EPOLLIN

EPOLLIN is a flag used with the Linux epoll I/O event notification facility. It indicates that a monitored file descriptor is currently ready for reading and that a read operation will not block. In practice, EPOLLIN is set for sockets, pipes, or other file descriptors when there is data available to be read, or when the other end has closed the connection (end-of-file).

EPOLLIN is specified in the events mask of an epoll_event structure used with epoll_ctl to add or modify interest in a file descriptor, and is returned by epoll_wait when the condition is met. When epoll_wait reports EPOLLIN, a subsequent read call on the descriptor will usually return immediately with the data. If the peer has performed an orderly shutdown, a read may return 0.

Edge-triggered vs level-triggered: In level-triggered mode (default), EPOLLIN remains set as long as there is data to read, which may cause repeated events if you don't drain the data. In edge-triggered mode (EPOLLET), epoll_wait emits a single notification when the state changes to ready, requiring the code to drain the file descriptor completely, repeatedly reading until EAGAIN.

EPOLLIN is Linux-specific and defined in include/linux/epoll.h or sys/epoll.h, part of the epoll API introduced in Linux 2.6. Related flags include EPOLLOUT (ready for write), EPOLLERR (error), EPOLLHUP (hang up), EPOLLRDHUP (remote hang up), and EPOLLET (edge-triggered behavior).