eventfd2

Eventfd2 is a Linux kernel facility that provides a file descriptor for event notification between processes or threads. It is a variant of the eventfd mechanism that allows creation-time specification of flags, enabling different modes of operation and behavior without requiring subsequent fcntl calls.

The object created by eventfd2 represents a 64-bit unsigned counter. It is initialized to a user-specified value (initval). Writing to the descriptor increments the counter by the given 64-bit value; reading retrieves the current counter value and typically resets the counter to zero, except when read with semaphore semantics. If the descriptor is created with the EFD_SEMAPHORE flag, reads decrement the counter by 1 and return a value of 1, providing a counting-semaphore style interface. Without EFD_SEMAPHORE, a read returns the full counter value and clears it to zero. Writes can block if an operation would overflow the 64-bit counter, unless the descriptor is opened with EFD_NONBLOCK.

Eventfd2 accepts creation flags that can be combined from among EFD_SEMAPHORE, EFD_NONBLOCK, and EFD_CLOEXEC. EFD_NONBLOCK makes I/O non-blocking and returns EAGAIN on a would-block operation. EFD_CLOEXEC sets the close-on-exec flag, so the descriptor is automatically closed on execve. EFD_SEMAPHORE changes the read semantics to a semaphore-like behavior as described above.

The descriptor is pollable and compatible with epoll, select, and similar mechanisms, making it suitable for integrating into event loops or signaling workflows. Typical use cases include signaling completion of asynchronous work, waking threads, and notifying other processes without shared memory. Eventfd2 is Linux-specific and is used as part of the eventfd family for lightweight, race-free inter-thread or inter-process notifications.