diff --git a/gnu/fcntl.in.h b/gnu/fcntl.in.h
index 95298de..a9a8ddd 100644
--- a/gnu/fcntl.in.h
+++ b/gnu/fcntl.in.h
@@ -216,6 +216,10 @@ _GL_WARN_ON_USE (openat, "openat is not portable - "
 # define O_SYNC 0
 #endif
 
+#ifndef __O_SYNC
+# define __O_SYNC 04000000
+#endif
+
 #ifndef O_TTY_INIT
 # define O_TTY_INIT 0
 #endif