SIGCHLD not caught in epoll_wait?

Question

I wanted to understand the behavior of signals on fork. I wrote a small program to catch SIGCHLD with epoll_wait but when I do a "kill -9" on the forked child, I am not getting any signal and the child is in defunct state (I have a handler that does a wait()).

Here is the code.

//....
sigemptyset(&mask);
sigaddset(&mask, SIGCHLD);
pthread_sigmask(SIG_BLOCK, &mask, NULL);

signal_fd = signalfd(-1, &mask, 0);

memset(&tev, 0, sizeof(tev));
tev.events = EPOLLIN | EPOLLONESHOT;
tev.data.fd = signal_fd;

epoll_ctl(efd_, EPOLL_CTL_MOD, signal_fd, &tev);

while (1) {
    child_pid_ = fork();

    if (child_pid_ == 0) {
        close(signal_fd);
        close(efd_);
        make_grand_child(); //just sleeps in while(1) and never returns.
    } else {
        memset(&event, 0, sizeof(event));
        while (1) {
            epoll_wait(efd_, &event, 1, -1);
            deliver_events = (event.events & EPOLLERR|EPOLLHUP|EPOLLIN|EPOLLONESHOT);
            if (deliver_events) {
                parent_sig_handler(SIGCHLD);
                break;
            }
        }
    }
}

UPDATE:

Used a EPOLL_CTL_MOD without first doing an add (EPOLL_CTL_ADD). After I changed that, it worked like a charm.

Answer 1

The default disposition of SIGCHLD is to be ignored. Blocking the signal won't change the disposition it just gives signalfd the opportunity to process it synchronously.

Add a signal handler for SIGCHLD with signal or sigaction. It doesn't have to do anything since it won't be executed. Because SIGCHLD is blocked, signalfd will consume and process it before the handler.