Index: include/__os_support
===================================================================
--- include/__os_support
+++ include/__os_support
@@ -186,7 +186,71 @@
     pthread_setspecific(__key, __p);
 }
 
-#else // !_LIBCPP_THREAD_API_PTHREAD
+#elif defined(_LIBCPP_THREAD_API_EXTERNAL)
+
+// Mutex
+#define __OS_MUTEX_INITIALIZER 0
+struct __libcpp_mutex_external;
+typedef __libcpp_mutex_external* __libcpp_mutex;
+
+int __os_mutex_init(__libcpp_mutex* __m, bool is_recursive);
+
+int __os_mutex_lock(__libcpp_mutex* __m);
+
+int __os_mutex_trylock(__libcpp_mutex* __m);
+
+int __os_mutex_unlock(__libcpp_mutex* __m);
+
+int __os_mutex_destroy(__libcpp_mutex* __m);
+
+// Condition variable
+#define __OS_COND_INITIALIZER 0
+struct __libcpp_condvar_external;
+typedef __libcpp_condvar_external* __libcpp_condvar;
+
+int __os_condvar_signal(__libcpp_condvar* __cv);
+
+int __os_condvar_broadcast(__libcpp_condvar* __cv);
+
+int __os_condvar_wait(__libcpp_condvar* __cv, __libcpp_mutex* __m);
+
+int __os_condvar_timedwait(__libcpp_condvar* __cv, __libcpp_mutex* __m, timespec* __ts);
+
+int __os_condvar_destroy(__libcpp_condvar* __cv);
+
+// Thread id
+typedef unsigned long __libcpp_thread_id;
+
+int __os_thread_id_compare(__libcpp_thread_id t1, __libcpp_thread_id t2);
+
+// Thread
+struct __libcpp_thread_external;
+typedef __libcpp_thread_external* __libcpp_thread;
+
+template<class _Func, class _Arg>
+int __os_thread_create(__libcpp_thread* __t, _Func&& __f, _Arg&& __arg);
+
+__libcpp_thread_id __os_thread_get_current_id();
+
+__libcpp_thread_id __os_thread_get_id(const __libcpp_thread* __t);
+
+int __os_thread_join(__libcpp_thread* __t);
+
+int __os_thread_detach(__libcpp_thread* __t);
+
+void __os_thread_yield();
+
+// Thread local storage
+typedef unsigned long __libcpp_tl_key;
+
+template<class _Func>
+int __os_tl_create(__libcpp_tl_key* __key, _Func&& __at_exit);
+
+void* __os_tl_get(__libcpp_tl_key __key);
+
+void __os_tl_set(__libcpp_tl_key __key, void* __p);
+
+#else
   #error "No thread API selected."
 #endif