Index: tests/sys/sys/Makefile
===================================================================
--- tests/sys/sys/Makefile
+++ tests/sys/sys/Makefile
@@ -4,7 +4,7 @@
 
 TESTSDIR=	${TESTSBASE}/sys/sys
 
-ATF_TESTS_C=	arb_test bitstring_test qmath_test rb_test splay_test
+ATF_TESTS_C=	arb_test bitstring_test qmath_test rb_test splay_test sysctl_h_self
 
 .if ${COMPILER_TYPE} == "gcc"
 CFLAGS.bitstring_test=	-fno-strict-overflow
Index: tests/sys/sys/sysctl_h_self.c
===================================================================
--- /dev/null
+++ tests/sys/sys/sysctl_h_self.c
@@ -0,0 +1,32 @@
+/*-
+ * Copyright (c) 2021 Warner Losh <imp@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+/*
+ * Tests to make sure that sys/sysctl.h can be included by itself.
+ */
+
+#include <sys/sysctl.h>
+
+#include <atf-c.h>
+
+ATF_TC(sysctl_h_self);
+ATF_TC_HEAD(sysctl_h_self, tc)
+{
+	atf_tc_set_md_var(tc, "descr", "Test that <sys/sysctl.h> is standalone");
+}
+
+ATF_TC_BODY(sysctl_h_self, tc)
+{
+	/* Always succeeds -- this is a build test. */
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+
+	ATF_TP_ADD_TC(tp, sysctl_h_self);
+
+	return atf_no_error();
+}