diff --git a/lib/libc/tests/stdio/Makefile b/lib/libc/tests/stdio/Makefile
index f7383e5b3ef4..2a8a4e69a6c4 100644
--- a/lib/libc/tests/stdio/Makefile
+++ b/lib/libc/tests/stdio/Makefile
@@ -1,48 +1,49 @@
 
 .include <bsd.own.mk>
 
 ATF_TESTS_C+=		eintr_test
 ATF_TESTS_C+=		fdopen_test
 ATF_TESTS_C+=		fmemopen2_test
 ATF_TESTS_C+=		fopen2_test
 ATF_TESTS_C+=		freopen_test
 ATF_TESTS_C+=		getdelim_test
 ATF_TESTS_C+=		gets_s_test
 ATF_TESTS_C+=		mkostemp_test
 ATF_TESTS_C+=		open_memstream2_test
 ATF_TESTS_C+=		open_wmemstream_test
 ATF_TESTS_C+=		perror_test
 ATF_TESTS_C+=		print_positional_test
 ATF_TESTS_C+=		printbasic_test
 ATF_TESTS_C+=		printfloat_test
 ATF_TESTS_C+=		scanfloat_test
 ATF_TESTS_C+=		snprintf_test
 ATF_TESTS_C+=		sscanf_test
+ATF_TESTS_C+=		swprintf_test
 ATF_TESTS_C+=		swscanf_test
 
 SRCS.fopen2_test=	fopen_test.c
 
 NETBSD_ATF_TESTS_C=	clearerr_test
 NETBSD_ATF_TESTS_C+=	fflush_test
 NETBSD_ATF_TESTS_C+=	fmemopen_test
 NETBSD_ATF_TESTS_C+=	fopen_test
 NETBSD_ATF_TESTS_C+=	fputc_test
 NETBSD_ATF_TESTS_C+=	mktemp_test
 NETBSD_ATF_TESTS_C+=	open_memstream_test
 NETBSD_ATF_TESTS_C+=	popen_test
 NETBSD_ATF_TESTS_C+=	printf_test
 NETBSD_ATF_TESTS_C+=	scanf_test
 
 LIBADD.eintr_test+=	 md
 LIBADD.printfloat_test+= m
 LIBADD.scanfloat_test+=  m
 
 .if ${COMPILER_TYPE} == "gcc"
 # 90: use of assignment suppression and length modifier together in scanf format
 PROG_OVERRIDE_VARS+=	NO_WFORMAT
 NO_WFORMAT.scanfloat_test=
 .endif
 
 .include "../Makefile.netbsd-tests"
 
 .include <bsd.test.mk>
diff --git a/lib/libc/tests/stdio/snprintf_test.c b/lib/libc/tests/stdio/snprintf_test.c
index 0c335bc1a512..10d938435b96 100644
--- a/lib/libc/tests/stdio/snprintf_test.c
+++ b/lib/libc/tests/stdio/snprintf_test.c
@@ -1,138 +1,139 @@
 /*-
  * Copyright (c) 2023 Dag-Erling Smørgrav
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <assert.h>
 #include <limits.h>
 #include <locale.h>
 #include <stdint.h>
 #include <stdio.h>
 
 #include <atf-c.h>
 
-#define SNPRINTF_TEST(output, format, ...) \
+#ifndef nitems
+#define nitems(a) (sizeof(a) / sizeof(a[0]))
+#endif
+
+#define SNPRINTF_TEST(output, format, ...)				\
 	do {								\
 		char buf[256];						\
-		assert(output == NULL || strlen(output) < sizeof(buf));	\
-		int ret = snprintf(buf, sizeof(buf), format, __VA_ARGS__); \
-		if (output == NULL) {					\
-			ATF_CHECK_EQ(-1, ret);				\
-		} else {						\
-			ATF_CHECK_EQ(strlen(output), ret);		\
-			if (ret > 0) {					\
-				ATF_CHECK_STREQ(output, buf);		\
-			}						\
+		assert(strlen(output) < nitems(buf));			\
+		int ret = snprintf(buf, nitems(buf), format,		\
+		    __VA_ARGS__);					\
+		ATF_CHECK_EQ(strlen(output), ret);			\
+		if (ret > 0) {						\
+			ATF_CHECK_EQ(0, strcmp(output, buf));		\
 		}							\
 	} while (0)
 
 ATF_TC_WITHOUT_HEAD(snprintf_b);
 ATF_TC_BODY(snprintf_b, tc)
 {
 	SNPRINTF_TEST("0", "%b", 0);
 	SNPRINTF_TEST("           0", "%12b", 0);
 	SNPRINTF_TEST("000000000000", "%012b", 0);
 	SNPRINTF_TEST("1", "%b", 1);
 	SNPRINTF_TEST("           1", "%12b", 1);
 	SNPRINTF_TEST("000000000001", "%012b", 1);
 	SNPRINTF_TEST("1111111111111111111111111111111", "%b", INT_MAX);
 	SNPRINTF_TEST("0", "%#b", 0);
 	SNPRINTF_TEST("           0", "%#12b", 0);
 	SNPRINTF_TEST("000000000000", "%#012b", 0);
 	SNPRINTF_TEST("0b1", "%#b", 1);
 	SNPRINTF_TEST("         0b1", "%#12b", 1);
 	SNPRINTF_TEST("0b0000000001", "%#012b", 1);
 	SNPRINTF_TEST("0b1111111111111111111111111111111", "%#b", INT_MAX);
 }
 
 ATF_TC_WITHOUT_HEAD(snprintf_B);
 ATF_TC_BODY(snprintf_B, tc)
 {
 	SNPRINTF_TEST("0", "%B", 0);
 	SNPRINTF_TEST("           0", "%12B", 0);
 	SNPRINTF_TEST("000000000000", "%012B", 0);
 	SNPRINTF_TEST("1", "%B", 1);
 	SNPRINTF_TEST("           1", "%12B", 1);
 	SNPRINTF_TEST("000000000001", "%012B", 1);
 	SNPRINTF_TEST("1111111111111111111111111111111", "%B", INT_MAX);
 	SNPRINTF_TEST("0", "%#B", 0);
 	SNPRINTF_TEST("           0", "%#12B", 0);
 	SNPRINTF_TEST("000000000000", "%#012B", 0);
 	SNPRINTF_TEST("0B1", "%#B", 1);
 	SNPRINTF_TEST("         0B1", "%#12B", 1);
 	SNPRINTF_TEST("0B0000000001", "%#012B", 1);
 	SNPRINTF_TEST("0B1111111111111111111111111111111", "%#B", INT_MAX);
 }
 
 ATF_TC_WITHOUT_HEAD(snprintf_d);
 ATF_TC_BODY(snprintf_d, tc)
 {
 	SNPRINTF_TEST("0", "%d", 0);
 	SNPRINTF_TEST("           0", "%12d", 0);
 	SNPRINTF_TEST("000000000000", "%012d", 0);
 	SNPRINTF_TEST("1", "%d", 1);
 	SNPRINTF_TEST("           1", "%12d", 1);
 	SNPRINTF_TEST("000000000001", "%012d", 1);
 	SNPRINTF_TEST("2147483647", "%d", INT_MAX);
 	SNPRINTF_TEST("  2147483647", "%12d", INT_MAX);
 	SNPRINTF_TEST("002147483647", "%012d", INT_MAX);
 	SNPRINTF_TEST("2,147,483,647", "%'d", INT_MAX);
 }
 
 ATF_TC_WITHOUT_HEAD(snprintf_x);
 ATF_TC_BODY(snprintf_x, tc)
 {
 	SNPRINTF_TEST("0", "%x", 0);
 	SNPRINTF_TEST("           0", "%12x", 0);
 	SNPRINTF_TEST("000000000000", "%012x", 0);
 	SNPRINTF_TEST("1", "%x", 1);
 	SNPRINTF_TEST("           1", "%12x", 1);
 	SNPRINTF_TEST("000000000001", "%012x", 1);
 	SNPRINTF_TEST("7fffffff", "%x", INT_MAX);
 	SNPRINTF_TEST("    7fffffff", "%12x", INT_MAX);
 	SNPRINTF_TEST("00007fffffff", "%012x", INT_MAX);
 	SNPRINTF_TEST("0", "%#x", 0);
 	SNPRINTF_TEST("           0", "%#12x", 0);
 	SNPRINTF_TEST("000000000000", "%#012x", 0);
 	SNPRINTF_TEST("0x1", "%#x", 1);
 	SNPRINTF_TEST("         0x1", "%#12x", 1);
 	SNPRINTF_TEST("0x0000000001", "%#012x", 1);
 	SNPRINTF_TEST("0x7fffffff", "%#x", INT_MAX);
 	SNPRINTF_TEST("  0x7fffffff", "%#12x", INT_MAX);
 	SNPRINTF_TEST("0x007fffffff", "%#012x", INT_MAX);
 }
 
 ATF_TC_WITHOUT_HEAD(snprintf_X);
 ATF_TC_BODY(snprintf_X, tc)
 {
 	SNPRINTF_TEST("0", "%X", 0);
 	SNPRINTF_TEST("           0", "%12X", 0);
 	SNPRINTF_TEST("000000000000", "%012X", 0);
 	SNPRINTF_TEST("1", "%X", 1);
 	SNPRINTF_TEST("           1", "%12X", 1);
 	SNPRINTF_TEST("000000000001", "%012X", 1);
 	SNPRINTF_TEST("7FFFFFFF", "%X", INT_MAX);
 	SNPRINTF_TEST("    7FFFFFFF", "%12X", INT_MAX);
 	SNPRINTF_TEST("00007FFFFFFF", "%012X", INT_MAX);
 	SNPRINTF_TEST("0", "%#X", 0);
 	SNPRINTF_TEST("           0", "%#12X", 0);
 	SNPRINTF_TEST("000000000000", "%#012X", 0);
 	SNPRINTF_TEST("0X1", "%#X", 1);
 	SNPRINTF_TEST("         0X1", "%#12X", 1);
 	SNPRINTF_TEST("0X0000000001", "%#012X", 1);
 	SNPRINTF_TEST("0X7FFFFFFF", "%#X", INT_MAX);
 	SNPRINTF_TEST("  0X7FFFFFFF", "%#12X", INT_MAX);
 	SNPRINTF_TEST("0X007FFFFFFF", "%#012X", INT_MAX);
 }
 
 ATF_TP_ADD_TCS(tp)
 {
 	setlocale(LC_NUMERIC, "en_US.UTF-8");
 	ATF_TP_ADD_TC(tp, snprintf_b);
 	ATF_TP_ADD_TC(tp, snprintf_B);
 	ATF_TP_ADD_TC(tp, snprintf_d);
 	ATF_TP_ADD_TC(tp, snprintf_x);
 	ATF_TP_ADD_TC(tp, snprintf_X);
 	return (atf_no_error());
 }
diff --git a/lib/libc/tests/stdio/swprintf_test.c b/lib/libc/tests/stdio/swprintf_test.c
new file mode 100644
index 000000000000..23859b5cd2e1
--- /dev/null
+++ b/lib/libc/tests/stdio/swprintf_test.c
@@ -0,0 +1,140 @@
+/*-
+ * Copyright (c) 2023 Dag-Erling Smørgrav
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <assert.h>
+#include <limits.h>
+#include <locale.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <wchar.h>
+
+#include <atf-c.h>
+
+#ifndef nitems
+#define nitems(a) (sizeof(a) / sizeof(a[0]))
+#endif
+
+#define SWPRINTF_TEST(output, format, ...)				\
+	do {								\
+		wchar_t buf[256];					\
+		assert(wcslen(L##output) < nitems(buf));		\
+		int ret = swprintf(buf, nitems(buf), L##format,		\
+		    __VA_ARGS__);					\
+		ATF_CHECK_EQ(wcslen(L##output), ret);			\
+		if (ret > 0) {						\
+			ATF_CHECK_EQ(0, wcscmp(L##output, buf));	\
+		}							\
+	} while (0)
+
+ATF_TC_WITHOUT_HEAD(swprintf_b);
+ATF_TC_BODY(swprintf_b, tc)
+{
+	SWPRINTF_TEST("0", "%b", 0);
+	SWPRINTF_TEST("           0", "%12b", 0);
+	SWPRINTF_TEST("000000000000", "%012b", 0);
+	SWPRINTF_TEST("1", "%b", 1);
+	SWPRINTF_TEST("           1", "%12b", 1);
+	SWPRINTF_TEST("000000000001", "%012b", 1);
+	SWPRINTF_TEST("1111111111111111111111111111111", "%b", INT_MAX);
+	SWPRINTF_TEST("0", "%#b", 0);
+	SWPRINTF_TEST("           0", "%#12b", 0);
+	SWPRINTF_TEST("000000000000", "%#012b", 0);
+	SWPRINTF_TEST("0b1", "%#b", 1);
+	SWPRINTF_TEST("         0b1", "%#12b", 1);
+	SWPRINTF_TEST("0b0000000001", "%#012b", 1);
+	SWPRINTF_TEST("0b1111111111111111111111111111111", "%#b", INT_MAX);
+}
+
+ATF_TC_WITHOUT_HEAD(swprintf_B);
+ATF_TC_BODY(swprintf_B, tc)
+{
+	SWPRINTF_TEST("0", "%B", 0);
+	SWPRINTF_TEST("           0", "%12B", 0);
+	SWPRINTF_TEST("000000000000", "%012B", 0);
+	SWPRINTF_TEST("1", "%B", 1);
+	SWPRINTF_TEST("           1", "%12B", 1);
+	SWPRINTF_TEST("000000000001", "%012B", 1);
+	SWPRINTF_TEST("1111111111111111111111111111111", "%B", INT_MAX);
+	SWPRINTF_TEST("0", "%#B", 0);
+	SWPRINTF_TEST("           0", "%#12B", 0);
+	SWPRINTF_TEST("000000000000", "%#012B", 0);
+	SWPRINTF_TEST("0B1", "%#B", 1);
+	SWPRINTF_TEST("         0B1", "%#12B", 1);
+	SWPRINTF_TEST("0B0000000001", "%#012B", 1);
+	SWPRINTF_TEST("0B1111111111111111111111111111111", "%#B", INT_MAX);
+}
+
+ATF_TC_WITHOUT_HEAD(swprintf_d);
+ATF_TC_BODY(swprintf_d, tc)
+{
+	SWPRINTF_TEST("0", "%d", 0);
+	SWPRINTF_TEST("           0", "%12d", 0);
+	SWPRINTF_TEST("000000000000", "%012d", 0);
+	SWPRINTF_TEST("1", "%d", 1);
+	SWPRINTF_TEST("           1", "%12d", 1);
+	SWPRINTF_TEST("000000000001", "%012d", 1);
+	SWPRINTF_TEST("2147483647", "%d", INT_MAX);
+	SWPRINTF_TEST("  2147483647", "%12d", INT_MAX);
+	SWPRINTF_TEST("002147483647", "%012d", INT_MAX);
+	SWPRINTF_TEST("2,147,483,647", "%'d", INT_MAX);
+}
+
+ATF_TC_WITHOUT_HEAD(swprintf_x);
+ATF_TC_BODY(swprintf_x, tc)
+{
+	SWPRINTF_TEST("0", "%x", 0);
+	SWPRINTF_TEST("           0", "%12x", 0);
+	SWPRINTF_TEST("000000000000", "%012x", 0);
+	SWPRINTF_TEST("1", "%x", 1);
+	SWPRINTF_TEST("           1", "%12x", 1);
+	SWPRINTF_TEST("000000000001", "%012x", 1);
+	SWPRINTF_TEST("7fffffff", "%x", INT_MAX);
+	SWPRINTF_TEST("    7fffffff", "%12x", INT_MAX);
+	SWPRINTF_TEST("00007fffffff", "%012x", INT_MAX);
+	SWPRINTF_TEST("0", "%#x", 0);
+	SWPRINTF_TEST("           0", "%#12x", 0);
+	SWPRINTF_TEST("000000000000", "%#012x", 0);
+	SWPRINTF_TEST("0x1", "%#x", 1);
+	SWPRINTF_TEST("         0x1", "%#12x", 1);
+	SWPRINTF_TEST("0x0000000001", "%#012x", 1);
+	SWPRINTF_TEST("0x7fffffff", "%#x", INT_MAX);
+	SWPRINTF_TEST("  0x7fffffff", "%#12x", INT_MAX);
+	SWPRINTF_TEST("0x007fffffff", "%#012x", INT_MAX);
+}
+
+ATF_TC_WITHOUT_HEAD(swprintf_X);
+ATF_TC_BODY(swprintf_X, tc)
+{
+	SWPRINTF_TEST("0", "%X", 0);
+	SWPRINTF_TEST("           0", "%12X", 0);
+	SWPRINTF_TEST("000000000000", "%012X", 0);
+	SWPRINTF_TEST("1", "%X", 1);
+	SWPRINTF_TEST("           1", "%12X", 1);
+	SWPRINTF_TEST("000000000001", "%012X", 1);
+	SWPRINTF_TEST("7FFFFFFF", "%X", INT_MAX);
+	SWPRINTF_TEST("    7FFFFFFF", "%12X", INT_MAX);
+	SWPRINTF_TEST("00007FFFFFFF", "%012X", INT_MAX);
+	SWPRINTF_TEST("0", "%#X", 0);
+	SWPRINTF_TEST("           0", "%#12X", 0);
+	SWPRINTF_TEST("000000000000", "%#012X", 0);
+	SWPRINTF_TEST("0X1", "%#X", 1);
+	SWPRINTF_TEST("         0X1", "%#12X", 1);
+	SWPRINTF_TEST("0X0000000001", "%#012X", 1);
+	SWPRINTF_TEST("0X7FFFFFFF", "%#X", INT_MAX);
+	SWPRINTF_TEST("  0X7FFFFFFF", "%#12X", INT_MAX);
+	SWPRINTF_TEST("0X007FFFFFFF", "%#012X", INT_MAX);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+	setlocale(LC_NUMERIC, "en_US.UTF-8");
+	ATF_TP_ADD_TC(tp, swprintf_b);
+	ATF_TP_ADD_TC(tp, swprintf_B);
+	ATF_TP_ADD_TC(tp, swprintf_d);
+	ATF_TP_ADD_TC(tp, swprintf_x);
+	ATF_TP_ADD_TC(tp, swprintf_X);
+	return (atf_no_error());
+}