Bug#682972: ttyname() (and /bin/tty) on /dev/tty return "/dev/tty"

[Ian Jackson <ijackson@chiark.greenend.org.uk>]

Ian Jackson writes ("ttyname() (and /bin/tty) on /dev/tty return "/dev/tty""):
> Package: libc6
> Version: 2.11.2-10
> 
> ttyname() should return the name of the tty device in a form than can
> be used by other processes (with different controlling terminals) to
> open it.

Having considered and discussed this some more I think it will be
difficult to fix this without a kernel change.

Consider the following case:

    $ exec 4<>/dev/tty
    $ xterm&
    [1] 15604
    $

And then in the resulting xterm:

    $ tty <&4
    /dev/tty
    $ echo hi >&4      [ prints hi on the original terminal, as expected ]
    $ echo hi >`tty <&4`
    hi                 [ this is wrong, it should go to the original terminal ]
    $

AFAICT Linux (2.6.32-5-686-bigmem at least) don't provide any way to
get the right answer.

My friends report that the behaviour across other operating systems is
not consistent, but I think the behaviour we see here is clearly
wrong.  (I don't know what Debian/kFreeBSD and Hurd do.)

SuSv3 says that ttyname returns
    a string containing a null-terminated pathname of the terminal 
    associated with file descriptor fildes

I can't see how that can be satisfied by returning a pathname which,
when opened, gives a new descriptor referring to a different terminal.
(Notwithstanding the non-normative "Application Usage" section giving
"/dev/tty" as an example of a possible return value.)

There is similar problem with ctermid, which the glibc info manual
documents as always returning "/dev/tty".

Thanks,
Ian.