Using Private Routines

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So far, symbol resolution for kernel extensions has been concerned with importing and exporting symbols from and to the kernel name space. Exported symbols are global in the kernel, and can be referenced by any subsequently loaded kernel extension.

Kernel extensions can also consist of several separately link-edited modules. This is particularly useful for device drivers, where a kernel extension contains the top (pageable) half of the driver and a dependent module contains the bottom (pinned) half of the driver. Using a dependent module also makes sense when several kernel extensions use common routines. In both cases, the symbols exported by the dependent modules are not added to the global kernel name space. Instead, these symbols are only available to the kernel extension being loaded.

When link-editing a kernel extension that depends on another module, an import file should be specified listing the symbols exported by the dependent module. Before any symbols are listed, the import file should contain one of the following lines:


#! path/file


#! path/file(member)

Note: This import file can also be used as an export file when building the dependent module.

Dependent modules can be found in an archive file. In this case, the member name must be specified in the #! line.

While a kernel extension is being loaded, any dependent modules are only loaded a single time. This allows modules to depend on each other in a complicated way, without causing multiple instances of a module to be loaded.

Note: The loader uses the pathname of a module to determine whether it has already been loaded. Another copy of the module can be loaded if different path names are used for the same module.

The symbols exported by dependent modules are not added to the kernel name space. These symbols can only be used by a kernel extension and its other dependent modules. If another kernel extension is loaded that uses the same dependent modules, these dependent modules will be loaded a second time.