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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 (“Japanese mode” |
| and “Latin-1 mode”), 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—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 “back up” |
| 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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