diff --git a/libc/include/threads.h b/libc/include/threads.h
index 1b00b8f..b1008de 100644
--- a/libc/include/threads.h
+++ b/libc/include/threads.h
@@ -51,9 +51,9 @@
 typedef pthread_mutex_t mtx_t;
 
 /** The type for a thread-specific storage destructor. */
-typedef void (*tss_dtor_t)(void*);
+typedef void (*tss_dtor_t)(void* _Nullable);
 /** The type of the function passed to thrd_create() to create a new thread. */
-typedef int (*thrd_start_t)(void*);
+typedef int (*thrd_start_t)(void* _Nullable);
 
 /** The type used by call_once(). */
 typedef pthread_once_t once_flag;
@@ -82,72 +82,72 @@
 // This file is implemented as static inlines before API level 30.
 
 /** Uses `__flag` to ensure that `__function` is called exactly once. */
-void call_once(once_flag* __flag, void (*__function)(void)) __INTRODUCED_IN(30);
+void call_once(once_flag* _Nonnull __flag, void (* _Nonnull __function)(void)) __INTRODUCED_IN(30);
 
 
 
 /**
  * Unblocks all threads blocked on `__cond`.
  */
-int cnd_broadcast(cnd_t* __cond) __INTRODUCED_IN(30);
+int cnd_broadcast(cnd_t* _Nonnull __cond) __INTRODUCED_IN(30);
 
 /**
  * Destroys a condition variable.
  */
-void cnd_destroy(cnd_t* __cond) __INTRODUCED_IN(30);
+void cnd_destroy(cnd_t* _Nonnull __cond) __INTRODUCED_IN(30);
 
 /**
  * Creates a condition variable.
  */
-int cnd_init(cnd_t* __cond) __INTRODUCED_IN(30);
+int cnd_init(cnd_t* _Nonnull __cond) __INTRODUCED_IN(30);
 
 /**
  * Unblocks one thread blocked on `__cond`.
  */
-int cnd_signal(cnd_t* __cond) __INTRODUCED_IN(30);
+int cnd_signal(cnd_t* _Nonnull __cond) __INTRODUCED_IN(30);
 
 /**
  * Unlocks `__mutex` and blocks until `__cond` is signaled or `__timeout` occurs.
  */
-int cnd_timedwait(cnd_t* __cond, mtx_t* __mutex, const struct timespec* __timeout)
+int cnd_timedwait(cnd_t* _Nonnull __cond, mtx_t* _Nonnull __mutex, const struct timespec* _Nonnull __timeout)
     __INTRODUCED_IN(30);
 
 /**
  * Unlocks `__mutex` and blocks until `__cond` is signaled.
  */
-int cnd_wait(cnd_t* __cond, mtx_t* __mutex) __INTRODUCED_IN(30);
+int cnd_wait(cnd_t* _Nonnull __cond, mtx_t* _Nonnull __mutex) __INTRODUCED_IN(30);
 
 
 
 /**
  * Destroys a mutex.
  */
-void mtx_destroy(mtx_t* __mutex) __INTRODUCED_IN(30);
+void mtx_destroy(mtx_t* _Nonnull __mutex) __INTRODUCED_IN(30);
 
 /**
  * Creates a mutex.
  */
-int mtx_init(mtx_t* __mutex, int __type) __INTRODUCED_IN(30);
+int mtx_init(mtx_t* _Nonnull __mutex, int __type) __INTRODUCED_IN(30);
 
 /**
  * Blocks until `__mutex` is acquired.
  */
-int mtx_lock(mtx_t* __mutex) __INTRODUCED_IN(30);
+int mtx_lock(mtx_t* _Nonnull __mutex) __INTRODUCED_IN(30);
 
 /**
  * Blocks until `__mutex` is acquired or `__timeout` expires.
  */
-int mtx_timedlock(mtx_t* __mutex, const struct timespec* __timeout) __INTRODUCED_IN(30);
+int mtx_timedlock(mtx_t* _Nonnull __mutex, const struct timespec* _Nonnull __timeout) __INTRODUCED_IN(30);
 
 /**
  * Acquires `__mutex` or returns `thrd_busy`.
  */
-int mtx_trylock(mtx_t* __mutex) __INTRODUCED_IN(30);
+int mtx_trylock(mtx_t* _Nonnull __mutex) __INTRODUCED_IN(30);
 
 /**
  * Unlocks `__mutex`.
  */
-int mtx_unlock(mtx_t* __mutex) __INTRODUCED_IN(30);
+int mtx_unlock(mtx_t* _Nonnull __mutex) __INTRODUCED_IN(30);
 
 
 
@@ -155,7 +155,7 @@
  * Creates a new thread running `__function(__arg)`, and sets `*__thrd` to
  * the new thread.
  */
-int thrd_create(thrd_t* __thrd, thrd_start_t __function, void* __arg) __INTRODUCED_IN(30);
+int thrd_create(thrd_t* _Nonnull __thrd, thrd_start_t _Nonnull __function, void* _Nullable __arg) __INTRODUCED_IN(30);
 
 /**
  * Returns the `thrd_t` corresponding to the caller.
@@ -181,7 +181,7 @@
  * Blocks until `__thrd` terminates. If `__result` is not null, `*__result`
  * is set to the exiting thread's result.
  */
-int thrd_join(thrd_t __thrd, int* __result) __INTRODUCED_IN(30);
+int thrd_join(thrd_t __thrd, int* _Nullable __result) __INTRODUCED_IN(30);
 
 /**
  * Blocks the caller for at least `__duration` unless a signal is delivered.
@@ -190,7 +190,7 @@
  *
  * Returns 0 on success, or -1 if a signal was delivered.
  */
-int thrd_sleep(const struct timespec* __duration, struct timespec* __remaining) __INTRODUCED_IN(30);
+int thrd_sleep(const struct timespec* _Nonnull __duration, struct timespec* _Nullable __remaining) __INTRODUCED_IN(30);
 
 /**
  * Request that other threads should be scheduled.
@@ -203,7 +203,7 @@
  * Creates a thread-specific storage key with the associated destructor (which
  * may be null).
  */
-int tss_create(tss_t* __key, tss_dtor_t __dtor) __INTRODUCED_IN(30);
+int tss_create(tss_t* _Nonnull __key, tss_dtor_t _Nullable __dtor) __INTRODUCED_IN(30);
 
 /**
  * Destroys a thread-specific storage key.
@@ -214,13 +214,13 @@
  * Returns the value for the current thread held in the thread-specific storage
  * identified by `__key`.
  */
-void* tss_get(tss_t __key) __INTRODUCED_IN(30);
+void* _Nullable tss_get(tss_t __key) __INTRODUCED_IN(30);
 
 /**
  * Sets the current thread's value for the thread-specific storage identified
  * by `__key` to `__value`.
  */
-int tss_set(tss_t __key, void* __value) __INTRODUCED_IN(30);
+int tss_set(tss_t __key, void* _Nonnull __value) __INTRODUCED_IN(30);
 
 #endif