Data Model Concepts

The term data model is used here to describe the model for addresses and data that is used by the kernel and applications.

On the 32-bit kernel, the same data model is used by both kernel and applications: ILP32. There is no 64-bit application support on the 32-bit kernel.

On the 64-bit kernel, two different kinds of applications are supported concurrently: 32-bit applications using the ILP32 data model, and 64-bit applications using the LP64 data model. The 64-bit kernel itself uses the LP64 data model.

These concepts are captured in three flags that are associated with every system call, including ioctl(2):

• Application data model is ILP32.

• Application data model is LP64.

• Application data model is native.

At first sight, the most useful question to answer about the application invoking the kernel is: "Is it ILP32 or LP64?" However, a better test is: "Is the application using the same model as the kernel, or a different model?" The concept of native data model serves to answer that question; it is conditionally defined to match the data model of the kernel implementation. This approach enables you to write substantially cleaner code.