Ctrl + C interrupt event handling in Linux

When you press Ctr + C, the operating system sends a signal to the process. There are many signals and one of them is SIGINT. The SIGINT ("program interrupt") is one of the Termination Signals.

There are a few more kinds of Termination Signals, but the interesting thing about SIGINT is that it can be handled (caught) by your program. The default action of SIGINT is program termination. That is, if your program doesn't specifically handle this signal, when you press Ctr + C your program terminates as the default action.

To change the default action of a signal you have to register the signal to be caught. To register a signal in a C program (at least under POSIX systems) there are two functions

1. signal(int signum, sighandler_t handler);
2. sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);.

These functions require the header signal.h to be included in your C code. I have provide a simple example of the signal function below with comments.

#include <stdio.h>
#include <stdlib.h>
#include <signal.h> //  our new library 
volatile sig_atomic_t flag = 0;
void my_function(int sig){ // can be called asynchronously
  flag = 1; // set flag
}

int main(){
  // Register signals 
  signal(SIGINT, my_function); 
  //      ^          ^
  //  Which-Signal   |-- which user defined function registered
  while(1)  
    if(flag){ // my action when signal set it 1
        printf("\n Signal caught!\n");
        printf("\n default action it not termination!\n");
        flag = 0;
    }     
  return 0;
}  

Note: you should only call safe/authorized functions in signal handler. For example avoid calling printf in signal handler.

You can compile this code with gcc and execute it from the shell. There is an infinite loop in the code and it will run until you send a SIGINT signal by pressing Ctr + C.

You can use the signal macro.

Here is an example of how to deal with it:

#include <signal.h>
#include <stdio.h>
void sigint(int a)
{
    printf("^C caught\n");
}
int main()
{
    signal(SIGINT, sigint);
    for (;;) {}
}

Sample output:

Ethans-MacBook-Pro:~ phyrrus9$ ./a.out
^C^C caught
^C^C caught
^C^C caught
^C^C caught
^C^C caught

Typing CtrlC normally causes the shell to send SIGINT to your program. Add a handler for that signal (via signal(2) or sigaction(2)), and you can do what you like when CtrlC is pressed.

Alternately, if you only care about doing cleanup before your program exits, setting up an exit handler via atexit(3) might be more appropriate.