Does pressing ctrl-c several times make the running program close more quickly

killprocesssignals

I often start to read a huge file and then want to quit after a while, but there is a lag from pressing
Ctrl+C to the program stops. Is there a chance of shortening the lag by pressing the Ctrl+C key several times? Or am I wasting my keypresses?

Best Answer

After the first Ctrl-C, the program will receive SIGINT and usually starts cleaning up (deleting tmp files, closing sockets, etc.). If you hit Ctrl-C again while that is going on, it may happen that you interrupt the clean up routine (i.e. the additional signal might be acted upon instead of being left alone), leaving a mess behind. While this usually is not the case, more commonly the additional signals are in fact sent after the process finished (because of the inherent delays in the interaction of the operator with the system). That means that signals are received by another process (often shell, but not always). If that recipient doesn't handle this signal properly (like shell usually does - see Jenny D's answer) you may be unpleasantly surprised by the outcome of such an action.

Related Solutions

Networking – Use LD_PRELOAD to Prevent Network Access

Under Linux, try to use a network namespace, e.g:

sudo ip netns add namespace-name
sudo ip netns exec namespace-name executable

This should prevent the program from accessing the network.

SSH – Handling Ctrl-C in SSH Sessions

ssh can be invoked in a few different ways, each resulting slightly different treatment of terminal-initiated signals like Ctrl-C.

ssh remotehost will run an interactive session on remotehost. On the client side, ssh will try to set the tty used by stdin to "raw" mode, and sshd on the remote host will allocate a pseudo-tty and run your shell as a login shell (e.g. -bash).

Setting raw mode means that characters that would normally send signals (such as Ctrl-C and Ctrl-\) are instead just inserted into the input stream. ssh will send such characters as-is to the remote host, where they will likely send SIGINT or SIGQUIT and, typically, kill any command and return you to a shell on the remote host. The ssh connection will stay alive, as long as the remote shell is alive.
ssh -t remotehost command args ... will run an interactive session on remotehost, just like the above, except on the remote side, your_shell -c "command args ..." will be run. As above, if you type Ctrl-C, it will be sent to the remote host, where the command will likely receive SIGINT and immediately exit, and then the remote shell will exit. The remote sshd then closes the connection, and ssh reports Connection to remotehost closed.
ssh remotehost command args ... will run a non-interactive session on remotehost. On the client side, ssh will not set the tty to raw mode (well, except to read in a password or passphrase). If you type Ctrl-C, ssh will get sent SIGINT and will immediately be terminated, without even issuing a Connection to remotehost closed message.

The your_shell -c "command args ..." processes will likely remain running on the remote host. Either they will exit on their own, or one process will try to write data to the now-closed ssh socket, which will cause a (typically) fatal SIGPIPE signal to be sent to it.

Best Answer

Related Solutions

Networking – Use LD_PRELOAD to Prevent Network Access

SSH – Handling Ctrl-C in SSH Sessions

Related Question