diff --git a/source/turtle/io.cpp b/source/turtle/io.cpp
index c7ba938..e7cd7d8 100644
--- a/source/turtle/io.cpp
+++ b/source/turtle/io.cpp
@@ -34,9 +34,14 @@ export namespace turtle {
 	};
 
 	struct system_allocator : public coral::allocator {
-		system_allocator() = default;
+		enum class thread_safety {
+			none,
+			mutex,
+		};
 
-		// TODO: implement thread-safety.
+		system_allocator(thread_safety allocator_thread_safety) {
+			// TODO: optimize allocator with thread-unsafe variant.
+		}
 
 		coral::u8 * reallocate(coral::u8 * maybe_allocation, coral::usize requested_size) override {
 			if (maybe_allocation != nullptr) coral::unreachable();