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Installed KEXTs

The Kernel Extension Manager (KEXT Manager) is responsible for loading and unloading all installed KEXTs (commands such as kextload are used only during development). Installed KEXTs are dynamically added to the running Mac OS X kernel as part of the kernel’s address space. An installed and enabled KEXT is invoked as needed.

KEXTs are usually installed in the folder /System/Libraries/Extensions. The Kernel Extension Manager (in the form of a daemon, kextd), always checks here. KEXTs can also be installed in ROM or inside an application bundle.

Installing KEXTs in an application bundle allows an application to register those KEXTs without the need to install them permanently elsewhere within the system hierarchy. This may be more convenient and allows the KEXT to be associated with a specific, running application. When it starts, the application can register the KEXT and, if desired, unregister it on exit.

For example, a network packet sniffer application might employ a Network Kernel Extension (NKE). A tape backup application would require that a tape driver be loaded during the duration of the backup process. When the application exits, the kernel extension is no longer needed and can be unloaded.

Note that, although the application is responsible for registering the KEXT, this is no guarantee that the corresponding KEXTs are actually ever loaded. It is still up to a kernel component, such as the I/O Kit, to determine a need, such as matching a piece of hardware to a desired driver, thus causing the appropriate KEXTs (and their dependencies) to be loaded.

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