diff --git a/libc/include/sys/endian.h b/libc/include/sys/endian.h
index 60cc030..449e0d7 100644
--- a/libc/include/sys/endian.h
+++ b/libc/include/sys/endian.h
@@ -49,10 +49,10 @@
 
 /* glibc compatibility. */
 __BEGIN_DECLS
-uint32_t htonl(uint32_t) __pure2;
-uint16_t htons(uint16_t) __pure2;
-uint32_t ntohl(uint32_t) __pure2;
-uint16_t ntohs(uint16_t) __pure2;
+uint32_t htonl(uint32_t) __pure2 __INTRODUCED_IN(21);
+uint16_t htons(uint16_t) __pure2 __INTRODUCED_IN(21);
+uint32_t ntohl(uint32_t) __pure2 __INTRODUCED_IN(21);
+uint16_t ntohs(uint16_t) __pure2 __INTRODUCED_IN(21);
 __END_DECLS
 
 #define htonl(x) __swap32(x)