diff --git a/common/hw-file.ads b/common/hw-file.ads
index 9dbfcf7..bc2242c 100644
--- a/common/hw-file.ads
+++ b/common/hw-file.ads
@@ -39,4 +39,8 @@
       Pre => (Readable or Writable) and
              (if Map_Copy then Readable and not Writable);
 
+   -- Sets `Length` to the size of the file given by `Path` or 0 if an
+   -- error occurs.
+   procedure Size (Length : out Natural; Path : String);
+
 end HW.File;