diff --git a/src/shims/foreign_items/posix.rs b/src/shims/foreign_items/posix.rs
index 352e38113ab..db2fab526c1 100644
--- a/src/shims/foreign_items/posix.rs
+++ b/src/shims/foreign_items/posix.rs
@@ -331,42 +331,52 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_condattr_init" => {
-                let result = this.pthread_condattr_init(args[0])?;
+                let &[attr] = check_arg_count(args)?;
+                let result = this.pthread_condattr_init(attr)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_condattr_setclock" => {
-                let result = this.pthread_condattr_setclock(args[0], args[1])?;
+                let &[attr, clock_id] = check_arg_count(args)?;
+                let result = this.pthread_condattr_setclock(attr, clock_id)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_condattr_getclock" => {
-                let result = this.pthread_condattr_getclock(args[0], args[1])?;
+                let &[attr, clock_id] = check_arg_count(args)?;
+                let result = this.pthread_condattr_getclock(attr, clock_id)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_condattr_destroy" => {
-                let result = this.pthread_condattr_destroy(args[0])?;
+                let &[attr] = check_arg_count(args)?;
+                let result = this.pthread_condattr_destroy(attr)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_cond_init" => {
-                let result = this.pthread_cond_init(args[0], args[1])?;
+                let &[cond, attr] = check_arg_count(args)?;
+                let result = this.pthread_cond_init(cond, attr)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_cond_signal" => {
-                let result = this.pthread_cond_signal(args[0])?;
+                let &[cond] = check_arg_count(args)?;
+                let result = this.pthread_cond_signal(cond)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_cond_broadcast" => {
-                let result = this.pthread_cond_broadcast(args[0])?;
+                let &[cond] = check_arg_count(args)?;
+                let result = this.pthread_cond_broadcast(cond)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_cond_wait" => {
-                let result = this.pthread_cond_wait(args[0], args[1])?;
+                let &[cond, mutex] = check_arg_count(args)?;
+                let result = this.pthread_cond_wait(cond, mutex)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
             "pthread_cond_timedwait" => {
-                this.pthread_cond_timedwait(args[0], args[1], args[2], dest)?;
+                let &[cond, mutex, abstime] = check_arg_count(args)?;
+                this.pthread_cond_timedwait(cond, mutex, abstime, dest)?;
             }
             "pthread_cond_destroy" => {
-                let result = this.pthread_cond_destroy(args[0])?;
+                let &[cond] = check_arg_count(args)?;
+                let result = this.pthread_cond_destroy(cond)?;
                 this.write_scalar(Scalar::from_i32(result), dest)?;
             }
 
@@ -430,6 +440,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
             | "pthread_attr_init"
             | "pthread_attr_destroy"
+            if this.frame().instance.to_string().starts_with("std::sys::unix::") => {
+                let &[_] = check_arg_count(args)?;
+                this.write_null(dest)?;
+            }
             | "pthread_attr_setstacksize"
             if this.frame().instance.to_string().starts_with("std::sys::unix::") => {
                 let &[_, _] = check_arg_count(args)?;
diff --git a/src/shims/sync.rs b/src/shims/sync.rs
index 95092a042d3..5b0de43e546 100644
--- a/src/shims/sync.rs
+++ b/src/shims/sync.rs
@@ -129,14 +129,14 @@ fn mutex_set_kind<'mir, 'tcx: 'mir>(
 fn mutex_get_id<'mir, 'tcx: 'mir>(
     ecx: &MiriEvalContext<'mir, 'tcx>,
     mutex_op: OpTy<'tcx, Tag>,
-) -> InterpResult<'tcx, ScalarMaybeUndef<Tag>> {
+) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
     get_at_offset(ecx, mutex_op, 4, ecx.machine.layouts.u32, PTHREAD_MUTEX_T_MIN_SIZE)
 }
 
 fn mutex_set_id<'mir, 'tcx: 'mir>(
     ecx: &mut MiriEvalContext<'mir, 'tcx>,
     mutex_op: OpTy<'tcx, Tag>,
-    id: impl Into<ScalarMaybeUndef<Tag>>,
+    id: impl Into<ScalarMaybeUninit<Tag>>,
 ) -> InterpResult<'tcx, ()> {
     set_at_offset(ecx, mutex_op, 4, id, ecx.machine.layouts.u32, PTHREAD_MUTEX_T_MIN_SIZE)
 }
@@ -176,7 +176,7 @@ fn rwlock_get_id<'mir, 'tcx: 'mir>(
 fn rwlock_set_id<'mir, 'tcx: 'mir>(
     ecx: &mut MiriEvalContext<'mir, 'tcx>,
     rwlock_op: OpTy<'tcx, Tag>,
-    id: impl Into<ScalarMaybeUndef<Tag>>,
+    id: impl Into<ScalarMaybeUninit<Tag>>,
 ) -> InterpResult<'tcx, ()> {
     set_at_offset(ecx, rwlock_op, 4, id, ecx.machine.layouts.u32, PTHREAD_RWLOCK_T_MIN_SIZE)
 }
@@ -208,14 +208,14 @@ const PTHREAD_CONDATTR_T_MIN_SIZE: u64 = 4;
 fn condattr_get_clock_id<'mir, 'tcx: 'mir>(
     ecx: &MiriEvalContext<'mir, 'tcx>,
     attr_op: OpTy<'tcx, Tag>,
-) -> InterpResult<'tcx, ScalarMaybeUndef<Tag>> {
+) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
     get_at_offset(ecx, attr_op, 0, ecx.machine.layouts.i32, PTHREAD_CONDATTR_T_MIN_SIZE)
 }
 
 fn condattr_set_clock_id<'mir, 'tcx: 'mir>(
     ecx: &mut MiriEvalContext<'mir, 'tcx>,
     attr_op: OpTy<'tcx, Tag>,
-    clock_id: impl Into<ScalarMaybeUndef<Tag>>,
+    clock_id: impl Into<ScalarMaybeUninit<Tag>>,
 ) -> InterpResult<'tcx, ()> {
     set_at_offset(ecx, attr_op, 0, clock_id, ecx.machine.layouts.i32, PTHREAD_CONDATTR_T_MIN_SIZE)
 }
@@ -234,14 +234,14 @@ const PTHREAD_COND_T_MIN_SIZE: u64 = 12;
 fn cond_get_id<'mir, 'tcx: 'mir>(
     ecx: &MiriEvalContext<'mir, 'tcx>,
     cond_op: OpTy<'tcx, Tag>,
-) -> InterpResult<'tcx, ScalarMaybeUndef<Tag>> {
+) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
     get_at_offset(ecx, cond_op, 4, ecx.machine.layouts.u32, PTHREAD_COND_T_MIN_SIZE)
 }
 
 fn cond_set_id<'mir, 'tcx: 'mir>(
     ecx: &mut MiriEvalContext<'mir, 'tcx>,
     cond_op: OpTy<'tcx, Tag>,
-    id: impl Into<ScalarMaybeUndef<Tag>>,
+    id: impl Into<ScalarMaybeUninit<Tag>>,
 ) -> InterpResult<'tcx, ()> {
     set_at_offset(ecx, cond_op, 4, id, ecx.machine.layouts.u32, PTHREAD_COND_T_MIN_SIZE)
 }
@@ -265,14 +265,14 @@ fn cond_get_or_create_id<'mir, 'tcx: 'mir>(
 fn cond_get_clock_id<'mir, 'tcx: 'mir>(
     ecx: &MiriEvalContext<'mir, 'tcx>,
     cond_op: OpTy<'tcx, Tag>,
-) -> InterpResult<'tcx, ScalarMaybeUndef<Tag>> {
+) -> InterpResult<'tcx, ScalarMaybeUninit<Tag>> {
     get_at_offset(ecx, cond_op, 8, ecx.machine.layouts.i32, PTHREAD_COND_T_MIN_SIZE)
 }
 
 fn cond_set_clock_id<'mir, 'tcx: 'mir>(
     ecx: &mut MiriEvalContext<'mir, 'tcx>,
     cond_op: OpTy<'tcx, Tag>,
-    clock_id: impl Into<ScalarMaybeUndef<Tag>>,
+    clock_id: impl Into<ScalarMaybeUninit<Tag>>,
 ) -> InterpResult<'tcx, ()> {
     set_at_offset(ecx, cond_op, 8, clock_id, ecx.machine.layouts.i32, PTHREAD_COND_T_MIN_SIZE)
 }
@@ -518,8 +518,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             throw_ub_format!("destroyed a locked mutex");
         }
 
-        mutex_set_kind(this, mutex_op, ScalarMaybeUndef::Undef)?;
-        mutex_set_id(this, mutex_op, ScalarMaybeUndef::Undef)?;
+        mutex_set_kind(this, mutex_op, ScalarMaybeUninit::Uninit)?;
+        mutex_set_id(this, mutex_op, ScalarMaybeUninit::Uninit)?;
 
         Ok(0)
     }
@@ -643,7 +643,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             throw_ub_format!("destroyed a locked rwlock");
         }
 
-        rwlock_set_id(this, rwlock_op, ScalarMaybeUndef::Undef)?;
+        rwlock_set_id(this, rwlock_op, ScalarMaybeUninit::Uninit)?;
 
         Ok(0)
     }
@@ -696,7 +696,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     fn pthread_condattr_destroy(&mut self, attr_op: OpTy<'tcx, Tag>) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
 
-        condattr_set_clock_id(this, attr_op, ScalarMaybeUndef::Undef)?;
+        condattr_set_clock_id(this, attr_op, ScalarMaybeUninit::Uninit)?;
 
         Ok(0)
     }
@@ -835,8 +835,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         if this.condvar_is_awaited(id) {
             throw_ub_format!("destroyed an awaited conditional variable");
         }
-        cond_set_id(this, cond_op, ScalarMaybeUndef::Undef)?;
-        cond_set_clock_id(this, cond_op, ScalarMaybeUndef::Undef)?;
+        cond_set_id(this, cond_op, ScalarMaybeUninit::Uninit)?;
+        cond_set_clock_id(this, cond_op, ScalarMaybeUninit::Uninit)?;
 
         Ok(0)
     }