diff --git a/sys/amd64/amd64/db_trace.c b/sys/amd64/amd64/db_trace.c
--- a/sys/amd64/amd64/db_trace.c
+++ b/sys/amd64/amd64/db_trace.c
@@ -205,7 +205,7 @@
 	 */
 	tf_addr = *fp + 16;
 
-	if (!__is_aligned(tf_addr, _Alignof(*tf)) || !INKERNEL(tf_addr)) {
+	if (!__is_aligned(tf_addr, __alignof__(*tf)) || !INKERNEL(tf_addr)) {
 		db_printf("--- invalid trapframe %p\n", (void *)tf_addr);
 		*ip = 0;
 		*fp = 0;
diff --git a/sys/arm64/arm64/db_trace.c b/sys/arm64/arm64/db_trace.c
--- a/sys/arm64/arm64/db_trace.c
+++ b/sys/arm64/arm64/db_trace.c
@@ -92,7 +92,7 @@
 			struct trapframe *tf;
 
 			tf = (struct trapframe *)(uintptr_t)frame->fp - 1;
-			if (!__is_aligned(tf, _Alignof(*tf)) ||
+			if (!__is_aligned(tf, __alignof__(*tf)) ||
 			    !kstack_contains(td, (vm_offset_t)tf,
 			    sizeof(*tf))) {
 				db_printf("--- invalid trapframe %p\n", tf);
diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -274,7 +274,7 @@
 .PHONY: ${PHONY_NOTMAIN}
 .NOTMAIN: ${PHONY_NOTMAIN}
 
-CSTD?=		gnu99
+CSTD?=		gnu17
 
 .if ${CSTD} == "k&r"
 CFLAGS+=        -traditional
diff --git a/sys/contrib/zstd/lib/freebsd/assert.h b/sys/contrib/zstd/lib/freebsd/assert.h
new file mode 100644
--- /dev/null
+++ b/sys/contrib/zstd/lib/freebsd/assert.h
@@ -0,0 +1,77 @@
+/*-
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright (c) 1992, 1993
+ *	The Regents of the University of California.  All rights reserved.
+ * (c) UNIX System Laboratories, Inc.
+ * All or some portions of this file are derived from material licensed
+ * to the University of California by American Telephone and Telegraph
+ * Co. or Unix System Laboratories, Inc. and are reproduced herein with
+ * the permission of UNIX System Laboratories, Inc.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the University nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+
+/*
+ * Unlike other ANSI header files, <assert.h> may usefully be included
+ * multiple times, with and without NDEBUG defined.
+ */
+
+#undef assert
+#undef _assert
+
+#ifdef NDEBUG
+#define	assert(e)	((void)0)
+#define	_assert(e)	((void)0)
+#else
+#define	_assert(e)	assert(e)
+
+#define	assert(e)	((e) ? (void)0 : __assert(__func__, __FILE__, \
+			    __LINE__, #e))
+#endif /* NDEBUG */
+
+#ifndef _ASSERT_H_
+#define _ASSERT_H_
+
+/*
+ * Static assertions.  In principle we could define static_assert for
+ * C++ older than C++11, but this breaks if _Static_assert is
+ * implemented as a macro.
+ *
+ * C++ template parameters may contain commas, even if not enclosed in
+ * parentheses, causing the _Static_assert macro to be invoked with more
+ * than two parameters.
+ */
+#if __ISO_C_VISIBLE >= 2011 && !defined(__cplusplus)
+#define	static_assert	_Static_assert
+#endif
+
+__BEGIN_DECLS
+void __assert(const char *, const char *, int, const char *) __dead2;
+__END_DECLS
+
+#endif /* !_ASSERT_H_ */
diff --git a/sys/riscv/riscv/db_trace.c b/sys/riscv/riscv/db_trace.c
--- a/sys/riscv/riscv/db_trace.c
+++ b/sys/riscv/riscv/db_trace.c
@@ -83,7 +83,7 @@
 			struct trapframe *tf;
 
 			tf = (struct trapframe *)(uintptr_t)frame->sp;
-			if (!__is_aligned(tf, _Alignof(*tf)) ||
+			if (!__is_aligned(tf, __alignof__(*tf)) ||
 			    !kstack_contains(td, (vm_offset_t)tf,
 			    sizeof(*tf))) {
 				db_printf("--- invalid trapframe %p\n", tf);