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|
<?xml version='1.0'?>
<!DOCTYPE sconsdoc [
<!ENTITY % scons SYSTEM "../scons.mod">
%scons;
<!ENTITY % builders-mod SYSTEM "../generated/builders.mod">
%builders-mod;
<!ENTITY % functions-mod SYSTEM "../generated/functions.mod">
%functions-mod;
<!ENTITY % tools-mod SYSTEM "../generated/tools.mod">
%tools-mod;
<!ENTITY % variables-mod SYSTEM "../generated/variables.mod">
%variables-mod;
]>
<chapter id="chap-hierarchical"
xmlns="http://www.scons.org/dbxsd/v1.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 scons.xsd">
<title>Hierarchical Builds</title>
<!--
__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 The Build command
By default, Cons does not change its working directory to the directory
containing a subsidiary F<Conscript> file it is including. This behavior
can be enabled for a build by specifying, in the top-level F<Construct>
file:
Conscript_chdir 1;
When enabled, Cons will change to the subsidiary F<Conscript> file's
containing directory while reading in that file, and then change back
to the top-level directory once the file has been processed.
It is expected that this behavior will become the default in some future
version of Cons. To prepare for this transition, builds that expect
Cons to remain at the top of the build while it reads in a subsidiary
F<Conscript> file should explicitly disable this feature as follows:
Conscript_chdir 0;
=head2 Relative, top-relative, and absolute file names
(There is another file prefix, ``!'', that is interpreted specially by
Cons. See discussion of the C<Link> command, below, for details.)
=head2 Using modules in build scripts
You may pull modules into each F<Conscript> file using the normal Perl
C<use> or C<require> statements:
use English;
require My::Module;
Each C<use> or C<require> only affects the one F<Conscript> file in which
it appears. To use a module in multiple F<Conscript> files, you must
put a C<use> or C<require> statement in each one that needs the module.
=head2 Scope of variables
The top-level F<Construct> file and all F<Conscript> files begin life in
a common, separate Perl package. B<Cons> controls the symbol table for
the package so that, the symbol table for each script is empty, except
for the F<Construct> file, which gets some of the command line arguments.
All of the variables that are set or used, therefore, are set by the
script itself, not by some external script.
Variables can be explicitly B<imported> by a script from its parent
script. To import a variable, it must have been B<exported> by the parent
and initialized (otherwise an error will occur).
=head2 The Export command
The C<Export> command is used as in the following example:
$env = new cons();
$INCLUDE = "#export/include";
$LIB = "#export/lib";
Export qw( env INCLUDE LIB );
Build qw( util/Conscript );
The values of the simple variables mentioned in the C<Export> list will be
squirreled away by any subsequent C<Build> commands. The C<Export> command
will only export Perl B<scalar> variables, that is, variables whose name
begins with C<$>. Other variables, objects, etc. can be exported by
reference, but all scripts will refer to the same object, and this object
should be considered to be read-only by the subsidiary scripts and by the
original exporting script. It's acceptable, however, to assign a new value
to the exported scalar variable, that won't change the underlying variable
referenced. This sequence, for example, is OK:
$env = new cons();
Export qw( env INCLUDE LIB );
Build qw( util/Conscript );
$env = new cons(CFLAGS => '-O');
Build qw( other/Conscript );
It doesn't matter whether the variable is set before or after the C<Export>
command. The important thing is the value of the variable at the time the
C<Build> command is executed. This is what gets squirreled away. Any
subsequent C<Export> commands, by the way, invalidate the first: you must
mention all the variables you wish to export on each C<Export> command.
=head2 The Import command
Variables exported by the C<Export> command can be imported into subsidiary
scripts by the C<Import> command. The subsidiary script always imports
variables directly from the superior script. Consider this example:
Import qw( env INCLUDE );
This is only legal if the parent script exported both C<$env> and
C<$INCLUDE>. It also must have given each of these variables values. It is
OK for the subsidiary script to only import a subset of the exported
variables (in this example, C<$LIB>, which was exported by the previous
example, is not imported).
All the imported variables are automatically re-exported, so the sequence:
Import qw ( env INCLUDE );
Build qw ( beneath-me/Conscript );
will supply both C<$env> and C<$INCLUDE> to the subsidiary file. If only
C<$env> is to be exported, then the following will suffice:
Import qw ( env INCLUDE );
Export qw ( env );
Build qw ( beneath-me/Conscript );
Needless to say, the variables may be modified locally before invoking
C<Build> on the subsidiary script.
=head2 Build script evaluation order
The only constraint on the ordering of build scripts is that superior
scripts are evaluated before their inferior scripts. The top-level
F<Construct> file, for instance, is evaluated first, followed by any
inferior scripts. This is all you really need to know about the evaluation
order, since order is generally irrelevant. Consider the following C<Build>
command:
Build qw(
drivers/display/Conscript
drivers/mouse/Conscript
parser/Conscript
utilities/Conscript
);
We've chosen to put the script names in alphabetical order, simply because
that's the most convenient for maintenance purposes. Changing the order will
make no difference to the build.
-->
<para>
The source code for large software projects
rarely stays in a single directory,
but is nearly always divided into a
hierarchy of directories.
Organizing a large software build using &SCons;
involves creating a hierarchy of build scripts
using the &SConscript; function.
</para>
<section>
<title>&SConscript; Files</title>
<para>
As we've already seen,
the build script at the top of the tree is called &SConstruct;.
The top-level &SConstruct; file can
use the &SConscript; function to
include other subsidiary scripts in the build.
These subsidiary scripts can, in turn,
use the &SConscript; function
to include still other scripts in the build.
By convention, these subsidiary scripts are usually
named &SConscript;.
For example, a top-level &SConstruct; file might
arrange for four subsidiary scripts to be included
in the build as follows:
</para>
<sconstruct>
SConscript(['drivers/display/SConscript',
'drivers/mouse/SConscript',
'parser/SConscript',
'utilities/SConscript'])
</sconstruct>
<para>
In this case, the &SConstruct; file
lists all of the &SConscript; files in the build explicitly.
(Note, however, that not every directory in the tree
necessarily has an &SConscript; file.)
Alternatively, the <literal>drivers</literal>
subdirectory might contain an intermediate
&SConscript; file,
in which case the &SConscript; call in
the top-level &SConstruct; file
would look like:
</para>
<sconstruct>
SConscript(['drivers/SConscript',
'parser/SConscript',
'utilities/SConscript'])
</sconstruct>
<para>
And the subsidiary &SConscript; file in the
<literal>drivers</literal> subdirectory
would look like:
</para>
<sconstruct>
SConscript(['display/SConscript',
'mouse/SConscript'])
</sconstruct>
<para>
Whether you list all of the &SConscript; files in the
top-level &SConstruct; file,
or place a subsidiary &SConscript; file in
intervening directories,
or use some mix of the two schemes,
is up to you and the needs of your software.
</para>
</section>
<section>
<title>Path Names Are Relative to the &SConscript; Directory</title>
<para>
Subsidiary &SConscript; files make it easy to create a build
hierarchy because all of the file and directory names
in a subsidiary &SConscript; files are interpreted
relative to the directory in which the &SConscript; file lives.
Typically, this allows the &SConscript; file containing the
instructions to build a target file
to live in the same directory as the source files
from which the target will be built,
making it easy to update how the software is built
whenever files are added or deleted
(or other changes are made).
</para>
<para>
For example, suppose we want to build two programs
&prog1; and &prog2; in two separate directories
with the same names as the programs.
One typical way to do this would be
with a top-level &SConstruct; file like this:
</para>
<scons_example name="hierarchy_ex1">
<file name="SConstruct" printme="1">
SConscript(['prog1/SConscript',
'prog2/SConscript'])
</file>
<file name="prog1/SConscript">
env = Environment()
env.Program('prog1', ['main.c', 'foo1.c', 'foo2.c'])
</file>
<file name="prog2/SConscript">
env = Environment()
env.Program('prog2', ['main.c', 'bar1.c', 'bar2.c'])
</file>
<directory name="prog1"></directory>
<file name="prog1/main.c">
x
</file>
<file name="prog1/foo1.c">
x
</file>
<file name="prog1/foo2.c">
x
</file>
<directory name="prog2"></directory>
<file name="prog2/main.c">
x
</file>
<file name="prog2/bar1.c">
x
</file>
<file name="prog2/bar2.c">
x
</file>
</scons_example>
<para>
And subsidiary &SConscript; files that look like this:
</para>
<scons_example_file example="hierarchy_ex1" name="prog1/SConscript">
</scons_example_file>
<para>
And this:
</para>
<scons_example_file example="hierarchy_ex1" name="prog2/SConscript">
</scons_example_file>
<para>
Then, when we run &SCons; in the top-level directory,
our build looks like:
</para>
<scons_output example="hierarchy_ex1" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
Notice the following:
First, you can have files with the same names
in multiple directories, like main.c in the above example.
Second, unlike standard recursive use of &Make;,
&SCons; stays in the top-level directory
(where the &SConstruct; file lives)
and issues commands that use the path names
from the top-level directory to the
target and source files within the hierarchy.
</para>
</section>
<section>
<title>Top-Level Path Names in Subsidiary &SConscript; Files</title>
<para>
If you need to use a file from another directory,
it's sometimes more convenient to specify
the path to a file in another directory
from the top-level &SConstruct; directory,
even when you're using that file in
a subsidiary &SConscript; file in a subdirectory.
You can tell &SCons; to interpret a path name
as relative to the top-level &SConstruct; directory,
not the local directory of the &SConscript; file,
by appending a &hash; (hash mark)
to the beginning of the path name:
</para>
<scons_example name="hierarchy_ex2">
<file name="SConstruct">
SConscript('src/prog/SConscript')
</file>
<file name="src/prog/SConscript" printme="1">
env = Environment()
env.Program('prog', ['main.c', '#lib/foo1.c', 'foo2.c'])
</file>
<file name="src/prog/main.c">
x
</file>
<file name="lib/foo1.c">
x
</file>
<file name="src/prog/foo2.c">
x
</file>
</scons_example>
<para>
In this example,
the <literal>lib</literal> directory is
directly underneath the top-level &SConstruct; directory.
If the above &SConscript; file is in a subdirectory
named <literal>src/prog</literal>,
the output would look like:
</para>
<scons_output example="hierarchy_ex2" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
(Notice that the <literal>lib/foo1.o</literal> object file
is built in the same directory as its source file.
See <xref linkend="chap-separate"></xref>, below,
for information about
how to build the object file in a different subdirectory.)
</para>
</section>
<section>
<title>Absolute Path Names</title>
<para>
Of course, you can always specify
an absolute path name for a file--for example:
</para>
<scons_example name="hierarchy_ex3">
<file name="SConstruct">
SConscript('src/prog/SConscript')
</file>
<file name="src/prog/SConscript" printme="1">
env = Environment()
env.Program('prog', ['main.c', '__ROOT__/usr/joe/lib/foo1.c', 'foo2.c'])
</file>
<file name="src/prog/main.c">
x
</file>
<file name="__ROOT__/usr/joe/lib/foo1.c">
x
</file>
<file name="src/prog/foo2.c">
x
</file>
</scons_example>
<para>
Which, when executed, would yield:
</para>
<scons_output example="hierarchy_ex3" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
(As was the case with top-relative path names,
notice that the <literal>/usr/joe/lib/foo1.o</literal> object file
is built in the same directory as its source file.
See <xref linkend="chap-separate"></xref>, below,
for information about
how to build the object file in a different subdirectory.)
</para>
</section>
<section>
<title>Sharing Environments (and Other Variables) Between &SConscript; Files</title>
<para>
In the previous example,
each of the subsidiary &SConscript; files
created its own construction environment
by calling &Environment; separately.
This obviously works fine,
but if each program must be built
with the same construction variables,
it's cumbersome and error-prone to initialize
separate construction environments
in the same way over and over in each subsidiary
&SConscript; file.
</para>
<para>
&SCons; supports the ability to <emphasis>export</emphasis> variables
from a parent &SConscript; file
to its subsidiary &SConscript; files,
which allows you to share common initialized
values throughout your build hierarchy.
</para>
<section>
<title>Exporting Variables</title>
<para>
There are two ways to export a variable,
such as a construction environment,
from an &SConscript; file,
so that it may be used by other &SConscript; files.
First, you can call the &Export;
function with a list of variables,
or a string of white-space separated variable names.
Each call to &Export; adds one
or more variables to a global list
of variables that are available for import
by other &SConscript; files.
</para>
<sconstruct>
env = Environment()
Export('env')
</sconstruct>
<para>
You may export more than one variable name at a time:
</para>
<sconstruct>
env = Environment()
debug = ARGUMENTS['debug']
Export('env', 'debug')
</sconstruct>
<para>
Because white space is not legal in Python variable names,
the &Export; function will even automatically split
a string into separate names for you:
</para>
<sconstruct>
Export('env debug')
</sconstruct>
<para>
Second, you can specify a list of
variables to export as a second argument
to the &SConscript; function call:
</para>
<sconstruct>
SConscript('src/SConscript', 'env')
</sconstruct>
<para>
Or as the &exports; keyword argument:
</para>
<sconstruct>
SConscript('src/SConscript', exports='env')
</sconstruct>
<para>
These calls export the specified variables
to only the listed &SConscript; files.
You may, however, specify more than one
&SConscript; file in a list:
</para>
<sconstruct>
SConscript(['src1/SConscript',
'src2/SConscript'], exports='env')
</sconstruct>
<para>
This is functionally equivalent to
calling the &SConscript; function
multiple times with the same &exports; argument,
one per &SConscript; file.
</para>
</section>
<section>
<title>Importing Variables</title>
<para>
Once a variable has been exported from a calling
&SConscript; file,
it may be used in other &SConscript; files
by calling the &Import; function:
</para>
<sconstruct>
Import('env')
env.Program('prog', ['prog.c'])
</sconstruct>
<para>
The &Import; call makes the <literal>env</literal> construction
environment available to the &SConscript; file,
after which the variable can be used to build
programs, libraries, etc.
</para>
<para>
Like the &Export; function,
the &Import; function can be used
with multiple variable names:
</para>
<sconstruct>
Import('env', 'debug')
env = env.Clone(DEBUG = debug)
env.Program('prog', ['prog.c'])
</sconstruct>
<para>
And the &Import; function will similarly
split a string along white-space
into separate variable names:
</para>
<sconstruct>
Import('env debug')
env = env.Clone(DEBUG = debug)
env.Program('prog', ['prog.c'])
</sconstruct>
<para>
Lastly, as a special case,
you may import all of the variables that
have been exported by supplying an asterisk
to the &Import; function:
</para>
<sconstruct>
Import('*')
env = env.Clone(DEBUG = debug)
env.Program('prog', ['prog.c'])
</sconstruct>
<para>
If you're dealing with a lot of &SConscript; files,
this can be a lot simpler than keeping
arbitrary lists of imported variables in each file.
</para>
</section>
<section>
<title>Returning Values From an &SConscript; File</title>
<para>
Sometimes, you would like to be able to
use information from a subsidiary
&SConscript; file in some way.
For example,
suppose that you want to create one
library from source files
scattered throughout a number
of subsidiary &SConscript; files.
You can do this by using the &Return;
function to return values
from the subsidiary &SConscript; files
to the calling file.
</para>
<para>
If, for example, we have two subdirectories
&foo; and &bar;
that should each contribute a source
file to a Library,
what we'd like to be able to do is
collect the object files
from the subsidiary &SConscript; calls
like this:
</para>
<scons_example name="hierarchy_Return">
<file name="SConstruct" printme="1">
env = Environment()
Export('env')
objs = []
for subdir in ['foo', 'bar']:
o = SConscript('%s/SConscript' % subdir)
objs.append(o)
env.Library('prog', objs)
</file>
<directory name="foo"></directory>
<directory name="bar"></directory>
<file name="foo/SConscript">
Import('env')
obj = env.Object('foo.c')
Return('obj')
</file>
<file name="bar/SConscript">
Import('env')
obj = env.Object('bar.c')
Return('obj')
</file>
<file name="foo/foo.c">
void foo(void) { printf("foo/foo.c\n"); }
</file>
<file name="bar/bar.c">
void bar(void) { printf("bar/bar.c\n"); }
</file>
</scons_example>
<para>
We can do this by using the &Return;
function in the
<literal>foo/SConscript</literal> file like this:
</para>
<scons_example_file example="hierarchy_Return" name="foo/SConscript">
</scons_example_file>
<para>
(The corresponding
<literal>bar/SConscript</literal>
file should be pretty obvious.)
Then when we run &SCons;,
the object files from the subsidiary subdirectories
are all correctly archived in the desired library:
</para>
<scons_output example="hierarchy_Return" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<!--
XXX Return(stop=False)
-->
</section>
</section>
<!--
<section>
<title>Executing From a Subdirectory: the -D, -u and -U Options</title>
<para>
XXX -D, -u and -U
</para>
</section>
-->
</chapter>
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