diff --git a/libc/include/sys/errno.h b/libc/include/sys/errno.h
index 339f4fc..77526a9 100644
--- a/libc/include/sys/errno.h
+++ b/libc/include/sys/errno.h
@@ -1 +1,10 @@
+#pragma once
+
+/**
+ * @file sys/errno.h
+ * @brief Historical synonym for `<errno.h>`.
+ *
+ * New code should use `<errno.h>` directly.
+ */
+
 #include <errno.h>