getlogin_r (name, name_len)
     char *name;
     size_t name_len;
{
  char tty_pathname[2 + 2 * NAME_MAX];
  char *real_tty_path = tty_pathname;
  int result = 0;
  struct utmp *ut, line, buffer;

  /* Get name of tty connected to fd 0.  Return if not a tty or
     if fd 0 isn't open.  Note that a lot of documentation says that
     getlogin() is based on the controlling terminal---what they
     really mean is "the terminal connected to standard input".  The
     getlogin() implementation of DEC Unix, SunOS, Solaris, HP-UX all
     return NULL if fd 0 has been closed, so this is the compatible
     thing to do.  Note that ttyname(open("/dev/tty")) on those
     systems returns /dev/tty, so that is not a possible solution for
     getlogin().  */

  result = __ttyname_r (0, real_tty_path, sizeof (tty_pathname));
  
  if (result != 0)
    return result;

  real_tty_path += 5;		/* Remove "/dev/".  */

  __setutent ();
  strncpy (line.ut_line, real_tty_path, sizeof line.ut_line);
  if (__getutline_r (&line, &buffer, &ut) < 0)
    {
      if (errno == ESRCH)
	/* The caller expects ENOENT if nothing is found.  */
	result = ENOENT;
      else
	result = errno;
    }
  else
    {
      size_t needed = strlen (ut->ut_user) + 1;

      if (needed > name_len)
	{
	  __set_errno (ERANGE);
	  result = ERANGE;
	}
      else
	{
	  memcpy (name, ut->ut_user, needed);
	  result = 0;
	}
    }
  __endutent ();

  return result;
}
The software does not check the return value from a method or function, which can prevent it from detecting unexpected states and conditions.