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
@@ -318,6 +318,8 @@
             ..
             dynamicpie
             ..
+            errno
+            ..
             static
             ..
         ..
diff --git a/lib/csu/tests/Makefile b/lib/csu/tests/Makefile
--- a/lib/csu/tests/Makefile
+++ b/lib/csu/tests/Makefile
@@ -4,6 +4,7 @@
 TESTS_SUBDIRS=	dynamic
 TESTS_SUBDIRS+=	dynamiclib
 TESTS_SUBDIRS+=	dynamicpie
+TESTS_SUBDIRS+=	errno
 TESTS_SUBDIRS+=	static
 
 SUBDIR_DEPEND_dynamiclib=dso
diff --git a/lib/csu/tests/errno/Makefile b/lib/csu/tests/errno/Makefile
new file mode 100644
--- /dev/null
+++ b/lib/csu/tests/errno/Makefile
@@ -0,0 +1,18 @@
+PLAIN_TESTS_C=	errno_test		\
+		errno_static_test	\
+		errno_thr_test		\
+		errno_thr_static_test
+
+SRCS.errno_static_test= errno_test.c
+LDFLAGS.errno_static_test= -static
+
+SRCS.errno_thr_test= errno_test.c
+LIBADD.errno_thr_test= pthread
+
+SRCS.errno_thr_static_test= errno_test.c
+LDFLAGS.errno_thr_static_test= -static
+LIBADD.errno_thr_static_test= pthread
+
+MK_PIE:= no
+
+.include <bsd.test.mk>
diff --git a/lib/csu/tests/errno/errno_test.c b/lib/csu/tests/errno/errno_test.c
new file mode 100644
--- /dev/null
+++ b/lib/csu/tests/errno/errno_test.c
@@ -0,0 +1,23 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2025 Mark Johnston <markj@FreeBSD.org>
+ */
+
+#include <errno.h>
+#include <stdlib.h>
+
+static void __attribute__((constructor))
+f(void)
+{
+	errno = 42;
+}
+
+int
+main(void)
+{
+	/* errno must be zero upon program startup. */
+	if (errno != 0)
+		exit(1);
+	exit(0);
+}