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`mbtowc'--minimal multibyte to wide char converter

     #include <stdlib.h>
     int mbtowc(wchar_t *PWC, const char *S, size_t N);
When MB_CAPABLE is not defined, this is a minimal ANSI-conforming
implementation of `mbtowc'.  In this case, only "multi-byte character
sequences" recognized are single bytes, and they are "converted" to
themselves.  Each call to `mbtowc' copies one character from `*S' to
`*PWC', unless S is a null pointer.  The argument n is ignored.
   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 `mbtowc' returns `0' if S is `NULL' or is the
empty string; it returns `1' if not MB_CAPABLE or the character is a
single-byte character; it returns `-1' if n is `0' or the multi-byte
character is invalid; otherwise it returns the number of bytes in the
multibyte character.  If the return value is -1, no changes are made to
the `pwc' output string.  If the input is the empty string, a wchar_t
nul is placed in the output string and 0 is returned.  If the input has
a length of 0, no changes are made to the `pwc' output string.
`mbtowc' is required in the ANSI C standard.  However, the precise
effects vary with the locale.
   `mbtowc' requires no supporting OS subroutines.