How do I attach a terminal to a detached process?

Yes, it is. First, create a pipe: mkfifo /tmp/fifo. Use gdb to attach to the process: gdb -p PID

Then close stdin: call close (0); and open it again: call open ("/tmp/fifo", 0600)

Finally, write away (from a different terminal, as gdb will probably hang):

echo blah > /tmp/fifo

When original terminal is no longer accessible...

reptyr might be what you want, see https://serverfault.com/a/284795/187998

Quote from there:

Have a look at reptyr, which does exactly that. The github page has all the information.

reptyr - A tool for "re-ptying" programs.

reptyr is a utility for taking an existing running program and attaching it to a new terminal. Started a long-running process over ssh, but have to leave and don't want to interrupt it? Just start a screen, use reptyr to grab it, and then kill the ssh session and head on home.

USAGE

reptyr PID

"reptyr PID" will grab the process with id PID and attach it to your current terminal.

After attaching, the process will take input from and write output to the new terminal, including ^C and ^Z. (Unfortunately, if you background it, you will still have to run "bg" or "fg" in the old terminal. This is likely impossible to fix in a reasonable way without patching your shell.)

I am quite sure you can not.

Check using ps x. If a process has a ? as controlling tty, you can not send input to it any more.

9942 ?        S      0:00 tail -F /var/log/messages
9947 pts/1    S      0:00 tail -F /var/log/messages

In this example, you can send input to 9947 doing something like echo "test" > /dev/pts/1. The other process (9942) is not reachable.

Next time, you could use screen or tmux to avoid this situation.