Return_Values

   x                  Upon successful completion, ttyname returns a
                      pointer to a null-terminated string.
   NULL               Upon failure, ttyname returns a NULL pointer
                      and sets errno to indicate the failure:

                      o  EBADF - The fildes argument is not a valid
                         file descriptor.

                      o  ENOTTY - The fildes argument does not refer
                         to a terminal device.

   0                  Upon successful completion, ttyname_r returns
                      0.
   n                  Upon failure, ttyname_r sets errno to indicate
                      the failure, and returns the same errno code:

                      o  EBADF - The fildes argument is not a valid
                         file descriptor.

                      o  ENOTTY - The fildes argument does not refer
                         to a TTY device.

                      o  ERANGE - The value of namesize is smaller
                         than the length of the string to be
                         returned including the terminating null
                         character.

   0                  For the legacy ttyname, indicates that
                      SYS$INPUT is not a TTY device.