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### 2.29 `mbstowcs`—minimal multibyte string to wide char converter

Synopsis

    #include <stdlib.h>
    int mbstowcs(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 `mbstowcs`. In this case, the only “multi-byte character sequences” recognized are single bytes, and they are “converted” to wide-char versions simply by byte extension.

When _MB_CAPABLE is defined, this routine calls `_mbstowcs_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 `mbstowcs` returns `0` if s is `NULL` or is the empty string; it returns `-1` if _MB_CAPABLE and one of the multi-byte characters is invalid or incomplete; otherwise it returns the minimum of: `n` or the number of multi-byte characters in `s` plus 1 (to compensate for the nul character). If the return value is -1, the state of the `pwc` string is indeterminate. If the input has a length of 0, the output string will be modified to contain a wchar_t nul terminator.

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

`mbstowcs` requires no supporting OS subroutines.