--- a/src/header/semaphore/mod.rs	2026-04-25 17:07:53.742796721 +0100
+++ b/src/header/semaphore/mod.rs	2026-04-25 17:08:54.527084219 +0100
@@ -2,12 +2,24 @@
 //!
 //! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/semaphore.h.html>.
 
+use core::mem::size_of;
+
 use crate::{
+    c_str::CStr,
     header::{
         bits_timespec::timespec,
+        errno::{EEXIST, EINVAL},
+        fcntl::{O_CREAT, O_EXCL, O_RDWR},
+        sys_mman::{
+            mmap, munmap, shm_open, shm_unlink, MAP_SHARED, MAP_FAILED, PROT_READ, PROT_WRITE,
+        },
         time::{CLOCK_MONOTONIC, CLOCK_REALTIME},
+        unistd::{close, ftruncate},
+    },
+    platform::{
+        ERRNO,
+        types::{c_char, c_int, c_long, c_uint, clockid_t, c_void, mode_t, off_t, size_t},
     },
-    platform::types::{c_char, c_int, c_long, c_uint, clockid_t},
 };
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/semaphore.h.html>.
@@ -18,12 +30,17 @@
     pub size: [c_char; 4],
     pub align: c_long,
 }
+
+/// Pointer value returned by `sem_open` on failure.
+/// cbindgen:ignore
+pub const SEM_FAILED: *mut sem_t = usize::MAX as *mut sem_t;
+
 pub type RlctSempahore = crate::sync::Semaphore;
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/sem_close.html>.
-// #[unsafe(no_mangle)]
+#[unsafe(no_mangle)]
 pub unsafe extern "C" fn sem_close(sem: *mut sem_t) -> c_int {
-    todo!("named semaphores")
+    unsafe { munmap(sem.cast::<c_void>(), size_of::<sem_t>()) }
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/sem_destroy.html>.
@@ -50,13 +67,105 @@
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/sem_open.html>.
-// TODO: va_list
-// #[unsafe(no_mangle)]
+#[unsafe(no_mangle)]
 pub unsafe extern "C" fn sem_open(
     name: *const c_char,
-    oflag: c_int, /* (va_list) value: c_uint */
+    oflag: c_int,
+    mut __valist: ...
 ) -> *mut sem_t {
-    todo!("named semaphores")
+    // Validate name: must start with '/', no embedded '/'.
+    if name.is_null() {
+        ERRNO.set(EINVAL);
+        return SEM_FAILED;
+    }
+
+    let name_c = unsafe { CStr::from_ptr(name) };
+    let name_bytes = name_c.to_bytes();
+    if name_bytes.is_empty() || name_bytes[0] != b'/' {
+        ERRNO.set(EINVAL);
+        return SEM_FAILED;
+    }
+    if name_bytes[1..].iter().any(|&b| b == b'/') {
+        ERRNO.set(EINVAL);
+        return SEM_FAILED;
+    }
+
+    let creat = oflag & O_CREAT == O_CREAT;
+    let excl = oflag & O_EXCL == O_EXCL;
+
+    let (mode, value): (mode_t, c_uint) = if creat {
+        (
+            unsafe { __valist.arg::<mode_t>() },
+            unsafe { __valist.arg::<c_uint>() },
+        )
+    } else {
+        (0, 0)
+    };
+
+    // Open or create the shared memory backing.
+    let (fd, created) = if creat && excl {
+        // O_CREAT | O_EXCL: must create exclusively.
+        let fd = unsafe { shm_open(name, O_CREAT | O_EXCL | O_RDWR, mode) };
+        if fd < 0 {
+            return SEM_FAILED;
+        }
+        (fd, true)
+    } else if creat {
+        // O_CREAT without O_EXCL: try exclusive first, fall back to open.
+        let fd = unsafe { shm_open(name, O_CREAT | O_EXCL | O_RDWR, mode) };
+        if fd >= 0 {
+            (fd, true)
+        } else if ERRNO.get() == EEXIST {
+            let fd = unsafe { shm_open(name, O_RDWR, 0) };
+            if fd < 0 {
+                return SEM_FAILED;
+            }
+            (fd, false)
+        } else {
+            return SEM_FAILED;
+        }
+    } else {
+        // No O_CREAT: open existing.
+        let fd = unsafe { shm_open(name, O_RDWR, 0) };
+        if fd < 0 {
+            return SEM_FAILED;
+        }
+        (fd, false)
+    };
+
+    // Set size if we created the backing.
+    if created {
+        if unsafe { ftruncate(fd, size_of::<sem_t>() as off_t) } < 0 {
+            let _ = unsafe { close(fd) };
+            return SEM_FAILED;
+        }
+    }
+
+    // Map the shared memory.
+    let ptr = unsafe {
+        mmap(
+            core::ptr::null_mut(),
+            size_of::<sem_t>(),
+            PROT_READ | PROT_WRITE,
+            MAP_SHARED,
+            fd,
+            0,
+        )
+    };
+    let _ = unsafe { close(fd) };
+
+    if ptr == MAP_FAILED {
+        return SEM_FAILED;
+    }
+
+    let sem_ptr = ptr.cast::<sem_t>();
+
+    // Initialize the semaphore value if we created the backing.
+    if created {
+        unsafe { sem_ptr.cast::<RlctSempahore>().write(RlctSempahore::new(value)) };
+    }
+
+    sem_ptr
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/sem_post.html>.
@@ -76,10 +185,10 @@
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/sem_unlink.html>.
-// #[unsafe(no_mangle)]
+#[unsafe(no_mangle)]
 pub unsafe extern "C" fn sem_unlink(name: *const c_char) -> c_int {
-    todo!("named semaphores")
+    unsafe { shm_unlink(name) }
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/sem_trywait.html>.
--- a/src/header/semaphore/cbindgen.toml	2026-04-25 17:07:53.743979154 +0100
+++ b/src/header/semaphore/cbindgen.toml	2026-04-25 17:09:18.310792692 +0100
@@ -3,6 +3,9 @@
 after_includes = """
 #include <bits/timespec.h> // for timespec
 """
+trailer = """
+#define SEM_FAILED ((sem_t *) -1)
+"""
 language = "C"
 style = "Type"
 no_includes = true