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An introduction to file creator/type codes and what they mean to the MacOS

There is a hidden database on every Mac volume called the DesktopDB. Every new application added to the drive will register with this database. An application tells the database three things:

• the application's creator code
• the types of documents it can open.
• which icons are associated with document types.

This information is initially stored in a program's BNDL resource. A free application to let you browse this info is Desktop DB Diver.

Audion's creator code is Audn. Some of the documents it can open are of type: MPEG and LIST. These are not naming extensions like .mp3 and .txt, these are type codes and are usually hidden from the end user.

For the file type MPEG, a document can have an extensions mp3, mpeg3, or no extension! The MacOS doesn't care too much about naming extensions because every document has a creator code and file type associated with it. When you double-click a document, it's creator/type info is compared to the database and the appropriate application is launched. Cool huh?

What about documents downloaded from the internet, how do they get associated with an application?

In the Internet Control Panel under the "Advanced" tab is a section called File Mappings that equates naming extensions with creator/type codes in the database. Many applications not add their own entries to the Internet file mappings, or you can edit them manually. (If you are not seeing the "Advanced" tab, from the Edit menu, select User Level and click the Advanced.)

In my case, downloading a file with the naming extension mp3 automatically associates that document with Audion - even if it was originally created on a PC using "Audio Grabber".

For a document, the creator/type code is not written to the data or resource fork, it is instead placed in the directory of a volume, along with a file's length, its location on the disk, its modification date and other properties of a file. In this way, the document file can be sent to another OS with its original data intact.

This is why on a Mac, you can move most folders and files around with immunity. The file system is not based on naming extension and file paths, but on creator/type codes that are stored in the desktop database.

Sometimes installing competing applications can cause documents to battle over which creator code gets used; other times an applications BNDL resources are not correctly set. This can lead to a corruption in the DesktopDB. When this happens, you can rebuild the databases on all your mounted volumes by holding the Command-Option keys during startup.

The assigning of application creator codes to a developer is not a matter of personal preference. They must be registered with Apple! They provide a form to fill out and a database to make sure your choice has not already been taken. If you are curious, there are currently over 33,000 creator codes collected in the shareware database application TCDB. To test how easy this free service is, I registered two creators codes and am now the proud owner of ResX and RESX.

Creator codes and file types are an aspect of the MacOS that make it vastly superior to end users than Windows or Linux. It's very forgiving of a file's location in the folder hierarchy, and in conjunction with the Internet control panel, allows associations to be made when an unknown file is downloaded to the drive. While we should know shortly after the release of the beta, I fear this ease of use will be missing in MacOS X. Historically, UNIX based file systems place a high value on security, with the ability to adjust a file's owner and permissions to a high degree, something the current MacOS 9 file system does not do well. Apple appears to have blended the Aqua User Interface onto a UNIX core, it will be interesting to see how they blend the best of the current file system with the best of the UNIX-style BSD core.