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Kernel Extension Dependencies

Any KEXT can declare that it is dependent upon any other KEXT. The developer lists these dependencies in the “Requires” field of the module’s property list file.

Before a KEXT is loaded, all of its requirements are checked. Those required extensions (and their requirements) are loaded first, iterating back through the lists until there are no more required extensions to load. Only after all requirements are met, is the requested KEXT loaded as well.

For example, device drivers (a type of KEXT) are dependent upon (require) certain families (another type of KEXT). When a driver is loaded, its required families are also loaded to provide necessary, common functionality. To ensure that all requirements are met, each device driver should list all of its requirements (families and other drivers) in its property list. See the chapter “I/O Kit Overview”, for an explanation of drivers and families.

It is important to list all dependencies for each KEXT. If your KEXT fails to do so, your KEXT may not load due to unrecognized symbols, thus rendering the KEXT useless. Dependencies in KEXTs can be considered analogous to required header files or libraries in code development; in fact, the Kernel Extension Manager uses the standard linker to resolve KEXT requirements.

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