Why does epoll_ctl need the filedescriptor twice?

Question

In the example:

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

int ret = epoll_ctl(epoll_fd, EPOLL_CTL_ADD, event.data.fd, &event);

I pass the file descriptor in as both a member of event.data and as an argument in its own right.

What does epoll_ctl need the file descriptor twice?

Answer 1

This is a duplicate of about epoll_ctl()

The reason it needs it twice is that data inside event is a union. epoll_ctl does not know whether you actually provided a file descriptor or something else.

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