eventfd

Eventfd is a Linux kernel synchronization primitive that provides a file descriptor-based event counter for signaling between processes or threads. It is commonly used to notify one execution context about events occurring in another without using heavier synchronization primitives, and it integrates with standard I/O multiplexing mechanisms such as poll, select, or epoll.

An eventfd object maintains a 64-bit unsigned counter. The constructor is int eventfd(unsigned int initval, int flags) (or the related eventfd2 for combined flag specification); the return value is a file descriptor referring to the eventfd object. Writing to the descriptor adds the provided 64-bit value to the internal counter. Reading retrieves the current counter value and resets the counter to zero; if the EFD_SEMAPHORE flag is set, a read returns 1 and decrements the counter by one, enabling semaphore-like semantics. If the counter is zero, reads block unless the nonblocking flag (EFD_NONBLOCK) is set, in which case a read returns the error EAGAIN.

Eventfd supports integration with I/O multiplexing. A thread or process can wait on the descriptor using poll, select, or epoll, and a producer writing to the descriptor will wake waiters once the counter becomes nonzero.

Flags commonly used with eventfd include EFD_SEMAPHORE for semaphore-like behavior, EFD_NONBLOCK for non-blocking operations, and EFD_CLOEXEC to set close-on-exec semantics. Eventfd2 provides a single call to specify these flags at creation.

Common use cases include signaling the completion of work, waking an event loop, or implementing lightweight inter-thread or inter-process notifications. Eventfd is Linux-specific and is not a part of the POSIX standard, but it is widely available on modern Linux systems.