diff --git a/include/uv-private/uv-unix.h b/include/uv-private/uv-unix.h
index 34246a8f..99537347 100644
--- a/include/uv-private/uv-unix.h
+++ b/include/uv-private/uv-unix.h
@@ -44,6 +44,9 @@ typedef struct {
 
 typedef int uv_file;
 
+#define UV_ONCE_INIT PTHREAD_ONCE_INIT
+
+typedef pthread_once_t uv_once_t;
 typedef pthread_t uv_thread_t;
 typedef pthread_mutex_t uv_mutex_t;
 typedef pthread_rwlock_t uv_rwlock_t;
diff --git a/include/uv-private/uv-win.h b/include/uv-private/uv-win.h
index a87ede73..b4b89096 100644
--- a/include/uv-private/uv-win.h
+++ b/include/uv-private/uv-win.h
@@ -152,6 +152,16 @@ typedef union {
   } fallback_;
 } uv_rwlock_t;
 
+#define UV_ONCE_INIT { 0, NULL, NULL }
+
+typedef struct uv_once_s {
+  unsigned char ran;
+  /* The actual event handle must be aligned to sizeof(HANDLE), so in */
+  /* practice it might overlap padding a little. */
+  HANDLE event;
+  HANDLE padding;
+} uv_once_t;
+
 /* Platform-specific definitions for uv_dlopen support. */
 typedef HMODULE uv_lib_t;
 #define UV_DYNAMIC FAR WINAPI
diff --git a/include/uv.h b/include/uv.h
index 4d2417c5..7e089ef7 100644
--- a/include/uv.h
+++ b/include/uv.h
@@ -1339,6 +1339,12 @@ UV_EXTERN void uv_rwlock_wrlock(uv_rwlock_t* rwlock);
 UV_EXTERN int uv_rwlock_trywrlock(uv_rwlock_t* rwlock);
 UV_EXTERN void uv_rwlock_wrunlock(uv_rwlock_t* rwlock);
 
+/* Runs a function once and only once. Concurrent calls to uv_once() with the
+ * same guard will block all callers except one (it's unspecified which one).
+ * The guard should be initialized statically with the UV_ONCE_INIT macro.
+ */
+UV_EXTERN void uv_once(uv_once_t* guard, void (*callback)(void));
+
 UV_EXTERN int uv_thread_create(uv_thread_t *tid,
     void (*entry)(void *arg), void *arg);
 UV_EXTERN int uv_thread_join(uv_thread_t *tid);
diff --git a/src/unix/thread.c b/src/unix/thread.c
index 3cf9ac80..9a6b3d1b 100644
--- a/src/unix/thread.c
+++ b/src/unix/thread.c
@@ -151,3 +151,8 @@ int uv_rwlock_trywrlock(uv_rwlock_t* rwlock) {
 void uv_rwlock_wrunlock(uv_rwlock_t* rwlock) {
   CHECK(pthread_rwlock_unlock(rwlock));
 }
+
+
+void uv_once(uv_once_t* guard, void (*callback)(void)) {
+  CHECK(pthread_once(guard, callback));
+}
diff --git a/src/win/internal.h b/src/win/internal.h
index deb8972c..e7d6c974 100644
--- a/src/win/internal.h
+++ b/src/win/internal.h
@@ -343,22 +343,4 @@ extern int uv_allow_ipv6;
 extern struct sockaddr_in uv_addr_ip4_any_;
 extern struct sockaddr_in6 uv_addr_ip6_any_;
 
-
-/*
- * Threads and synchronization
- */
-typedef struct uv_once_s {
-  unsigned char ran;
-  /* The actual event handle must be aligned to sizeof(HANDLE), so in */
-  /* practice it might overlap padding a little. */
-  HANDLE event;
-  HANDLE padding;
-} uv_once_t;
-
-#define UV_ONCE_INIT \
-  { 0, NULL, NULL }
-
-void uv_once(uv_once_t* guard, void (*callback)(void));
-
-
 #endif /* UV_WIN_INTERNAL_H_ */