diff --git a/libc/include/sys/signal.h b/libc/include/sys/signal.h
index 2e602da..fde0819 100644
--- a/libc/include/sys/signal.h
+++ b/libc/include/sys/signal.h
@@ -1 +1,10 @@
+#pragma once
+
+/**
+ * @file sys/signal.h
+ * @brief Historical synonym for `<signal.h>`.
+ *
+ * New code should use `<signal.h>` directly.
+ */
+
 #include <signal.h>