According to the documentation, the following should work

len = mbstowcs(NULL, s, 0);

However, this is implemented in libc/ansi/locale/mbstowcs.c, which is:

size_t
mbstowcs(wchar_t *wcs, const char *s, size_t n)
{
  int i;
  for (i=0; s[i] && (i<n-1); i++)
    wcs[i] = s[i];
  wcs[i] = 0;
  return i;
}

and should be

size_t
mbstowcs(wchar_t *wcs, const char *s, size_t n)
{
  int i;
  for (i=0; s[i] && (i<n-1); i++)
    if (wcs) wcs[i] = s[i];
  if (wcs) wcs[i] = 0;
  return i;
}

size_t
mbstowcs(wchar_t *wcs, const char *s, size_t n)
{
  int i;
  for (i=0; s[i] && (i<n-1); i++)
    if (wcs) wcs[i] = s[i];
  if (wcs) wcs[i] = 0;
  return i;
}

Or maybe, the solution should be

size_t
mbstowcs(wchar_t *wcs, const char *s, size_t n)
{
  int i;
  for (i=0; s[i]; i++)
    if ((wcs) && (i<n-1)) wcs[i] = s[i];
  if (wcs) 
    if (i<n-1) wcs[i] = 0; else wcs[n-1] = 0;
  return i;
}