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<h4 class="subsection">6.4.3 States in Non-reentrant Functions</h4>

<p>In some multibyte character codes, the <em>meaning</em> of any particular
byte sequence is not fixed; it depends on what other sequences have come
earlier in the same string.  Typically there are just a few sequences that
can change the meaning of other sequences; these few are called
<dfn>shift sequences</dfn> and we say that they set the <dfn>shift state</dfn> for
other sequences that follow.

   <p>To illustrate shift state and shift sequences, suppose we decide that
the sequence <code>0200</code> (just one byte) enters Japanese mode, in which
pairs of bytes in the range from <code>0240</code> to <code>0377</code> are single
characters, while <code>0201</code> enters Latin-1 mode, in which single bytes
in the range from <code>0240</code> to <code>0377</code> are characters, and
interpreted according to the ISO Latin-1 character set.  This is a
multibyte code that has two alternative shift states (&ldquo;Japanese mode&rdquo;
and &ldquo;Latin-1 mode&rdquo;), and two shift sequences that specify particular
shift states.

   <p>When the multibyte character code in use has shift states, then
<code>mblen</code>, <code>mbtowc</code>, and <code>wctomb</code> must maintain and update
the current shift state as they scan the string.  To make this work
properly, you must follow these rules:

     <ul>
<li>Before starting to scan a string, call the function with a null pointer
for the multibyte character address&mdash;for example, <code>mblen (NULL,
0)</code>.  This initializes the shift state to its standard initial value.

     <li>Scan the string one character at a time, in order.  Do not &ldquo;back up&rdquo;
and rescan characters already scanned, and do not intersperse the
processing of different strings. 
</ul>

   <p>Here is an example of using <code>mblen</code> following these rules:

<pre class="smallexample">     void
     scan_string (char *s)
     {
       int length = strlen (s);
     
       /* <span class="roman">Initialize shift state.</span>  */
       mblen (NULL, 0);
     
       while (1)
         {
           int thischar = mblen (s, length);
           /* <span class="roman">Deal with end of string and invalid characters.</span>  */
           if (thischar == 0)
             break;
           if (thischar == -1)
             {
               error ("invalid multibyte character");
               break;
             }
           /* <span class="roman">Advance past this character.</span>  */
           s += thischar;
           length -= thischar;
         }
     }
</pre>
   <p>The functions <code>mblen</code>, <code>mbtowc</code> and <code>wctomb</code> are not
reentrant when using a multibyte code that uses a shift state.  However,
no other library functions call these functions, so you don't have to
worry that the shift state will be changed mysteriously.

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