EPOLLERR

EPOLLERR is a flag used by the Linux epoll I/O event notification facility to indicate that an error has occurred on the associated file descriptor. It is one of the event flags that epoll_wait may report for a given event and is distinct from readiness indicators like EPOLLIN or EPOLLOUT.

When EPOLLERR is reported, it signals an error condition on the descriptor’s underlying resource. This condition is independent of the current level or edge triggering configuration and may occur on sockets, pipes, or other file descriptors. The EPOLLERR flag itself does not convey the specific error value; the exact error is retrieved from the underlying operation or, for sockets, by querying SO_ERROR with getsockopt.

Handling EPOLLERR typically involves stopping normal I/O on the descriptor, determining the cause of the error, and taking appropriate action (such as closing the descriptor or reestablishing a connection). In practice, a common pattern is to treat EPOLLERR as a cue to inspect the pending error with getsockopt(SO_ERROR) on sockets, perform any necessary cleanup, and then remove the descriptor from the epoll set if recovery is not possible.

Notes and related concepts: EPOLLERR is reported even if no other events are requested; it can appear alongside other events. It is often considered together with EPOLLHUP (hang up) to signal terminal conditions on a descriptor. Understanding EPOLLERR is important for robust error handling in event-driven I/O using epoll.