01 — `epoll`

Event-driven I/O multiplexing. One epoll_fd watches many fds for readiness; epoll_wait blocks the loop until at least one fires. Foundation of the whole runtime — every other primitive on this list is an fd that lands on an epoll loop.

Kernel source

Path	What
`reference/linux/fs/eventpoll.c`	All three syscalls (`epoll_create1`, `epoll_ctl`, `epoll_wait`) live here. Grep for `SYSCALL_DEFINE`.
`reference/linux/include/uapi/linux/eventpoll.h`	`struct epoll_event`, `EPOLL_*` flags, the userspace-facing ABI.

Man pages

man 7 epoll (overview), man 2 epoll_create1, man 2 epoll_ctl, man 2 epoll_wait.

Rust FFI via `libc`

use libc::{epoll_create1, epoll_ctl, epoll_wait, epoll_event, EFD_CLOEXEC};
use libc::{EPOLL_CLOEXEC, EPOLL_CTL_ADD, EPOLL_CTL_MOD, EPOLL_CTL_DEL};
use libc::{EPOLLIN, EPOLLOUT, EPOLLET, EPOLLRDHUP, EPOLLHUP, EPOLLERR};

extern "C" {
    // all three already wrapped in libc — no SYS_* workaround needed
}

Direct-syscall example

unsafe {
    let epfd = libc::epoll_create1(libc::EPOLL_CLOEXEC);
    if epfd < 0 { return Err(io::Error::last_os_error()); }

    let mut ev = libc::epoll_event {
        events: (libc::EPOLLIN | libc::EPOLLET) as u32,
        u64:    token_value,                        // your own dispatch key
    };
    if libc::epoll_ctl(epfd, libc::EPOLL_CTL_ADD, watched_fd, &mut ev) < 0 {
        return Err(io::Error::last_os_error());
    }

    let mut events: [libc::epoll_event; 64] = std::mem::zeroed();
    let n = libc::epoll_wait(epfd, events.as_mut_ptr(), 64, timeout_ms);
    for ev in &events[..n as usize] {
        let token = ev.u64;
        // dispatch based on token
    }
}

Key flags

Flag	Meaning
`EPOLL_CLOEXEC`	Close the `epoll_fd` on `exec` — always set it.
`EPOLLIN` / `EPOLLOUT`	Readable / writable readiness.
`EPOLLET`	Edge-triggered. Reads must drain until `EAGAIN`; no re-arm needed. Required for the single-threaded loop's correctness.
`EPOLLRDHUP`	Peer half-closed — distinguishes "client closed" from transient.
`EPOLLHUP` / `EPOLLERR`	Always implicitly set; you don't ask for them but you must handle.

Gotchas

Edge-triggered means drain. A handler that reads once and returns leaks readiness; the next epoll_wait won't re-fire until more bytes arrive. Always read() in a loop until EAGAIN.
timeout = -1 blocks forever; 0 polls; positive milliseconds cap the wait.
epoll_wait can return fewer events than you asked. Normal. Don't assume full batches.
Modifying a watched fd's interest requires EPOLL_CTL_MOD, not delete-then-add — the atomic update avoids races with pending events.

Used by

Every runtime phase that touches I/O: 02-event-loop-epoll.md, 03-hand-rolled-http.md, 07-inotify-content-watcher.md, 08-sendfile-static-assets.md.

v1 port source

reference/crates/wo-event/src/epoll.rs (183 LOC) — already wraps all three syscalls with a safe EventLoop { register, deregister, wait_once } facade.

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01 — epoll