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?

Pires

do you use the same epoll_fd ? – Faith

yes, I use the same epoll_fd. – Pires

Can you give more details about your usage? Why do you have to use the same epoll fd? If you are doing an RX/TX structure, you can have one epoll fd corrsepond to the IN events and one correspond to the out events (this is what I use in a few apps) – Burka

Hi, I use epll io multiplexing with the thread pool, and I want to use epoll_ctl to change the events on other threads. But I do not know whether I need to use some synchronization mechanism to ensure thread safety. – Pires

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.)

Antique

Answer 2

Yes, it is thread-safe according to the manual page for epoll_wait. It 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.

Unshapen