Writing Device Drivers

How Do the Structure Macros Work?

In a 64-bit device driver, structure macros enable the use of the same piece of kernel memory by data structures of both sizes. The memory buffer holds the contents of the native form of the data structure, that is, the LP64 form, and the ILP32 form. Each structure access is implemented by a conditional expression. When compiled as a 32-bit driver, only one data model, the native form, is supported. No conditional expression is used.

The 64-bit versions of the macros depend on the definition of a shadow version of the data structure. The shadow version describes the 32-bit interface with fixed-width types. The name of the shadow data structure is formed by appending “32” to the name of the native data structure. For convenience, place the definition of the shadow structure in the same file as the native structure to ease future maintenance costs.

The macros can take the following arguments:


The structure name of the native form of the data structure as entered after the struct keyword.


A flag word that contains the user data model, such as FILP32 or FLP64, extracted from the mode parameter of ioctl(9E).


The name used to refer to a particular instance of a structure that is manipulated by these macros.


The name of the field within the structure.