diff --git a/src/header/semaphore/cbindgen.toml b/src/header/semaphore/cbindgen.toml
index a96854c7ad..2b04643716 100644
--- a/src/header/semaphore/cbindgen.toml
+++ b/src/header/semaphore/cbindgen.toml
@@ -5,12 +5,21 @@ after_includes = """
 """
 trailer = """
 #define SEM_FAILED ((sem_t *) -1)
+
+/* POSIX variadic named semaphore opener. cbindgen cannot emit variadic
+   functions, so the prototype is declared manually here. */
+sem_t *sem_open(const char *name, int oflag, ...);
 """
 language = "C"
 style = "Type"
 no_includes = true
 cpp_compat = true
 
+[fn]
+# cbindgen cannot emit variadic functions automatically; write the sem_open
+# prototype by hand in the trailer so C callers see the POSIX signature.
+exclude = ["sem_open"]
+
 [enum]
 prefix_with_name = true
 
diff --git a/src/header/semaphore/mod.rs b/src/header/semaphore/mod.rs
index a334c7e074..623a4ce399 100644
--- a/src/header/semaphore/mod.rs
+++ b/src/header/semaphore/mod.rs
@@ -144,7 +144,8 @@ pub unsafe extern "C" fn sem_init(sem: *mut sem_t, _pshared: c_int, value: c_uin
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn sem_open(
     name: *const c_char,
-    oflag: c_int, /* (va_list) mode: mode_t, value: c_uint */
+    oflag: c_int,
+    mut __valist: ...
 ) -> *mut sem_t {
     if name.is_null() { ERRNO.set(EINVAL); return SEM_FAILED_PTR; }
     let name_bytes = unsafe { CStr::from_ptr(name) }.to_bytes().to_vec();
@@ -156,11 +157,10 @@ pub unsafe extern "C" fn sem_open(
         with_named_sems(|map| { if let Some(e) = map.get(&name_bytes) { e.refs.fetch_add(1, Ordering::Relaxed); } });
         return ptr as *mut NamedSemaphore as *mut sem_t;
     }
-    let (mode, value): (mode_t, c_uint) = {
-        let oflag_ptr: *const c_int = &oflag;
-        let mode_ptr = unsafe { oflag_ptr.add(1) as *const mode_t };
-        let value_ptr = unsafe { oflag_ptr.add(2) as *const c_uint };
-        (unsafe { *mode_ptr }, if create { unsafe { *value_ptr } } else { 0 })
+    let (mode, value): (mode_t, c_uint) = if create {
+        (unsafe { __valist.arg::<mode_t>() }, unsafe { __valist.arg::<c_uint>() })
+    } else {
+        (0, 0)
     };
     let ptr = unsafe { map_named_semaphore(name, oflag, mode, value) };
     if ptr == SEM_FAILED_PTR { return SEM_FAILED_PTR; }