fasync and SIGIO¶

fasync allows a driver to notify a user-space process asynchronously by sending SIGIO. This is another notification model besides blocking read(), poll, and epoll.

Basic Flow¶

user space sets F_SETOWN
        ↓
user space enables O_ASYNC
        ↓
driver .fasync callback registers file
        ↓
driver event occurs
        ↓
kill_fasync(..., SIGIO, POLL_IN)
        ↓
user-space SIGIO handler runs

When It Is Useful¶

SIGIO can be useful for simple asynchronous notification, but it is usually less structured than fd-based event loops.

In this repo, SIGIO was later bridged into an eventfd so the final user-space architecture could still use epoll.

SIGIO handler
        ↓
write(eventfd)
        ↓
epoll_wait(eventfd)

Driver-Side Components¶

Component	Purpose
`.fasync`	Registers or unregisters async notification target
`fasync_helper()`	Maintains async notification list
`kill_fasync()`	Sends `SIGIO` to registered process

Common Pitfalls¶

Warning

Signal handlers should do as little work as possible. Writing to eventfd is a safer pattern than doing complex processing inside the handler.

Warning

SIGIO is process-signal based, while epoll is fd-based. Mixing both models requires careful design.

Day52 - fasync / SIGIO / eventfd Integration