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-NAME="OVERVIEW.APPROACHES">Approaches to Configurability</H1
-><P
->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.</P
-><P
->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.</P
-><P
->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 <TT
-CLASS="CLASSNAME"
->Button</TT
->
-and derived classes <TT
-CLASS="CLASSNAME"
->TextButton</TT
-> and
-<TT
-CLASS="CLASSNAME"
->PictureButton</TT
->. If an application only uses text
-buttons then it will only create objects of type
-<TT
-CLASS="CLASSNAME"
->TextButton</TT
->, and the code for the
-<TT
-CLASS="CLASSNAME"
->PictureButton</TT
-> 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
-<TT
-CLASS="CLASSNAME"
->TextButtonUsingABorderWidthOfOnePlusAWhiteBackgroundAndBlackForegroundAndATwelvePointTimesFontAndNoErrorCheckingOrAssertions</TT
->
-is not particularly sensible as far as most application developers are
-concerned.</P
-><P
->The <SPAN
-CLASS="APPLICATION"
->eCos</SPAN
-> 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.</P
-><P
->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
-<TT
-CLASS="FUNCTION"
->pthread_cancel</TT
->; the equivalent routine in µITRON
-is <TT
-CLASS="FUNCTION"
->ter_tsk</TT
->. 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.</P
-><P
->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 <SPAN
-CLASS="APPLICATION"
->eCos</SPAN
-> 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.</P
-><P
->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.</P
-><P
->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 <TT
-CLASS="FUNCTION"
->rand</TT
-> and
-<TT
-CLASS="FUNCTION"
->lrand48</TT
->; 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.</P
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