### 2.29 `mbtowc`—minimal multibyte to wide char converter

Synopsis

         #include <stdlib.h>
         int mbtowc(wchar_t *restrict pwc, const char *restrict s, size_t n);

Description
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.

Returns
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.

Portability
`mbtowc` is required in the ANSI C standard. However, the precise effects vary with the locale.

`mbtowc` requires no supporting OS subroutines.