diff -ruN a/src/header/sys_sem/mod.rs b/src/header/sys_sem/mod.rs
--- a/src/header/sys_sem/mod.rs	1970-01-01 00:00:00.000000000 +0000
+++ b/src/header/sys_sem/mod.rs	2026-04-15 09:52:34.039784454 +0100
@@ -0,0 +1,176 @@
+//! `sys/sem.h` implementation.
+
+use alloc::{collections::BTreeMap, ffi::CString, format};
+use core::{mem, ptr, sync::atomic::{AtomicI32, Ordering}};
+
+use crate::{
+    header::{
+        errno::{EFAULT, EINVAL, ENOENT},
+        fcntl::{O_CREAT, O_EXCL, O_RDWR},
+        sys_ipc::{IPC_CREAT, IPC_EXCL, IPC_NOWAIT, IPC_PRIVATE, IPC_RMID, ipc_perm, key_t},
+        sys_mman::{MAP_FAILED, MAP_SHARED, PROT_READ, PROT_WRITE, mmap, munmap, shm_open, shm_unlink},
+        unistd::close,
+    },
+    out::Out,
+    platform::{ERRNO, Pal, Sys, types::{c_int, c_short, c_ushort, c_void}},
+    sync::{Mutex, Semaphore},
+};
+
+pub const GETVAL: c_int = 12;
+pub const SETVAL: c_int = 16;
+pub const SEM_UNDO: c_short = 0x1000;
+
+#[repr(C)]
+#[derive(Clone, Copy, Default)]
+pub struct sembuf {
+    pub sem_num: c_ushort,
+    pub sem_op: c_short,
+    pub sem_flg: c_short,
+}
+
+#[repr(C)]
+#[derive(Clone, Copy, Default)]
+pub struct semid_ds {
+    pub sem_perm: ipc_perm,
+    pub sem_nsems: c_ushort,
+}
+
+#[repr(C)]
+pub union semun {
+    pub val: c_int,
+    pub buf: *mut semid_ds,
+}
+
+#[repr(C)]
+struct SharedSemSet {
+    sem: Semaphore,
+}
+
+#[derive(Clone)]
+struct SemMeta {
+    path: CString,
+    nsems: c_ushort,
+}
+
+static NEXT_SEMID: AtomicI32 = AtomicI32::new(1);
+static SEM_REGISTRY: Mutex<BTreeMap<c_int, SemMeta>> = Mutex::new(BTreeMap::new());
+static SEM_ATTACHMENTS: Mutex<BTreeMap<c_int, (usize, c_int)>> = Mutex::new(BTreeMap::new());
+
+fn key_path(key: key_t) -> CString { CString::new(format!("/relibc-sysv-sem-{key:x}")).unwrap() }
+fn id_path(id: c_int) -> CString { CString::new(format!("/relibc-sysv-sem-id-{id:x}")).unwrap() }
+
+fn open_flags_from_ipc(semflg: c_int) -> c_int {
+    let mut flags = O_RDWR;
+    if semflg & IPC_CREAT == IPC_CREAT { flags |= O_CREAT; }
+    if semflg & IPC_EXCL == IPC_EXCL { flags |= O_EXCL; }
+    flags
+}
+
+unsafe fn map_sem(path: &CString, create: bool, initial: c_int) -> Result<*mut SharedSemSet, c_int> {
+    let fd = unsafe { shm_open(path.as_ptr(), if create { O_CREAT | O_RDWR } else { O_RDWR }, 0o600) };
+    if fd < 0 { return Err(ERRNO.get()); }
+    let mut st = crate::header::sys_stat::stat::default();
+    if Sys::fstat(fd, Out::from_mut(&mut st)).is_err() { let _ = close(fd); return Err(ERRNO.get()); }
+    if st.st_size == 0 {
+        if Sys::ftruncate(fd, mem::size_of::<SharedSemSet>() as i64).is_err() { let _ = close(fd); return Err(ERRNO.get()); }
+    }
+    let ptr = unsafe { mmap(ptr::null_mut(), mem::size_of::<SharedSemSet>(), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0) };
+    let _ = close(fd);
+    if ptr == MAP_FAILED { return Err(ERRNO.get()); }
+    let set = ptr.cast::<SharedSemSet>();
+    if create && st.st_size == 0 { unsafe { set.write(SharedSemSet { sem: Semaphore::new(initial as u32) }) }; }
+    Ok(set)
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn semget(key: key_t, nsems: c_int, semflg: c_int) -> c_int {
+    if nsems != 1 { ERRNO.set(EINVAL); return -1; }
+    let semid = NEXT_SEMID.fetch_add(1, Ordering::SeqCst);
+    let path = if key == IPC_PRIVATE { id_path(semid) } else { key_path(key) };
+    let fd = unsafe { shm_open(path.as_ptr(), open_flags_from_ipc(semflg), (semflg & 0o777) as c_int) };
+    if fd < 0 { return -1; }
+    let _ = close(fd);
+    SEM_REGISTRY.lock().insert(semid, SemMeta { path, nsems: 1 });
+    semid
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn semop(semid: c_int, sops: *mut sembuf, nsops: usize) -> c_int {
+    if sops.is_null() { ERRNO.set(EFAULT); return -1; }
+    if nsops == 0 { return 0; }
+    let meta = { let registry = SEM_REGISTRY.lock(); registry.get(&semid).cloned() };
+    let Some(meta) = meta else { ERRNO.set(ENOENT); return -1; };
+    let set = {
+        let mut attachments = SEM_ATTACHMENTS.lock();
+        if let Some((ptr, _)) = attachments.get(&semid) { *ptr as *mut SharedSemSet } else {
+            let ptr = match unsafe { map_sem(&meta.path, false, 0) } { Ok(ptr) => ptr, Err(_) => return -1 };
+            attachments.insert(semid, (ptr as usize, 1));
+            ptr
+        }
+    };
+    let ops = unsafe { core::slice::from_raw_parts(sops, nsops) };
+    for op in ops {
+        if op.sem_num != 0 { ERRNO.set(EINVAL); return -1; }
+        if op.sem_op > 0 { unsafe { (*set).sem.post(op.sem_op as u32) }; continue; }
+        if op.sem_op == 0 {
+            if op.sem_flg & IPC_NOWAIT as c_short == IPC_NOWAIT as c_short {
+                if unsafe { (*set).sem.value() } != 0 { ERRNO.set(crate::header::errno::EAGAIN); return -1; }
+            } else if unsafe { (*set).sem.value() } != 0 {
+                ERRNO.set(crate::header::errno::EAGAIN);
+                return -1;
+            }
+            continue;
+        }
+        for _ in 0..(-op.sem_op) {
+            if op.sem_flg & IPC_NOWAIT as c_short == IPC_NOWAIT as c_short {
+                if unsafe { (*set).sem.try_wait() } == 0 { ERRNO.set(crate::header::errno::EAGAIN); return -1; }
+            } else if unsafe { (*set).sem.wait(None, crate::header::time::CLOCK_MONOTONIC) }.is_err() {
+                ERRNO.set(crate::header::errno::EINTR);
+                return -1;
+            }
+        }
+    }
+    0
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn semctl(semid: c_int, semnum: c_int, cmd: c_int, mut args: ...) -> c_int {
+    if semnum != 0 { ERRNO.set(EINVAL); return -1; }
+    let meta = { let registry = SEM_REGISTRY.lock(); registry.get(&semid).cloned() };
+    let Some(meta) = meta else { ERRNO.set(ENOENT); return -1; };
+    match cmd {
+        IPC_RMID => {
+            let _ = unsafe { shm_unlink(meta.path.as_ptr()) };
+            SEM_REGISTRY.lock().remove(&semid);
+            if let Some((ptr, _)) = SEM_ATTACHMENTS.lock().remove(&semid) {
+                let _ = unsafe { munmap((ptr as *mut SharedSemSet).cast::<c_void>(), mem::size_of::<SharedSemSet>()) };
+            }
+            0
+        }
+        GETVAL => {
+            let set = match unsafe { map_sem(&meta.path, false, 0) } { Ok(set) => set, Err(errno) => { ERRNO.set(errno); return -1; } };
+            let value = unsafe { (*set).sem.value() as c_int };
+            let _ = unsafe { munmap(set.cast::<c_void>(), mem::size_of::<SharedSemSet>()) };
+            value
+        }
+        SETVAL => {
+            let val = unsafe { args.arg::<c_int>() };
+            let set = match unsafe { map_sem(&meta.path, false, 0) } { Ok(set) => set, Err(errno) => { ERRNO.set(errno); return -1; } };
+            let current = unsafe { (*set).sem.value() as c_int };
+            if val > current { unsafe { (*set).sem.post((val - current) as u32) }; } else {
+                for _ in 0..(current - val) {
+                    if unsafe { (*set).sem.try_wait() } == 0 { break; }
+                }
+            }
+            let _ = unsafe { munmap(set.cast::<c_void>(), mem::size_of::<SharedSemSet>()) };
+            0
+        }
+        crate::header::sys_ipc::IPC_STAT => {
+            let buf = unsafe { args.arg::<*mut semid_ds>() };
+            if buf.is_null() { ERRNO.set(EFAULT); return -1; }
+            unsafe { *buf = semid_ds { sem_perm: ipc_perm::default(), sem_nsems: meta.nsems }; }
+            0
+        }
+        _ => { ERRNO.set(EINVAL); -1 }
+    }
+}