CODEWITHSUNDEEP
csocketsepoll
jinreal
jinreal

Reputation: 1565

about epoll_ctl()

when using epoll_ctl(), I found that the third parameter "fd" is another file descriptor besides the epoll file descriptor "epfd". And I saw an example like this:

event.data.fd = sfd; //sfd is a fd for listening
event.events = EPOLLIN | EPOLLET;
s = epoll_ctl (efd, EPOLL_CTL_ADD, sfd, &event);

As I saw, file descriptor in event.data.fd is the same as the third parameter in epoll_ctl, why need to pass this descriptor twice? is there any difference?

Upvotes: 15

Views: 7732

Answers (2)

chuleimo
chuleimo

Reputation: 1

I think there is.

This is my test demo.

When using ev.data.fd = STDIN_FILENO

In the terminal, you press the Enter button to end the input and printf “welcome...”

When using ev.data.ptr = stdin

You need to press ctrl+d to end the input and printf “welcome...”

Does it mean that ev.data is not only used for recording?

    int main()
    {
        int epfd, nfds;
        struct epoll_event ev, events[5];
        epfd = epoll_create(1); 
    
        //ev.data.fd = STDIN_FILENO;
        ev.data.ptr = stdin;
        ev.events = EPOLLIN | EPOLLET; 
        epoll_ctl(epfd, EPOLL_CTL_ADD, STDIN_FILENO, &ev); 
    
        for (;;)
        {   
            nfds = epoll_wait(epfd, events, 5, -1);
            for (int i = 0; i < nfds; i++)
            {
                // if (events[i].data.fd == STDIN_FILENO)
                if (events[i].data.ptr == stdin)
                   printf("welcome to epoll's word!\n");
            }
        } 
    }

Upvotes: 0

cnicutar
cnicutar

Reputation: 182744

Actually you don't have to set event.data.fd. It's a union, you can set other members. When epoll_wait returns you get the event.data associated with the descriptor that became interesting:

typedef union epoll_data {
    void    *ptr;
    int      fd;
    uint32_t u32;
    uint64_t u64;
} epoll_data_t;

Which means you're completely free not to put anything in fd and put something in ptr instead (for example).

In conclusion, epoll_ctl can't rely on the fact you'll fill fd in, that's why it has a separate explicit parameter.

Upvotes: 21

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