When UpdateMaker builds a new document it first examines the old and the new files. It records their attributes, their resources, and the contents of their data forks (which are treated just like resources). It then generates difference information that specifies which resources should be added, deleted, copied, or modified to make the new file, given the old file. An UpdateMaker document consists of this difference information plus the new file's attributes and the added and modified resources.
UpdateMaker generates checksums for every resource that is copied or modified from the original file. It will not allow an update to proceed if these are different. This guarantees that the file generated by an update is identical to the new file that was used to make the update document.
SIZE 0 resources are an exception. Because they are added to applications by the Finder when the user adjusts the preferred memory size, many users will have different SIZE 0's . UpdateMaker will never try to modify an old SIZE 0 resource, and will therefore never depend on its contents. It will always replace or ignore it, as appropriate.
Some care may be needed to ensure that the same problem does not occur with applications that record the user's name when they are first used. The solution is to ensure that the new file when the UpdateMaker document is built has no name installed. The corresponding resource will therefore be missing and UpdateMaker will simply ignore any such resource in the original file when the document is used.
