wcsrtombs
extern size_t wcsrtombs(char *_RESTRICT_KYWD, const wchar_t **_RESTRICT_KYWD,
using std::wcsrtombs;
cnt = wcsrtombs(mbs, &wcs, sizeof (mbs), &ms);
cnt = wcsrtombs(dst, &wcs, 1, &ms);
cnt = wcsrtombs(dst, &wcs, 2, &ms);
cnt = wcsrtombs(dst, &wcs, 4, &ms);
cnt = wcsrtombs(dst, &wcs, sizeof (mbs) - strlen(mbs), &ms);
cnt = wcsrtombs(dst, &wcs, sizeof (mbs), &ms);