The purpose of configurability is to control the behavior of components. A scheduler component may or may not support time slicing; it may or may not support multiple priorities; it may or may not perform error checking on arguments passed to the scheduler routines. In the context of a desktop application a button widget may contain some text or it may contain a picture; the text may be displayed in a variety of fonts; the foreground and background color may vary. When an application uses a component there must be some way of specifying the desired behavior. The component writer has no way of knowing in advance exactly how a particular component will end up being used.
One way to control the behavior is at run time. The application creates an instance of a button object, and then instructs this object to display either text or a picture. No special effort by the application developer is required, since a button can always support all desired behavior. There is of course a major disadvantage in terms of the size of the final application image: the code that gets linked with the application has to provide support for all possible behavior, even if the application does not require it.
Another approach is to control the behavior at link-time, typically
by using inheritance in an object-oriented language. The button
library provides an abstract base class Button
and derived classes TextButton and
PictureButton. If an application only uses text
buttons then it will only create objects of type
TextButton, and the code for the
PictureButton class does not get used. In
many cases this approach works rather well and reduces the final image
size, but there are limitations. The main one is that you can only
have so many derived classes before the system gets unmanageable: a
derived class
TextButtonUsingABorderWidthOfOnePlusAWhiteBackgroundAndBlackForegroundAndATwelvePointTimesFontAndNoErrorCheckingOrAssertions
is not particularly sensible as far as most application developers are
concerned.
The eCos component framework allows the behavior of components to be controlled at an even earlier time: when the component source code gets compiled and turned into a library. The button component could provide options, for example an option that only text buttons need to be supported. The component gets built and becomes part of a library intended specifically for the application, and the library will contain only the code that is required by this application and nothing else. A different application with different requirements would need its own version of the library, configured separately.
In theory compile-time configurability should give the best possible
results in terms of code size, because it allows code to be controlled
at the individual statement level rather than at the function or
object level. Consider an example more closely related to embedded
systems, a package to support multi-threading. A standard routine
within such a package allows applications to kill threads
asynchronously: the POSIX routine for this is
pthread_cancel; the equivalent routine in µITRON
is ter_tsk. These routines themselves tend to
involve a significant amount of code, but that is not the real
problem: other parts of the system require extra code and data for the
kill routine to be able to function correctly. For example if a thread
is blocked while waiting on a mutex and is killed off by another
thread then the kill operation may have to do two things: remove the
thread from the mutex's queue of waiting threads; and undo the
effects, if any, of priority inheritance. The implementation requires
extra fields in the thread data structure so that the kill routine
knows about the thread's current state, and extra code in the mutex
routines to fill in and clear these extra fields correctly.
Most embedded applications do not require the ability to kill off a thread asynchronously, and hence the kill routine will not get linked into the final application image. Without compile-time configurability this would still mean that the mutex code and similar parts of the system contain code and data that serve no useful purpose in this application. The eCos approach allows the user to select that the thread kill functionality is not required, and all the components can adapt to this at compile-time. For example the code in the mutex lock routine contains statements to support the killing of threads, but these statements will only get compiled in if that functionality is required. The overall result is that the final application image contains only the code and data that is really needed for the application to work, and nothing else.
Of course there are complications. To return to the button example, the application code might only use text buttons directly, but it might also use some higher-level widget such as a file selector and this file selector might require buttons with pictures. Therefore the button code must still be compiled to support pictures as well as text. The configuration tools must be aware of the dependencies between components and ensure that the internal constraints are met, as well as the external requirements of the application code. An area of particular concern is conflicting requirements: a button component might be written in such a way that it can only support either text buttons or picture buttons, but not both in one application; this would represent a weakness in the component itself rather than in the component framework as a whole.
Compile-time configurability is not intended to replace the other
approaches but rather to complement them. There will be times when
run-time selection of behavior is desirable: for example an
application may need to be able to change the baud rate of a serial
line, and the system must then provide a way of doing this at
run-time. There will also be times when link-time selection is
desirable: for example a C library might provide two different random
number routines rand and
lrand48; these do not affect other code so there
is no good reason for the C library component not to provide both of
these, and allow the application code to use none, one, or both of
them as appropriate; any unused functions will just get eliminated at
link-time. Compile-time selection of behavior is another option, and
it can be the most powerful one of the three and the best suited to
embedded systems development.