Send and trap SIGTTIN SIGTTOU in foreground process?

Question

From the description of signals (reference), it seems like SIGTTIN and SIGTTOU are sent to a process only if it is in background. Pressing Ctrl-s does stop printing in terminal, if such flow control is enabled. So the question is, is it possible to actually pass SIGTTIN/SIGTTOU signals to a process currently in foreground and trap it?

As an example, save the following code in a file, chmod +x and run it. Then press Ctrl-s/Ctrl-q. You would notice that Ctrl-s does stop and Ctrl-q does resume printing. But the file ./trap.log file will be empty.

#!/usr/bin/env zsh
touch ./trap.log
trap 'echo TSTP >>./trap.log' TSTP
trap 'echo TTIN >>./trap.log' TTIN
trap 'echo TTOU >>./trap.log' TTOU
trap 'echo CONT >>./trap.log' CONT
for ((i=1;i>0;i=i+1)); do
    printf "print $i\n"
    sleep 1
done

Answer 1

You can send SIGTTIN or SIGTTOU with kill, like any other signal. You can trap them with trap (in sh), like any other (trappable) signal. But Ctrl+S and Ctrl+Q have nothing to do with signals. They're handled by the terminal, not by the application.

If you want your application to treat Ctrl+S as a “pause output” command, make sure the terminal does not capture these characters (stty -ixoff or the tcsetattr equivalent), and handle them like any other input.