Index: include/__threading_support
===================================================================
--- include/__threading_support
+++ include/__threading_support
@@ -194,7 +194,73 @@
     pthread_setspecific(__key, __p);
 }
 
-#else // !_LIBCPP_THREAD_API_PTHREAD
+#elif defined(_LIBCPP_THREAD_API_EXTERNAL)
+
+// Mutex
+#define _LIBCPP_MUTEX_INITIALIZER 0
+struct __libcpp_mutex_external;
+typedef __libcpp_mutex_external* __libcpp_mutex_t;
+
+int __libcpp_recursive_mutex_init(__libcpp_mutex_t* __m);
+
+int __libcpp_mutex_lock(__libcpp_mutex_t* __m);
+
+int __libcpp_mutex_trylock(__libcpp_mutex_t* __m);
+
+int __libcpp_mutex_unlock(__libcpp_mutex_t* __m);
+
+int __libcpp_mutex_destroy(__libcpp_mutex_t* __m);
+
+// Condition variable
+#define _LIBCPP_CONDVAR_INITIALIZER 0
+struct __libcpp_condvar_external;
+typedef __libcpp_condvar_external* __libcpp_condvar_t;
+
+int __libcpp_condvar_signal(__libcpp_condvar_t* __cv);
+
+int __libcpp_condvar_broadcast(__libcpp_condvar_t* __cv);
+
+int __libcpp_condvar_wait(__libcpp_condvar_t* __cv, __libcpp_mutex_t* __m);
+
+int __libcpp_condvar_timedwait(__libcpp_condvar_t* __cv, __libcpp_mutex_t* __m, timespec* __ts);
+
+int __libcpp_condvar_destroy(__libcpp_condvar_t* __cv);
+
+// Thread id
+typedef unsigned long __libcpp_thread_id;
+
+// Returns non-zero if the thread ids are equal, otherwise 0
+int __libcpp_thread_id_equal(__libcpp_thread_id t1, __libcpp_thread_id t2);
+
+// Returns non-zero if t1 < t2, otherwise 0
+int __libcpp_thread_id_less(__libcpp_thread_id t1, __libcpp_thread_id t2);
+
+// Thread
+struct __libcpp_thread_external;
+typedef __libcpp_thread_external* __libcpp_thread_t;
+
+int __libcpp_thread_create(__libcpp_thread_t* __t, void* (*__f)(void*), void* __arg);
+
+__libcpp_thread_id __libcpp_thread_get_current_id();
+
+__libcpp_thread_id __libcpp_thread_get_id(const __libcpp_thread_t* __t);
+
+int __libcpp_thread_join(__libcpp_thread_t* __t);
+
+int __libcpp_thread_detach(__libcpp_thread_t* __t);
+
+void __libcpp_thread_yield();
+
+// Thread local storage
+typedef unsigned long __libcpp_tl_key;
+
+int __libcpp_tl_create(__libcpp_tl_key* __key, void (*__at_exit) (void*));
+
+void* __libcpp_tl_get(__libcpp_tl_key __key);
+
+void __libcpp_tl_set(__libcpp_tl_key __key, void* __p);
+
+#else
   #error "No thread API selected."
 #endif
 
Index: src/algorithm.cpp
===================================================================
--- src/algorithm.cpp
+++ src/algorithm.cpp
@@ -48,14 +48,22 @@
 template unsigned __sort5<__less<long double>&, long double*>(long double*, long double*, long double*, long double*, long double*, __less<long double>&);
 
 #ifndef _LIBCPP_HAS_NO_THREADS
+# if defined(_LIBCPP_THREAD_API_EXTERNAL)
+static mutex __rs_mut;
+#  define __LOCK_LOCK() __rs_mut.lock();
+#  define __LOCK_UNLOCK() __rs_mut.unlock();
+# else
 static __libcpp_mutex_t __rs_mut = _LIBCPP_MUTEX_INITIALIZER;
+#  define __LOCK_LOCK() __libcpp_mutex_lock(&__rs_mut);
+#  define __LOCK_UNLOCK() __libcpp_mutex_unlock(&__rs_mut);
+# endif
 #endif
 unsigned __rs_default::__c_ = 0;
 
 __rs_default::__rs_default()
 {
 #ifndef _LIBCPP_HAS_NO_THREADS
-    __libcpp_mutex_lock(&__rs_mut);
+    __LOCK_LOCK()
 #endif
     __c_ = 1;
 }
@@ -69,7 +77,7 @@
 {
 #ifndef _LIBCPP_HAS_NO_THREADS
     if (--__c_ == 0)
-       __libcpp_mutex_unlock(&__rs_mut);
+       __LOCK_UNLOCK()
 #else
     --__c_;
 #endif
@@ -88,4 +96,9 @@
     return __rs_default();
 }
 
+#ifndef _LIBCPP_HAS_NO_THREADS
+# undef __LOCK_LOCK
+# undef __LOCK_UNLOCK
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
Index: src/memory.cpp
===================================================================
--- src/memory.cpp
+++ src/memory.cpp
@@ -127,6 +127,9 @@
 #if defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
 
 static const std::size_t __sp_mut_count = 16;
+#if defined(_LIBCPP_THREAD_API_EXTERNAL)
+static mutex mut_back_imp[__sp_mut_count];
+#else
 static __libcpp_mutex_t mut_back_imp[__sp_mut_count] =
 {
     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
@@ -134,6 +137,7 @@
     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER
 };
+#endif
 
 static mutex* mut_back = reinterpret_cast<std::mutex*>(mut_back_imp);
 
Index: src/mutex.cpp
===================================================================
--- src/mutex.cpp
+++ src/mutex.cpp
@@ -195,8 +195,23 @@
 // keep in sync with:  7741191.
 
 #ifndef _LIBCPP_HAS_NO_THREADS
+# if defined(_LIBCPP_THREAD_API_EXTERNAL)
+static mutex mut;
+static condition_variable cv;
+#  define __LOCK_INIT() unique_lock<mutex> lk(mut);
+#  define __LOCK_LOCK() lk.lock();
+#  define __LOCK_UNLOCK() lk.unlock();
+#  define __CV_WAIT_LOCK() cv.wait(lk);
+#  define __CV_NOTIFY_ALL() cv.notify_all();
+# else
 static __libcpp_mutex_t mut = _LIBCPP_MUTEX_INITIALIZER;
 static __libcpp_condvar_t cv = _LIBCPP_CONDVAR_INITIALIZER;
+#  define __LOCK_INIT() __libcpp_mutex_lock(&mut);
+#  define __LOCK_LOCK() __libcpp_mutex_lock(&mut);
+#  define __LOCK_UNLOCK() __libcpp_mutex_unlock(&mut);
+#  define __CV_WAIT_LOCK() __libcpp_condvar_wait(&cv, &mut);
+#  define __CV_NOTIFY_ALL() __libcpp_condvar_broadcast(&cv);
+# endif
 #endif
 
 /// NOTE: Changes to flag are done via relaxed atomic stores
@@ -225,9 +240,9 @@
 #endif  // _LIBCPP_NO_EXCEPTIONS
     }
 #else // !_LIBCPP_HAS_NO_THREADS
-    __libcpp_mutex_lock(&mut);
+    __LOCK_INIT()
     while (flag == 1)
-        __libcpp_condvar_wait(&cv, &mut);
+        __CV_WAIT_LOCK()
     if (flag == 0)
     {
 #ifndef _LIBCPP_NO_EXCEPTIONS
@@ -235,28 +250,36 @@
         {
 #endif  // _LIBCPP_NO_EXCEPTIONS
             __libcpp_relaxed_store(&flag, 1ul);
-            __libcpp_mutex_unlock(&mut);
+            __LOCK_UNLOCK()
             func(arg);
-            __libcpp_mutex_lock(&mut);
+            __LOCK_LOCK()
             __libcpp_relaxed_store(&flag, ~0ul);
-            __libcpp_mutex_unlock(&mut);
-            __libcpp_condvar_broadcast(&cv);
+            __LOCK_UNLOCK()
+            __CV_NOTIFY_ALL()
 #ifndef _LIBCPP_NO_EXCEPTIONS
         }
         catch (...)
         {
-            __libcpp_mutex_lock(&mut);
+            __LOCK_LOCK()
             __libcpp_relaxed_store(&flag, 0ul);
-            __libcpp_mutex_unlock(&mut);
-            __libcpp_condvar_broadcast(&cv);
+            __LOCK_UNLOCK()
+            __CV_NOTIFY_ALL()
             throw;
         }
 #endif  // _LIBCPP_NO_EXCEPTIONS
     }
     else
-        __libcpp_mutex_unlock(&mut);
+        __LOCK_UNLOCK()
 #endif // !_LIBCPP_HAS_NO_THREADS
 
 }
 
+#ifndef _LIBCPP_HAS_NO_THREADS
+# undef __LOCK_INIT
+# undef __LOCK_LOCK
+# undef __LOCK_UNLOCK
+# undef __CV_WAIT_LOCK
+# undef __CV_NOTIFY_ALL
+#endif
+
 _LIBCPP_END_NAMESPACE_STD