All the options in a given configuration live in the same namespace.
Furthermore it is not possible for two separate options to have the
same name, because this would make any references to those options in
CDL expressions ambiguous. A naming convention exists to avoid
problems. It is recommended that component writers observe some or all
of this convention to reduce the probability of name clashes with
other packages.
There is an important restriction on option names. Typically the
component framework will output a #define for every
active and enabled option, using the name as the symbol being defined.
This requires that all names are valid C preprocessor symbols, a
limitation that is enforced even for options which have the
no_define property. Preprocessor symbols can be any sequence of
lower case letters a-z, upper
case letters, A-Z, the
underscore character _, and the digits
0-9. The first character must be
a non-digit. Using an underscore as the first character is
discouraged, because that may clash with reserved language
identifiers. In addition there is a convention that preprocessor
symbols only use upper case letters, and some component writers may
wish to follow this convention.
A typical option name could be something like
CYGSEM_KERNEL_SCHED_BITMAP. This name consists of
several different parts:
The first few characters, in this case the three letters
CYG, are used to identify the organization that
produced the package. For historical reasons packages produced by Red
Hat tend to use the prefix CYG rather than
RHAT. Component writers should use their own
prefix: even when cutting and pasting from an existing CDL script
the prefix should be changed to something appropriate to their
organization.
It can be argued that a short prefix, often limited to upper case
letters, is not sufficiently long to eliminate the possibility of
name clashes. A longer prefix could be used, for example one based on
internet domain names. However the C preprocessor has no concept of
namespaces or import directives, so it would always
be necessary to use the full option name in component source code
which gets tedious - option names tend to be long enough as it is.
There is a small increased risk of name clashes, but this risk is felt
to be acceptable.
The next three characters indicate the nature of the option, for
example whether it affects the interface or just the implementation. A
list of common tags is given below.
The KERNEL_SCHED part indicates the location of the
option within the overall hierarchy. In this case the option is part of
the scheduling component of the kernel package. Having the hierarchy
details as part of the option name can help in understanding
configurable code and further reduces the probability of a name clash.
The final part, BITMAP, identifies the option
itself.
The three-character tag is intended to provide some additional
information about the nature of the option. There are a number of
pre-defined tags. However for many options there is a choice:
options related to the platform should normally use
HWR, but numerical options should normally use
NUM; a platform-related numerical option such as
the size of an interrupt stack could therefore use either tag.
There are no absolute rules, and it is left to component writers to
interpret the following guidelines:
xxxARC_
The ARC tag is intended for options related
to the processor architecture. Typically such options will only occur
in architectural or variant HAL packages.
xxxHWR_
The HWR tag is intended for options related to
the specific target board. Typically such options will only occur in
platform HAL packages.
xxxPKG_
This tag is intended for packages or components, in other words
options which extend the configuration hierarchy. Arguably a
COM tag would be more appropriate for
components, but this could be confusing because of the considerable
number of computing terms that begin with com.
xxxGLO_
This is intended for global configuration options, especially
preferences.
xxxDBG_
The DBG tag indicates that the option is in
some way related to debugging, for example it may enable assertions in
some part of the system.
xxxTST_
This tag is for testing-related options. Typically these do not
affect actual application code, instead they control the interaction
between target-side test cases and a host-side testing infrastructure.
xxxFUN_
This is for configuration options which affect the interface of a
package. There are a number of related tag which are also
interface-related. xxxFUN_ is intended primarily
for options that control whether or not one or more functions are
provided by the package, but can also be used if none of the other
interface-related tags is applicable.
xxxVAR_
This is analogous to FUN but controls the presence
or absence of one or more variables or objects.
xxxCLS_
The CLS tag is intended only for packages that
provide an object-oriented interface, and controls the presence or
absence of an entire class.
xxxMFN_
This is also for object-orientated interfaces, and indicates the
presence or absence of a member function rather than an entire class.
xxxSEM_
A SEM option does not affect the interface (or if
does affect the interface, this is incidental). Instead it is used for
options which have a fundamental effect on the semantic behavior of a
package. For example the choice of kernel schedulers is semantic in
nature: it does not affect the interface, in particular the function
cyg_thread_create exists irrespective of which
scheduler has been selected. However it does have a major impact on
the system's behavior.
xxxIMP_
IMP is for implementation options. These do not
affect either the interface or the semantic behavior (with the
possible exception of timing-related changes). A typical
implementation option controls whether or not a particular function or
set of functions should get inlined.
xxxNUM_
This tag is for numerical options, for example the number of
scheduling priority levels.
xxxDAT_
This is for data items that are not numerical in nature, for example a
device name.
xxxBLD_
The BLD tag indicates an option that affects
the build process, for example compiler flag settings.
xxxINT_
This should normally be used for CDL interfaces, which is a language
construct that is largely independent from the interface exported by a
package via its header files. For more details of CDL interfaces
see the Section called Interfaces.
xxxPRI_
This tag is not normally used for configuration options. Instead
it is used by CDL scripts to pass additional private information to
the source code via the configuration header files, typically inside a
define_proc property.
xxxSRC_
This tag is not normally used for configuration options. Instead
it can be used by package source code to interact with such options,
especially in the context of the if_define property.
There is one special case of a potential name clash that is worth
mentioning here. When the component framework generates a
configuration header file for a given package, by default it will use
a name derived from the package name (the define_header property can
be used to override this). The file name is constructed from the
package name by removing everything up to and including the first
underscore, converting the remainder of the name to lower case, and
appending a .h suffix. For example the kernel
package CYGPKG_KERNEL will involve a header file
pkgconf/kernel.h. If a
configuration contained some other package
XYZPKG_KERNEL then this would attempt to use the
same configuration header file, with unfortunate effects. Case
sensitivity could introduce problems as well, so a package
xyzpkg_kernel would involve the same problem. Even
if the header file names preserved the case of the package name, not
all file systems are case sensitive. There is no simple solution to
this problem. Changing the names of the generated configuration header
files would involve a major incompatible change to the interface, to
solve a problem which is essentially hypothetical in nature.
