Is epoll thread-safe?

Question

There are two functions in epoll:

1. epoll_ctl
2. epoll_wait

Are they thread-safe when I use the same epoll_fd?
What will happen if one thread calls epoll_wait and others call epoll_ctl at the same time?

Answer 1

It is thread-safe, but there isn't much documentation that explicitly states that. See here

BTW, you can also have multiple threads waiting on a single epoll_fd, but in that case it can get a bit tricky. (I.e. you might want to use edge-triggered EPOLLET or oneshot mode EPOLLONESHOT. See here.)

Answer 2

_{Note: The existing accepted answer is correct, but it mentions "there isn't much documentation that explicitly states that", while epoll documentation does state it.}

The manual page for epoll_wait explicitly allows adding a file descriptor to an epoll set while it is being waited for in another thread:

Section "Notes":

While one thread is blocked in a call to epoll_wait(), it is possible for another thread to add a file descriptor to the waited-upon epoll instance. If the new file descriptor becomes ready, it will cause the epoll_wait() call to unblock.