diff --git a/util/nvmutil/Makefile b/util/nvmutil/Makefile
index 91b5ba1c..c6611075 100644
--- a/util/nvmutil/Makefile
+++ b/util/nvmutil/Makefile
@@ -3,7 +3,7 @@
 # SPDX-FileCopyrightText: 2023 Riku Viitanen <riku.viitanen@protonmail.com>
 
 CC?=cc
-CFLAGS?=-Os -Wall -Wextra -Werror -pedantic
+CFLAGS?=-Os -Wall -Wextra -Werror -pedantic -std=c99 -D_POSIX_C_SOURCE=200809L
 DESTDIR?=
 PREFIX?=/usr/local
 INSTALL?=install
diff --git a/util/nvmutil/nvmutil.c b/util/nvmutil/nvmutil.c
index 3ec92048..5aa000a2 100644
--- a/util/nvmutil/nvmutil.c
+++ b/util/nvmutil/nvmutil.c
@@ -2,6 +2,8 @@
 /* Copyright (c) 2022-2026 Leah Rowe <leah@libreboot.org> */
 /* Copyright (c) 2023 Riku Viitanen <riku.viitanen@protonmail.com> */
 
+#define _POSIX_C_SOURCE 200809L
+
 #ifdef __OpenBSD__
 #include <sys/param.h>
 #endif