04 — `signalfd`

Unix signals as file descriptors. signalfd(fd, mask) installs a mask on the process and returns an fd that becomes readable when any masked signal arrives. Replaces signal handlers entirely — no sigaction, no re-entrancy minefield, no interrupted syscalls. The whole wo binary's shutdown path is one signalfd on the event loop.

Kernel source

Path	What
`.dev/reference/linux/fs/signalfd.c`	`SYSCALL_DEFINE4(signalfd4, ...)` + `signalfd_dequeue`.
`.dev/reference/linux/include/uapi/linux/signalfd.h`	`struct signalfd_siginfo`, `SFD_*` flags.
`.dev/reference/linux/kernel/signal.c`	Background: `sigprocmask`, pending-signal dequeue.

Man pages

man 2 signalfd, man 7 signal.

Rust FFI via `libc`

use libc::{signalfd, signalfd_siginfo, sigset_t};
use libc::{sigemptyset, sigaddset, sigprocmask};
use libc::{SFD_CLOEXEC, SFD_NONBLOCK, SIG_BLOCK, SIG_UNBLOCK};
use libc::{SIGINT, SIGTERM, SIGHUP, SIGQUIT};

Direct-syscall example

unsafe {
    // 1. Block the signals in the thread so the kernel delivers them via the fd instead
    let mut mask: sigset_t = std::mem::zeroed();
    libc::sigemptyset(&mut mask);
    libc::sigaddset(&mut mask, libc::SIGINT);
    libc::sigaddset(&mut mask, libc::SIGTERM);
    if libc::sigprocmask(libc::SIG_BLOCK, &mask, std::ptr::null_mut()) < 0 {
        return Err(io::Error::last_os_error());
    }

    // 2. Create the fd
    let fd = libc::signalfd(-1, &mask, libc::SFD_CLOEXEC | libc::SFD_NONBLOCK);
    if fd < 0 { return Err(io::Error::last_os_error()); }

    // 3. Register fd on epoll. On EPOLLIN:
    let mut info: signalfd_siginfo = std::mem::zeroed();
    let n = libc::read(fd, &mut info as *mut _ as *mut _, std::mem::size_of::<signalfd_siginfo>());
    if n as usize == std::mem::size_of::<signalfd_siginfo>() {
        match info.ssi_signo as i32 {
            libc::SIGINT | libc::SIGTERM => begin_graceful_shutdown(),
            _ => {}
        }
    }
}

Key flags

Flag	Meaning
`SFD_CLOEXEC`	Close on exec. Always set.
`SFD_NONBLOCK`	Non-blocking reads. Required when on `epoll`.
`SIG_BLOCK`	Passed to `sigprocmask` to add the set to the current mask. The corresponding `SIG_UNBLOCK` / `SIG_SETMASK` are available if you need them.

Gotchas

Blocking the signal with sigprocmask is mandatory. Otherwise the default handler runs and kills the process before the fd ever becomes readable. Block once at boot, never unblock.
Per-thread mask. sigprocmask is per-thread. In a single-threaded runtime that's fine; if you ever spawn threads, use pthread_sigmask on each so the main loop gets the signals.
signalfd_siginfo is big (128 bytes). Read into an aligned buffer; partial reads return EINVAL.
Doesn't catch SIGKILL or SIGSTOP. Nothing does. Those bypass everything.
Child exit notifications (SIGCHLD) work via signalfd but pidfd (see 10-pidfd.md) is usually the better fit for clean child supervision.

Used by

04-cutover-remove-tokio-axum.md — replaces tokio::signal::ctrl_c() for graceful shutdown. Every subsequent phase inherits this pattern.

v1 port source

.dev/reference/crates/wo-event/src/signalfd.rs (62 LOC) — SignalFd::new(&[SIGINT, SIGTERM]) -> SignalFd with a safe read_signo() helper.

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04 — signalfd