diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5c5b927..8f51aa6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,6 +1,7 @@
 cmake_minimum_required(VERSION 3.13)
 
 project(xsystem35 LANGUAGES C)
+set(CMAKE_C_STANDARD 99)
 enable_testing()
 
 include(CheckIncludeFile)