!-- GNU -->

<div class="defun">
&mdash; Function: void <b>argp_error</b> (<var>const struct argp_state *state, const char *fmt, <small class="dots">...</small></var>)<var><a name="index-argp_005ferror-3061"></a></var><br>
<blockquote><p>Prints the printf format string <var>fmt</var> and following args, preceded
by the program name and &lsquo;<samp><span class="samp">:</span></samp>&rsquo;, and followed by a &lsquo;<samp><span class="samp">Try&nbsp;...&nbsp;--help</span></samp>&rsquo;<!-- /@w --> message, and terminates the program with an exit status of
<code>argp_err_exit_status</code>.  See <a href="Argp-Global-Variables.html#Argp-Global-Variables">Argp Global Variables</a>. 
</p></blockquote></div>

   <p><a name="index-error-messages_002c-in-argp-3062"></a><!-- argp.h -->
<!-- GNU -->

<div class="defun">
&mdash; Function: void <b>argp_failure</b> (<var>const struct argp_state *state, int status, int errnum, const char *fmt, <small class="dots">...</small></var>)<var><a name="index-argp_005ffailure-3063"></a></var><br>
<blockquote><p>Similar to the standard gnu error-reporting function <code>error</code>, this
prints the program name and &lsquo;<samp><span class="samp">:</span></samp>&rsquo;, the printf format string
<var>fmt</var>, and the appropriate following args.  If it is non-zero, the
standard unix error text for <var>errnum</var> is printed.  If <var>status</var> is
non-zero, it terminates the program with that value as its exit status.

        <p>The difference between <code>argp_failure</code> and <code>argp_error</code> is that
<code>argp_error</code> is for <em>parsing errors</em>, whereas
<code>argp_failure</code> is for other problems that occur during parsing but
don't reflect a syntactic problem with the input, such as illegal values
for options, bad phase of the moon, etc. 
</p></blockquote></div>

<!-- argp.h -->
<!-- GNU -->
<div class="defun">
&mdash; Function: void <b>argp_state_help</b> (<var>const struct argp_state *state, FILE *stream, unsigned flags</var>)<var><a name="index-argp_005fstate_005fhelp-3064"></a></var><br>
<blockquote><p>Outputs a help message for the argp parser referred to by <var>state</var>,
to <var>stream</var>.  The <var>flags</var> argument determines what sort of help
message is produced.  See <a href="Argp-Help-Flags.html#Argp-Help-Flags">Argp Help Flags</a>. 
</p></blockquote></div>

   <p>Error output is sent to <var>state</var><code>-&gt;err_stream</code>, and the program
name printed is <var>state</var><code>-&gt;name</code>.

   <p>The output or program termination behavior of these functions may be
suppressed if the <code>ARGP_NO_EXIT</code> or <code>ARGP_NO_ERRS</code> flags are
passed to <code>argp_parse</code>.  See <a href="Argp-Flags.html#Argp-Flags">Argp Flags</a>.

   <p>This behavior is useful if an argp parser is exported for use by other
programs (e.g., by a library), and may be used in a context where it is
not desirable to terminate the program in response to parsing errors.  In
argp parsers intended for such general use, and for the case where the
program <em>doesn't</em> terminate, calls to any of these functions should
be followed by code that returns the appropriate error code:

<pre class="smallexample">     if (<var>bad argument syntax</var>)
       {
          argp_usage (<var>state</var>);
          return EINVAL;
       }
</pre>
   <p class="noindent">If a parser function will <em>only</em> be used when <code>ARGP_NO_EXIT</code>
is not set, the return may be omitted.

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