epoll_wait 直到按下 enter 才会唤醒

Question

I'm new with epoll. My code is working fine. The epoll is storing my file-descriptor and wait until file-descriptor is "ready". But, for some reason it will not wake up until I will press on Enter (even though data has already received to fd, and after enter I will immediately see all data that has been sent before). After one enter it will work as expected (no enters needed and when fd is ready again it will wake up again).

Here is am essence of my code:

    int nEventCountReady = 0;
    epoll_event event, events[EPOLL_MAX_EVENTS];
    int epoll_fd = epoll_create1(0);

    if(epoll_fd == -1)
    {
        std::cout << "Error: Failed to create EPoll" << std::endl;
        return ;
    }

    event.events = EPOLLIN;
    event.data.fd = myfd;

    if(epoll_ctl(epoll_fd, EPOLL_CTL_ADD, 0, &event))
    {
        fprintf(stderr, "Failed to add file descriptor to epoll\n");
        close(epoll_fd);
        return ;
    }

    while(true)
    {
        std::cout << "Waiting for messages" << std::endl;
        nEventCountReady = epoll_wait(epoll_fd, events, EPOLL_MAX_EVENTS, 30000); << Stuck until Enter will be pressed (at first while loop)
        for(int i=0; i<nEventCountReady; i++)
        {
            msgrcv(events[i].data.fd, oIpCMessageContent, sizeof(SIPCMessageContent), 1, 0);
            std::cout << oIpCMessageContent.buff << std::endl;
        }
    }

Answer 1

This

if(epoll_ctl(epoll_fd, EPOLL_CTL_ADD, 0, &event))

should probably be

if(epoll_ctl(epoll_fd, EPOLL_CTL_ADD, myfd, &event))

In the first line you tell epoll to monitor fd 0 which is typically the standard input. That's why it waits for it, eg for your Enter.

Note that your original code works only by coincidence. It just happens that when you Enter there is data in your myfd (and even if there's none msgrcv blocks). And once you pressed Enter it will wake up all the time since epoll knows that STDIN is ready but you didn't read from it.

Answer 2

Thanks to kamilCuk, I noticed that msgget doesn't return a file descriptor as I thought. It returns a "System V message queue identifier".

And as freakish said before, System V message queues don't work with selectors like epoll.