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`mblen'--minimal multibyte length function

     #include <stdlib.h>
     int mblen(const char *S, size_t N);
When MB_CAPABLE is not defined, this is a minimal ANSI-conforming
implementation of `mblen'.  In this case, the only "multi-byte
character sequences" recognized are single bytes, and thus `1' is
returned unless S is the null pointer or has a length of 0 or is the
empty string.
   When MB_CAPABLE is defined, this routine calls `_mbtowc_r' to perform
the conversion, passing a state variable to allow state dependent
decoding.  The result is based on the locale setting which may be
restricted to a defined set of locales.
This implementation of `mblen' returns `0' if S is `NULL' or the empty
string; it returns `1' if not MB_CAPABLE or the character is a
single-byte character; it returns `-1' if the multi-byte character is
invalid; otherwise it returns the number of bytes in the multibyte
`mblen' is required in the ANSI C standard.  However, the precise
effects vary with the locale.
   `mblen' requires no supporting OS subroutines.