^S and ^Q don't send signals; they make the tty driver stop delivering output. If you ^S a process that's not generating output, it will continue to run until i

^S and ^Q don't send signals; they make the tty driver stop delivering output. If you ^S a process that's not generating output, it will continue to run until it generates some output (until it fills an output buffer? not sure; the original intent, to match a fast process with a slow output device, would be well served by buffering some output). SIGSTOP is like SIGTSTP, the signal usually generated by ^Z, except (like SIGKILL, signal number 9) it's not possible to catch it. It makes the process stop doing anything at all until something sends SIGCONT (which shows up in ps output as state T).