diff --git a/CMakeLists.txt b/CMakeLists.txt
index e7c7d10..d9956f9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,7 +3,7 @@ CMAKE_MINIMUM_REQUIRED(VERSION 3.15)
 
 SET(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
-SET(CMAKE_C_STANDARD 90)
+SET(CMAKE_C_STANDARD 99)
 
 IF(CMAKE_C_COMPILER_ID MATCHES "Clang" OR CMAKE_COMPILER_ID MATCHES "GNU")
   IF(UNIX AND CMAKE_GENERATOR STREQUAL "Ninja")
diff --git a/bin/makefile-header b/bin/makefile-header
index 0756251..6f3a3d5 100755
--- a/bin/makefile-header
+++ b/bin/makefile-header
@@ -14,8 +14,8 @@ AR = ar rcv
 ANSICC = $(CC)
 
 # Compiler flags
-CLANG_CFLAGS = -std=gnu89 -funsigned-char -fno-strict-aliasing -fwrapv
-GCC_CFLAGS = -std=gnu89 -funsigned-char -fno-strict-aliasing -fwrapv
+CLANG_CFLAGS = -std=gnu99 -funsigned-char -fno-strict-aliasing -fwrapv
+GCC_CFLAGS = -std=gnu99 -funsigned-char -fno-strict-aliasing -fwrapv
 
 # for the files that need to be included in byte-swapped implementations:
 BYTESWAPFILES =