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?

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.