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<HTML
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><HEAD
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><TITLE
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>Option Naming Convention</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="The CDL Language"
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HREF="language.html"><LINK
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REL="PREVIOUS"
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TITLE="CDL Properties"
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HREF="language.properties.html"><LINK
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REL="NEXT"
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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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>Prev</A
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 3. The CDL Language</TD
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WIDTH="10%"
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ALIGN="right"
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><H1
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CLASS="SECT1"
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><A
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NAME="LANGUAGE.NAMING">Option Naming Convention</H1
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><P
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>All the options in a given configuration live in the same namespace.
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Furthermore it is not possible for two separate options to have the
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same name, because this would make any references to those options in
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<SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> expressions ambiguous. A naming convention exists to avoid
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problems. It is recommended that component writers observe some or all
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of this convention to reduce the probability of name clashes with
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other packages.</P
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><P
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>There is an important restriction on option names. Typically the
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component framework will output a <TT
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CLASS="LITERAL"
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>#define</TT
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> for every
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active and enabled option, using the name as the symbol being defined.
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This requires that all names are valid C preprocessor symbols, a
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limitation that is enforced even for options which have the
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<SPAN
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CLASS="PROPERTY"
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>no_define</SPAN
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> property. Preprocessor symbols can be any sequence of
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lower case letters <TT
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CLASS="LITERAL"
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>a</TT
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>-<TT
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CLASS="LITERAL"
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>z</TT
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>, upper
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case letters, <TT
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CLASS="LITERAL"
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>A</TT
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>-<TT
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CLASS="LITERAL"
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>Z</TT
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>, the
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underscore character <TT
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CLASS="LITERAL"
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>_</TT
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>, and the digits
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<TT
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CLASS="LITERAL"
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>0</TT
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>-<TT
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CLASS="LITERAL"
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>9</TT
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>. The first character must be
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a non-digit. Using an underscore as the first character is
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discouraged, because that may clash with reserved language
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identifiers. In addition there is a convention that preprocessor
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symbols only use upper case letters, and some component writers may
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wish to follow this convention.</P
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><P
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>A typical option name could be something like
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<TT
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CLASS="VARNAME"
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>CYGSEM_KERNEL_SCHED_BITMAP</TT
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>. This name consists of
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several different parts:</P
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><P
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></P
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><OL
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TYPE="1"
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><LI
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><P
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>The first few characters, in this case the three letters
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<TT
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CLASS="LITERAL"
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>CYG</TT
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>, are used to identify the organization that
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produced the package. For historical reasons packages produced by Red
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Hat tend to use the prefix <TT
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CLASS="LITERAL"
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>CYG</TT
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> rather than
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<TT
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CLASS="LITERAL"
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>RHAT</TT
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>. Component writers should use their own
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prefix: even when cutting and pasting from an existing <SPAN
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CLASS="APPLICATION"
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>CDL</SPAN
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> script
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the prefix should be changed to something appropriate to their
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organization. </P
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><P
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>It can be argued that a short prefix, often limited to upper case
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letters, is not sufficiently long to eliminate the possibility of
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name clashes. A longer prefix could be used, for example one based on
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internet domain names. However the C preprocessor has no concept of
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namespaces or <TT
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CLASS="LITERAL"
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>import</TT
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> directives, so it would always
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be necessary to use the full option name in component source code
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which gets tedious - option names tend to be long enough as it is.
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There is a small increased risk of name clashes, but this risk is felt
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to be acceptable.</P
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></LI
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><LI
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><P
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>The next three characters indicate the nature of the option, for
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example whether it affects the interface or just the implementation. A
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list of common tags is given below.</P
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></LI
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><LI
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><P
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>The <TT
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CLASS="LITERAL"
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>KERNEL_SCHED</TT
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> part indicates the location of the
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option within the overall hierarchy. In this case the option is part of
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the scheduling component of the kernel package. Having the hierarchy
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details as part of the option name can help in understanding
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configurable code and further reduces the probability of a name clash.</P
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></LI
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|
|
><LI
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|
|
><P
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|
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>The final part, <TT
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CLASS="LITERAL"
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>BITMAP</TT
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>, identifies the option
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itself. </P
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></LI
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></OL
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|
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><P
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|
|
>The three-character tag is intended to provide some additional
|
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information about the nature of the option. There are a number of
|
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pre-defined tags. However for many options there is a choice:
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options related to the platform should normally use
|
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<TT
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CLASS="LITERAL"
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>HWR</TT
|
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>, but numerical options should normally use
|
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<TT
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CLASS="LITERAL"
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|
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>NUM</TT
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>; a platform-related numerical option such as
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the size of an interrupt stack could therefore use either tag.
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There are no absolute rules, and it is left to component writers to
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interpret the following guidelines:</P
|
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|
><P
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></P
|
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><DIV
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| 235 |
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CLASS="VARIABLELIST"
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|
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><DL
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><DT
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><TT
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CLASS="LITERAL"
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|
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>xxxARC_</TT
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|
|
></DT
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|
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><DD
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><P
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>The <TT
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CLASS="LITERAL"
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|
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>ARC</TT
|
| 247 |
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> tag is intended for options related
|
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to the processor architecture. Typically such options will only occur
|
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in architectural or variant HAL packages.</P
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| 250 |
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxHWR_</TT
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></DT
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><DD
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><P
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>The <TT
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CLASS="LITERAL"
|
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|
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>HWR</TT
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|
> tag is intended for options related to
|
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the specific target board. Typically such options will only occur in
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platform HAL packages.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxPKG_</TT
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></DT
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><DD
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><P
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>This tag is intended for packages or components, in other words
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options which extend the configuration hierarchy. Arguably a
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<TT
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CLASS="LITERAL"
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>COM</TT
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> tag would be more appropriate for
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components, but this could be confusing because of the considerable
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number of computing terms that begin with com.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxGLO_</TT
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></DT
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><DD
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><P
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>This is intended for global configuration options, especially
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preferences.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxDBG_</TT
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></DT
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><DD
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><P
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>The <TT
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CLASS="LITERAL"
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>DBG</TT
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> tag indicates that the option is in
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some way related to debugging, for example it may enable assertions in
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some part of the system.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxTST_</TT
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></DT
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><DD
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><P
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>This tag is for testing-related options. Typically these do not
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affect actual application code, instead they control the interaction
|
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between target-side test cases and a host-side testing infrastructure.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxFUN_</TT
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></DT
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><DD
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><P
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>This is for configuration options which affect the interface of a
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package. There are a number of related tag which are also
|
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interface-related. <TT
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CLASS="LITERAL"
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>xxxFUN_</TT
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> is intended primarily
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for options that control whether or not one or more functions are
|
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provided by the package, but can also be used if none of the other
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interface-related tags is applicable.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxVAR_</TT
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></DT
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><DD
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><P
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>This is analogous to <TT
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CLASS="LITERAL"
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>FUN</TT
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> but controls the presence
|
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or absence of one or more variables or objects.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxCLS_</TT
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></DT
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><DD
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><P
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>The <TT
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CLASS="LITERAL"
|
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|
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>CLS</TT
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|
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> tag is intended only for packages that
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provide an object-oriented interface, and controls the presence or
|
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absence of an entire class.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxMFN_</TT
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></DT
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><DD
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><P
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>This is also for object-orientated interfaces, and indicates the
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presence or absence of a member function rather than an entire class.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxSEM_</TT
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></DT
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><DD
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>A <TT
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>SEM</TT
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> option does not affect the interface (or if
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does affect the interface, this is incidental). Instead it is used for
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options which have a fundamental effect on the semantic behavior of a
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package. For example the choice of kernel schedulers is semantic in
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nature: it does not affect the interface, in particular the function
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<TT
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CLASS="FUNCTION"
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>cyg_thread_create</TT
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> exists irrespective of which
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scheduler has been selected. However it does have a major impact on
|
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the system's behavior.</P
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></DD
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><DT
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><TT
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>xxxIMP_</TT
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></DT
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><DD
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><P
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><TT
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CLASS="LITERAL"
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>IMP</TT
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> is for implementation options. These do not
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affect either the interface or the semantic behavior (with the
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| 404 |
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possible exception of timing-related changes). A typical
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implementation option controls whether or not a particular function or
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set of functions should get inlined.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxNUM_</TT
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></DT
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><DD
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><P
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>This tag is for numerical options, for example the number of
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scheduling priority levels.</P
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></DD
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><DT
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><TT
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></DT
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><DD
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><P
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>This is for data items that are not numerical in nature, for example a
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device name.</P
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></DD
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><DT
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><TT
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>xxxBLD_</TT
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></DT
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><DD
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><P
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>The <TT
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CLASS="LITERAL"
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>BLD</TT
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> tag indicates an option that affects
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the build process, for example compiler flag settings.</P
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><DT
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><TT
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>xxxINT_</TT
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></DT
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><DD
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><P
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>This should normally be used for <SPAN
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| 449 |
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CLASS="APPLICATION"
|
| 450 |
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>CDL</SPAN
|
| 451 |
|
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> interfaces, which is a language
|
| 452 |
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|
construct that is largely independent from the interface exported by a
|
| 453 |
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package via its header files. For more details of <SPAN
|
| 454 |
|
|
CLASS="APPLICATION"
|
| 455 |
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>CDL</SPAN
|
| 456 |
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> interfaces
|
| 457 |
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see <A
|
| 458 |
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HREF="language.interface.html"
|
| 459 |
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>the Section called <I
|
| 460 |
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>Interfaces</I
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| 461 |
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></A
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>.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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| 467 |
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>xxxPRI_</TT
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></DT
|
| 469 |
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><DD
|
| 470 |
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><P
|
| 471 |
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>This tag is not normally used for configuration options. Instead
|
| 472 |
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it is used by <SPAN
|
| 473 |
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CLASS="APPLICATION"
|
| 474 |
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>CDL</SPAN
|
| 475 |
|
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> scripts to pass additional private information to
|
| 476 |
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|
the source code via the configuration header files, typically inside a
|
| 477 |
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<SPAN
|
| 478 |
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CLASS="PROPERTY"
|
| 479 |
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>define_proc</SPAN
|
| 480 |
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> property.</P
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></DD
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><DT
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><TT
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CLASS="LITERAL"
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>xxxSRC_</TT
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></DT
|
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><DD
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| 488 |
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><P
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| 489 |
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>This tag is not normally used for configuration options. Instead
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| 490 |
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it can be used by package source code to interact with such options,
|
| 491 |
|
|
especially in the context of the <SPAN
|
| 492 |
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CLASS="PROPERTY"
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>if_define</SPAN
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> property.</P
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></DD
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></DL
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></DIV
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><P
|
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>There is one special case of a potential name clash that is worth
|
| 500 |
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|
mentioning here. When the component framework generates a
|
| 501 |
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configuration header file for a given package, by default it will use
|
| 502 |
|
|
a name derived from the package name (the <SPAN
|
| 503 |
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CLASS="PROPERTY"
|
| 504 |
|
|
>define_header</SPAN
|
| 505 |
|
|
> property can
|
| 506 |
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|
be used to override this). The file name is constructed from the
|
| 507 |
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|
package name by removing everything up to and including the first
|
| 508 |
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|
underscore, converting the remainder of the name to lower case, and
|
| 509 |
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appending a <TT
|
| 510 |
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CLASS="LITERAL"
|
| 511 |
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>.h</TT
|
| 512 |
|
|
> suffix. For example the kernel
|
| 513 |
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|
package <TT
|
| 514 |
|
|
CLASS="VARNAME"
|
| 515 |
|
|
>CYGPKG_KERNEL</TT
|
| 516 |
|
|
> will involve a header file
|
| 517 |
|
|
<TT
|
| 518 |
|
|
CLASS="FILENAME"
|
| 519 |
|
|
>pkgconf/kernel.h</TT
|
| 520 |
|
|
>. If a
|
| 521 |
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|
configuration contained some other package
|
| 522 |
|
|
<TT
|
| 523 |
|
|
CLASS="VARNAME"
|
| 524 |
|
|
>XYZPKG_KERNEL</TT
|
| 525 |
|
|
> then this would attempt to use the
|
| 526 |
|
|
same configuration header file, with unfortunate effects. Case
|
| 527 |
|
|
sensitivity could introduce problems as well, so a package
|
| 528 |
|
|
<TT
|
| 529 |
|
|
CLASS="VARNAME"
|
| 530 |
|
|
>xyzpkg_kernel</TT
|
| 531 |
|
|
> would involve the same problem. Even
|
| 532 |
|
|
if the header file names preserved the case of the package name, not
|
| 533 |
|
|
all file systems are case sensitive. There is no simple solution to
|
| 534 |
|
|
this problem. Changing the names of the generated configuration header
|
| 535 |
|
|
files would involve a major incompatible change to the interface, to
|
| 536 |
|
|
solve a problem which is essentially hypothetical in nature.</P
|
| 537 |
|
|
></DIV
|
| 538 |
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><DIV
|
| 539 |
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|
CLASS="NAVFOOTER"
|
| 540 |
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="language.properties.html"
|
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ACCESSKEY="P"
|
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>Prev</A
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="cdl-guide.html"
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ACCESSKEY="H"
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>Home</A
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="language.tcl.html"
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ACCESSKEY="N"
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>Next</A
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ALIGN="left"
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>CDL Properties</TD
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WIDTH="34%"
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ALIGN="center"
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>An Introduction to Tcl</TD
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