process catch SIGINT signal using sigint_handler

Question

I encountered this problem when doing shell lab from the book CSAPP, the lab ask you to implement your own version of shell with some specification,one of them is

Typing ctrl-c (ctrl-z) should cause a SIGINT (SIGTSTP) signal to be sent to the current foreground job, as well as any descendents of that job (eg, any child processes that it forked). If there is no foreground job, then the signal should have no effect.

so you should complete one of the given functions called sigint_handler which supposed to catch SIGINT signal and send it along to the foreground job. below is a piece of code I find online(the code passed the correctness check)

void sigint_handler(int sig) 
{
    int olderrno = errno;
    pid_t pid = fgpid(jobs);
    if (pid != 0)
        kill(-pid, sig);
    errno = olderrno;
    return;
}

what I don't understand is if SIGINT is sent using kill,then the descendents of foreground job will also use this handler to catch SIGINT signal right? so it's kind of a recursive call to me.so how does this actually work? thanks for helping me.

handler is installed in the main function

signal(SIGINT,  sigint_handler);   /* ctrl-c */

and fgpid return PID of current foreground job, 0 if no such job

Answer 1

Once a child process calls execve() , the child's (usually short-lived) initial address space is freed/released and replaced with space for the specified executable image; now the child no longer has a copy or access to the parent's data or text, like signal handlers.

Now consider a process-group associated with the control-terminal (tty). When a user types a CTRL-C (or CTRL-\ or CTRL-Z), the tty driver posts a signal to 1+ processes as members of the associated process-group. The result of delivering a signal would be the system default action unless a process established a different signal disposition ( signal() , sigaction() , or related).

The posted code excerpt indicates a relayed event: user types a CTRL-C, tty driver posts a SIGINT to the shell, shell's handler looks for a foreground job, calls kill() with a negative pid to post a signal to members of that process-group.

For related info see these man pages:

man setpgrp
man tty_ioctl  (symbols: TIOCSCTTY, TIOCGPGRP, TIOCSPGRP)

alternates:
man tcgetpgrp tcsetpgrp