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<!--
__COPYRIGHT__
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
-->
<!--
=head2 Adding new methods
For slightly more demanding changes, you may wish to add new methods to the
C<cons> package. Here's an example of a very simple extension,
C<InstallScript>, which installs a tcl script in a requested location, but
edits the script first to reflect a platform-dependent path that needs to be
installed in the script:
# cons::InstallScript - Create a platform dependent version of a shell
# script by replacing string ``#!your-path-here'' with platform specific
# path $BIN_DIR.
sub cons::InstallScript {
my ($env, $dst, $src) = @_;
Command $env $dst, $src, qq(
sed s+your-path-here+$BIN_DIR+ %< > %>
chmod oug+x %>
);
}
Notice that this method is defined directly in the C<cons> package (by
prefixing the name with C<cons::>). A change made in this manner will be
globally visible to all environments, and could be called as in the
following example:
InstallScript $env "$BIN/foo", "foo.tcl";
For a small improvement in generality, the C<BINDIR> variable could be
passed in as an argument or taken from the construction environment-,-as
C<%BINDIR>.
=head2 Overriding methods
Instead of adding the method to the C<cons> name space, you could define a
new package which inherits existing methods from the C<cons> package and
overrides or adds others. This can be done using Perl's inheritance
mechanisms.
The following example defines a new package C<cons::switch> which
overrides the standard C<Library> method. The overridden method builds
linked library modules, rather than library archives. A new
constructor is provided. Environments created with this constructor
will have the new library method; others won't.
package cons::switch;
BEGIN {@ISA = 'cons'}
sub new {
shift;
bless new cons(@_);
}
sub Library {
my($env) = shift;
my($lib) = shift;
my(@objs) = Objects $env @_;
Command $env $lib, @objs, q(
%LD -r %LDFLAGS %< -o %>
);
}
This functionality could be invoked as in the following example:
$env = new cons::switch(@overrides);
...
Library $env 'lib.o', 'foo.c', 'bar.c';
-->
<para>
Although &SCons; provides many useful methods
for building common software products:
programs, libraries, documents.
you frequently want to be
able to build some other type of file
not supported directly by &SCons;
Fortunately, &SCons; makes it very easy
to define your own &Builder; objects
for any custom file types you want to build.
(In fact, the &SCons; interfaces for creating
&Builder; objects are flexible enough and easy enough to use
that all of the the &SCons; built-in &Builder; objects
are created the mechanisms described in this section.)
</para>
<section>
<title>Writing Builders That Execute External Commands</title>
<para>
The simplest &Builder; to create is
one that executes an external command.
For example, if we want to build
an output file by running the contents
of the input file through a command named
<literal>foobuild</literal>,
creating that &Builder; might look like:
</para>
<programlisting>
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
</programlisting>
<para>
All the above line does is create a free-standing
&Builder; object.
The next section will show us how to actually use it.
</para>
</section>
<section>
<title>Attaching a Builder to a &ConsEnv;</title>
<para>
A &Builder; object isn't useful
until it's attached to a &consenv;
so that we can call it to arrange
for files to be built.
This is done through the &cv-link-BUILDERS;
&consvar; in an environment.
The &cv-BUILDERS; variable is a Python dictionary
that maps the names by which you want to call
various &Builder; objects to the objects themselves.
For example, if we want to call the
&Builder; we just defined by the name
<function>Foo</function>,
our &SConstruct; file might look like:
</para>
<programlisting>
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
</programlisting>
<para>
With the &Builder; so attached to our &consenv;
we can now actually call it like so:
</para>
<programlisting>
env.Foo('file.foo', 'file.input')
</programlisting>
<para>
Then when we run &SCons; it looks like:
</para>
<screen>
% <userinput>scons -Q</userinput>
foobuild < file.input > file.foo
</screen>
<para>
Note, however, that the default &cv-BUILDERS;
variable in a &consenv;
comes with a default set of &Builder; objects
already defined:
&b-link-Program;, &b-link-Library;, etc.
And when we explicitly set the &cv-BUILDERS; variable
when we create the &consenv;,
the default &Builder;s are no longer part of
the environment:
</para>
<!--
The ToolSurrogate stuff that's used to capture output initializes
SCons.Defaults.ConstructionEnvironment with its own list of TOOLS.
In this next example, we want to show the user that when they
set the BUILDERS explicitly, the call to env.Program() generates
an AttributeError. This won't happen with all of the default
ToolSurrogates in the default construction environment. To make the
AttributeError show up, we have to overwite the default construction
environment's TOOLS variable so Program() builder doesn't show up.
We do this by executing a slightly different SConstruct file than the
one we print in the guide, with two extra statements at the front
that overwrite the TOOLS variable as described. Note that we have
to jam those statements on to the first line to keep the line number
in the generated error consistent with what the user will see in the
User's Guide.
-->
<programlisting>
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
</programlisting>
<screen>
% <userinput>scons -Q</userinput>
AttributeError: SConsEnvironment instance has no attribute 'Program':
File "/home/my/project/SConstruct", line 4:
env.Program('hello.c')
</screen>
<para>
To be able to use both our own defined &Builder; objects
and the default &Builder; objects in the same &consenv;,
you can either add to the &cv-BUILDERS; variable
using the &Append; function:
</para>
<programlisting>
env = Environment()
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env.Append(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
</programlisting>
<para>
Or you can explicitly set the appropriately-named
key in the &cv-BUILDERS; dictionary:
</para>
<programlisting>
env = Environment()
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env['BUILDERS']['Foo'] = bld
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
</programlisting>
<para>
Either way, the same &consenv;
can then use both the newly-defined
<function>Foo</function> &Builder;
and the default &b-link-Program; &Builder;:
</para>
<screen>
% <userinput>scons -Q</userinput>
foobuild < file.input > file.foo
cc -o hello.o -c hello.c
cc -o hello hello.o
</screen>
</section>
<section>
<title>Letting &SCons; Handle The File Suffixes</title>
<para>
By supplying additional information
when you create a &Builder;,
you can let &SCons; add appropriate file
suffixes to the target and/or the source file.
For example, rather than having to specify
explicitly that you want the <literal>Foo</literal>
&Builder; to build the <literal>file.foo</literal>
target file from the <literal>file.input</literal> source file,
you can give the <literal>.foo</literal>
and <literal>.input</literal> suffixes to the &Builder;,
making for more compact and readable calls to
the <literal>Foo</literal> &Builder;:
</para>
<programlisting>
bld = Builder(action = 'foobuild < $SOURCE > $TARGET',
suffix = '.foo',
src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file1')
env.Foo('file2')
</programlisting>
<screen>
% <userinput>scons -Q</userinput>
foobuild < file1.input > file1.foo
foobuild < file2.input > file2.foo
</screen>
<para>
You can also supply a <literal>prefix</literal> keyword argument
if it's appropriate to have &SCons; append a prefix
to the beginning of target file names.
</para>
</section>
<section>
<title>Builders That Execute Python Functions</title>
<para>
In &SCons;, you don't have to call an external command
to build a file.
You can, instead, define a Python function
that a &Builder; object can invoke
to build your target file (or files).
Such a &buildfunc; definition looks like:
</para>
<programlisting>
def build_function(target, source, env):
# Code to build "target" from "source"
return None
</programlisting>
<para>
The arguments of a &buildfunc; are:
</para>
<variablelist>
<varlistentry>
<term>target</term>
<listitem>
<para>
A list of Node objects representing
the target or targets to be
built by this builder function.
The file names of these target(s)
may be extracted using the Python &str; function.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>source</term>
<listitem>
<para>
A list of Node objects representing
the sources to be
used by this builder function to build the targets.
The file names of these source(s)
may be extracted using the Python &str; function.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>env</term>
<listitem>
<para>
The &consenv; used for building the target(s).
The builder function may use any of the
environment's construction variables
in any way to affect how it builds the targets.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
The builder function must
return a <literal>0</literal> or <literal>None</literal> value
if the target(s) are built successfully.
The builder function
may raise an exception
or return any non-zero value
to indicate that the build is unsuccessful,
</para>
<para>
Once you've defined the Python function
that will build your target file,
defining a &Builder; object for it is as
simple as specifying the name of the function,
instead of an external command,
as the &Builder;'s
<literal>action</literal>
argument:
</para>
<programlisting>
def build_function(target, source, env):
# Code to build "target" from "source"
return None
bld = Builder(action = build_function,
suffix = '.foo',
src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
</programlisting>
<para>
And notice that the output changes slightly,
reflecting the fact that a Python function,
not an external command,
is now called to build the target file:
</para>
<screen>
% <userinput>scons -Q</userinput>
build_function(["file.foo"], ["file.input"])
</screen>
</section>
<section>
<title>Builders That Create Actions Using a &Generator;</title>
<para>
&SCons; Builder objects can create an action "on the fly"
by using a function called a &generator;.
This provides a great deal of flexibility to
construct just the right list of commands
to build your target.
A &generator; looks like:
</para>
<programlisting>
def generate_actions(source, target, env, for_signature):
return 'foobuild < %s > %s' % (target[0], source[0])
</programlisting>
<para>
The arguments of a &generator; are:
</para>
<variablelist>
<varlistentry>
<term>source</term>
<listitem>
<para>
A list of Node objects representing
the sources to be built
by the command or other action
generated by this function.
The file names of these source(s)
may be extracted using the Python &str; function.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>target</term>
<listitem>
<para>
A list of Node objects representing
the target or targets to be built
by the command or other action
generated by this function.
The file names of these target(s)
may be extracted using the Python &str; function.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>env</term>
<listitem>
<para>
The &consenv; used for building the target(s).
The generator may use any of the
environment's construction variables
in any way to determine what command
or other action to return.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>for_signature</term>
<listitem>
<para>
A flag that specifies whether the
generator is being called to contribute to a build signature,
as opposed to actually executing the command.
<!-- XXX NEED MORE HERE, describe generators use in signatures -->
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
The &generator; must return a
command string or other action that will be used to
build the specified target(s) from the specified source(s).
</para>
<para>
Once you've defined a &generator;,
you create a &Builder; to use it
by specifying the generator keyword argument
instead of <literal>action</literal>.
</para>
<programlisting>
def generate_actions(source, target, env, for_signature):
return 'foobuild < %s > %s' % (source[0], target[0])
bld = Builder(generator = generate_actions,
suffix = '.foo',
src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
</programlisting>
<screen>
% <userinput>scons -Q</userinput>
foobuild < file.input > file.foo
</screen>
<para>
Note that it's illegal to specify both an
<literal>action</literal>
and a
<literal>generator</literal>
for a &Builder;.
</para>
</section>
<section>
<title>Builders That Modify the Target or Source Lists Using an &Emitter;</title>
<para>
&SCons; supports the ability for a Builder to modify the
lists of target(s) from the specified source(s).
</para>
<programlisting>
def modify_targets(target, source, env):
target.append('new_target')
source.append('new_source')
return target, source
bld = Builder(action = 'foobuild $TARGETS - $SOURCES',
suffix = '.foo',
src_suffix = '.input',
emitter = modify_targets)
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
</programlisting>
<screen>
% <userinput>scons -Q</userinput>
foobuild file.foo new_target - file.input new_source
</screen>
<programlisting>
bld = Builder(action = 'my_command',
suffix = '.foo',
src_suffix = '.input',
emitter = 'MY_EMITTER')
def modify1(target, source, env):
return target, source
def modify2(target, source, env):
return target, source
env1 = Environment(BUILDERS = {'Foo' : bld},
MY_EMITTER = modify1)
env2 = Environment(BUILDERS = {'Foo' : bld},
MY_EMITTER = modify2)
env1.Foo('file1')
env2.Foo('file2')
</programlisting>
</section>
<!--
<section>
<title>Builders That Use Other Builders</title>
<para>
XXX para
</para>
<scons_example name="ex8">
<file name="SConstruct" printme="1">
env = Environment()
#env.SourceCode('.', env.BitKeeper('my_command'))
env.Program('hello.c')
</file>
<file name="hello.c">
hello.c
</file>
</scons_example>
<scons_output example="ex8">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
</section>
-->
|