diff --git a/CMakeLists.txt b/CMakeLists.txt
index 41dbde5d..93012159 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -16,8 +16,16 @@ execute_process(
 
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY bin)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY lib)
-set(CMAKE_C_FLAGS "-Wall -Wextra -std=c99")
+set(CMAKE_C_FLAGS "-Wall -Wextra")
 set(CMAKE_C_FLAGS_DEBUG "-g -DDEBUG=1")
+
+option(USE_C11_FEATURE "Enable c11 feature" OFF)
+if(USE_C11_FEATURE)
+  add_definitions(-std=c11)
+else()
+  add_definitions(-std=c99)     # at least c99 is required
+endif()
+
 include_directories(include extlib)
 
 # build picrin
diff --git a/README.md b/README.md
index 4152ea6b..f7cfe12b 100644
--- a/README.md
+++ b/README.md
@@ -35,12 +35,12 @@ There is a chat room on chat.freenode.org, channel #picrin.
 
 - make `Makefile`
 
-	Change directory to `build` then run `cmake` to create Makefile. Once `Makefile` is generated you can run `make` command to build picrin.
+	Change directory to `build` then run `ccmake` to create Makefile. Once `Makefile` is generated you can run `make` command to build picrin.
 
 		$ cd build
-        $ cmake ..
+        $ ccmake ..
 
-	Actually you don't necessarily need to move to `build` directory before running `cmake` (in that case `$ cmake .`), but I strongly recommend to follow above instruction.
+	Actually you don't necessarily need to move to `build` directory before running `ccmake` (in that case `$ ccmake .`), but I strongly recommend to follow above instruction.
     
 - build
 
diff --git a/docs/intro.rst b/docs/intro.rst
index 06e75b88..e81632df 100644
--- a/docs/intro.rst
+++ b/docs/intro.rst
@@ -21,13 +21,13 @@ Installation
 
 - make `Makefile`
 
-Change directory to `build` then run `cmake` to create Makefile. Once `Makefile` is generated you can run `make` command to build picrin::
+Change directory to `build` then run `ccmake` to create Makefile. Once `Makefile` is generated you can run `make` command to build picrin::
 
   $ cd build
 
-  $ cmake ..
+  $ ccmake ..
 
-Actually you don't necessarily need to move to `build` directory before running `cmake` (in that case `$ cmake .`), but I strongly recommend to follow above instruction.
+Actually you don't necessarily need to move to `build` directory before running `ccmake` (in that case `$ ccmake .`), but I strongly recommend to follow above instruction.
 
 - build
 
diff --git a/include/picrin/config.h b/include/picrin/config.h
index 2d5fd2a2..2acfe0ea 100644
--- a/include/picrin/config.h
+++ b/include/picrin/config.h
@@ -45,6 +45,10 @@
 /* #define GC_DEBUG 1 */
 /* #define GC_DEBUG_DETAIL 1 */
 
+#if __STDC_VERSION__ < 199901L
+# error please activate c99 features
+#endif
+
 #ifndef PIC_CONTRIB_INITS
 # define PIC_CONTRIB_INITS
 #endif
@@ -56,7 +60,7 @@
 #endif
 
 #ifndef PIC_NAN_BOXING
-# if __x86_64__
+# if __x86_64__ && __STDC_VERSION__ >= 201112L
 #  define PIC_NAN_BOXING 1
 # endif
 #endif