EVFILTWRITE

EVFILT_WRITE is a filter in the BSD kqueue I/O event notification system. It reports when a file descriptor is ready for writing, meaning a non-blocking write to the descriptor would not block. This filter is commonly used with sockets, pipes, and other streaming descriptors to implement asynchronous or event-driven I/O.

Usage typically involves creating a kqueue object and registering the target descriptor with the EVFILT_WRITE filter using EV_ADD and, often, EV_ENABLE. When the system indicates a write event for that descriptor, the application performs write operations. After writing, the application may continue to receive EVFILT_WRITE events as long as the descriptor remains writable, or until it no longer is (for example, if the peer back-pressures the connection or the send buffer fills). The data field in the returned kevent may convey the amount of space available in the send buffer, and EV_ERROR may be returned to indicate an error condition to check via errno.

EVFILT_WRITE is frequently used in conjunction with EVFILT_READ to implement asynchronous I/O loops, enabling the program to react to both readiness for reading and readiness for writing on multiple descriptors. It is a BSD-specific mechanism and is not part of the Linux epoll family; other platforms provide different interfaces for similar functionality.

Notes and considerations include the need to handle partial writes, non-blocking error conditions, and re-arming the event after each operation when appropriate. Proper error handling and cleanup are essential to avoid busy loops or resource leaks.