I've got this working!
The proof of concept program is a bit longish to include the source in
this email, and certainly longish for such a seemingly obvious bit of
data that an ordinary program would want to know about its file
descriptors. Here is the output:
[nix-shell:~/src/ac_incidental/src]$ cc identifying_tty.c
[nix-shell:~/src/ac_incidental/src]$ ./a.out
/proc/self/stat ctty dev is 136 1
/dev/tty dev is 5 0
0 is the ctty called /dev/pts/1 device ID 136 1
1 is the ctty called /dev/pts/1 device ID 136 1
2 is the ctty called /dev/pts/1 device ID 136 1
3 is the ctty called /dev/tty device ID 5 0
[nix-shell:~/src/ac_incidental/src]$ ./a.out </dev/tty
/proc/self/stat ctty dev is 136 1
/dev/tty dev is 5 0
0 is the ctty called /dev/tty device ID 5 0
1 is the ctty called /dev/pts/1 device ID 136 1
2 is the ctty called /dev/pts/1 device ID 136 1
3 is the ctty called /dev/tty device ID 5 0
And I also noticed that the linux device list says this about major
numbers 5 and 136:
5 char Alternate TTY devices
0 = /dev/tty Current TTY device
1 = /dev/console System console
2 = /dev/ptmx PTY master multiplex
64 = /dev/cua0 Callout device for ttyS0
...
255 = /dev/cua191 Callout device for ttyS191
136-143 char Unix98 PTY slaves
0 = /dev/pts/0 First Unix98 pseudo-TTY
1 = /dev/pts/1 Second Unix98 pesudo-TTY
...
There are just two ways to determine if a file descriptor might be the
controlling terminal on Linux, (/dev/tty, /proc/self/stat field 7), and
you need to check both. I consider this puzzle solved, but I'd welcome a
simpler solution.
--
Anthony Carrico
