diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4376a37..3d72108 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,6 +5,8 @@ project (lpac
     HOMEPAGE_URL "https://github.com/estkme-group/lpac"
     DESCRIPTION "C-based eUICC LPA."
     LANGUAGES C)
+set(CMAKE_C_STANDARD 99)
+set(CMAKE_C_STANDARD_REQUIRED ON)
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
diff --git a/README.md b/README.md
index 656b185..18ebfad 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # lpac
 
-lpac is a cross-platform local profile agent program.
+lpac is a cross-platform local profile agent program, compatible with [SGP.22 version 2.2.2](https://www.gsma.com/solutions-and-impact/technologies/esim/wp-content/uploads/2020/06/SGP.22-v2.2.2.pdf).
 
 Features:
 
diff --git a/documents/DEVELOPERS.md b/documents/DEVELOPERS.md
index bc1d6a3..2998001 100644
--- a/documents/DEVELOPERS.md
+++ b/documents/DEVELOPERS.md
@@ -1,3 +1,7 @@
+## Standard
+
+lpac is written with C99 and compatible with [SGP.22 version 2.2.2](https://www.gsma.com/solutions-and-impact/technologies/esim/wp-content/uploads/2020/06/SGP.22-v2.2.2.pdf).
+
 ## Compile
 
 ### Linux