Why Linux always output "^C" upon pressing of Ctrl+C?

Question

I have been studying signals in Linux. And I've done a test program to capture SIGINT.

#include <unistd.h>
#include <signal.h>
#include <iostream>
void signal_handler(int signal_no);
int main() {
  signal(SIGINT, signal_handler);
  for (int i = 0; i < 10; ++i) {
  std::cout << "I'm sleeping..." << std::endl;
  unsigned int one_ms = 1000;
  usleep(200* one_ms);
  }
  return 0;
}
void signal_handler(int signal_no) {
  if (signal_no == SIGINT)
    std::cout << "Oops, you pressed Ctrl+C!\n";
  return;
}

While the output looks like this:

I'm sleeping...
I'm sleeping...
^COops, you pressed Ctrl+C!
I'm sleeping...
I'm sleeping...
^COops, you pressed Ctrl+C!
I'm sleeping...
^COops, you pressed Ctrl+C!
I'm sleeping...
^COops, you pressed Ctrl+C!
I'm sleeping...
^COops, you pressed Ctrl+C!
I'm sleeping...
I'm sleeping...
I'm sleeping...

I understand that when pressing Ctrl+C, processes in foreground process group all receives a SIGINT(if no process chooses to ignore it).

So is it that the shell(bash) AND the instance of the above program both received the signal? Where does the "^C" before each "Oops" come from?

The OS is CentOS, and the shell is bash.

Answer 1

It is the terminal (driver) that intercepts the ^C and translates it to a signal sent to the attached process (which is the shell) stty intr ^B would instruct the terminal driver to intercept a ^B instead. It is also the terminal driver that echoes the ^C back to the terminal.

The shell is just a process that sits at the other end of the line, and receives it's stdin from your terminal via the terminal driver (such as /dev/ttyX), and it's stdout (and stderr) are also attached to the same tty.

Note that (if echoing is enabled) the terminal sends the keystrokes to both the process (group) and back to the terminal. The stty command is just wrapper around the ioctl()s for the tty driver for the processes "controlling" tty.

UPDATE: to demonstrate that the shell is not involved, I created the following small program. It should be executed by its parent shell via exec ./a.out (it appears an interactive shell will fork a daughter shell, anyway) The program sets the key that generates the SIGINTR to ^B, switches echo off, and than waits for input from stdin.

#include <stdio.h>
#include <string.h>
#include <termios.h>
#include <unistd.h>
#include <signal.h>
#include <errno.h>

int thesignum = 0;
void handler(int signum);

void handler(int signum)
{ thesignum = signum;}

#define THE_KEY 2 /* ^B */

int main(void)
{
int rc;
struct termios mytermios;

rc = tcgetattr(0 , &mytermios);
printf("tcgetattr=%d\n", rc );

mytermios.c_cc[VINTR] = THE_KEY; /* set intr to ^B */
mytermios.c_lflag &= ~ECHO ; /* Dont echo */
rc = tcsetattr(0 , TCSANOW, &mytermios);
printf("tcsetattr(intr,%d) =%d\n", THE_KEY, rc );

printf("Setting handler()\n" );
signal(SIGINT, handler);

printf("entering pause()\n... type something followed by ^%c\n", '@'+THE_KEY );
rc = pause();
printf("Rc=%d: %d(%s), signum=%d\n", rc, errno , strerror(errno), thesignum );

// mytermios.c_cc[VINTR] = 3; /* reset intr to ^C */
mytermios.c_lflag |= ECHO ; /* Do echo */
rc = tcsetattr(0 , TCSANOW, &mytermios);
printf("tcsetattr(intr,%d) =%d\n", THE_KEY, rc );

return 0;
}

intr.sh:

#!/bin/sh
echo $$
exec ./a.out
echo I am back.