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(libc.info)Selecting the Conversion

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Selecting the conversion and its properties
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   We already said above that the currently selected locale for the
`LC_CTYPE' 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 `localedef') and this is the one
assumed as the external multibyte encoding.  The wide character
character set always is UCS-4, at least on GNU systems.

   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
standard defines two macros that provide this information.

 - Macro: int MB_LEN_MAX
     `MB_LEN_MAX' 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 `limits.h'.

 - Macro: int MB_CUR_MAX
     `MB_CUR_MAX' 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 `MB_LEN_MAX'.  Unlike
     `MB_LEN_MAX' this macro need not be a compile-time constant, and in
     the GNU C library it is not.

     `MB_CUR_MAX' is defined in `stdlib.h'.

   Two different macros are necessary since strictly ISO C90 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:

     {
       char buf[MB_LEN_MAX];
       ssize_t len = 0;
     
       while (! feof (fp))
         {
           fread (&buf[len], 1, MB_CUR_MAX - len, fp);
           /* ... process buf */
           len -= used;
         }
     }

   The code in the inner loop is expected to have always enough bytes in
the array BUF to convert one multibyte character.  The array BUF has to
be sized statically since many compilers do not allow a variable size.
The `fread' call makes sure that `MB_CUR_MAX' bytes are always
available in BUF.  Note that it isn't a problem if `MB_CUR_MAX' is not
a compile-time constant.