diff --git a/etc/mtree/BSD.tests.dist b/etc/mtree/BSD.tests.dist
--- a/etc/mtree/BSD.tests.dist
+++ b/etc/mtree/BSD.tests.dist
@@ -278,6 +278,8 @@
             ..
         ..
     ..
+    include
+    ..
     lib
         atf
             libatf-c
diff --git a/tests/Makefile b/tests/Makefile
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -9,6 +9,7 @@
 
 SUBDIR+= etc
 SUBDIR+= examples
+SUBDIR+= include
 SUBDIR+= sys
 SUBDIR+= atf_python
 
diff --git a/tests/include/Makefile b/tests/include/Makefile
new file mode 100644
--- /dev/null
+++ b/tests/include/Makefile
@@ -0,0 +1,7 @@
+.include <bsd.own.mk>
+
+TESTSDIR=	${TESTSBASE}/include
+
+ATF_TESTS_C+=		stdckdint_test
+
+.include <bsd.test.mk>
diff --git a/tests/include/stdckdint_test.c b/tests/include/stdckdint_test.c
new file mode 100644
--- /dev/null
+++ b/tests/include/stdckdint_test.c
@@ -0,0 +1,52 @@
+/*-
+ * Copyright (c) 2023 Dag-Erling Smørgrav
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <limits.h>
+#include <stdckdint.h>
+
+#include <atf-c.h>
+
+ATF_TC_WITHOUT_HEAD(ckd_add);
+ATF_TC_BODY(ckd_add, tc)
+{
+	int result;
+
+	ATF_CHECK(!ckd_add(&result, INT_MAX, 0));
+	ATF_CHECK_EQ(INT_MAX, result);
+	ATF_CHECK(ckd_add(&result, INT_MAX, 1));
+	ATF_CHECK_EQ(INT_MIN, result);
+}
+
+ATF_TC_WITHOUT_HEAD(ckd_sub);
+ATF_TC_BODY(ckd_sub, tc)
+{
+	int result;
+
+	ATF_CHECK(!ckd_sub(&result, INT_MIN, 0));
+	ATF_CHECK_EQ(INT_MIN, result);
+	ATF_CHECK(ckd_sub(&result, INT_MIN, 1));
+	ATF_CHECK_EQ(INT_MAX, result);
+}
+
+ATF_TC_WITHOUT_HEAD(ckd_mul);
+ATF_TC_BODY(ckd_mul, tc)
+{
+	int result;
+
+	ATF_CHECK(!ckd_mul(&result, INT_MAX / 2, 2));
+	ATF_CHECK_EQ(INT_MAX - 1, result);
+	ATF_CHECK(ckd_mul(&result, INT_MAX / 2 + 1, 2));
+	ATF_CHECK_EQ(INT_MIN, result);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	ATF_TP_ADD_TC(tp, ckd_add);
+	ATF_TP_ADD_TC(tp, ckd_sub);
+	ATF_TP_ADD_TC(tp, ckd_mul);
+	return (atf_no_error());
+
+}