diff --git a/src/to-json/src/ffi.rs b/src/to-json/src/ffi.rs
new file mode 100644
index 0000000..fef3e3a
--- /dev/null
+++ b/src/to-json/src/ffi.rs
@@ -0,0 +1,18 @@
+use std::ffi::{c_char, CStr, CString};
+
+#[no_mangle]
+pub unsafe extern "C" fn free_rust_string(string: *const c_char) {
+    let _ = CString::from_raw(string as *mut _);
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn to_json_rust(content: *const c_char) -> *const c_char {
+    let content: &str = CStr::from_ptr(content).to_str().unwrap();
+
+    // TODO: remove test code
+    println!("Raw content -> {}", content);
+
+    // TODO: convert to JSON format
+
+    CString::new(content).unwrap().into_raw()
+}
diff --git a/src/to-json/src/lib.rs b/src/to-json/src/lib.rs
index 1ece23a..57ae9b9 100644
--- a/src/to-json/src/lib.rs
+++ b/src/to-json/src/lib.rs
@@ -1,15 +1 @@
-use std::ffi::{c_char, CStr, CString};
-
-#[no_mangle]
-pub unsafe extern "C" fn to_json_rust(content: *const c_char) -> *const c_char {
-    let content: &str = CStr::from_ptr(content).to_str().unwrap();
-
-    println!("Raw content -> {}", content);
-
-    CString::new(content).unwrap().into_raw()
-}
-
-#[no_mangle]
-pub unsafe extern "C" fn free_rust_string(string: *const c_char) {
-    let _ = CString::from_raw(string as *mut _);
-}
+mod ffi;