diff --git a/conform/data/threads.h-data b/conform/data/threads.h-data
index bb5ca75..70b2fe0 100644
--- a/conform/data/threads.h-data
+++ b/conform/data/threads.h-data
@@ -1,5 +1,7 @@
 #if defined ISO11
 
+macro ONCE_FLAG_INIT
+
 constant thrd_success
 constant thrd_busy
 constant thrd_error
@@ -13,6 +15,7 @@ constant mtx_timed
 type thrd_t
 type thrd_start_t
 type mtx_t
+type once_flag
 
 function int thrd_create (thrd_t*, thrd_start_t, void*)
 function int thrd_equal (thrd_t, thrd_t)
@@ -30,6 +33,8 @@ function int mtx_trylock (mtx_t*)
 function int mtx_unlock (mtx_t*)
 function void mtx_destroy (mtx_t*)
 
+function void call_once (once_flag*, void (*)(void))
+
 #include "time.h-data"
 
 #endif
diff --git a/nptl/Makefile b/nptl/Makefile
index c4f6970..7d2b83f 100644
--- a/nptl/Makefile
+++ b/nptl/Makefile
@@ -142,7 +142,7 @@ libpthread-routines = nptl-init vars events version pt-interp \
 		      thrd_create thrd_current thrd_detach thrd_equal \
 		      thrd_exit thrd_join thrd_sleep thrd_yield \
 		      mtx_destroy mtx_init mtx_lock mtx_timedlock \
-		      mtx_trylock mtx_unlock
+		      mtx_trylock mtx_unlock call_once
 #		      pthread_setuid pthread_seteuid pthread_setreuid \
 #		      pthread_setresuid \
 #		      pthread_setgid pthread_setegid pthread_setregid \
diff --git a/nptl/Versions b/nptl/Versions
index 5cf581a..b055860 100644
--- a/nptl/Versions
+++ b/nptl/Versions
@@ -269,7 +269,7 @@ libpthread {
   GLIBC_2.27 {
     thrd_create; thrd_current; thrd_detach; thrd_equal; thrd_exit; thrd_join;
     thrd_sleep; thrd_yield; mtx_init; mtx_lock; mtx_timedlock; mtx_trylock;
-    mtx_unlock; mtx_destroy;
+    mtx_unlock; mtx_destroy; call_once;
   }
 
   GLIBC_PRIVATE {
diff --git a/nptl/call_once.c b/nptl/call_once.c
new file mode 100644
index 0000000..8a15f1f
--- /dev/null
+++ b/nptl/call_once.c
@@ -0,0 +1,33 @@
+/* C11 threads call once implementation.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <stdalign.h>
+
+#include "thrd_priv.h"
+
+/* Call function func exactly once, even if invoked from several threads.
+   All calls must be made with the same flag object.  */
+void
+call_once (once_flag *flag, void (*func)(void))
+{
+  _Static_assert (sizeof (once_flag) == sizeof (pthread_once_t),
+		  "once_flag size");
+  _Static_assert (alignof (once_flag) == alignof (pthread_once_t),
+		  "once_flag alignment");
+  __pthread_once (flag, func);
+}
diff --git a/sysdeps/nptl/threads.h b/sysdeps/nptl/threads.h
index cd811f4..53fe731 100644
--- a/sysdeps/nptl/threads.h
+++ b/sysdeps/nptl/threads.h
@@ -27,8 +27,11 @@ __BEGIN_DECLS
 #include <bits/pthreadtypes-arch.h>
 #include <bits/types/struct_timespec.h>
 
+#define ONCE_FLAG_INIT 0
+
 typedef unsigned long int thrd_t;
 typedef int (*thrd_start_t) (void*);
+typedef int __ONCE_ALIGNMENT once_flag;
 
 /* Exit and error codes.  */
 enum
@@ -129,6 +132,10 @@ extern int mtx_unlock (mtx_t *__mutex);
 /* Destroy the mutex object pointed by __mutex.  */
 extern void mtx_destroy (mtx_t *__mutex);
 
+/* Call function func exactly once, even if invoked from several threads.
+   All calls must be made with the same __flag object.  */
+extern void call_once (once_flag *__flag, void (*__func)(void));
+
 __END_DECLS
 
 #endif /* _THREADS_H */
