Index: src/crc32.h
==================================================================
--- src/crc32.h
+++ src/crc32.h
@@ -24,9 +24,10 @@
 #import "macros.h"
 
 #ifdef __cplusplus
 extern "C" {
 #endif
-extern uint32_t of_crc32(uint32_t crc, unsigned char *bytes, size_t length);
+extern uint32_t of_crc32(uint32_t crc, unsigned char *_Nonnull bytes,
+    size_t length);
 #ifdef __cplusplus
 }
 #endif

Index: src/runtime/runtime-private.h
==================================================================
--- src/runtime/runtime-private.h
+++ src/runtime/runtime-private.h
@@ -17,146 +17,155 @@
 #include "config.h"
 
 #include "platform.h"
 
 struct objc_abi_class {
-	struct objc_abi_class *metaclass;
-	const char *superclass;
-	const char *name;
+	struct objc_abi_class *_Nonnull metaclass;
+	const char *_Nullable superclass;
+	const char *_Nonnull name;
 	unsigned long version;
 	unsigned long info;
 	long instance_size;
-	void *ivars;
-	struct objc_abi_method_list *methodlist;
-	void *dtable;
-	void *subclass_list;
-	void *sibling_class;
-	void *protocols;
-	void *gc_object_type;
+	void *_Nullable ivars;
+	struct objc_abi_method_list *_Nullable methodlist;
+	void *_Nullable dtable;
+	void *_Nullable subclass_list;
+	void *_Nullable sibling_class;
+	void *_Nullable protocols;
+	void *_Nullable gc_object_type;
 	long abi_version;
-	int32_t **ivar_offsets;
-	void *properties;
+	int32_t *_Nonnull *_Nullable ivar_offsets;
+	void *_Nullable properties;
+};
+
+struct objc_abi_selector {
+	const char *_Nonnull name;
+	const char *_Nullable types;
 };
 
 struct objc_abi_method {
-	const char *name;
-	const char *types;
-	IMP imp;
+	struct objc_abi_selector sel;
+	IMP _Nonnull imp;
 };
 
 struct objc_abi_method_list {
-	struct objc_abi_method_list *next;
+	struct objc_abi_method_list *_Nullable next;
 	unsigned int count;
 	struct objc_abi_method methods[1];
 };
 
-struct objc_abi_selector {
-	const char *name;
-	const char *types;
-};
-
 struct objc_abi_category {
-	const char *category_name;
-	const char *class_name;
-	struct objc_abi_method_list *instance_methods;
-	struct objc_abi_method_list *class_methods;
-	struct objc_protocol_list *protocols;
+	const char *_Nonnull category_name;
+	const char *_Nonnull class_name;
+	struct objc_abi_method_list *_Nullable instance_methods;
+	struct objc_abi_method_list *_Nullable class_methods;
+	struct objc_protocol_list *_Nullable protocols;
 };
 
 struct objc_abi_method_description {
-	const char *name;
-	const char *types;
+	const char *_Nonnull name;
+	const char *_Nonnull types;
 };
 
 struct objc_abi_method_description_list {
 	int count;
 	struct objc_abi_method_description list[1];
 };
 
 struct objc_abi_static_instances {
-	const char *class_name;
-	id instances[1];
+	const char *_Nonnull class_name;
+	id _Nullable instances[1];
 };
 
 struct objc_abi_symtab {
 	unsigned long unknown;
-	struct objc_abi_selector *sel_refs;
+	struct objc_abi_selector *_Nullable sel_refs;
 	uint16_t cls_def_cnt;
 	uint16_t cat_def_cnt;
-	void *defs[1];
+	void *_Nonnull defs[1];
 };
 
 struct objc_abi_module {
 	unsigned long version;	/* 9 = non-fragile */
 	unsigned long size;
-	const char *name;
-	struct objc_abi_symtab *symtab;
+	const char *_Nullable name;
+	struct objc_abi_symtab *_Nonnull symtab;
 };
 
 struct objc_hashtable_bucket {
-	const void *key, *obj;
+	const void *_Nonnull key, *_Nonnull obj;
 	uint32_t hash;
 };
 
 struct objc_hashtable {
-	uint32_t (*hash)(const void *key);
-	bool (*equal)(const void *key1, const void *key2);
+	uint32_t (*_Nonnull hash)(const void *_Nonnull key);
+	bool (*_Nonnull equal)(const void *_Nonnull key1,
+	    const void *_Nonnull key2);
 	uint32_t count, size;
-	struct objc_hashtable_bucket **data;
+	struct objc_hashtable_bucket *_Nonnull *_Nullable data;
 };
 
 struct objc_sparsearray {
 	struct objc_sparsearray_data {
-		void *next[256];
-	} *data;
+		void *_Nullable next[256];
+	} *_Nonnull data;
 	uint8_t index_size;
 };
 
 struct objc_dtable {
 	struct objc_dtable_level2 {
 #ifdef OF_SELUID24
 		struct objc_dtable_level3 {
-			IMP buckets[256];
-		} *buckets[256];
+			IMP _Nullable buckets[256];
+		} *_Nonnull buckets[256];
 #else
-		IMP buckets[256];
+		IMP _Nullable buckets[256];
 #endif
-	} *buckets[256];
+	} *_Nonnull buckets[256];
 };
 
-extern void objc_register_all_categories(struct objc_abi_symtab *);
-extern struct objc_category **objc_categories_for_class(Class);
+extern void objc_register_all_categories(struct objc_abi_symtab *_Nonnull);
+extern struct objc_category *_Nullable *_Nullable
+    objc_categories_for_class(Class _Nonnull);
 extern void objc_unregister_all_categories(void);
-extern void objc_initialize_class(Class);
-extern void objc_update_dtable(Class);
-extern void objc_register_all_classes(struct objc_abi_symtab *);
-extern Class objc_classname_to_class(const char *, bool);
-extern void objc_unregister_class(Class);
+extern void objc_initialize_class(Class _Nonnull);
+extern void objc_update_dtable(Class _Nonnull);
+extern void objc_register_all_classes(struct objc_abi_symtab *_Nonnull);
+extern Class _Nullable objc_classname_to_class(const char *_Nonnull, bool);
+extern void objc_unregister_class(Class _Nonnull);
 extern void objc_unregister_all_classes(void);
-extern uint32_t objc_hash_string(const void *);
-extern bool objc_equal_string(const void *, const void *);
-extern struct objc_hashtable *objc_hashtable_new(uint32_t (*)(const void *),
-    bool (*)(const void *, const void *), uint32_t);
+extern uint32_t objc_hash_string(const void *_Nonnull);
+extern bool objc_equal_string(const void *_Nonnull, const void *_Nonnull);
+extern struct objc_hashtable *_Nonnull objc_hashtable_new(
+    uint32_t (*_Nonnull)(const void *_Nonnull),
+    bool (*_Nonnull)(const void *_Nonnull, const void *_Nonnull),
+    uint32_t);
 extern struct objc_hashtable_bucket objc_deleted_bucket;
-extern void objc_hashtable_set(struct objc_hashtable *, const void *,
-    const void *);
-extern void *objc_hashtable_get(struct objc_hashtable *, const void *);
-extern void objc_hashtable_delete(struct objc_hashtable *, const void *);
-extern void objc_hashtable_free(struct objc_hashtable *);
-extern void objc_register_selector(struct objc_abi_selector *);
-extern void objc_register_all_selectors(struct objc_abi_symtab *);
+extern void objc_hashtable_set(struct objc_hashtable *_Nonnull,
+    const void *_Nonnull, const void *_Nonnull);
+extern void *_Nullable objc_hashtable_get(struct objc_hashtable *_Nonnull,
+    const void *_Nonnull);
+extern void objc_hashtable_delete(struct objc_hashtable *_Nonnull,
+    const void *_Nonnull);
+extern void objc_hashtable_free(struct objc_hashtable *_Nonnull);
+extern void objc_register_selector(struct objc_abi_selector *_Nonnull);
+extern void objc_register_all_selectors(struct objc_abi_symtab *_Nonnull);
 extern void objc_unregister_all_selectors(void);
-extern struct objc_sparsearray *objc_sparsearray_new(uint8_t);
-extern void *objc_sparsearray_get(struct objc_sparsearray *, uintptr_t);
-extern void objc_sparsearray_set(struct objc_sparsearray *, uintptr_t, void *);
-extern void objc_sparsearray_free(struct objc_sparsearray *);
-extern struct objc_dtable *objc_dtable_new(void);
-extern void objc_dtable_copy(struct objc_dtable *, struct objc_dtable *);
-extern void objc_dtable_set(struct objc_dtable *, uint32_t, IMP);
-extern void objc_dtable_free(struct objc_dtable *);
+extern struct objc_sparsearray *_Nonnull objc_sparsearray_new(uint8_t);
+extern void *_Nullable objc_sparsearray_get(struct objc_sparsearray *_Nonnull,
+    uintptr_t);
+extern void objc_sparsearray_set(struct objc_sparsearray *_Nonnull, uintptr_t,
+    void *_Nullable);
+extern void objc_sparsearray_free(struct objc_sparsearray *_Nonnull);
+extern struct objc_dtable *_Nonnull objc_dtable_new(void);
+extern void objc_dtable_copy(struct objc_dtable *_Nonnull,
+    struct objc_dtable *_Nonnull);
+extern void objc_dtable_set(struct objc_dtable *_Nonnull, uint32_t,
+    IMP _Nullable);
+extern void objc_dtable_free(struct objc_dtable *_Nonnull);
 extern void objc_dtable_cleanup(void);
-extern void objc_init_static_instances(struct objc_abi_symtab *);
+extern void objc_init_static_instances(struct objc_abi_symtab *_Nonnull);
 extern void objc_forget_pending_static_instances(void);
 #ifdef OF_HAVE_THREADS
 extern void objc_global_mutex_lock(void);
 extern void objc_global_mutex_unlock(void);
 extern void objc_global_mutex_free(void);
@@ -164,12 +173,12 @@
 # define objc_global_mutex_lock()
 # define objc_global_mutex_unlock()
 # define objc_global_mutex_free()
 #endif
 
-static inline IMP
-objc_dtable_get(const struct objc_dtable *dtable, uint32_t idx)
+static inline IMP _Nullable
+objc_dtable_get(const struct objc_dtable *_Nonnull dtable, uint32_t idx)
 {
 #ifdef OF_SELUID24
 	uint8_t i = idx >> 16;
 	uint8_t j = idx >>  8;
 	uint8_t k = idx;

Index: src/threading.h
==================================================================
--- src/threading.h
+++ src/threading.h
@@ -136,11 +136,11 @@
 }
 
 static OF_INLINE bool
 of_tlskey_set(of_tlskey_t key, void *ptr)
 {
-	return !pthread_setspecific(key, ptr);
+	return (pthread_setspecific(key, ptr) == 0);
 }
 #elif defined(OF_WINDOWS)
 static OF_INLINE void *
 of_tlskey_get(of_tlskey_t key)
 {
@@ -171,11 +171,11 @@
 {
 #if defined(OF_HAVE_ATOMIC_OPS)
 	*spinlock = 0;
 	return true;
 #elif defined(OF_HAVE_PTHREAD_SPINLOCKS)
-	return !pthread_spin_init(spinlock, 0);
+	return (pthread_spin_init(spinlock, 0) == 0);
 #else
 	return of_mutex_new(spinlock);
 #endif
 }
 
@@ -188,11 +188,11 @@
 		return true;
 	}
 
 	return false;
 #elif defined(OF_HAVE_PTHREAD_SPINLOCKS)
-	return !pthread_spin_trylock(spinlock);
+	return (pthread_spin_trylock(spinlock) == 0);
 #else
 	return of_mutex_trylock(spinlock);
 #endif
 }
 
@@ -209,11 +209,11 @@
 	while (!of_spinlock_trylock(spinlock))
 		of_thread_yield();
 
 	return true;
 #elif defined(OF_HAVE_PTHREAD_SPINLOCKS)
-	return !pthread_spin_lock(spinlock);
+	return (pthread_spin_lock(spinlock) == 0);
 #else
 	return of_mutex_lock(spinlock);
 #endif
 }
 
@@ -225,11 +225,11 @@
 
 	of_memory_barrier_release();
 
 	return ret;
 #elif defined(OF_HAVE_PTHREAD_SPINLOCKS)
-	return !pthread_spin_unlock(spinlock);
+	return (pthread_spin_unlock(spinlock) == 0);
 #else
 	return of_mutex_unlock(spinlock);
 #endif
 }
 
@@ -237,10 +237,10 @@
 of_spinlock_free(of_spinlock_t *spinlock)
 {
 #if defined(OF_HAVE_ATOMIC_OPS)
 	return true;
 #elif defined(OF_HAVE_PTHREAD_SPINLOCKS)
-	return !pthread_spin_destroy(spinlock);
+	return (pthread_spin_destroy(spinlock) == 0);
 #else
 	return of_mutex_free(spinlock);
 #endif
 }

Index: src/unicode.h
==================================================================
--- src/unicode.h
+++ src/unicode.h
@@ -22,16 +22,16 @@
 #define OF_UNICODE_CASEFOLDING_TABLE_SIZE 0x1EA
 
 #ifdef __cplusplus
 extern "C" {
 #endif
-extern const of_unichar_t *const
+extern const of_unichar_t *const _Nonnull
     of_unicode_uppercase_table[OF_UNICODE_UPPERCASE_TABLE_SIZE];
-extern const of_unichar_t *const
+extern const of_unichar_t *const _Nonnull
     of_unicode_lowercase_table[OF_UNICODE_LOWERCASE_TABLE_SIZE];
-extern const of_unichar_t *const
+extern const of_unichar_t *const _Nonnull
     of_unicode_titlecase_table[OF_UNICODE_TITLECASE_TABLE_SIZE];
-extern const of_unichar_t *const
+extern const of_unichar_t *const _Nonnull
     of_unicode_casefolding_table[OF_UNICODE_CASEFOLDING_TABLE_SIZE];
 #ifdef __cplusplus
 }
 #endif