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| <a name="Keeping-the-state"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Converting-a-Character.html#Converting-a-Character">Converting a Character</a>, |
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| <h4 class="subsection">6.3.2 Representing the state of the conversion</h4> |
| |
| <p><a name="index-stateful-638"></a>In the introduction of this chapter it was said that certain character |
| sets use a <dfn>stateful</dfn> encoding. That is, the encoded values depend |
| in some way on the previous bytes in the text. |
| |
| <p>Since the conversion functions allow converting a text in more than one |
| step we must have a way to pass this information from one call of the |
| functions to another. |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Data type: <b>mbstate_t</b><var><a name="index-mbstate_005ft-639"></a></var><br> |
| <blockquote><p><a name="index-shift-state-640"></a>A variable of type <code>mbstate_t</code> can contain all the information |
| about the <dfn>shift state</dfn> needed from one call to a conversion |
| function to another. |
| |
| <p><a name="index-wchar_002eh-641"></a><code>mbstate_t</code> is defined in <samp><span class="file">wchar.h</span></samp>. It was introduced in |
| Amendment 1<!-- /@w --> to ISO C90<!-- /@w -->. |
| </p></blockquote></div> |
| |
| <p>To use objects of type <code>mbstate_t</code> the programmer has to define such |
| objects (normally as local variables on the stack) and pass a pointer to |
| the object to the conversion functions. This way the conversion function |
| can update the object if the current multibyte character set is stateful. |
| |
| <p>There is no specific function or initializer to put the state object in |
| any specific state. The rules are that the object should always |
| represent the initial state before the first use, and this is achieved by |
| clearing the whole variable with code such as follows: |
| |
| <pre class="smallexample"> { |
| mbstate_t state; |
| memset (&state, '\0', sizeof (state)); |
| /* <span class="roman">from now on </span><var>state</var><span class="roman"> can be used.</span> */ |
| ... |
| } |
| </pre> |
| <p>When using the conversion functions to generate output it is often |
| necessary to test whether the current state corresponds to the initial |
| state. This is necessary, for example, to decide whether to emit |
| escape sequences to set the state to the initial state at certain |
| sequence points. Communication protocols often require this. |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>mbsinit</b> (<var>const mbstate_t *ps</var>)<var><a name="index-mbsinit-642"></a></var><br> |
| <blockquote><p>The <code>mbsinit</code> function determines whether the state object pointed |
| to by <var>ps</var> is in the initial state. If <var>ps</var> is a null pointer or |
| the object is in the initial state the return value is nonzero. Otherwise |
| it is zero. |
| |
| <p><a name="index-wchar_002eh-643"></a><code>mbsinit</code> was introduced in Amendment 1<!-- /@w --> to ISO C90<!-- /@w --> and is |
| declared in <samp><span class="file">wchar.h</span></samp>. |
| </p></blockquote></div> |
| |
| <p>Code using <code>mbsinit</code> often looks similar to this: |
| |
| <!-- Fix the example to explicitly say how to generate the escape sequence --> |
| <!-- to restore the initial state. --> |
| <pre class="smallexample"> { |
| mbstate_t state; |
| memset (&state, '\0', sizeof (state)); |
| /* <span class="roman">Use </span><var>state</var><span class="roman">.</span> */ |
| ... |
| if (! mbsinit (&state)) |
| { |
| /* <span class="roman">Emit code to return to initial state.</span> */ |
| const wchar_t empty[] = L""; |
| const wchar_t *srcp = empty; |
| wcsrtombs (outbuf, &srcp, outbuflen, &state); |
| } |
| ... |
| } |
| </pre> |
| <p>The code to emit the escape sequence to get back to the initial state is |
| interesting. The <code>wcsrtombs</code> function can be used to determine the |
| necessary output code (see <a href="Converting-Strings.html#Converting-Strings">Converting Strings</a>). Please note that on |
| GNU systems it is not necessary to perform this extra action for the |
| conversion from multibyte text to wide character text since the wide |
| character encoding is not stateful. But there is nothing mentioned in |
| any standard that prohibits making <code>wchar_t</code> using a stateful |
| encoding. |
| |
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