What is CTRL-C?

When you are running a process, you can use Control+C to abort the program. But what does this do exactly?

Control+C sends the SIGINT signal to interrupt a process.

The default behavior is for the receiving program to terminate itself. However, you can override the default behavior to do whatever you want by handling the signal.

Overriding CTRL-C

I wrote a small program to demonstrate this behavior:

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

void signal_handler(int signal)
{
  printf("Received SIGINT from CTRL-C but not quitting because it is overridden.\n");
}

int main()
{
  // Install the signal handler for SIGINT
  signal(SIGINT,  signal_handler);

  // After 5 seconds, switch the signal handler back to the default one
  int count = 0;
  while(1) {
    printf("Count: %d\n", count);
    sleep(1);
    count++;
    if (count == 5) {
      signal(SIGINT, SIG_DFL);
    }
  }
}

In this example, we set up a signal handler to override the default behavior. After 5 seconds, we switch the signal handler back to the default one.

$ ./a.out
Count: 0
^CReceived SIGINT from CTRL-C but not quitting because it is overridden.
Count: 1
Count: 2
^CReceived SIGINT from CTRL-C but not quitting because it is overridden.
Count: 3
^CReceived SIGINT from CTRL-C but not quitting because it is overridden.
Count: 4
Count: 5
^C

Why would you override CTRL-C?

The example from the GNU libc manual on Basic Signal Handling shows that the custom signal handler is being used to delete temporary files.