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<h4 class="subsection">12.13.4 <code>printf</code> Extension Example</h4>

<p>Here is an example showing how to define a <code>printf</code> handler function. 
This program defines a data structure called a <code>Widget</code> and
defines the &lsquo;<samp><span class="samp">%W</span></samp>&rsquo; conversion to print information about <code>Widget&nbsp;*</code><!-- /@w -->
arguments, including the pointer value and the name stored in the data
structure.  The &lsquo;<samp><span class="samp">%W</span></samp>&rsquo; conversion supports the minimum field width and
left-justification options, but ignores everything else.

<pre class="smallexample">     #include &lt;stdio.h&gt;
     #include &lt;stdlib.h&gt;
     #include &lt;printf.h&gt;
     
     typedef struct
     {
       char *name;
     }
     Widget;
     
     int
     print_widget (FILE *stream,
                   const struct printf_info *info,
                   const void *const *args)
     {
       const Widget *w;
       char *buffer;
       int len;
     
       /* <span class="roman">Format the output into a string.</span> */
       w = *((const Widget **) (args[0]));
       len = asprintf (&amp;buffer, "&lt;Widget %p: %s&gt;", w, w-&gt;name);
       if (len == -1)
         return -1;
     
       /* <span class="roman">Pad to the minimum field width and print to the stream.</span> */
       len = fprintf (stream, "%*s",
                      (info-&gt;left ? -info-&gt;width : info-&gt;width),
                      buffer);
     
       /* <span class="roman">Clean up and return.</span> */
       free (buffer);
       return len;
     }
     
     
     int
     print_widget_arginfo (const struct printf_info *info, size_t n,
                           int *argtypes)
     {
       /* <span class="roman">We always take exactly one argument and this is a pointer to the
          structure..</span> */
       if (n &gt; 0)
         argtypes[0] = PA_POINTER;
       return 1;
     }
     
     
     int
     main (void)
     {
       /* <span class="roman">Make a widget to print.</span> */
       Widget mywidget;
       mywidget.name = "mywidget";
     
       /* <span class="roman">Register the print function for widgets.</span> */
       register_printf_function ('W', print_widget, print_widget_arginfo);
     
       /* <span class="roman">Now print the widget.</span> */
       printf ("|%W|\n", &amp;mywidget);
       printf ("|%35W|\n", &amp;mywidget);
       printf ("|%-35W|\n", &amp;mywidget);
     
       return 0;
     }
</pre>
   <p>The output produced by this program looks like:

<pre class="smallexample">     |&lt;Widget 0xffeffb7c: mywidget&gt;|
     |      &lt;Widget 0xffeffb7c: mywidget&gt;|
     |&lt;Widget 0xffeffb7c: mywidget&gt;      |
</pre>
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