--- /dev/null	2026-05-03 20:55:05.750445686 +0100
+++ b/src/header/sys_eventfd/mod.rs
@@ -0,0 +1,37 @@
+//! `sys/eventfd.h` implementation — eventfd() with EFD_SEMAPHORE/CLOEXEC/NONBLOCK.
+
+use alloc::format;
+use crate::c_str::{CStr, CString};
+use crate::error::{Errno, ResultExt};
+use crate::header::fcntl::{O_CLOEXEC, O_NONBLOCK, O_RDWR};
+use crate::header::errno::EINVAL;
+use crate::platform::{Pal, Sys, types::{c_int, c_uint}};
+
+pub const EFD_SEMAPHORE: c_int = 1;
+pub const EFD_CLOEXEC: c_int = 0x80000;
+pub const EFD_NONBLOCK: c_int = 0x800;
+
+pub type eventfd_t = u64;
+
+#[unsafe(no_mangle)]
+pub extern "C" fn eventfd(initval: c_uint, flags: c_int) -> c_int {
+    let supported = EFD_CLOEXEC | EFD_NONBLOCK | EFD_SEMAPHORE;
+    if flags & !supported != 0 {
+        return Err::<c_int, _>(Errno(EINVAL)).or_minus_one_errno();
+    }
+    let sem = if flags & EFD_SEMAPHORE != 0 { 1 } else { 0 };
+    let path = format!("/scheme/event/eventfd/{}/{}", initval, sem);
+    let cpath = match CString::new(path) {
+        Ok(p) => p,
+        Err(_) => return Err::<c_int, _>(Errno(EINVAL)).or_minus_one_errno(),
+    };
+
+    let mut oflag = O_RDWR;
+    if flags & EFD_CLOEXEC == EFD_CLOEXEC {
+        oflag |= O_CLOEXEC;
+    }
+    if flags & EFD_NONBLOCK == EFD_NONBLOCK {
+        oflag |= O_NONBLOCK;
+    }
+    Sys::open(CStr::borrow(&cpath), oflag, 0).or_minus_one_errno()
+}