CMake 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 use the directive REQUIRES_IMPLEMENTATIONS in the register_fprime_module call.

Note

REQUIRES_IMPLEMENTATIONS is only needed for modules that have a direct dependency on the implementation, not on their dependents.

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 the CHOOSES_IMPLEMENTATIONS directive available to register_fprime_config. Platform developers should choose implementations as part of the platform definition.

https://github.com/nasa/fprime/blob/dfaf496263bdfff04461179eb99fb3f906a10009/cmake/platform/Linux.cmake#L15-L17

Overriding an Implementation Choice

Executables, unit tests, and deployments may wish to use a different implementation than that specified by the platform. This can be done by using CHOOSES_IMPLEMENTATIONS directive call in the deployment, executable, or unit test's registration. 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:

register_fprime_ut(
    ...
  CHOOSES_IMPLEMENTATIONS
    Os_File_Stub
)

Note

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 and supply the IMPLEMENTS directive to that call.

Conclusion

F´ provides for various implementations for certain packages needed by the framework.