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The Libraries of Mac OS X: /System/Library

I now take a more detailed look at the contents of each of the main Library folders in Mac OS X, starting with /System/Library. The number and variety of files and folders in any of these Library folders are far too great for me to cite even a bare majority of them. so I will limit the list to the ones that are most relevant for any troubleshooting you may have to do. Feel free to open these folders and browse around yourself, however. There is no fee for just looking.

Figure 4.8Figure 4.8 The /System/Library folder (partial view; items A through I seen).

Core Services

Core Services is the most critical folder in the /System/Library folder. Like the System folder itself, it has an X on it to denote its special status. It contains the BootX file, required for starting up from Mac OS X (as described in Chapter 5).

The folder also contains the Dock, Finder Classic Startup, Help Viewer, and Software Update applications, as well as the loginwindow process (also covered in Chapter 5) and the Menu Extras (covered in Chapter 3). There are also fake Mac OS 9 Finder and System files, created so that Carbon applications that expect to see these Mac OS 9-type files will "find" them. Yes, you will see the word Fake used in the Version description in the files' Show Info windows.

The SystemVersion.plist file determines what Mac OS X version is listed in the About This Mac window.

Figure 4.9Figure 4.9 Some of the files and directories in the Core Services folder.


CFMSupport contains software used for running Carbon applications. The CarbonLib file is in this folder, for example.


This folder contains the kext (for kernel extension) files that load at startup, primarily acting as driver software for hardware peripherals (covered in Chapter 5). As their name implies, kext files are extensions of the basic kernel software that loads at startup.


This folder is one of several Fonts folders in Mac OS X. This one contains the fonts that are considered to be essential for Mac OS X.


"Take Note: Multiple Folders of the Same Name in Multiple Library Folders," earlier in this chapter.


Frameworks are an important component of Mac OS X, but you will have little reason to work with them directly in troubleshooting.

Briefly, frameworks are the Mac OS X equivalents of Mac OS 9's dynamic shared libraries, which means that they contain code that can be used by more than one application simultaneously. The basic idea is to eliminate the need to repeat code that will be used by multiple applications.

Frameworks have the structure of package files, although they appear to be ordinary folders and can be opened without the Show Package Contents contextual menu. A framework package can contain multiple versions of the shared software; applications that require the newer version can access it, and those that are incompatible with the newer version will be able to access the older version.

Frameworks can occur in other locations besides this folder. The ones in this folder are simply the ones that are most essential for the OS.


Technically Speaking:"Understanding Packages," in Chapter 2, for more details on packages.

Preference Panes

This folder contains the panes that you access via the System Preferences application.


"System Preferences," in Chapter 3.


This folder contains files needed for printers to work with Mac OS X. Among other things, it contains the PPD and PDE files required for LaserWriter printers in Mac OS X.


Chapter 7 for more information on printing, including details on PPD and PDE files.


This folder contains some QuickTime-related software, such as the QuickTime Updater application.


This folder contains the basic screen-saver options (Beach, Forest, and so on), which you accessed from the Screen Saver System Preferences window.


This folder contains software needed for some of Mac OS X's Services feature, which allows you to access certain features of one application while you are in another application. You typically access this feature via the Services command in the menu that has the name of the active application (such as Finder, if Finder is the active application). If this feature is working, it allows you to open TextEdit with the selected text of your open application already pasted into an untitled TextEdit document, for example. This feature works only if the given applications support Services technology. Most do not, so most often, these options will be dimmed.

In the current context, Services also refers to some options that can be incorporated into any Cocoa application. As of Mac OS X 10.1, just two such Services are stored here: AppleSpell and Summary. AppleSpell, for example, allows a developer to include a spelling-checking feature in his application without having to write his own code.


This folder contains the sound files (in AIFF format) that are listed in the Alerts tab of the Sound System Preferences window.

Note: AIFF is one of several sound formats supported by Mac OS X. Another is the well-known MP3 format, commonly used for music files stored on your drive and used by iTunes and iPod.


This important folder contains the various protocols that load at startup while you wait for the log-in window and Desktop to appear. These items include the Apache Web server, AppleShare, AppleTalk, Networking, and Network Time.


Chapter 5 for more information on the startup sequence.

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