C SIGINT signal in Linux

Question

I want to make a specific combination of keyboard keys in order to terminate a process e.x I want to terminate the process by pressin CTRL + C ^ 3 (pressing three times C: CTRL +CCC).

So basically I want to replace CTRL + C with CTRL + CCC

Answer 1

The default behavior of Ctrl+C is a combination of two things. The terminal driver¹ does not transmit this key press, but instead sends a SIGINT signal to the foreground process². By default, a process dies when it receives a SIGINT, but the process can set a signal handler, and then it'll run the signal handler when it receives SIGINT.

There's no way to configure the terminal driver to only convert the third Ctrl+C in a row to kill the foreground process. To do that, you need to count to three in your program. There are two ways you can do that, which will behave differently if the user presses something else between the Ctrl+C's.

One way is to disable Ctrl+C's behavior of sending a signal and telling the terminal driver to instead pass it through. You can do that by calling stty intr \^- from a shell script or tcsetattr(fd, &termios) with termios.c_cc[VINTR] set to _POSIX_VDISABLE from C. Then, in your program's input processing loop, exit when you've seen three Ctrl+C's in a row.

The other way is to set a signal handler for SIGINT that counts how many times it's been invoked and terminates the program the third time. You may want to reset the counter if there's normal input in between.

¹ _{Not the terminal emulator, the generic part of the operating system that handles all terminals.}
² _{I'm only explaining the simple case. This is not a treatise on how the terminal driver works.}

Answer 2

You can't change key binding in your code because a signal is a software generated interrupt that is sent to a process by the OS. when user press ctrl-c another process tell something to your process.

There are fix set of signals that can be sent to a process. signals are similar to interrupts, the difference being that interrupts are mediated by the processor and handled by the kernel while signals are mediated by the kernel (possibly via system calls) and handled by processes. The kernel may pass an interrupt as a signal to the process that caused it (typical examples are SIGSEGV, SIGBUS, SIGILL and SIGFPE).

you can remap signal key binding in your computer with stty

Enable Ctrl+C for copy and Ctrl+Shift+C for interrput

https://docstore.mik.ua/orelly/unix3/upt/ch05_08.htm

if you want exactly 3 times ctrl-c. you can count SIGINT and break the program when 3 times ctrl-c pressed. (pressing three times C: CTRL +CCC).

#include<stdio.h>
#include<signal.h>
#include<unistd.h>

void sig_handler(int signo)
{
    static int counter=0;
    if (signo == SIGINT)
        counter++;
    printf("received SIGINT %d times\n", counter);
    if (counter == 3)
        exit(0);
}

int main() 
{ 
    if (signal(SIGINT, sig_handler) == SIG_ERR)
        printf("\ncan't catch SIGINT\n");
    while(1) 
        sleep(1);
    return 0;
}