75 lines
3.3 KiB
Markdown
75 lines
3.3 KiB
Markdown
# 01 — `epoll`
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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.
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## Kernel source
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| Path | What |
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| --- | --- |
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| [`reference/linux/fs/eventpoll.c`](../../../reference/linux/fs/eventpoll.c) | All three syscalls (`epoll_create1`, `epoll_ctl`, `epoll_wait`) live here. Grep for `SYSCALL_DEFINE`. |
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| [`reference/linux/include/uapi/linux/eventpoll.h`](../../../reference/linux/include/uapi/linux/eventpoll.h) | `struct epoll_event`, `EPOLL_*` flags, the userspace-facing ABI. |
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## Man pages
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`man 7 epoll` (overview), `man 2 epoll_create1`, `man 2 epoll_ctl`, `man 2 epoll_wait`.
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## Rust FFI via `libc`
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```rust
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use libc::{epoll_create1, epoll_ctl, epoll_wait, epoll_event, EFD_CLOEXEC};
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use libc::{EPOLL_CLOEXEC, EPOLL_CTL_ADD, EPOLL_CTL_MOD, EPOLL_CTL_DEL};
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use libc::{EPOLLIN, EPOLLOUT, EPOLLET, EPOLLRDHUP, EPOLLHUP, EPOLLERR};
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extern "C" {
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// all three already wrapped in libc — no SYS_* workaround needed
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}
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```
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## Direct-syscall example
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```rust
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unsafe {
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let epfd = libc::epoll_create1(libc::EPOLL_CLOEXEC);
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if epfd < 0 { return Err(io::Error::last_os_error()); }
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let mut ev = libc::epoll_event {
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events: (libc::EPOLLIN | libc::EPOLLET) as u32,
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u64: token_value, // your own dispatch key
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};
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if libc::epoll_ctl(epfd, libc::EPOLL_CTL_ADD, watched_fd, &mut ev) < 0 {
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return Err(io::Error::last_os_error());
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}
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let mut events: [libc::epoll_event; 64] = std::mem::zeroed();
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let n = libc::epoll_wait(epfd, events.as_mut_ptr(), 64, timeout_ms);
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for ev in &events[..n as usize] {
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let token = ev.u64;
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// dispatch based on token
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}
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}
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```
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## Key flags
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| Flag | Meaning |
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| --- | --- |
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| `EPOLL_CLOEXEC` | Close the `epoll_fd` on `exec` — always set it. |
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| `EPOLLIN` / `EPOLLOUT` | Readable / writable readiness. |
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| `EPOLLET` | **Edge-triggered.** Reads must drain until `EAGAIN`; no re-arm needed. Required for the single-threaded loop's correctness. |
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| `EPOLLRDHUP` | Peer half-closed — distinguishes "client closed" from transient. |
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| `EPOLLHUP` / `EPOLLERR` | Always implicitly set; you don't ask for them but you must handle. |
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## Gotchas
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- **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`.
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- **`timeout = -1` blocks forever**; `0` polls; positive milliseconds cap the wait.
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- **`epoll_wait` can return fewer events than you asked.** Normal. Don't assume full batches.
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- **Modifying a watched fd's interest requires `EPOLL_CTL_MOD`**, not delete-then-add — the atomic update avoids races with pending events.
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## Used by
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Every runtime phase that touches I/O: [`02-event-loop-epoll.md`](../02-event-loop-epoll.md), [`03-hand-rolled-http.md`](../03-hand-rolled-http.md), [`07-inotify-content-watcher.md`](../07-inotify-content-watcher.md), [`08-sendfile-static-assets.md`](../08-sendfile-static-assets.md).
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## v1 port source
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[`reference/crates/wo-event/src/epoll.rs`](../../../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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