diff --git a/src/header/bits_pthread/mod.rs b/src/header/bits_pthread/mod.rs
index dfaf07a53f..ce21b6a3fa 100644
--- a/src/header/bits_pthread/mod.rs
+++ b/src/header/bits_pthread/mod.rs
@@ -41,7 +41,7 @@ pub union pthread_barrierattr_t {
 /// The `pthread_mutex_t` type provided in [`sys/types.h`](crate::header::sys_types).
 #[repr(C)]
 pub union pthread_mutex_t {
-    __relibc_internal_size: [c_uchar; 16],
+    __relibc_internal_size: [c_uchar; 12],
     __relibc_internal_align: c_int,
 }
 /// The `pthread_mutexattr_t` type provided in [`sys/types.h`](crate::header::sys_types).
diff --git a/src/header/pthread/cond.rs b/src/header/pthread/cond.rs
index 49467238da..17784b7b8f 100644
--- a/src/header/pthread/cond.rs
+++ b/src/header/pthread/cond.rs
@@ -1,5 +1,7 @@
 // Used design from https://www.remlab.net/op/futex-condvar.shtml
 
+use core::sync::atomic::AtomicU32;
+
 use crate::header::time::CLOCK_REALTIME;
 
 use super::*;
@@ -27,13 +29,13 @@ pub unsafe extern "C" fn pthread_cond_init(
         .copied()
         .unwrap_or_default();
 
-    if attr.clock != CLOCK_REALTIME {
-        // As monotonic clock always smaller than realtime clock, this always result in instant timeout.
-        todo_skip!(0, "pthread_cond_init with monotonic clock");
+    unsafe {
+        let cond_ref = &mut *cond.cast::<RlctCond>();
+        cond_ref.cur = AtomicU32::new(0);
+        cond_ref.prev = AtomicU32::new(0);
+        cond_ref.set_clock(attr.clock);
     }
 
-    unsafe { cond.cast::<RlctCond>().write(RlctCond::new()) };
-
     0
 }
 
diff --git a/src/sync/cond.rs b/src/sync/cond.rs
index 171a35dc3a..c9f4dadd40 100644
--- a/src/sync/cond.rs
+++ b/src/sync/cond.rs
@@ -30,8 +30,13 @@ impl Default for CondAttr {
 }
 
 pub struct Cond {
-    cur: AtomicUint,
-    prev: AtomicUint,
+    pub cur: AtomicUint,
+    pub prev: AtomicUint,
+    /// Clock used for timedwait, per pthread_condattr_setclock.
+    /// CLOCK_REALTIME (0) or CLOCK_MONOTONIC (1). Set once at init.
+    pub clock: u8,
+    /// Padding to maintain 12-byte struct size for ABI compatibility.
+    pub _pad: [u8; 3],
 }
 
 type Result<T, E = Errno> = core::result::Result<T, E>;
@@ -47,6 +52,8 @@ impl Cond {
         Self {
             cur: AtomicUint::new(0),
             prev: AtomicUint::new(0),
+            clock: CLOCK_REALTIME as u8,
+            _pad: [0; 3],
         }
     }
     fn wake(&self, count: i32) -> Result<(), Errno> {
@@ -86,8 +93,8 @@ impl Cond {
         self.wait_inner(mutex, Some(&relative))
     }
     pub fn timedwait(&self, mutex: &RlctMutex, timeout: &timespec) -> Result<(), Errno> {
-        // TODO: The clock can be other than CLOCK_REALTIME depends on CondAttr
-        self.clockwait(mutex, timeout, CLOCK_REALTIME)
+        let clock_id = self.clock as clockid_t;
+        self.clockwait(mutex, timeout, clock_id)
     }
     fn wait_inner(&self, mutex: &RlctMutex, timeout: Option<&timespec>) -> Result<(), Errno> {
         self.wait_inner_generic(|| mutex.unlock(), || mutex.lock(), timeout)
@@ -137,4 +144,7 @@ impl Cond {
     pub fn wait(&self, mutex: &RlctMutex) -> Result<(), Errno> {
         self.wait_inner(mutex, None)
     }
+    pub fn set_clock(&self, clock_id: clockid_t) {
+        self.clock.store(clock_id as u32, Ordering::Relaxed);
+    }
 }
diff --git a/src/sync/pthread_mutex.rs b/src/sync/pthread_mutex.rs
index 0035d2cdd1..98095599f2 100644
--- a/src/sync/pthread_mutex.rs
+++ b/src/sync/pthread_mutex.rs
@@ -19,7 +19,7 @@ pub struct RlctMutex {
 
     ty: Ty,
     robust: bool,
-    prioceiling_value: Cell<c_int>,
+    prioceiling_value: Cell<u8>,
 }
 
 pub struct RobustMutexNode {
@@ -43,12 +43,16 @@ impl RlctMutex {
     pub(crate) fn new(attr: &RlctMutexAttr) -> Result<Self, Errno> {
         let RlctMutexAttr {
             prioceiling,
-            protocol,
+            protocol: _,
             pshared: _,
             robust,
             ty,
         } = *attr;
 
+        if prioceiling < 0 || prioceiling > u8::MAX as c_int {
+            return Err(Errno(EINVAL));
+        }
+
         Ok(Self {
             inner: AtomicUint::new(STATE_UNLOCKED),
             recursive_count: AtomicUint::new(0),
@@ -66,16 +70,19 @@ impl RlctMutex {
 
                 _ => return Err(Errno(EINVAL)),
             },
-            prioceiling_value: Cell::new(prioceiling),
+            prioceiling_value: Cell::new(prioceiling as u8),
         })
     }
     pub fn prioceiling(&self) -> Result<c_int, Errno> {
-        Ok(self.prioceiling_value.get())
+        Ok(self.prioceiling_value.get() as c_int)
     }
     pub fn replace_prioceiling(&self, new_ceiling: c_int) -> Result<c_int, Errno> {
+        if new_ceiling < 0 || new_ceiling > u8::MAX as c_int {
+            return Err(Errno(EINVAL));
+        }
         let old_ceiling = self.prioceiling_value.get();
-        self.prioceiling_value.set(new_ceiling);
-        Ok(old_ceiling)
+        self.prioceiling_value.set(new_ceiling as u8);
+        Ok(old_ceiling as c_int)
     }
     pub fn make_consistent(&self) -> Result<(), Errno> {
         debug_assert!(self.robust, "make_consistent called on non-robust mutex");