How to fork the youngest process - c

Question

Let's say that whenever main program receives SIGINT signal, the youngest (i.e. the lastly forked) process is forked. In other words:
p1 SIGINT
p1->p2 SIGINT
p1->p2->p3
and so on
My problem is that I don't know how to tell the grand-grand-...-children to fork.

Answer 1

My problem is that I don't know how to tell the
grand-grand-...-children to fork.
Parent should install SIGINT handler using SA_SIGINFO flag.
When a grandchild is created it should send signal to parent(p1).
P1 through its signal handler(si_pid field of handler's siginfo_t argument) reads pid of grandchild.
When Parent receives SIGINT then it send signal to grand child.
When grand child receives signal it should fork a new child.
Repeat from 2-5 for newly created child.
One more approach:
Let parent poll for SIGINT signal.
When SIGINT received by parent, create child.
Mask SIGINT in parents context and wait for SIGINT in child context.
repeat step 1 to 3 in childs context.
Example:
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#define CHILD_NEEDED 10
static volatile sig_atomic_t got_sigint;
static volatile sig_atomic_t child_created;
void sigint_handler(int sig, siginfo_t *f, void *main_context) {
ucontext_t *uc = main_context;
// to mask SIGINT while returning from handler.
sigaddset(&uc->uc_sigmask, SIGINT);
++child_created;
got_sigint = 1;
}
void create_child(void)
{
pid_t pid = fork();
sigset_t set;
sigemptyset(&set);
if (-1 == pid) {
perror("Fork failed\n");
exit(1);
} else if (0 == pid) {
// Exit if expected child are created
if (child_created == CHILD_NEEDED) {
printf("New Child\nTotal Child: %d\n", child_created);
exit(0);
}
got_sigint = 0;
// As parent mask is copied to child so clear child mask set.
sigprocmask(SIG_SETMASK, &set, NULL);
printf("New Child\n Please send signal to create child\n");
while (!got_sigint);
create_child();
}
}
int main(int argc, const char *argv[]) {
struct sigaction act = {.sa_sigaction = sigint_handler, .sa_mask = 0, .sa_flags = SA_SIGINFO};
sigset_t mask;
int status;
parent = getpid();
sigaction(SIGINT, &act, NULL);
printf("parent\n Please send signal to create child\n");
while (!got_sigint);
// As we have returned from signal so SIGINT will be mask for parent.
create_child();
// parent will wait here.
wait(&status);
exit(0);
}

Answer 2

All your processes will receive the SIGINT signal, so the main process doesn't need to tell the youngest process to fork; just let the process that hasn't forked yet fork, and all other processes do nothing.