SIGCHLD没有陷入epoll_wait吗？

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.