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<HTML
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><HEAD
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><TITLE
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>Approaches to Configurability</TITLE
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"><LINK
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REL="HOME"
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TITLE="The eCos Component Writer's Guide"
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HREF="cdl-guide.html"><LINK
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REL="UP"
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TITLE="Overview"
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HREF="overview.html"><LINK
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REL="PREVIOUS"
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TITLE="Why Configurability?"
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TITLE="Degrees of Configurability"
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><TH
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ALIGN="center"
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>The <SPAN
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CLASS="APPLICATION"
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>eCos</SPAN
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> Component Writer's Guide</TH
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></TR
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><TR
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><TD
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><A
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HREF="overview.configurability.html"
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>Prev</A
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WIDTH="80%"
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VALIGN="bottom"
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>Chapter 1. Overview</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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><A
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NAME="OVERVIEW.APPROACHES">Approaches to Configurability</H1
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><P
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>The purpose of configurability is to control the behavior of
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components. A scheduler component may or may not support time slicing;
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it may or may not support multiple priorities; it may or may not
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perform error checking on arguments passed to the scheduler routines.
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In the context of a desktop application a button widget may contain
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some text or it may contain a picture; the text may be displayed in a
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variety of fonts; the foreground and background color may vary. When
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an application uses a component there must be some way of specifying
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the desired behavior. The component writer has no way of knowing in
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advance exactly how a particular component will end up being used.</P
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><P
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>One way to control the behavior is at run time. The application
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creates an instance of a button object, and then instructs this object
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to display either text or a picture. No special effort by the
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application developer is required, since a button can always support
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all desired behavior. There is of course a major disadvantage in
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terms of the size of the final application image: the code that gets
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linked with the application has to provide support for all possible
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behavior, even if the application does not require it.</P
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><P
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>Another approach is to control the behavior at link-time, typically
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by using inheritance in an object-oriented language. The button
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library provides an abstract base class <TT
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CLASS="CLASSNAME"
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>Button</TT
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>
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and derived classes <TT
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CLASS="CLASSNAME"
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>TextButton</TT
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> and
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<TT
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CLASS="CLASSNAME"
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>PictureButton</TT
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>. If an application only uses text
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buttons then it will only create objects of type
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<TT
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CLASS="CLASSNAME"
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>TextButton</TT
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>, and the code for the
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<TT
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CLASS="CLASSNAME"
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>PictureButton</TT
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> class does not get used. In
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many cases this approach works rather well and reduces the final image
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size, but there are limitations. The main one is that you can only
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have so many derived classes before the system gets unmanageable: a
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derived class
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<TT
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CLASS="CLASSNAME"
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>TextButtonUsingABorderWidthOfOnePlusAWhiteBackgroundAndBlackForegroundAndATwelvePointTimesFontAndNoErrorCheckingOrAssertions</TT
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>
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is not particularly sensible as far as most application developers are
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concerned.</P
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><P
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>The <SPAN
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CLASS="APPLICATION"
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>eCos</SPAN
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> component framework allows the behavior of components to
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be controlled at an even earlier time: when the component source code
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gets compiled and turned into a library. The button component could
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provide options, for example an option that only text buttons need to
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be supported. The component gets built and becomes part of a library
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intended specifically for the application, and the library will
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contain only the code that is required by this application and nothing
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else. A different application with different requirements would need
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its own version of the library, configured separately.</P
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><P
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>In theory compile-time configurability should give the best possible
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results in terms of code size, because it allows code to be controlled
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at the individual statement level rather than at the function or
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object level. Consider an example more closely related to embedded
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systems, a package to support multi-threading. A standard routine
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within such a package allows applications to kill threads
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asynchronously: the POSIX routine for this is
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<TT
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CLASS="FUNCTION"
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>pthread_cancel</TT
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>; the equivalent routine in µITRON
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is <TT
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CLASS="FUNCTION"
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>ter_tsk</TT
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>. These routines themselves tend to
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involve a significant amount of code, but that is not the real
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problem: other parts of the system require extra code and data for the
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kill routine to be able to function correctly. For example if a thread
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is blocked while waiting on a mutex and is killed off by another
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thread then the kill operation may have to do two things: remove the
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thread from the mutex's queue of waiting threads; and undo the
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effects, if any, of priority inheritance. The implementation requires
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extra fields in the thread data structure so that the kill routine
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knows about the thread's current state, and extra code in the mutex
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routines to fill in and clear these extra fields correctly.</P
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><P
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>Most embedded applications do not require the ability to kill off a
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thread asynchronously, and hence the kill routine will not get linked
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into the final application image. Without compile-time configurability
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this would still mean that the mutex code and similar parts of the
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system contain code and data that serve no useful purpose in this
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application. The <SPAN
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CLASS="APPLICATION"
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>eCos</SPAN
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> approach allows the user to select that the
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thread kill functionality is not required, and all the components can
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adapt to this at compile-time. For example the code in the mutex lock
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routine contains statements to support the killing of threads, but
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these statements will only get compiled in if that functionality is
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required. The overall result is that the final application image
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contains only the code and data that is really needed for the
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application to work, and nothing else.</P
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><P
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>Of course there are complications. To return to the button example,
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the application code might only use text buttons directly, but it
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might also use some higher-level widget such as a file selector and
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this file selector might require buttons with pictures. Therefore the
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button code must still be compiled to support pictures as well as
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text. The configuration tools must be aware of the dependencies
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between components and ensure that the internal constraints are met,
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as well as the external requirements of the application code. An area
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of particular concern is conflicting requirements: a button component
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might be written in such a way that it can only support either text
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buttons or picture buttons, but not both in one application; this
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would represent a weakness in the component itself rather than in the
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component framework as a whole.</P
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><P
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>Compile-time configurability is not intended to replace the other
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approaches but rather to complement them. There will be times when
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run-time selection of behavior is desirable: for example an
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application may need to be able to change the baud rate of a serial
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line, and the system must then provide a way of doing this at
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run-time. There will also be times when link-time selection is
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desirable: for example a C library might provide two different random
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number routines <TT
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CLASS="FUNCTION"
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>rand</TT
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> and
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<TT
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CLASS="FUNCTION"
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>lrand48</TT
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>; these do not affect other code so there
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is no good reason for the C library component not to provide both of
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these, and allow the application code to use none, one, or both of
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them as appropriate; any unused functions will just get eliminated at
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link-time. Compile-time selection of behavior is another option, and
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it can be the most powerful one of the three and the best suited to
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embedded systems development.</P
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>Why Configurability?</TD
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>Degrees of Configurability</TD
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