### 3.27 `mblen`—minimal multibyte length function ¶

Synopsis

    #include <stdlib.h>
    int mblen(const char *s, size_t n);

Description
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.

Returns
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 character.

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

`mblen` requires no supporting OS subroutines.