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I think there is some general confusion on the relation between all four of these items scattered throughout the web. I wanted to get this straightened out.

  1. Are named forks the same as extended attributes? If not, what are extended attributes?
  2. Is the resource fork still implemented as a fork or as an extended attribute? If it is implemented some other way, then how?
  3. The metadata stored with a file (creator, date modified...) is there any relation or overlap in terminology between the other three mentioned. An example would be, are extended attributes just extra metadata pairs (key + value) stored on a file.

Any answers that can clarify how all four of these are interrelated, especially regarding the three question would be greatly appreciated, and contribute to settling disputes I see occurring across different resources.

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For learning about HFS Plus: fileXray User Guide and Reference (PDF, 1.8 MB, 173 pages) is very useful. I'm a user of fileXray. Also: Technical Note TN1150: HFS Plus Volume Format is legacy, retired by Apple, but still useful for many purposes. –  Graham Perrin Dec 25 '12 at 7:00
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up vote 5 down vote accepted

Confusion arises from the fact that the relationship between these concepts is complex and has changed over time. In current systems the difference between a named fork and an extended attribute is largely academic.

For an extended attribute, the actual data is stored in the attribute data record.

For a fork, what's stored is the list of disk allocation blocks that contain the data. A resource fork is still a fork.

Basic file system metadata is stored in dedicated elements of the file system record itself, independent of the attributes and named fork references.

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But is an extended attribute actually stored as it's own fork, or is there a fork for extended attributes? How is it related in the most recent version of OSX? –  rubixibuc Feb 16 '12 at 20:27
I don't understand, when you say attribute do u mean "extended attribute." If so where is the attribute record kept. Is it stored as part of the file system structure HFS+ or is stored in some sort of database? Please respond to the first comment as well when you can. –  rubixibuc Feb 17 '12 at 4:10
Sorry. I'm not here frequently and didn't see your follow-up questions until I came here looking for something else. An extended attribute is a named fork. I was using attribute as shorthand for "extended attribute." The attribute records are kept in a private part of the file system called the attributes tree. You could consider it a form of database. The data for each attribute may or may not be in that tree depending on how large it is. –  GregW Mar 21 '12 at 12:39
"… file system metadata is stored in dedicated elements of the file system record …" – instead, I'd probably say "… file metadata is stored in dedicated elements of the record for the file …" –  Graham Perrin Dec 25 '12 at 6:35
Maybe worth nothing that whilst com.apple.FinderInfo may appear as an extended attribute, Extended Finder Info (typically date_added, extended_flags plus three reserved fields) is not an extended attribute. –  Graham Perrin Dec 25 '12 at 6:46
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