Module eventfd 

eventfd – Event notification file descriptor

Provides a file descriptor for event wait/notify, commonly used by epoll-based event loops (Qt6, glib). Supports both counter and semaphore semantics.

Syscall Interface

• eventfd_create(initval, flags) -> fd (syscall 330)
• Read/write via standard read(2) / write(2) on returned fd

Semantics

• write: Adds the 8-byte unsigned integer to the internal counter. Blocks (or returns EAGAIN) if the counter would overflow u64::MAX - 1.
• read: Returns the current counter as an 8-byte unsigned integer and resets it to zero. In semaphore mode, returns 1 and decrements by 1. Blocks (or returns EAGAIN) if the counter is zero.

Structs

EventFdNode

VfsNode wrapper around an eventfd instance.

Constants

EFD_CLOEXEC

EFD_CLOEXEC flag: set close-on-exec (tracked but not enforced in kernel).

EFD_NONBLOCK

EFD_NONBLOCK flag: reads/writes return EAGAIN instead of blocking.

EFD_SEMAPHORE

EFD_SEMAPHORE flag: read returns 1 and decrements (instead of draining).

Functions

eventfd_close

Close (destroy) an eventfd instance.

eventfd_create

Create a new eventfd.

eventfd_read

Read from an eventfd. Returns the counter value as a u64.

eventfd_write

Write to an eventfd. Adds value to the internal counter.

is_readable

Query whether an eventfd is readable (counter > 0). Used by epoll to check readiness without consuming data.

is_writable

Query whether an eventfd is writable (counter < u64::MAX - 1).