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| <h3 class="section">12.6 Streams in Internationalized Applications</h3> |
| |
| <p>ISO C90<!-- /@w --> introduced the new type <code>wchar_t</code> to allow handling |
| larger character sets. What was missing was a possibility to output |
| strings of <code>wchar_t</code> directly. One had to convert them into |
| multibyte strings using <code>mbstowcs</code> (there was no <code>mbsrtowcs</code> |
| yet) and then use the normal stream functions. While this is doable it |
| is very cumbersome since performing the conversions is not trivial and |
| greatly increases program complexity and size. |
| |
| <p>The Unix standard early on (I think in XPG4.2) introduced two additional |
| format specifiers for the <code>printf</code> and <code>scanf</code> families of |
| functions. Printing and reading of single wide characters was made |
| possible using the <code>%C</code> specifier and wide character strings can be |
| handled with <code>%S</code>. These modifiers behave just like <code>%c</code> and |
| <code>%s</code> only that they expect the corresponding argument to have the |
| wide character type and that the wide character and string are |
| transformed into/from multibyte strings before being used. |
| |
| <p>This was a beginning but it is still not good enough. Not always is it |
| desirable to use <code>printf</code> and <code>scanf</code>. The other, smaller and |
| faster functions cannot handle wide characters. Second, it is not |
| possible to have a format string for <code>printf</code> and <code>scanf</code> |
| consisting of wide characters. The result is that format strings would |
| have to be generated if they have to contain non-basic characters. |
| |
| <p><a name="index-C_002b_002b-streams-952"></a><a name="index-streams_002c-C_002b_002b-953"></a>In the Amendment 1<!-- /@w --> to ISO C90<!-- /@w --> a whole new set of functions was |
| added to solve the problem. Most of the stream functions got a |
| counterpart which take a wide character or wide character string instead |
| of a character or string respectively. The new functions operate on the |
| same streams (like <code>stdout</code>). This is different from the model of |
| the C++ runtime library where separate streams for wide and normal I/O |
| are used. |
| |
| <p><a name="index-orientation_002c-stream-954"></a><a name="index-stream-orientation-955"></a>Being able to use the same stream for wide and normal operations comes |
| with a restriction: a stream can be used either for wide operations or |
| for normal operations. Once it is decided there is no way back. Only a |
| call to <code>freopen</code> or <code>freopen64</code> can reset the |
| <dfn>orientation</dfn>. The orientation can be decided in three ways: |
| |
| <ul> |
| <li>If any of the normal character functions is used (this includes the |
| <code>fread</code> and <code>fwrite</code> functions) the stream is marked as not |
| wide oriented. |
| |
| <li>If any of the wide character functions is used the stream is marked as |
| wide oriented. |
| |
| <li>The <code>fwide</code> function can be used to set the orientation either way. |
| </ul> |
| |
| <p>It is important to never mix the use of wide and not wide operations on |
| a stream. There are no diagnostics issued. The application behavior |
| will simply be strange or the application will simply crash. The |
| <code>fwide</code> function can help avoiding this. |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>fwide</b> (<var>FILE *stream, int mode</var>)<var><a name="index-fwide-956"></a></var><br> |
| <blockquote> |
| <p>The <code>fwide</code> function can be used to set and query the state of the |
| orientation of the stream <var>stream</var>. If the <var>mode</var> parameter has |
| a positive value the streams get wide oriented, for negative values |
| narrow oriented. It is not possible to overwrite previous orientations |
| with <code>fwide</code>. I.e., if the stream <var>stream</var> was already |
| oriented before the call nothing is done. |
| |
| <p>If <var>mode</var> is zero the current orientation state is queried and |
| nothing is changed. |
| |
| <p>The <code>fwide</code> function returns a negative value, zero, or a positive |
| value if the stream is narrow, not at all, or wide oriented |
| respectively. |
| |
| <p>This function 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>It is generally a good idea to orient a stream as early as possible. |
| This can prevent surprise especially for the standard streams |
| <code>stdin</code>, <code>stdout</code>, and <code>stderr</code>. If some library |
| function in some situations uses one of these streams and this use |
| orients the stream in a different way the rest of the application |
| expects it one might end up with hard to reproduce errors. Remember |
| that no errors are signal if the streams are used incorrectly. Leaving |
| a stream unoriented after creation is normally only necessary for |
| library functions which create streams which can be used in different |
| contexts. |
| |
| <p>When writing code which uses streams and which can be used in different |
| contexts it is important to query the orientation of the stream before |
| using it (unless the rules of the library interface demand a specific |
| orientation). The following little, silly function illustrates this. |
| |
| <pre class="smallexample"> void |
| print_f (FILE *fp) |
| { |
| if (fwide (fp, 0) > 0) |
| /* <span class="roman">Positive return value means wide orientation.</span> */ |
| fputwc (L'f', fp); |
| else |
| fputc ('f', fp); |
| } |
| </pre> |
| <p>Note that in this case the function <code>print_f</code> decides about the |
| orientation of the stream if it was unoriented before (will not happen |
| if the advise above is followed). |
| |
| <p>The encoding used for the <code>wchar_t</code> values is unspecified and the |
| user must not make any assumptions about it. For I/O of <code>wchar_t</code> |
| values this means that it is impossible to write these values directly |
| to the stream. This is not what follows from the ISO C<!-- /@w --> locale model |
| either. What happens instead is that the bytes read from or written to |
| the underlying media are first converted into the internal encoding |
| chosen by the implementation for <code>wchar_t</code>. The external encoding |
| is determined by the <code>LC_CTYPE</code> category of the current locale or |
| by the ‘<samp><span class="samp">ccs</span></samp>’ part of the mode specification given to <code>fopen</code>, |
| <code>fopen64</code>, <code>freopen</code>, or <code>freopen64</code>. How and when the |
| conversion happens is unspecified and it happens invisible to the user. |
| |
| <p>Since a stream is created in the unoriented state it has at that point |
| no conversion associated with it. The conversion which will be used is |
| determined by the <code>LC_CTYPE</code> category selected at the time the |
| stream is oriented. If the locales are changed at the runtime this |
| might produce surprising results unless one pays attention. This is |
| just another good reason to orient the stream explicitly as soon as |
| possible, perhaps with a call to <code>fwide</code>. |
| |
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