diff --git a/libc/include/sys/swap.h b/libc/include/sys/swap.h
index e4c1819..b1f9295 100644
--- a/libc/include/sys/swap.h
+++ b/libc/include/sys/swap.h
@@ -38,8 +38,8 @@
 #define SWAP_FLAG_PRIO_MASK 0x7fff
 #define SWAP_FLAG_PRIO_SHIFT 0
 
-extern int swapon(const char*, int) __nonnull((1));
-extern int swapoff(const char*) __nonnull((1));
+extern int swapon(const char*, int) __nonnull((1)) __INTRODUCED_IN(21);
+extern int swapoff(const char*) __nonnull((1)) __INTRODUCED_IN(21);
 
 __END_DECLS