Design Guidelines

Module developers should follow these guidelines:

• If a module does not understand the message types, the message types must be passed to the next module.

• The module that acts on an M_IOCTL message sends an M_IOCACK or M_IOCNAK message in response to the ioctl(2). If the module does not understand the ioctl(2), it passes the M_IOCTL message to the next module.

• Design modules so that they don't pertain to any particular driver but can be used with all drivers.

• In general, modules should not require the data in an M_DATA message to follow a particular format, such as a specific alignment. This makes it easier to arbitrarily push modules on top of each other in a sensible fashion. Not following this rule can limit module usability.

• Filter modules pushed between a service user and a service provider do not alter the contents of the M_PROTO or M_PCPROTO block in messages. The contents of the data blocks can be changed, but the message boundaries must be preserved.