Inspecting files of type “NeXT/Apple typedstream” version 4 (NSArchiver)

Question

For a data recovery program I need to be able to extract the values+types from files written by NSArchiver, without having access to Apple\'s CF / NS frameworks.

The OS

Answer 1

Part of the issue here is that each class in Cocoa/NeXTSTEP/OPENSTEP knows how to archive itself. In each class there is an initWithCoder:/encodeWithCoder: method and inside there is a section for typedstream and another section for keyed archives. Keyed archives are more modern and are usually expressed as XML plists. These can be encoded in binary form, but, make no mistake, this binary form is NOT the same as a typedstream archive. Further they are keyed so that it's easy to pull out individual pieces of data without having to read all of the data which came before. Typedstream archives don't work this way. They are order based which means that each element in each object is written one after the other. First the class name, then the version, then each of the pieces of data. The reason GNUstep never implemented this is because the order of encoding is nearly impossible to discover.

When you archive the root object of an object graph it calls the encodeWithCoder: method on that object which in turn calls the encodeWithCoder: methods on each of the objects it contains and so on recursively until the entire object graph is archived. When this is done using keyed archives (NSKeyedArchiver) the archive is built and keyed appropriately. When it is done with a typed stream archive (NSArchiver) the same recursion happens but each time an object is encoded it just dumps each element out into the archive in whatever order the developer deemed appropriate at the time.

I hope this explanation clears things up a little. You have a hard road ahead of you. There were reasons doing this was avoided in GNUstep. If we had, we would STILL be trying to figure it out.