To build an interactive application you can open /dev/tty, it will return a file descriptor to the controlling terminal:

int ttyfd = open("/dev/tty", O_RDWR);

You can use it instead of STDIN_FILENO or STDOUT_FILENO (those could be redirected to something different than the terminal when the program is started).

Here is some example:

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main() {
  int ttyfd = open("/dev/tty", O_RDWR);
  printf("fd: %d\n", ttyfd);
  write(ttyfd, "hello tty!\n", 11);
  return 0;
}

When invoked with ./test >out, it should print the hello message on the terminal and something like fd: 3 in the out file.

If using bash, the following should do the trick:

TOLASTLINE=$(tput cup "$LINES")
PS1="\[$TOLASTLINE\]$PS1"

Or (less efficient as it runs one tput command before each prompt, but works after the terminal window has been resized):

PS1='\[$(tput cup "$LINES")\]'$PS1

To prevent tput from changing the exit code, you can explicitly save and reset it:

PS1='\[$(retval=$?;tput cup "$LINES";exit $retval)\]'$PS1

Note that the variable retval is local; it doesn't affect any retval variable you might have defined otherwise in the shell.

Since most terminals cup capability is the same \e[y;xH, you could also hardcode it:

PS1='\[\e[$LINES;1H\]'$PS1

If you want it to be safe against later resetting of PS1, you can also utilize the PROMPT_COMMAND variable. If set, it is run as command before the prompt is output. So the effect can also be achieved by

PROMPT_COMMAND='(retval=$?;tput cup "$LINES";exit $retval)'

Of course, while resetting PS1 won't affect this, some other software might also change PROMPT_COMMAND.