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>The <SPAN
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>eCos</SPAN
> Component Writer's Guide</TH
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="BUILD.HEADERS">Configuration Header File Generation</H1
><P
>Configuration options can affect a build in two main ways. First,
enabling a configuration option or other <SPAN
CLASS="APPLICATION"
>CDL</SPAN
> entity can result in
various files being built and added to a library, thus providing
functionality to the application code. However this mechanism can only
operate at a rather coarse grain, at the level of entire source files.
Hence the component framework also generates configuration header
files containing mainly C preprocessor <TT
CLASS="LITERAL"
>#define</TT
>
directives. Package source code can then <TT
CLASS="LITERAL"
>#include</TT
>
the appropriate header files and use <TT
CLASS="LITERAL"
>#if</TT
>,
<TT
CLASS="LITERAL"
>#ifdef</TT
> and <TT
CLASS="LITERAL"
>#ifndef</TT
> directives to
adapt accordingly. In this way configuration options can be used to
enable or disable entire functions within a source file or just a
single line, whichever is appropriate.</P
><P
>The configuration header files end up in the <TT
CLASS="FILENAME"
>include/pkgconf</TT
> subdirectory of the
install tree. There will be one header file for the system as a whole,
<TT
CLASS="FILENAME"
>pkgconf/system.h</TT
>, and there will
be additional header files for each package, for example
<TT
CLASS="FILENAME"
>pkgconf/kernel.h</TT
>. The header files
are generated when creating or updating the build and install trees,
which needs to happen after every change to the configuration.</P
><P
>The component framework processes each package in the configuration
one at a time. The exact order in which the packages are processed is
not defined, so the order in which <TT
CLASS="LITERAL"
>#define's</TT
> will
end up in the global <TT
CLASS="FILENAME"
>pkgconf/system.h</TT
> header may vary. However
for any given configuration the order should remain consistent until
packages are added to or removed from the system. This avoids
unnecessary changes to the global header file and hence unnecessary
rebuilds of the packages and of application code because of header
file dependency handling.</P
><P
>Within a given package the various components, options and interfaces
will be processed in the order in which they were defined in the
corresponding <SPAN
CLASS="APPLICATION"
>CDL</SPAN
> scripts. Typically the data in the configuration
headers consists only of a sequence of <TT
CLASS="LITERAL"
>#define's</TT
> so
the order in which these are generated is irrelevant, but some
properties such as <SPAN
CLASS="PROPERTY"
>define_proc</SPAN
> can be used to add arbitrary data to
a configuration header and hence there may be dependencies on the
order. It should be noted that re-parenting an option below some other
package has no effect on which header file will contain the
corresponding <TT
CLASS="LITERAL"
>#define</TT
>: the preprocessor directives
will always end up in the header file for the package that defines the
option, or in the global configuration header.</P
><P
>There are six properties which affect the process of generating header
files:
<A
HREF="ref.define-header.html"
><SPAN
CLASS="PROPERTY"
>define_header</SPAN
></A
>,
<A
HREF="ref.no-define.html"
><SPAN
CLASS="PROPERTY"
>no_define</SPAN
></A
>,
<A
HREF="ref.define-format.html"
><SPAN
CLASS="PROPERTY"
>define_format</SPAN
></A
>,
<A
HREF="ref.define.html"
><SPAN
CLASS="PROPERTY"
>define</SPAN
></A
>,
<A
HREF="ref.if-define.html"
><SPAN
CLASS="PROPERTY"
>if_define</SPAN
></A
>, and
<A
HREF="ref.define-proc.html"
><SPAN
CLASS="PROPERTY"
>define_proc</SPAN
></A
>.</P
><P
>The <SPAN
CLASS="PROPERTY"
>define_header</SPAN
> property can only occur in the body of a
<TT
CLASS="LITERAL"
>cdl_package</TT
> command and specifies the name of the header file which
should contain the package's configuration data, for example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>cdl_package <some_package> {
…
define_header xyzzy.h
}</PRE
></TD
></TR
></TABLE
><P
>Given such a <SPAN
CLASS="PROPERTY"
>define_header</SPAN
> property the component framework will
use the file <TT
CLASS="FILENAME"
>pkgconf/xyzzy.h</TT
> for
the package's configuration data. If a package does not have
a <SPAN
CLASS="PROPERTY"
>define_header</SPAN
> property then a suitable file name is constructed
from the package's name. This involves:</P
><P
></P
><OL
TYPE="1"
><LI
><P
>All characters in the package name up to and including the first
underscore are removed. For example <TT
CLASS="VARNAME"
>CYGPKG_KERNEL</TT
>
is converted to <TT
CLASS="LITERAL"
>KERNEL</TT
>, and
<TT
CLASS="VARNAME"
>CYGPKG_HAL_ARM</TT
> is converted to
<TT
CLASS="LITERAL"
>HAL_ARM</TT
>.</P
></LI
><LI
><P
>Any upper case letters in the resulting string will be converted to
lower case, yielding e.g. <TT
CLASS="LITERAL"
>kernel</TT
> and
<TT
CLASS="LITERAL"
>hal_arm</TT
>.</P
></LI
><LI
><P
>A <TT
CLASS="LITERAL"
>.h</TT
> suffix is appended, yielding e.g.
<TT
CLASS="LITERAL"
>kernel.h</TT
> and <TT
CLASS="LITERAL"
>hal_arm.h</TT
>.</P
></LI
></OL
><P
>Because of the naming restrictions on configuration options, this
should result in a valid filename. There is a small possibility of a
file name class, for example <TT
CLASS="VARNAME"
>CYGPKG_PLUGH</TT
> and
<TT
CLASS="VARNAME"
>CYGPKG_plugh</TT
> would both end up trying to use the
same header file <TT
CLASS="FILENAME"
>pkgconf/plugh.h</TT
>,
but the use of lower case letters for package names violates the
naming conventions. It is not legal to use the <SPAN
CLASS="PROPERTY"
>define_header</SPAN
>
property to put the configuration data for several packages in a
single header file. The resulting behaviour is undefined.</P
><P
>Once the name of the package's header file has been determined and the
file has been opened, the various components, options and interfaces
in the package will be processed starting with the package itself. The
following steps are involved:</P
><P
></P
><OL
TYPE="1"
><LI
><P
>If the current option or other <SPAN
CLASS="APPLICATION"
>CDL</SPAN
> entity is inactive or disabled,
the option is ignored for the purposes of header file generation.
<TT
CLASS="LITERAL"
>#define's</TT
> are only generated for options that are
both active and enabled.</P
></LI
><LI
><P
>The next step is to generate a default <TT
CLASS="LITERAL"
>#define</TT
> for
the current option. If this option has a <SPAN
CLASS="PROPERTY"
>no_define</SPAN
> property then the
default <TT
CLASS="LITERAL"
>#define</TT
> is suppressed, and processing
continues for <SPAN
CLASS="PROPERTY"
>define</SPAN
>, <SPAN
CLASS="PROPERTY"
>if_define</SPAN
> and <SPAN
CLASS="PROPERTY"
>define_proc</SPAN
> properties.</P
><P
></P
><OL
TYPE="a"
><LI
><P
>The header file appropriate for the default <TT
CLASS="LITERAL"
>#define</TT
>
is determined. For a <TT
CLASS="LITERAL"
>cdl_package</TT
> this will be <TT
CLASS="FILENAME"
>pkgconf/system.h</TT
>, for any other option this
will be the package's own header file. The intention here is that
packages and application code can always determine which packages are
in the configuration by <TT
CLASS="LITERAL"
>#include'ing</TT
> <TT
CLASS="FILENAME"
>pkgconf/system.h</TT
>. The C preprocessor lacks
any facilities for including a header file only if it exists, and
taking appropriate action otherwise.</P
></LI
><LI
><P
>For options with the flavors <TT
CLASS="LITERAL"
>bool</TT
> or
<TT
CLASS="LITERAL"
>none</TT
>, a single <TT
CLASS="LITERAL"
>#define</TT
> will be
generated. This takes the form:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define <option> 1</PRE
></TD
></TR
></TABLE
><P
>For example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define CYGFUN_LIBC_TIME_POSIX 1</PRE
></TD
></TR
></TABLE
><P
>Package source code can check whether or not an option is active and
enabled by using the <TT
CLASS="LITERAL"
>#ifdef</TT
>,
<TT
CLASS="LITERAL"
>#ifndef</TT
> or <TT
CLASS="LITERAL"
>#if
defined(…)</TT
>directives.</P
></LI
><LI
><P
>For options with the flavors <TT
CLASS="LITERAL"
>data</TT
> or
<TT
CLASS="LITERAL"
>booldata</TT
>, either one or two
<TT
CLASS="LITERAL"
>#define's</TT
> will be generated. The first of these may
be affected by a <SPAN
CLASS="PROPERTY"
>define_format</SPAN
> property. If this property is not
defined then the first <TT
CLASS="LITERAL"
>#define</TT
> will take the form:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define <option> <value></PRE
></TD
></TR
></TABLE
><P
>For example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define CYGNUM_LIBC_ATEXIT_HANDLERS 32</PRE
></TD
></TR
></TABLE
><P
>Package source code can examine this value using the
<TT
CLASS="LITERAL"
>#if</TT
> directive, or by using the symbol in
code such as:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> for (i = 0; i < CYGNUM_LIBC_ATEXIT_HANDLERS; i++) {
…
}</PRE
></TD
></TR
></TABLE
><P
>It must be noted that the <TT
CLASS="LITERAL"
>#define</TT
> will be generated
only if the corresponding option is both active and enabled. Options
with the <TT
CLASS="LITERAL"
>data</TT
> flavor are always enabled but may not
be active. Code like the above should be written only if it is known
that the symbol will always be defined, for example if the
corresponding source file will only get built if the containing
component is active and enabled. Otherwise the use of additional
<TT
CLASS="LITERAL"
>#ifdef</TT
> or similar directives will be necessary.</P
></LI
><LI
><P
>If there is a <SPAN
CLASS="PROPERTY"
>define_format</SPAN
> property then this controls how the
option's value will appear in the header file. Given a format string
such as <TT
CLASS="LITERAL"
>%08x</TT
> and a value 42, the component
framework will execute the <SPAN
CLASS="APPLICATION"
>Tcl</SPAN
> command
<TT
CLASS="LITERAL"
>format %08x 42</TT
> and the result will be
used for the <TT
CLASS="LITERAL"
>#define's</TT
> value. It is the
responsibility of the component writer to make sure that this <SPAN
CLASS="APPLICATION"
>Tcl</SPAN
>
command will be valid given the format string and the legal values for
the option.</P
></LI
><LI
><P
>In addition a second <TT
CLASS="LITERAL"
>#define</TT
> may or may not be
generated. This will take the form:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define <option>_<value></PRE
></TD
></TR
></TABLE
><P
>For example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define CYGNUM_LIBC_ATEXIT_HANDLERS_32</PRE
></TD
></TR
></TABLE
><P
>The <TT
CLASS="LITERAL"
>#define</TT
> will be generated only if it would
result in a valid C preprocessor symbol. If the value is a string such
as <TT
CLASS="LITERAL"
>"/dev/ser0"</TT
> then the <TT
CLASS="LITERAL"
>#define</TT
>
would be suppressed. This second <TT
CLASS="LITERAL"
>#define</TT
> is not
particularly useful for numerical data, but can be valuable in other
circumstances. For example if the legal values for an option
<TT
CLASS="LITERAL"
>XXX_COLOR</TT
> are <TT
CLASS="LITERAL"
>red</TT
>,
<TT
CLASS="LITERAL"
>green</TT
> and <TT
CLASS="LITERAL"
>blue</TT
> then code like
the following can be used:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#ifdef XXX_COLOR_red
…
#endif
#ifdef XXX_COLOR_green
…
#endif
#ifdef XXX_COLOR_blue
…
#endif</PRE
></TD
></TR
></TABLE
><P
>The expression syntax provided by the C preprocessor is limited to
numerical data and cannot perform string comparisons. By generating
two <TT
CLASS="LITERAL"
>#define's</TT
> in this way it is possible to work
around this limitation of the C preprocessor. However some care has to
be taken: if a component writer also defined a configuration option
<TT
CLASS="LITERAL"
>XXX_COLOR_green</TT
> then there will be confusion. Since
such a configuration option violates the naming conventions, the
problem is unlikely to arise in practice.</P
></LI
></OL
></LI
><LI
><P
>For some options it may be useful to generate one or more additional
<TT
CLASS="LITERAL"
>#define's</TT
> or, in conjunction with the <SPAN
CLASS="PROPERTY"
>no_define</SPAN
>
property, to define a symbol with a name different from the option's
name. This can be achieved with the <SPAN
CLASS="PROPERTY"
>define</SPAN
> property, which takes the
following form:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> define [-file=<filename>] [-format=<format>] <symbol></PRE
></TD
></TR
></TABLE
><P
>For example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> define FOPEN_MAX</PRE
></TD
></TR
></TABLE
><P
>This will result in something like:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define FOPEN_MAX 8
#define FOPEN_MAX_8</PRE
></TD
></TR
></TABLE
><P
>The specified symbol must be a valid C preprocessor symbol. Normally
the <TT
CLASS="LITERAL"
>#define</TT
> will end up in the same header file as
the default one, in other words <TT
CLASS="FILENAME"
>pkgconf/system.h</TT
> in the case of a
<TT
CLASS="LITERAL"
>cdl_package</TT
>, or the package's own header file for any other option.
The <TT
CLASS="LITERAL"
>-file</TT
> option can be used to change this. At
present the only legal value is <TT
CLASS="LITERAL"
>system.h</TT
>, for
example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> define -file=system.h <symbol></PRE
></TD
></TR
></TABLE
><P
>This will cause the <TT
CLASS="LITERAL"
>#define</TT
> to end up in the global
configuration header rather than in the package's own header. Use of
this facility should be avoided since it is very rarely necessary to
make options globally visible.</P
><P
>The <SPAN
CLASS="PROPERTY"
>define</SPAN
> property takes another option,
<TT
CLASS="LITERAL"
>-format</TT
>, to provide a format string.</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> define -format=%08x <symbol></PRE
></TD
></TR
></TABLE
><P
>This should only be used for options with the <TT
CLASS="LITERAL"
>data</TT
>
or <TT
CLASS="LITERAL"
>booldata</TT
> flavor, and has the same effect as the
<SPAN
CLASS="PROPERTY"
>define_format</SPAN
> property has on the default
<TT
CLASS="LITERAL"
>#define</TT
>.</P
><P
><SPAN
CLASS="PROPERTY"
>define</SPAN
> properties are processed in the same way the default
<TT
CLASS="LITERAL"
>#define</TT
>. For options with the
<TT
CLASS="LITERAL"
>bool</TT
> or <TT
CLASS="LITERAL"
>none</TT
> flavors a single
<TT
CLASS="LITERAL"
>#define</TT
> will be generated using the value
<TT
CLASS="LITERAL"
>1</TT
>. For options with the <TT
CLASS="LITERAL"
>data</TT
> or
<TT
CLASS="LITERAL"
>booldata</TT
> flavors either one or two
<TT
CLASS="LITERAL"
>#define's</TT
> will be generated.</P
></LI
><LI
><P
>After processing all <SPAN
CLASS="PROPERTY"
>define</SPAN
> properties, the component framework will
look for any <SPAN
CLASS="PROPERTY"
>if_define</SPAN
> properties. These take the following form:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> if_define [-file=<filename>] <symbol1> <symbol2></PRE
></TD
></TR
></TABLE
><P
>For example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> if_define CYGSRC_KERNEL CYGDBG_USE_ASSERTS</PRE
></TD
></TR
></TABLE
><P
>The following will be generated in the configuration header file:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#ifdef CYGSRC_KERNEL
# define CYGDBG_USE_ASSERTS
#endif</PRE
></TD
></TR
></TABLE
><P
>Typical kernel source code would begin with the following construct:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define CYGSRC_KERNEL 1
#include <pkgconf/kernel.h>
#include <cyg/infra/cyg_ass.h></PRE
></TD
></TR
></TABLE
><P
>The infrastructure header file <TT
CLASS="FILENAME"
>cyg/infra/cyg_ass.h</TT
> only checks for symbols
such as <TT
CLASS="LITERAL"
>CYGDBG_USE_ASSERTS</TT
>, and has no special
knowledge of the kernel or any other package. The <SPAN
CLASS="PROPERTY"
>if_define</SPAN
> property
will only affect code that defines the symbol
<TT
CLASS="LITERAL"
>CYGSRC_KERNEL</TT
>, so typically only kernel source
code. If the option is enabled then assertion support will be enabled
for the kernel source code only. If the option is inactive or disabled
then kernel assertions will be disabled. Assertions in other packages
are not affected. Thus the <SPAN
CLASS="PROPERTY"
>if_define</SPAN
> property allows control over
assertions, tracing, and similar facilities at the level of individual
packages, or at finer levels such as components or even single source
files if desired.</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>Current <SPAN
CLASS="APPLICATION"
>eCos</SPAN
> packages do not yet make use of this facility. Instead
there is a single global configuration option
<TT
CLASS="VARNAME"
>CYGDBG_USE_ASSERTS</TT
> which is used to enable or
disable assertions for all packages. This issue should be addressed in
a future release of the system.</P
></BLOCKQUOTE
></DIV
><P
>As with the <SPAN
CLASS="PROPERTY"
>define</SPAN
> property, the <SPAN
CLASS="PROPERTY"
>if_define</SPAN
> property takes an
option <TT
CLASS="LITERAL"
>-file</TT
> with a single legal value
<TT
CLASS="LITERAL"
>system.h</TT
>. This allows the output to be redirected
to <TT
CLASS="FILENAME"
>pkgconf/system.h</TT
> if and when
necessary. </P
></LI
><LI
><P
>The final property that is relevant to configuration header file
generation is <SPAN
CLASS="PROPERTY"
>define_proc</SPAN
>. This takes a single argument, a <SPAN
CLASS="APPLICATION"
>Tcl</SPAN
>
fragment that can add arbitrary data to the global header <TT
CLASS="FILENAME"
>pkgconf/system.h</TT
> and to the package's own
header. When the <SPAN
CLASS="PROPERTY"
>define_proc</SPAN
> script is invoked two variables will be
set up to allow access to these headers: <TT
CLASS="LITERAL"
>cdl_header</TT
>
will be a channel to the package's own header file, for example
<TT
CLASS="FILENAME"
>pkgconf/kernel.h</TT
>;
<TT
CLASS="LITERAL"
>cdl_system_header</TT
> will be a channel to <TT
CLASS="FILENAME"
>pkgconf/system.h</TT
>. A typical <SPAN
CLASS="PROPERTY"
>define_proc</SPAN
>
script will use the <SPAN
CLASS="APPLICATION"
>Tcl</SPAN
> <TT
CLASS="LITERAL"
>puts</TT
> command to output
data to one of these channels, for example:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>cdl_option <name> {
…
define_proc {
puts $::cdl_header "#define XXX 1"
}
}</PRE
></TD
></TR
></TABLE
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>In the current implementation the use of <SPAN
CLASS="PROPERTY"
>define_proc</SPAN
> is limited
because the <SPAN
CLASS="APPLICATION"
>Tcl</SPAN
> script cannot access any of the configuration data.
Therefore the script is limited to writing constant data to the
configuration headers. This is a major limitation which will be
addressed in a future release of the component framework.</P
></BLOCKQUOTE
></DIV
></LI
></OL
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>Generating C header files with <TT
CLASS="LITERAL"
>#define's</TT
> for the
configuration data suffices for existing packages written in some
combination of C, C++ and assembler. It can also be used in
conjunction with some other languages, for example by first passing
the source code through the C preprocessor and feeding the result into
the appropriate compiler. In future versions of the component
framework additional programming languages such as Java may be
supported, and the configuration data may also be written to files in
some format other than C preprocessor directives. </P
></BLOCKQUOTE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>At present there is no way for application or package source code to
get hold of all the configuration details related to the current
hardware. Instead that information is spread over various different
configuration headers for the HAL and device driver packages, with
some of the information going into <TT
CLASS="FILENAME"
>pkgconf/system.h</TT
>. It is possible that in
some future release of the system there will be another global
configuration header file <TT
CLASS="FILENAME"
>pkgconf/hardware.h</TT
> which either contains the
configuration details for the various hardware-specific packages or
which <TT
CLASS="LITERAL"
>#include's</TT
> all the hardware-specific
configuration headers. The desirability and feasibility of such a
scheme are still to be determined. To avoid future incompatibility
problems as a result of any such changes, it is recommended that all
hardware packages (in other packages containing the <SPAN
CLASS="PROPERTY"
>hardware</SPAN
>
property) use the <SPAN
CLASS="PROPERTY"
>define_header</SPAN
> property to specify explicitly which
configuration header should be generated.</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="BUILD.HEADERS.SYSTEM.H">The <TT
CLASS="FILENAME"
>system.h</TT
> Header</H2
><P
>Typically configuration header files are <TT
CLASS="LITERAL"
>#include'd</TT
>
only by the package's source code at build time, or by a package's
exported header files if the interface provided by the package may be
affected by a configuration option. There should be no need for
application code to know the details of individual configuration
options, instead the configuration should specifically meet the needs
of the application.</P
><P
>There are always exceptions. Application code may want to adapt to
configuration options, for example to do different things for ROM and
RAM booting systems, or when it is necessary to support several
different target boards. This is especially true if the code in question
is really re-usable library code which has not been converted to an
eCos package, and hence cannot use any CDL facilities.</P
><P
>A major problem here is determining which packages are in the
configuration: attempting to <TT
CLASS="LITERAL"
>#include</TT
> a header file
such as <TT
CLASS="FILENAME"
>pkgconf/net.h</TT
>
when it is not known for certain that that particular package is part
of the configuration will result in compilation errors. The global
header file <TT
CLASS="FILENAME"
>pkgconf/system.h</TT
>
serves to provide such information, so application code can use
techniques like the following:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#include <pkgconf/system.h>
#ifdef CYGPKG_NET
# include <pkgconf/net.h>
#endif</PRE
></TD
></TR
></TABLE
><P
>This will compile correctly irrespective of the eCos configuration,
and subsequent code can use <TT
CLASS="LITERAL"
>#ifdef</TT
> or similar
directives on <TT
CLASS="LITERAL"
>CYGPKG_NET</TT
> or any of the
configuration options in that package. </P
><P
>In addition to determining whether or not a package is present, the
global configuration header file can also be used to find out the
specific version of a package that is being used. This can be useful
if a more recent version exports additional functionality. It may also
be necessary to adapt to incompatible changes in the exported
interface or to changes in behaviour. For each package the
configuration system will typically <TT
CLASS="LITERAL"
>#define</TT
> three
symbols, for example for a V1.3.1 release:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define CYGNUM_NET_VERSION_MAJOR 1
#define CYGNUM_NET_VERSION_MINOR 3
#define CYGNUM_NET_VERSION_RELEASE 1</PRE
></TD
></TR
></TABLE
><P
>There are a number of problems associated with such version
<TT
CLASS="LITERAL"
>#define's</TT
>. The first restriction is that the
package must follow the standard naming conventions, so the package
name must be of the form <TT
CLASS="LITERAL"
>xxxPKG_yyy</TT
>. The three
characters immediately preceding the first underscore must be
<TT
CLASS="LITERAL"
>PKG</TT
>, and will be replaced with
<TT
CLASS="LITERAL"
>NUM</TT
> when generating the version
<TT
CLASS="LITERAL"
>#define's</TT
>. If a package does not follow the naming
convention then no version <TT
CLASS="LITERAL"
>#define's</TT
> will be
generated. </P
><P
>Assuming the package does follow the naming conventions, the
configuration tools will always generate three version
<TT
CLASS="LITERAL"
>#define's</TT
> for the major, minor, and release
numbers. The symbol names are obtained from the package name by
replacing <TT
CLASS="LITERAL"
>PKG</TT
> with <TT
CLASS="LITERAL"
>NUM</TT
> and
appending <TT
CLASS="LITERAL"
>_VERSION_MAJOR</TT
>,
<TT
CLASS="LITERAL"
>_VERSION_MINOR</TT
> and
<TT
CLASS="LITERAL"
>_VERSION_RELEASE</TT
>. It is assumed that the resulting
symbols will not clash with any configuration option names. The values
for the <TT
CLASS="LITERAL"
>#define's</TT
> are determined by searching the
version string for sequences of digits, optionally preceded by a minus
sign. It is possible that some or all of the numbers are absent in any
given version string, in which case <TT
CLASS="LITERAL"
>-1</TT
> will be used
in the <TT
CLASS="LITERAL"
>#define</TT
>. For example, given a version string
of <TT
CLASS="LITERAL"
>V1.12beta</TT
>, the major version number is
<TT
CLASS="LITERAL"
>1</TT
>, the minor number is <TT
CLASS="LITERAL"
>12</TT
>, and
the release number is <TT
CLASS="LITERAL"
>-1</TT
>. Given a version string of
<TT
CLASS="LITERAL"
>beta</TT
> all three numbers would be set to
<TT
CLASS="LITERAL"
>-1</TT
>.</P
><P
>There is special case code for the version <TT
CLASS="LITERAL"
>current</TT
>,
which typically corresponds to a development version obtained via
anonymous CVS or similar means. The configuration system has special
built-in knowledge of this version, and will assume it is more recent
than any specific release number. The global configuration header
defines a special symbol <TT
CLASS="LITERAL"
>CYGNUM_VERSION_CURRENT</TT
>,
and this will be used as the major version number when version
<TT
CLASS="LITERAL"
>current</TT
> of a package is used:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#define CYGNUM_VERSION_CURRENT 0x7fffff00
...
#define CYGNUM_INFRA_VERSION_MAJOR CYGNUM_VERSION_CURRENT
#define CYGNUM_INFRA_VERSION_MINOR -1
#define CYGNUM_INFRA_VERSION_RELEASE -1</PRE
></TD
></TR
></TABLE
><P
>The large number used for <TT
CLASS="LITERAL"
>CYGNUM_VERSION_CURRENT</TT
>
should ensure that major version comparisons work as expected, while
still allowing for a small amount of arithmetic in case that proves
useful. </P
><P
>It should be noted that this implementation of version
<TT
CLASS="LITERAL"
>#define's</TT
> will not cope with all version number
schemes. However for many cases it should suffice.</P
></DIV
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