The epoll_event type is declared like this in C:

 struct epoll_event {
 uint32_t events; /* Epoll events */
 epoll_data_t data; /* User data variable */
 };
 union epoll_data {
 void *ptr;
 int fd;
 uint32_t u32;
 uint64_t u64;
 };
 typedef union epoll_data epoll_data_t;

However, the Rust version in libc is different -- it just uses u64 for the data field. That is, unfortunately, not equivalent -- u64 cannot carry pointer provenance, so programs that want to store a pointer in data (which is quite common) need to use exposed-provenance APIs (i.e., ptr/int casts). Tokio carries some funky hacks to avoid that.

Since the C API is actually compatible with strict provenance here, it would be nice to expose that in libc as well. Given this is a breaking change it can only happen for libc 1.0 -- though it would be possible to have a separate event_data type and event_ctl declaration in its own module even in libc 0.2.