创建，添加sdram

rt-thread/components/net/sal/include/socket/netinet/in.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2006-2021, RT-Thread Development Team
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Change Logs:
+ * Date           Author       Notes
+ * 2015-02-17     Bernard      First version
+ */
+
+#ifndef IN_H__
+#define IN_H__
+
+#include <sys/socket.h>
+
+#ifndef IN6_IS_ADDR_MULTICAST
+#define IN6_IS_ADDR_MULTICAST(a) (((uint8_t *) (a))[0] == 0xff)
+#endif
+
+#endif