Property active_if — Allow additional control over the active state of an option or other CDL entity.


cdl_option <name> {
    active_if <condition>


Configuration options or other entities may be either active or inactive. Typically this is controlled by the option's location in the overall hierarchy. Consider the option CYGDBG_INFRA_DEBUG_PRECONDITIONS , which exists below the component CYGDBG_USE_ASSERT . If the whole component is disabled then the options it contains are inactive: there is no point in enabling preconditions unless there is generic assertion support; any requires constraints associated with preconditions are irrelevant; any compile property or other build-related property is ignored.

In some cases the hierarchy does not provide sufficient control over whether or not a particular option should be active. For example, the math library could have support for floating point exceptions which is only worthwhile if the hardware implements appropriate functionality, as specified by the architectural HAL. The relevant math library configuration options should remain below the CYGPKG_LIBM package in the overall hierarchy, but should be inactive unless there is appropriate hardware support. In cases like this an active_if property is appropriate.

Another common use of active_if properties is to avoid excessive nesting in the configuration hierarchy. If some option B is only relevant if option A is enabled, it is possible to turn A into a component that contains B. However adding another level to the hierarchy for a component which will contain just one entry may be considered excessive. In such cases it is possible for B to have an active_if dependency on A.

active_if takes a goal expression as argument. For details of goal expression syntax see the section called Goal Expressions in Chapter 3 . In most cases the goal expression will be very simple, often involving just one other option, but more complicated expressions can be used when appropriate. It is also possible to have multiple active_if conditions in a single option, in which case all of the conditions have to be satisfied if the option is to be active.

The active_if and requires properties have certain similarities, but they serve a different purpose. Suppose there are two options A and B, and option B relies on functionality provided by A. This could be expressed as either active_if A or as requires A . The points to note are:


# Do not provide extra semaphore debugging if there are no semaphores

# Avoid another level in the configuration hierarchy

# Functionality that is only relevant if another package is loaded
    active_if CYGPKG_UITRON

# Check that the hardware or HAL provide the appropriate functionality

See Also

Property requires .