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| <h4 class="subsection">6.3.1 Selecting the conversion and its properties</h4> |
| |
| <p>We already said above that the currently selected locale for the |
| <code>LC_CTYPE</code> category decides about the conversion that is performed |
| by the functions we are about to describe. Each locale uses its own |
| character set (given as an argument to <code>localedef</code>) and this is the |
| one assumed as the external multibyte encoding. The wide character |
| set is always UCS-4, at least on GNU systems. |
| |
| <p>A characteristic of each multibyte character set is the maximum number |
| of bytes that can be necessary to represent one character. This |
| information is quite important when writing code that uses the |
| conversion functions (as shown in the examples below). |
| The ISO C<!-- /@w --> standard defines two macros that provide this information. |
| |
| <!-- limits.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Macro: int <b>MB_LEN_MAX</b><var><a name="index-MB_005fLEN_005fMAX-634"></a></var><br> |
| <blockquote><p><code>MB_LEN_MAX</code> specifies the maximum number of bytes in the multibyte |
| sequence for a single character in any of the supported locales. It is |
| a compile-time constant and is defined in <samp><span class="file">limits.h</span></samp>. |
| <a name="index-limits_002eh-635"></a></p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Macro: int <b>MB_CUR_MAX</b><var><a name="index-MB_005fCUR_005fMAX-636"></a></var><br> |
| <blockquote><p><code>MB_CUR_MAX</code> expands into a positive integer expression that is the |
| maximum number of bytes in a multibyte character in the current locale. |
| The value is never greater than <code>MB_LEN_MAX</code>. Unlike |
| <code>MB_LEN_MAX</code> this macro need not be a compile-time constant, and in |
| the GNU C library it is not. |
| |
| <p><a name="index-stdlib_002eh-637"></a><code>MB_CUR_MAX</code> is defined in <samp><span class="file">stdlib.h</span></samp>. |
| </p></blockquote></div> |
| |
| <p>Two different macros are necessary since strictly ISO C90<!-- /@w --> compilers |
| do not allow variable length array definitions, but still it is desirable |
| to avoid dynamic allocation. This incomplete piece of code shows the |
| problem: |
| |
| <pre class="smallexample"> { |
| char buf[MB_LEN_MAX]; |
| ssize_t len = 0; |
| |
| while (! feof (fp)) |
| { |
| fread (&buf[len], 1, MB_CUR_MAX - len, fp); |
| /* <span class="roman">... process</span> buf */ |
| len -= used; |
| } |
| } |
| </pre> |
| <p>The code in the inner loop is expected to have always enough bytes in |
| the array <var>buf</var> to convert one multibyte character. The array |
| <var>buf</var> has to be sized statically since many compilers do not allow a |
| variable size. The <code>fread</code> call makes sure that <code>MB_CUR_MAX</code> |
| bytes are always available in <var>buf</var>. Note that it isn't |
| a problem if <code>MB_CUR_MAX</code> is not a compile-time constant. |
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