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.)
Writing Builders That Execute External Commands
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
foobuild,
creating that &Builder; might look like:
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
All the above line does is create a free-standing
&Builder; object.
The next section will show us how to actually use it.
Attaching a Builder to a &ConsEnv;
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
Foo,
our &SConstruct; file might look like:
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
env.Foo('file.foo', 'file.input')
file.input
cat
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
With the &Builder; so attached to our &consenv;
we can now actually call it like so:
env.Foo('file.foo', 'file.input')
Then when we run &SCons; it looks like:
scons -Q
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:
import SCons.Defaults; SCons.Defaults.ConstructionEnvironment['TOOLS'] = {}; bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
file.input
hello.c
scons -Q
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:
env = Environment()
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env.Append(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
file.input
hello.c
cat
env = Environment()
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env.Append(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
Or you can explicitly set the appropriately-named
key in the &cv-BUILDERS; dictionary:
env = Environment()
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env['BUILDERS']['Foo'] = bld
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
Either way, the same &consenv;
can then use both the newly-defined
Foo &Builder;
and the default &b-link-Program; &Builder;:
scons -Q
Letting &SCons; Handle The File Suffixes
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 Foo
&Builder; to build the file.foo
target file from the file.input source file,
you can give the .foo
and .input suffixes to the &Builder;,
making for more compact and readable calls to
the Foo &Builder;:
bld = Builder(action = 'foobuild < $SOURCE > $TARGET',
suffix = '.foo',
src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
env.Foo('file1')
env.Foo('file2')
file1.input
file2.input
cat
bld = Builder(action = 'foobuild < $SOURCE > $TARGET',
suffix = '.foo',
src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file1')
env.Foo('file2')
scons -Q
You can also supply a prefix keyword argument
if it's appropriate to have &SCons; append a prefix
to the beginning of target file names.
Builders That Execute Python Functions
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:
def build_function(target, source, env):
# Code to build "target" from "source"
return None
The arguments of a &buildfunc; are:
target
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.
source
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.
env
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.
The builder function must
return a 0 or None 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,
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
action
argument:
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')
file.input
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:
scons -Q
Builders That Create Actions Using a &Generator;
&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:
def generate_actions(source, target, env, for_signature):
return 'foobuild < %s > %s' % (target[0], source[0])
The arguments of a &generator; are:
source
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.
target
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.
env
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.
for_signature
A flag that specifies whether the
generator is being called to contribute to a build signature,
as opposed to actually executing the command.
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).
Once you've defined a &generator;,
you create a &Builder; to use it
by specifying the generator keyword argument
instead of action.
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})
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
env.Foo('file')
file.input
cat
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')
scons -Q
Note that it's illegal to specify both an
action
and a
generator
for a &Builder;.
Builders That Modify the Target or Source Lists Using an &Emitter;
&SCons; supports the ability for a Builder to modify the
lists of target(s) from the specified source(s).
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})
import os
env['ENV']['PATH'] = env['ENV']['PATH'] + os.pathsep + os.getcwd()
env.Foo('file')
file.input
new_source
cat
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')
scons -Q
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')