pass capacity as max_events to epoll_wait

[ibraheemdev]

Up to maxevents are returned by epoll_wait(). The maxevents argument must be greater than zero.

Passing the length here doesn't really make sense. Passing Vec::with_capacity(1024) currently causes in invalid argument exception because the length is still 0.

[nathansizemore]

Passing a Vec would be a bad idea. Epoll will coerce to a pointer with a length it can write into. If you passed a Vec instead of an actual array you'd need to unsafely set the length. Because when the Vec comes back out, it will have no idea how many elements it actually has in it (what the len really is).

[ibraheemdev]

https://github.com/tokio-rs/mio/blob/441367ba70b78989f4de577917f6e8ddfd744063/src/sys/unix/selector/epoll.rs#L68

[nathansizemore]

That is because their underlying type is Vec. They can safely do this afterwards, because they know it's their Vec

.map(|n_events| {
     // This is safe because `epoll_wait` ensures that `n_events` are
     // assigned.
    unsafe { events.set_len(n_events as usize) };  

You cannot just do that to say, a mutable reference to an actual array that was passed in, which is what should be used.

[ibraheemdev]

Okay got it, so I should just pass an array of dummy events?

[nathansizemore]

If you wanna use a vec, you can do it like so...

let mut buf = Vec::<epoll::Event>::with_capacity(100);
unsafe { buf.set_len(100); }
epoll::wait(123, -1, &buf[0..100]);

You can then just take the slice of whatever wait returned. So, you'd use [0..num_returned]