diff --git a/reactos/cmake/gcc.cmake b/reactos/cmake/gcc.cmake
index c6da9b8c19e..97658829b5b 100644
--- a/reactos/cmake/gcc.cmake
+++ b/reactos/cmake/gcc.cmake
@@ -36,6 +36,7 @@ if(CMAKE_C_COMPILER_ID STREQUAL "Clang")
     add_compile_flags("-Wno-microsoft")
 endif()
 
+add_compile_flags_language("-Wold-style-declaration" "C")
 add_compile_flags_language("-fno-rtti -fno-exceptions" "CXX")
 
 #bug