Configuration items are the individual entities that form a configuration. Each item corresponds to the setting of a C pre-processor macro (for example, CYGHWR_HAL_ARM_PID_GDB_BAUD). The code of eCos itself is written to test such pre-processor macros so as to tailor the code. User code can do likewise.
Configuration items come in the following flavors:
None: such entities serve only as place holders in the hierarchy, allowing other entities to be grouped more easily.
Boolean entities are the most common flavor; they correspond to units of functionality that can be either enabled or disabled. If the entity is enabled then there will be a #define; code will check the setting using, for example, #ifdef
Data entities encapsulate some arbitrary data. Other properties such as a set or range of legal values can be used to constrain the actual values, for example to an integer or floating point value within a certain range.
Booldata entities combine the attributes of Boolean and Data: they can be enabled or disabled and, if enabled, will hold a data value.
Like packages, configuration items exist in a tree-based hierarchy: each configuration item has a parent which may be another configuration item or a package. Under some conditions (such as when packages are added or removed from a configuration), items may be “re-parented” such that their position in the tree changes.
Expressions are relationships between CDL items. There are three types of expression in CDL:
Table 24-1. CDL Expressions
| Expression Type | Result | Common Use (see Table 24-2) |
|---|---|---|
| Ordinary | A single value | legal_values property |
| List | A range of values (for example “1 to 10”) | legal_values property |
| Goal | True or False | requires and active_if properties |
Each configuration item has a set of properties. The following table describes the most commonly used:
Table 24-2. Configuration properties
| Property | Use |
|---|---|
| Flavor | The “type” of the item, as described above |
| Enabled | Whether the item is enabled |
| Current_value | The current value of the item |
| Default_value | An ordinary expression defining the default value of the item |
| Legal_values | A list expression defining the values the item may hold (for example, 1 to10) |
| Active_if | A goal expression denoting the requirement for this item to be active (see below: Inactive Items) |
| Requires | A goal expression denoting requirements this item places on others (see below: Conflicts) |
| Calculated | Whether the item as non-modifiable |
| Macro | The corresponding C pre-processor macro |
| File | The C header file in which the macro is defined |
| URL | The URL of a documentation page describing the item |
| Hardware | Indicates that a particular package is related to specific hardware |
A complete description of properties is contained in the Component Writer’s Guide.
Descendants of an item that is disabled are inactive: their values may not be changed. Items may also become inactive if an active_if expression is used to make the item dependent on an expression involving other items.