Why does epoll_ctl need the filedescriptor twice? - file-descriptor

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;

Answer 2

int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event)
Where:
epfd is the file descriptor returned by epoll_create which identifies the epoll instance in the kernel.
fd is the file descriptor we want to add to the epoll list/interest list.
op refers to the operation to be performed on the file descriptor fd. In general, three operations are supported:
Register fd with the epoll instance (EPOLL_CTL_ADD) and get notified about events that occur on fd
Delete/deregister fd from the epoll instance. This would mean that the process would no longer get any notifications about events on that file descriptor (EPOLL_CTL_DEL). If a file descriptor has been added to multiple epoll instances, then closing it will remove it from all of the epoll interest lists to which it was added.
Modify the events fd is monitoring (EPOLL_CTL_MOD)
event is a pointer to a structure called epoll_event which stores the event we actually want to monitor fd for.
The first field events of the epoll_event structure is a bitmask that indicates which events fd is being monitored for.
Like so, if fd is a socket, we might want to monitor it for the arrival of new data on the socket buffer (EPOLLIN). We might also want to monitor fd for edge-triggered notifications which is done by OR-ing EPOLLET with EPOLLIN. We might also want to monitor fd for the occurrence of a registered event but only once and stop monitoring fd for subsequent occurrences of that event. This can be accomplished by OR-ing the other flags (EPOLLET, EPOLLIN) we want to set for descriptor fd with the flag for only-once notification delivery EPOLLONESHOT. All possible flags can be found in the man page.
The second field of the epoll_event struct is a union field.
Source
Added some extra data apart from just what was asked for context.
Hope this helps!