On old Macs before OS X files already could have two "forks" - a data fork and a resource fork.

According to Wikipedia, at some point the HFS+ filesystem of OS X gained the ability to have an arbitrary number of forks, much like Microsofts "Alternate Data Streams" (ADS).

HFS Plus permits filenames up to 255 UTF-16 characters in length, and n-forked files similar to NTFS, though until recently, almost no systems software takes advantage of forks other than the data fork and resource fork.

I want to know what the official term is for these forks or streams. I guess it's possible they're just called "forks" but that term has several meanings already in computing. I've done some searching but can't find a definitive answer.

2 Answers 2


Apple uses "multiple fork" or the shortened form "multi-fork" consistently through its documentation, so it seems it is the official term. I list here a few examples:

https://developer.apple.com/library/mac/#documentation/General/Conceptual/SLGlobalGlossary/Glossary/Glossary.html (Last updated: 2010-07-09)

HFS+ Hierarchical File System Plus. The Mac OS Extended file-system format. This format adds support for filenames longer than 31 characters, Unicode representation of file and directory names, and efficient operation on very large disks. HFS+ is a multiple-fork volume format.

https://developer.apple.com/library/mac/#documentation/Porting/Conceptual/PortingUnix/io_porting/io_porting.html (Last updated: 2012-06-11)

(...) some Mac applications also take advantage of the HFS+ file system’s ability to handle multi-forked files.

https://developer.apple.com/library/ios/#documentation/CoreFoundation/Conceptual/CFBundles/AboutBundles/AboutBundles.html (Last updated: 2010-07-08)

Bundles can reside on volumes of many different formats, including multiple fork formats like HFS, HFS+, and AFP, and single-fork formats like UFS, SMB, and NFS.

When talking about "fork" in computing I can only think of either HFS+/HFS forks or the system call for creating a child process.

  • What a great answer - thanks for your effort! (-: Commented Oct 25, 2012 at 10:41
  • Yes there is forking child processes and also forking projects, especially open source projects, as happened to OpenOffice->LibreOffice and WikiTravel->WikiVoyage. There may be other uses I don't know of too... Commented Oct 25, 2012 at 10:44
  • 1
    Oh God, I forgot fork in software development, thanks for pointing that out.
    – jaume
    Commented Oct 25, 2012 at 13:03

I've just become interested in this topic again and started poking around.

This time I've noticed that the term "named fork" is widely used for forks other than the data fork and resource fork.

Here are some references:

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