diff --git a/21-shell/libc/function.h b/21-shell/libc/function.h
new file mode 100644
index 0000000..bf656ed
--- /dev/null
+++ b/21-shell/libc/function.h
@@ -0,0 +1,8 @@
+#ifndef FUNCTION_H
+#define FUNCTION_H
+
+/* Sometimes we want to keep parameters to a function for later use
+ * and this is a solution to avoid the 'unused parameter' compiler warning */
+#define UNUSED(x) (void)(x)
+
+#endif