diff --git a/src/c11/impl/threads_posix.c b/src/c11/impl/threads_posix.c
index 3b8692482bb..6db8c42f08c 100644
--- a/src/c11/impl/threads_posix.c
+++ b/src/c11/impl/threads_posix.c
@@ -46,12 +46,13 @@ impl_thrd_routine(void *p)
 
 /*--------------- 7.25.2 Initialization functions ---------------*/
 // 7.25.2.1
+#ifndef __once_flag_defined
 void
 call_once(once_flag *flag, void (*func)(void))
 {
     pthread_once(flag, func);
 }
-
+#endif
 
 /*------------- 7.25.3 Condition variable functions -------------*/
 // 7.25.3.1
diff --git a/src/c11/threads.h b/src/c11/threads.h
index dbcb3459a9b..c849f385e30 100644
--- a/src/c11/threads.h
+++ b/src/c11/threads.h
@@ -118,8 +118,10 @@ typedef pthread_cond_t  cnd_t;
 typedef pthread_t       thrd_t;
 typedef pthread_key_t   tss_t;
 typedef pthread_mutex_t mtx_t;
+#ifndef __once_flag_defined
 typedef pthread_once_t  once_flag;
 #  define ONCE_FLAG_INIT PTHREAD_ONCE_INIT
+#endif
 #  ifdef PTHREAD_DESTRUCTOR_ITERATIONS
 #    define TSS_DTOR_ITERATIONS PTHREAD_DESTRUCTOR_ITERATIONS
 #  else