eCos is a component architecture. The system comes as a number of packages which can be enabled or disabled as required, and new packages can be added as they become available. Unfortunately, the packages are not completely independent: for example the µITRON compatibility package relies almost entirely on functionality provided by the kernel package, and it would not make sense to try to build µITRON if the kernel was disabled. The C library has fewer dependencies: some parts of the C library rely on kernel functionality, but it is possible to disable these parts and thus build a system that has the C library but no kernel. The ecosconfig tool has the capability of checking that all the dependencies are satisfied, but it may still be possible to produce configurations that will not build or (conceivably) that will build but not run. Developers should be aware of this and take appropriate care.
By default, ecosconfig will include all packages that are appropriate for the specified hardware in the configuration. The common HAL package and the eCos infrastructure must be present in every configuration. In addition, it is always necessary to have one architectural HAL package and one platform HAL package. Other packages are optional, and can be added or removed from a configuration as required.
The application may not require all of the packages; for example, it might not need the µITRON compatibility package, or the floating point support provided by the math library. There is a slight overhead when eCos is built because the packages will get compiled, and there is also a small disk space penalty. However, any unused facilities will get stripped out at link-time, so having redundant packages will not affect the final executable.