diff --git a/lib/libc/tests/stdlib/Makefile b/lib/libc/tests/stdlib/Makefile
index bb9542b185e..a8209209e7e 100644
--- a/lib/libc/tests/stdlib/Makefile
+++ b/lib/libc/tests/stdlib/Makefile
@@ -2,6 +2,7 @@
 
 .include <src.opts.mk>
 
+ATF_TESTS_C+=		dynthr_test
 ATF_TESTS_C+=		heapsort_test
 ATF_TESTS_C+=		mergesort_test
 ATF_TESTS_C+=		qsort_test
@@ -13,6 +14,10 @@ ATF_TESTS_CXX+=		cxa_thread_atexit_test
 ATF_TESTS_CXX+=		cxa_thread_atexit_nothr_test
 .endif
 
+dynthr_mod.so: dynthr_test.c
+	${CC} -Wall -DMODULE -fPIC -shared -pthread -o ${.TARGET} \
+		${.CURDIR}/dynthr_test.c
+
 # Not sure why this isn't defined for all architectures, since most
 # have long double.
 .if ${MACHINE_CPUARCH} == "aarch64" || \
@@ -63,3 +68,6 @@ LIBADD.${t}+=	netbsd util
 LIBADD.strtod_test+=		m
 
 .include <bsd.test.mk>
+# Placed here because bsd.test.mk is where PACKAGE gets defaulted, if not
+# otherwise defined.
+${PACKAGE}FILES+=	dynthr_mod.so
diff --git a/lib/libc/tests/stdlib/dynthr_test.c b/lib/libc/tests/stdlib/dynthr_test.c
new file mode 100644
index 00000000000..cb984ea0d64
--- /dev/null
+++ b/lib/libc/tests/stdlib/dynthr_test.c
@@ -0,0 +1,115 @@
+/*
+ * Copyright (C) 2019 Andrew Gierth
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Though this file is initially distributed under the 2-clause BSD license,
+ * the author grants permission for its redistribution under alternative
+ * licenses as set forth at <https://rhodiumtoad.github.io/RELICENSE.txt>.
+ * This paragraph and the RELICENSE.txt file are not part of the license and
+ * may be omitted in redistributions.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdarg.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <dlfcn.h>
+
+#ifdef MODULE
+#include <pthread.h>
+#else
+#include <atf-c.h>
+#endif
+
+#define say(...)
+#define die(...) _exit(1)
+
+typedef void (modfunc_t)(int op);
+
+#ifdef MODULE
+
+static void *mod_thread(void *ptr)
+{
+	char *volatile dummy;
+	dummy = malloc(500);
+	dummy = malloc(500);
+	return NULL;
+}
+
+modfunc_t mod_main;
+
+static pthread_t thr;
+
+void mod_main(int op)
+{
+	char *volatile dummy;
+	switch (op)
+	{
+		case 1:
+		{
+			int rc = pthread_create(&thr, NULL, mod_thread, NULL);
+			if (rc)
+				die("thread creation failed: %s", strerror(rc));
+			dummy = malloc(500);
+			say("thread started");
+			return;
+		}
+		case 0:
+		{
+			say("waiting for thread");
+			pthread_join(thr, NULL);
+			say("thread stopped");
+			return;
+		}
+	}
+}
+
+#else
+
+ATF_TC(maintc);
+ATF_TC_HEAD(maintc, tc)
+{
+	atf_tc_set_md_var(tc, "timeout", "5");
+}
+
+ATF_TC_BODY(maintc, tc)
+{
+	const char *srcdir;
+	char *libpath;
+	char *volatile dummy;
+	say("starting");
+
+	srcdir = atf_tc_get_config_var(tc, "srcdir");
+#ifdef PRELOAD_LIBTHR
+	void *thr_handle = dlopen("libthr.so", RTLD_GLOBAL|RTLD_NOW);
+	if (!thr_handle)
+		atf_tc_fail("failed to open libthr.so: %s", dlerror());
+#endif
+	if (asprintf(&libpath, "%s/dynthr_mod.so", srcdir) < 0)
+		atf_tc_fail("failed to construct path to libthr");
+	void *mod_handle = dlopen(libpath, RTLD_LOCAL);
+	free(libpath);
+	if (!mod_handle)
+		atf_tc_fail("failed to open dynthr_mod.so: %s", dlerror());
+	say("module loaded");
+	dlfunc_t rawfunc = dlfunc(mod_handle, "mod_main");
+	if (!rawfunc)
+		atf_tc_fail("failed to resolve function mod_main");
+	say("invoking module");
+	modfunc_t *func = (modfunc_t *) rawfunc;
+	func(1);
+	dummy = malloc(500);
+	say("done; invoking module stop");
+	func(0);
+	say("stopped successfully");
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	ATF_TP_ADD_TC(tp, maintc);
+
+	return atf_no_error();
+}
+#endif