Catalog File

Written by John Harris, Updated on October 11, 2023

What is a Catalog File?

The catalog file is the main component of the Mac operating system. It is used by the file manager to store all the records of the file hierarchy containing files and folders that are stored on your Mac Volume. It is structured in the form of a B-Tree, containing header nodes, index nodes, leaf nodes, and map nodes.

Catalog Node ID

All the files and folders in the catalog file are identified by their unique ID called CNID, and Catalog Node ID. These CNIDs play a very useful role in accessing and finding the files from the HFS+ Volumes. In the case of folders, the CNID is the folder ID, and for files, then the CNID will be the file ID. The first 16 CNIDs are used by the Mac OS X to store system-related files.

CNID Components

The CNID contains: The catalog file key the parent folder’s CNID and the name of the file or folder and catalog file data (leaf node) contains the records of the files or folders which comprises of following records, explained below:

  • Folder Record:
  • It contains the information about the number of files and folders contained in that folder, folder ID, date of creation and modification of the folder, last access date, and Finder information and also the permissions allotted to these folders.

  • File Record:
  • Contains the information about the three timestamps like creation, modification, and last backup dates, data of the first file extents, pointers associated with the file’s first data, and also the resource forks. In addition, it also contains information for the File finders like Creator code and type code.

  • File Thread Record:
  • Used to link the CNID to the file or folder using or referencing that CNID.

  • Folder Thread Record:
  • It contains the name of the folders and the CNID of its parent folder.

    (These two Thread records just provides the link between its parent folder and itself)

  • Catalog Thread Record:
  • Used to link the CNID to the file or folder using or referencing that CNID

The thread records mainly contain the CNID of its parent file or folder which it references. In order to find or locate a file or a folder using its CNID, first you need to locate the thread record of that file or folder. Using this, you can find its parent folder CNID and also the name. Then, with the help of its parent folder name and ID, you can easily locate the file needed.

Catalog File Uses

Catalog File is incredibly important and allows iOS applications to be the best at what they do. A few examples are listed below-

  • Spotlight uses CNIDs to store and maintain the file directory allowing Spotlight to find files quickly even if they are located deep within the folder structure.
  • Time Machine uses CNIDs to create backups of files and folders and update the backup files when any modifications are detected.
  • Finder and other system services use CNIDs to troubleshoot errors that are related to the file system or folder structure.
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