Why does gnu readline require me to hit control c twice?

Question

Normally, Control-C sends a sigint to a program, and kills it if it's not caught. The gnureadline library will install handlers for sigint. However, even when disabling those handlers in haskell, I still need to hit Control-C twice to kill a program. What's going on?

import System.Console.Readline

main = do 
        setCatchSignals False
        mainLoop


mainLoop = do
        maybeLine <- readline ">"
        case maybeLine of
            Nothing -> putStrLn ":("
            Just line -> do 
                            putStr line 
                            putStr " catch:"
                            catch <- getCatchSignals
                            putStrLn $ show $ catch
        mainLoop

This may be related to cooked/uncooked/rare terminal modes; `^C` does not always send a signal. It could be that readline causes a SIGTERM only on two sequential `^C`s. — ehird, Dec 23 '11 at 19:47
Oh, interesting. I hadn't known that about terminal modes. I'll check and see if readline does something with that. Thank you. — archgoon, Dec 23 '11 at 19:49

score 8 · Accepted Answer · answered Dec 23 '11 at 19:59

This may be related to the cooked/uncooked/rare terminal modes; ^C does not always send a signal. It seems likely that readline uncooks the terminal, and thus any signals caused by keyboard input must be due to logic within readline itself; it seems plausible that it might only trigger a SIGINT on two sequential ^Cs (especially since for many programs that utilise readline such as shells and REPLs, the program exiting on a single ^C would be very annoying!).

You might be able to change this behaviour by using the readline API to rebind ^C to some of your own code that triggers a SIGINT. I haven't used readline from Haskell, just from C, so I'm not sure exactly how you'd go about this, but the binding seems rich enough to achieve it.

Why does gnu readline require me to hit control c twice?

1 Answers1