diff --git a/libc/include/stdatomic.h b/libc/include/stdatomic.h
new file mode 100644
index 0000000..43ec753
--- /dev/null
+++ b/libc/include/stdatomic.h
@@ -0,0 +1,420 @@
+/*-
+ * Copyright (c) 2011 Ed Schouten <ed@FreeBSD.org>
+ *                    David Chisnall <theraven@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#ifndef _STDATOMIC_H_
+#define	_STDATOMIC_H_
+
+#include <sys/cdefs.h>
+#include <sys/types.h>
+#include <stdbool.h>
+
+#if __has_extension(c_atomic) || __has_extension(cxx_atomic)
+#define	__CLANG_ATOMICS
+#elif __GNUC_PREREQ__(4, 7)
+#define	__GNUC_ATOMICS
+#elif defined(__GNUC__)
+#define	__SYNC_ATOMICS
+#else
+#error "stdatomic.h does not support your compiler"
+#endif
+
+/*
+ * 7.17.1 Atomic lock-free macros.
+ */
+
+#ifdef __GCC_ATOMIC_BOOL_LOCK_FREE
+#define	ATOMIC_BOOL_LOCK_FREE		__GCC_ATOMIC_BOOL_LOCK_FREE
+#endif
+#ifdef __GCC_ATOMIC_CHAR_LOCK_FREE
+#define	ATOMIC_CHAR_LOCK_FREE		__GCC_ATOMIC_CHAR_LOCK_FREE
+#endif
+#ifdef __GCC_ATOMIC_CHAR16_T_LOCK_FREE
+#define	ATOMIC_CHAR16_T_LOCK_FREE	__GCC_ATOMIC_CHAR16_T_LOCK_FREE
+#endif
+#ifdef __GCC_ATOMIC_CHAR32_T_LOCK_FREE
+#define	ATOMIC_CHAR32_T_LOCK_FREE	__GCC_ATOMIC_CHAR32_T_LOCK_FREE
+#endif
+#ifdef __GCC_ATOMIC_WCHAR_T_LOCK_FREE
+#define	ATOMIC_WCHAR_T_LOCK_FREE	__GCC_ATOMIC_WCHAR_T_LOCK_FREE
+#endif
+#ifdef __GCC_ATOMIC_SHORT_LOCK_FREE
+#define	ATOMIC_SHORT_LOCK_FREE		__GCC_ATOMIC_SHORT_LOCK_FREE
+#endif
+#ifdef __GCC_ATOMIC_INT_LOCK_FREE
+#define	ATOMIC_INT_LOCK_FREE		__GCC_ATOMIC_INT_LOCK_FREE
+#endif
+#ifdef __GCC_ATOMIC_LONG_LOCK_FREE
+#define	ATOMIC_LONG_LOCK_FREE		__GCC_ATOMIC_LONG_LOCK_FREE
+#endif
+#ifdef __GCC_ATOMIC_LLONG_LOCK_FREE
+#define	ATOMIC_LLONG_LOCK_FREE		__GCC_ATOMIC_LLONG_LOCK_FREE
+#endif
+#ifdef __GCC_ATOMIC_POINTER_LOCK_FREE
+#define	ATOMIC_POINTER_LOCK_FREE	__GCC_ATOMIC_POINTER_LOCK_FREE
+#endif
+
+/*
+ * 7.17.2 Initialization.
+ */
+
+#if defined(__CLANG_ATOMICS)
+#define	ATOMIC_VAR_INIT(value)		(value)
+#define	atomic_init(obj, value)		__c11_atomic_init(obj, value)
+#else
+#define	ATOMIC_VAR_INIT(value)		{ .__val = (value) }
+#define	atomic_init(obj, value)		((void)((obj)->__val = (value)))
+#endif
+
+/*
+ * Clang and recent GCC both provide predefined macros for the memory
+ * orderings.  If we are using a compiler that doesn't define them, use the
+ * clang values - these will be ignored in the fallback path.
+ */
+
+#ifndef __ATOMIC_RELAXED
+#define __ATOMIC_RELAXED		0
+#endif
+#ifndef __ATOMIC_CONSUME
+#define __ATOMIC_CONSUME		1
+#endif
+#ifndef __ATOMIC_ACQUIRE
+#define __ATOMIC_ACQUIRE		2
+#endif
+#ifndef __ATOMIC_RELEASE
+#define __ATOMIC_RELEASE		3
+#endif
+#ifndef __ATOMIC_ACQ_REL
+#define __ATOMIC_ACQ_REL		4
+#endif
+#ifndef __ATOMIC_SEQ_CST
+#define __ATOMIC_SEQ_CST		5
+#endif
+
+/*
+ * 7.17.3 Order and consistency.
+ *
+ * The memory_order_* constants that denote the barrier behaviour of the
+ * atomic operations.
+ */
+
+typedef enum {
+	memory_order_relaxed = __ATOMIC_RELAXED,
+	memory_order_consume = __ATOMIC_CONSUME,
+	memory_order_acquire = __ATOMIC_ACQUIRE,
+	memory_order_release = __ATOMIC_RELEASE,
+	memory_order_acq_rel = __ATOMIC_ACQ_REL,
+	memory_order_seq_cst = __ATOMIC_SEQ_CST
+} memory_order;
+
+/*
+ * 7.17.4 Fences.
+ */
+
+static __inline void
+atomic_thread_fence(memory_order __order __unused)
+{
+
+#ifdef __CLANG_ATOMICS
+	__c11_atomic_thread_fence(__order);
+#elif defined(__GNUC_ATOMICS)
+	__atomic_thread_fence(__order);
+#else
+	__sync_synchronize();
+#endif
+}
+
+static __inline void
+atomic_signal_fence(memory_order __order __unused)
+{
+
+#ifdef __CLANG_ATOMICS
+	__c11_atomic_signal_fence(__order);
+#elif defined(__GNUC_ATOMICS)
+	__atomic_signal_fence(__order);
+#else
+	__asm volatile ("" ::: "memory");
+#endif
+}
+
+/*
+ * 7.17.5 Lock-free property.
+ */
+
+#if defined(_KERNEL)
+/* Atomics in kernelspace are always lock-free. */
+#define	atomic_is_lock_free(obj) \
+	((void)(obj), (_Bool)1)
+#elif defined(__CLANG_ATOMICS)
+#define	atomic_is_lock_free(obj) \
+	__atomic_is_lock_free(sizeof(*(obj)), obj)
+#elif defined(__GNUC_ATOMICS)
+#define	atomic_is_lock_free(obj) \
+	__atomic_is_lock_free(sizeof((obj)->__val), &(obj)->__val)
+#else
+#define	atomic_is_lock_free(obj) \
+	((void)(obj), sizeof((obj)->__val) <= sizeof(void *))
+#endif
+
+/*
+ * 7.17.6 Atomic integer types.
+ */
+
+#if !__has_extension(c_atomic) && !__has_extension(cxx_atomic)
+/*
+ * No native support for _Atomic(). Place object in structure to prevent
+ * most forms of direct non-atomic access.
+ */
+#define _Atomic(T)              struct { T volatile __val; }
+#endif
+
+typedef _Atomic(bool)			atomic_bool;
+typedef _Atomic(char)			atomic_char;
+typedef _Atomic(signed char)		atomic_schar;
+typedef _Atomic(unsigned char)		atomic_uchar;
+typedef _Atomic(short)			atomic_short;
+typedef _Atomic(unsigned short)		atomic_ushort;
+typedef _Atomic(int)			atomic_int;
+typedef _Atomic(unsigned int)		atomic_uint;
+typedef _Atomic(long)			atomic_long;
+typedef _Atomic(unsigned long)		atomic_ulong;
+typedef _Atomic(long long)		atomic_llong;
+typedef _Atomic(unsigned long long)	atomic_ullong;
+typedef _Atomic(char16_t)		atomic_char16_t;
+typedef _Atomic(char32_t)		atomic_char32_t;
+typedef _Atomic(wchar_t)		atomic_wchar_t;
+typedef _Atomic(int_least8_t)		atomic_int_least8_t;
+typedef _Atomic(uint_least8_t)	atomic_uint_least8_t;
+typedef _Atomic(int_least16_t)	atomic_int_least16_t;
+typedef _Atomic(uint_least16_t)	atomic_uint_least16_t;
+typedef _Atomic(int_least32_t)	atomic_int_least32_t;
+typedef _Atomic(uint_least32_t)	atomic_uint_least32_t;
+typedef _Atomic(int_least64_t)	atomic_int_least64_t;
+typedef _Atomic(uint_least64_t)	atomic_uint_least64_t;
+typedef _Atomic(int_fast8_t)		atomic_int_fast8_t;
+typedef _Atomic(uint_fast8_t)		atomic_uint_fast8_t;
+typedef _Atomic(int_fast16_t)		atomic_int_fast16_t;
+typedef _Atomic(uint_fast16_t)	atomic_uint_fast16_t;
+typedef _Atomic(int_fast32_t)		atomic_int_fast32_t;
+typedef _Atomic(uint_fast32_t)	atomic_uint_fast32_t;
+typedef _Atomic(int_fast64_t)		atomic_int_fast64_t;
+typedef _Atomic(uint_fast64_t)	atomic_uint_fast64_t;
+typedef _Atomic(intptr_t)		atomic_intptr_t;
+typedef _Atomic(uintptr_t)		atomic_uintptr_t;
+typedef _Atomic(size_t)		atomic_size_t;
+typedef _Atomic(ptrdiff_t)		atomic_ptrdiff_t;
+typedef _Atomic(intmax_t)		atomic_intmax_t;
+typedef _Atomic(uintmax_t)		atomic_uintmax_t;
+
+/*
+ * 7.17.7 Operations on atomic types.
+ */
+
+/*
+ * Compiler-specific operations.
+ */
+
+#if defined(__CLANG_ATOMICS)
+#define	atomic_compare_exchange_strong_explicit(object, expected,	\
+    desired, success, failure)						\
+	__c11_atomic_compare_exchange_strong(object, expected, desired,	\
+	    success, failure)
+#define	atomic_compare_exchange_weak_explicit(object, expected,		\
+    desired, success, failure)						\
+	__c11_atomic_compare_exchange_weak(object, expected, desired,	\
+	    success, failure)
+#define	atomic_exchange_explicit(object, desired, order)		\
+	__c11_atomic_exchange(object, desired, order)
+#define	atomic_fetch_add_explicit(object, operand, order)		\
+	__c11_atomic_fetch_add(object, operand, order)
+#define	atomic_fetch_and_explicit(object, operand, order)		\
+	__c11_atomic_fetch_and(object, operand, order)
+#define	atomic_fetch_or_explicit(object, operand, order)		\
+	__c11_atomic_fetch_or(object, operand, order)
+#define	atomic_fetch_sub_explicit(object, operand, order)		\
+	__c11_atomic_fetch_sub(object, operand, order)
+#define	atomic_fetch_xor_explicit(object, operand, order)		\
+	__c11_atomic_fetch_xor(object, operand, order)
+#define	atomic_load_explicit(object, order)				\
+	__c11_atomic_load(object, order)
+#define	atomic_store_explicit(object, desired, order)			\
+	__c11_atomic_store(object, desired, order)
+#elif defined(__GNUC_ATOMICS)
+#define	atomic_compare_exchange_strong_explicit(object, expected,	\
+    desired, success, failure)						\
+	__atomic_compare_exchange_n(&(object)->__val, expected,		\
+	    desired, 0, success, failure)
+#define	atomic_compare_exchange_weak_explicit(object, expected,		\
+    desired, success, failure)						\
+	__atomic_compare_exchange_n(&(object)->__val, expected,		\
+	    desired, 1, success, failure)
+#define	atomic_exchange_explicit(object, desired, order)		\
+	__atomic_exchange_n(&(object)->__val, desired, order)
+#define	atomic_fetch_add_explicit(object, operand, order)		\
+	__atomic_fetch_add(&(object)->__val, operand, order)
+#define	atomic_fetch_and_explicit(object, operand, order)		\
+	__atomic_fetch_and(&(object)->__val, operand, order)
+#define	atomic_fetch_or_explicit(object, operand, order)		\
+	__atomic_fetch_or(&(object)->__val, operand, order)
+#define	atomic_fetch_sub_explicit(object, operand, order)		\
+	__atomic_fetch_sub(&(object)->__val, operand, order)
+#define	atomic_fetch_xor_explicit(object, operand, order)		\
+	__atomic_fetch_xor(&(object)->__val, operand, order)
+#define	atomic_load_explicit(object, order)				\
+	__atomic_load_n(&(object)->__val, order)
+#define	atomic_store_explicit(object, desired, order)			\
+	__atomic_store_n(&(object)->__val, desired, order)
+#else
+#define	__atomic_apply_stride(object, operand) \
+	(((__typeof__((object)->__val))0) + (operand))
+#define	atomic_compare_exchange_strong_explicit(object, expected,	\
+    desired, success, failure)	__extension__ ({			\
+	__typeof__(expected) __ep = (expected);				\
+	__typeof__(*__ep) __e = *__ep;					\
+	(void)(success); (void)(failure);				\
+	(bool)((*__ep = __sync_val_compare_and_swap(&(object)->__val,	\
+	    __e, desired)) == __e);					\
+})
+#define	atomic_compare_exchange_weak_explicit(object, expected,		\
+    desired, success, failure)						\
+	atomic_compare_exchange_strong_explicit(object, expected,	\
+		desired, success, failure)
+#if __has_builtin(__sync_swap)
+/* Clang provides a full-barrier atomic exchange - use it if available. */
+#define	atomic_exchange_explicit(object, desired, order)		\
+	((void)(order), __sync_swap(&(object)->__val, desired))
+#else
+/*
+ * __sync_lock_test_and_set() is only an acquire barrier in theory (although in
+ * practice it is usually a full barrier) so we need an explicit barrier before
+ * it.
+ */
+#define	atomic_exchange_explicit(object, desired, order)		\
+__extension__ ({							\
+	__typeof__(object) __o = (object);				\
+	__typeof__(desired) __d = (desired);				\
+	(void)(order);							\
+	__sync_synchronize();						\
+	__sync_lock_test_and_set(&(__o)->__val, __d);			\
+})
+#endif
+#define	atomic_fetch_add_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_add(&(object)->__val,		\
+	    __atomic_apply_stride(object, operand)))
+#define	atomic_fetch_and_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_and(&(object)->__val, operand))
+#define	atomic_fetch_or_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_or(&(object)->__val, operand))
+#define	atomic_fetch_sub_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_sub(&(object)->__val,		\
+	    __atomic_apply_stride(object, operand)))
+#define	atomic_fetch_xor_explicit(object, operand, order)		\
+	((void)(order), __sync_fetch_and_xor(&(object)->__val, operand))
+#define	atomic_load_explicit(object, order)				\
+	((void)(order), __sync_fetch_and_add(&(object)->__val, 0))
+#define	atomic_store_explicit(object, desired, order)			\
+	((void)atomic_exchange_explicit(object, desired, order))
+#endif
+
+/*
+ * Convenience functions.
+ *
+ * Don't provide these in kernel space. In kernel space, we should be
+ * disciplined enough to always provide explicit barriers.
+ */
+
+#ifndef _KERNEL
+#define	atomic_compare_exchange_strong(object, expected, desired)	\
+	atomic_compare_exchange_strong_explicit(object, expected,	\
+	    desired, memory_order_seq_cst, memory_order_seq_cst)
+#define	atomic_compare_exchange_weak(object, expected, desired)		\
+	atomic_compare_exchange_weak_explicit(object, expected,		\
+	    desired, memory_order_seq_cst, memory_order_seq_cst)
+#define	atomic_exchange(object, desired)				\
+	atomic_exchange_explicit(object, desired, memory_order_seq_cst)
+#define	atomic_fetch_add(object, operand)				\
+	atomic_fetch_add_explicit(object, operand, memory_order_seq_cst)
+#define	atomic_fetch_and(object, operand)				\
+	atomic_fetch_and_explicit(object, operand, memory_order_seq_cst)
+#define	atomic_fetch_or(object, operand)				\
+	atomic_fetch_or_explicit(object, operand, memory_order_seq_cst)
+#define	atomic_fetch_sub(object, operand)				\
+	atomic_fetch_sub_explicit(object, operand, memory_order_seq_cst)
+#define	atomic_fetch_xor(object, operand)				\
+	atomic_fetch_xor_explicit(object, operand, memory_order_seq_cst)
+#define	atomic_load(object)						\
+	atomic_load_explicit(object, memory_order_seq_cst)
+#define	atomic_store(object, desired)					\
+	atomic_store_explicit(object, desired, memory_order_seq_cst)
+#endif /* !_KERNEL */
+
+/*
+ * 7.17.8 Atomic flag type and operations.
+ *
+ * XXX: Assume atomic_bool can be used as an atomic_flag. Is there some
+ * kind of compiler built-in type we could use?
+ */
+
+typedef struct {
+	atomic_bool	__flag;
+} atomic_flag;
+
+#define	ATOMIC_FLAG_INIT		{ ATOMIC_VAR_INIT(0) }
+
+static __inline bool
+atomic_flag_test_and_set_explicit(volatile atomic_flag *__object,
+    memory_order __order)
+{
+	return (atomic_exchange_explicit(&__object->__flag, 1, __order));
+}
+
+static __inline void
+atomic_flag_clear_explicit(volatile atomic_flag *__object, memory_order __order)
+{
+
+	atomic_store_explicit(&__object->__flag, 0, __order);
+}
+
+#ifndef _KERNEL
+static __inline bool
+atomic_flag_test_and_set(volatile atomic_flag *__object)
+{
+
+	return (atomic_flag_test_and_set_explicit(__object,
+	    memory_order_seq_cst));
+}
+
+static __inline void
+atomic_flag_clear(volatile atomic_flag *__object)
+{
+
+	atomic_flag_clear_explicit(__object, memory_order_seq_cst);
+}
+#endif /* !_KERNEL */
+
+#endif /* !_STDATOMIC_H_ */
diff --git a/libc/include/sys/cdefs.h b/libc/include/sys/cdefs.h
index 685de32..0286616 100644
--- a/libc/include/sys/cdefs.h
+++ b/libc/include/sys/cdefs.h
@@ -38,6 +38,24 @@
 #define	_SYS_CDEFS_H_
 
 /*
+ * Testing against Clang-specific extensions.
+ */
+
+#ifndef __has_extension
+#define __has_extension         __has_feature
+#endif
+#ifndef __has_feature
+#define __has_feature(x)        0
+#endif
+#ifndef __has_include
+#define __has_include(x)        0
+#endif
+#ifndef __has_builtin
+#define __has_builtin(x)        0
+#endif
+
+
+/*
  * Macro to test if we're using a GNU C compiler of a specific vintage
  * or later, for e.g. features that appeared in a particular version
  * of GNU C.  Usage: