EVFILTREAD

EVFILT_READ is a filter type used by the kqueue I/O event notification mechanism on BSD-derived systems, including macOS and FreeBSD. It monitors a file descriptor for readability, delivering an event when data is available to read without blocking. The filter may also indicate end-of-file, such as when the other end of a pipe or socket has closed.

When a kevent with EVFILT_READ is triggered, the event’s data field typically reports how much data is available to read. The EV_EOF flag is set if the read side has reached end-of-file. Readability semantics vary by descriptor type: for sockets and pipes, it reflects buffered data; for regular files, readability behavior can differ and may be always available in some contexts.

Usage involves creating a kqueue, then registering the target file descriptor with EVFILT_READ using flags such as EV_ADD and EV_ENABLE; optional flags like EV_CLEAR enable edge-triggered notifications. Applications wait for events with kevent, and upon return examine the event. If EV_EOF is set, the peer has closed the connection and the descriptor may be closed or removed from monitoring. Otherwise, data can be read from the descriptor up to the amount indicated by the data field, using non-blocking I/O as appropriate, and the event may be re-armed according to the chosen semantics.

EVFILT_READ is commonly used for sockets, pipes, and other non-regular-file descriptors in event-driven I/O. It is a core component of the BSD kqueue API and differs from Linux’s epoll in platform and semantics, making it a key tool for scalable network servers and similar applications on supported systems.