EPOLLOUT

EPOLLOUT is a flag used by the Linux epoll I/O event notification facility. It indicates that a file descriptor is currently ready for write operations; a non-blocking write to the descriptor should not block. It is used with epoll_ctl to register the flag and with epoll_wait to retrieve events. Commonly applied to sockets and pipes, where the write queue has space.

When using non-blocking I/O, you may register for EPOLLOUT when you have data to send, or re-arm it after a failed write. If epoll_wait returns an event with EPOLLOUT set, you should attempt to write as much data as possible. If the write returns EAGAIN or EWOULDBLOCK, you should stop and wait for the next EPOLLOUT.

Notes and caveats: For regular files EPOLLOUT is typically always set; epoll may continuously report it, so it is less useful there. In edge-triggered mode you must drain the socket completely to avoid missing a future event; in level-triggered mode epoll_wait will continue to report EPOLLOUT as long as the descriptor remains writable. Practically, many implementations enable EPOLLOUT only when there is data to send, and disable it again once all data has been written.