Package Implementations

Certain parts of F´ have different implementations that a project may choose from. The canonical example of this is a file system implementation. Projects may wish to use the stubbed implementation of the file system, the platform supplied standard implementation, or something project-specific like a flash chip or SD card.

These packages must be fully specified. Thus, every platform must specify a specific implementation for every package. These specific implementations may be overridden for a specific executable deployment or unit test.

Requiring an Implementation

Modules that require an implementation should call the CMake API function require_fprime_implementation() to declare a requirement on a specific implementation. For example, the Os module require_fprime_implementation(Os/Task) declares that some implementation of Os/Task is required.

Only Os requires an implementation of Os/Task as users of the Os package get the implementation through Os and should not specify it directly.

Choosing an Implementation

Platform developers must specify an implementation of every package used in the system. Failing to do so means that a given functionality is undefined and impossible to link. Stub implementations are provided in the case that a platform does not support a given package’s functionality.

Choosing an implementation is done with a series of choose_fprime_implementation calls. One for each required implementation defined in the system. These calls are put in a platform CMake file.

Overriding an Implementation Choice

Some executables, unit tests, and deployments may wish to use a different implementation than that specified by the platform. This can be done by a choose_fprime_implementation call in the deployment, executable, or unit test’s module. For example, a unit test may wish to choose Os_File_Stub as an implementation of Os_File to disable platform file system support for the given unit test.

A CMake target with the name of the chosen implementor must be defined somewhere in F´, an F´ library used by the project, or by the project itself.

Defining an Implementation

To define an implementation, developers should declare an F´ module (using register_fprime_module) that implements the needed interface of the package. Then call register_fprime_implementation to specify this module is an implementation.

Conclusion

F´ provides for various implementations for certain packages needed by the framework. See cmake/API and Os for API descriptions and Os implementations.