Question

I’m learning the signal of inter process communication, I made the very simple test code below:

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

void sighup();
void sigint();
void sigquit();

int main(int argc, const char *argv[])
{
    int child_pid;
    if((child_pid = fork()) < 0) exit (1);
    if(child_pid == 0) {
        sleep(2);
        signal(SIGHUP, sighup);
        signal(SIGINT, sigint);
        signal(SIGQUIT, sigquit);
        puts("this is the end of the child process");
    } else {
        printf("\n Parent: sending SIGHUP signal to child\n\n");
        kill(child_pid, SIGHUP);
        printf("\n Parent: sending SIGINT signal to child\n\n");
        kill(child_pid, SIGINT);
        printf("\n Parent: sending SIGQUIT signal to child\n\n");
        kill(child_pid, SIGQUIT);
    }
}

void sighup() {
    signal(SIGHUP, sighup);
    printf("CHILD: I have received a SIGHUP\n");
}
void sigint() {
    signal(SIGINT, sigint);
    printf("CHILD: I have received a SIGINT\n");
}

void sigquit() {
    sleep(2);
    printf("CHILD: My parent process has killed me!!");
    printf("CHILD: cleaning up...\n");
    exit(0);
}

It seems like the child process doesn’t do anything, even doesn’t print the end of the process string. any idea?

Answer 1

Your signal handlers are not being invoked in the child because of a race condition. The parent thread sends the child thread a signal before the child calls signal() that overrides the signal handling behavior.

In this case, the child receives a SIGINT and performs its default behavior, which is to terminate. Thus the child terminates before executing the statements after sleep(2).