diff --git a/libc/include/sys/un.h b/libc/include/sys/un.h
index 3e1c429..03ef5e4 100644
--- a/libc/include/sys/un.h
+++ b/libc/include/sys/un.h
@@ -33,4 +33,9 @@
 #include <linux/un.h>
 #include <sys/cdefs.h>
 
+#if defined(__USE_BSD) || defined(__USE_GNU)
+#include <string.h>
+#define SUN_LEN(__ptr) (offsetof(struct sockaddr_un, sun_path) + strlen((__ptr)->sun_path))
+#endif
+
 #endif