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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-separate"
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>Separating Source and Build Directories</title>
<!--
__COPYRIGHT__
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permit persons to whom the Software is furnished to do so, subject to
the following conditions:
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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
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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.
-->
<!--
=head1 Separating source and build trees
It's often desirable to keep any derived files from the build completely
separate from the source files. This makes it much easier to keep track of
just what is a source file, and also makes it simpler to handle B<variant>
builds, especially if you want the variant builds to co-exist.
=head2 Separating build and source directories using the Link command
Cons provides a simple mechanism that handles all of these requirements. The
C<Link> command is invoked as in this example:
Link 'build' => 'src';
The specified directories are ``linked'' to the specified source
directory. Let's suppose that you setup a source directory, F<src>, with the
sub-directories F<world> and F<hello> below it, as in the previous
example. You could then substitute for the original build lines the
following:
Build qw(
build/world/Conscript
build/hello/Conscript
);
Notice that you treat the F<Conscript> file as if it existed in the build
directory. Now if you type the same command as before, you will get the
following results:
% cons export
Install build/world/world.h as export/include/world.h
cc -Iexport/include -c build/hello/hello.c -o build/hello/hello.o
cc -Iexport/include -c build/world/world.c -o build/world/world.o
ar r build/world/libworld.a build/world/world.o
ar: creating build/world/libworld.a
ranlib build/world/libworld.a
Install build/world/libworld.a as export/lib/libworld.a
cc -o build/hello/hello build/hello/hello.o -Lexport/lib -lworld
Install build/hello/hello as export/bin/hello
Again, Cons has taken care of the details for you. In particular, you will
notice that all the builds are done using source files and object files from
the build directory. For example, F<build/world/world.o> is compiled from
F<build/world/world.c>, and F<export/include/world.h> is installed from
F<build/world/world.h>. This is accomplished on most systems by the simple
expedient of ``hard'' linking the required files from each source directory
into the appropriate build directory.
The links are maintained correctly by Cons, no matter what you do to the
source directory. If you modify a source file, your editor may do this ``in
place'' or it may rename it first and create a new file. In the latter case,
any hard link will be lost. Cons will detect this condition the next time
the source file is needed, and will relink it appropriately.
You'll also notice, by the way, that B<no> changes were required to the
underlying F<Conscript> files. And we can go further, as we shall see in the
next section.
=head2 Explicit references to the source directory
When using the C<Link> command on some operating systems or with some
tool chains, it's sometimes useful to have a command actually use
the path name to the source directory, not the build directory. For
example, on systems that must copy, not "hard link," the F<src/> and
F<build/> copies of C<Linked> files, using the F<src/> path of a file
name might make an editor aware that a syntax error must be fixed in the
source directory, not the build directory.
You can tell Cons that you want to use the "source path" for a file by
preceding the file name with a ``!'' (exclamation point). For example,
if we add a ``!'' to the beginning of a source file:
Program $env "foo", "!foo.c"; # Notice initial ! on foo.c
Cons will compile the target as follows:
cc -c src/foo.c -o build/foo.o
cc -o build/foo build/foo.o
Notice that Cons has compiled the program from the the F<src/foo.c>
source file. Without the initial ``!'', Cons would have compiled the
program using the F<build/foo.c> path name.
-->
<para>
It's often useful to keep any built files completely
separate from the source files.
In &SCons;, this is usually done by creating one or more separate
<emphasis>variant directory trees</emphasis>
that are used to hold the built objects files, libraries,
and executable programs, etc.
for a specific flavor, or variant, of build.
&SCons; provides two ways to do this,
one through the &SConscript; function that we've already seen,
and the second through a more flexible &VariantDir; function.
</para>
<para>
One historical note: the &VariantDir; function
used to be called &BuildDir;.
That name is still supported
but has been deprecated
because the &SCons; functionality
differs from the model of a "build directory"
implemented by other build systems like the GNU Autotools.
</para>
<section>
<title>Specifying a Variant Directory Tree as Part of an &SConscript; Call</title>
<para>
The most straightforward way to establish a variant directory tree
uses the fact that the usual way to
set up a build hierarchy is to have an
&SConscript; file in the source subdirectory.
If you then pass a &variant_dir; argument to the
&SConscript; function call:
</para>
<scons_example name="ex1">
<file name="SConstruct" printme="1">
SConscript('src/SConscript', variant_dir='build')
</file>
<file name="src/SConscript">
env = Environment()
env.Program('hello.c')
</file>
<file name="src/hello.c">
int main() { printf("Hello, world!\n"); }
</file>
</scons_example>
<para>
&SCons; will then build all of the files in
the &build; subdirectory:
</para>
<scons_output example="ex1">
<scons_output_command>ls src</scons_output_command>
<scons_output_command>scons -Q</scons_output_command>
<scons_output_command>ls build</scons_output_command>
</scons_output>
<para>
But wait a minute--what's going on here?
&SCons; created the object file
<filename>build/hello.o</filename>
in the &build; subdirectory,
as expected.
But even though our &hello_c; file lives in the &src; subdirectory,
&SCons; has actually compiled a
<filename>build/hello.c</filename> file
to create the object file.
</para>
<para>
What's happened is that &SCons; has <emphasis>duplicated</emphasis>
the &hello_c; file from the &src; subdirectory
to the &build; subdirectory,
and built the program from there.
The next section explains why &SCons; does this.
</para>
</section>
<section>
<title>Why &SCons; Duplicates Source Files in a Variant Directory Tree</title>
<para>
&SCons; duplicates source files in variant directory trees
because it's the most straightforward way to guarantee a correct build
<emphasis>regardless of include-file directory paths,
relative references between files,
or tool support for putting files in different locations</emphasis>,
and the &SCons; philosophy is to, by default,
guarantee a correct build in all cases.
</para>
<para>
The most direct reason to duplicate source files
in variant directories
is simply that some tools (mostly older versions)
are written to only build their output files
in the same directory as the source files.
In this case, the choices are either
to build the output file in the source directory
and move it to the variant directory,
or to duplicate the source files in the variant directory.
</para>
<para>
Additionally,
relative references between files
can cause problems if we don't
just duplicate the hierarchy of source files
in the variant directory.
You can see this at work in
use of the C preprocessor <literal>#include</literal>
mechanism with double quotes, not angle brackets:
</para>
<sconstruct>
#include "file.h"
</sconstruct>
<para>
The <emphasis>de facto</emphasis> standard behavior
for most C compilers in this case
is to first look in the same directory
as the source file that contains the <literal>#include</literal> line,
then to look in the directories in the preprocessor search path.
Add to this that the &SCons; implementation of
support for code repositories
(described below)
means not all of the files
will be found in the same directory hierarchy,
and the simplest way to make sure
that the right include file is found
is to duplicate the source files into the variant directory,
which provides a correct build
regardless of the original location(s) of the source files.
</para>
<para>
Although source-file duplication guarantees a correct build
even in these end-cases,
it <emphasis>can</emphasis> usually be safely disabled.
The next section describes
how you can disable the duplication of source files
in the variant directory.
</para>
</section>
<section>
<title>Telling &SCons; to Not Duplicate Source Files in the Variant Directory Tree</title>
<para>
In most cases and with most tool sets,
&SCons; can place its target files in a build subdirectory
<emphasis>without</emphasis>
duplicating the source files
and everything will work just fine.
You can disable the default &SCons; behavior
by specifying <literal>duplicate=0</literal>
when you call the &SConscript; function:
</para>
<sconstruct>
SConscript('src/SConscript', variant_dir='build', duplicate=0)
</sconstruct>
<para>
When this flag is specified,
&SCons; uses the variant directory
like most people expect--that is,
the output files are placed in the variant directory
while the source files stay in the source directory:
</para>
<screen>
% <userinput>ls src</userinput>
SConscript
hello.c
% <userinput>scons -Q</userinput>
cc -c src/hello.c -o build/hello.o
cc -o build/hello build/hello.o
% <userinput>ls build</userinput>
hello
hello.o
</screen>
</section>
<section>
<title>The &VariantDir; Function</title>
<para>
Use the &VariantDir; function to establish that target
files should be built in a separate directory
from the source files:
</para>
<scons_example name="ex_builddir">
<file name="SConstruct" printme="1">
VariantDir('build', 'src')
env = Environment()
env.Program('build/hello.c')
</file>
<file name="src/hello.c">
int main() { printf("Hello, world!\n"); }
</file>
</scons_example>
<para>
Note that when you're not using
an &SConscript; file in the &src; subdirectory,
you must actually specify that
the program must be built from
the <filename>build/hello.c</filename>
file that &SCons; will duplicate in the
&build; subdirectory.
</para>
<para>
When using the &VariantDir; function directly,
&SCons; still duplicates the source files
in the variant directory by default:
</para>
<scons_output example="ex_builddir">
<scons_output_command>ls src</scons_output_command>
<scons_output_command>scons -Q</scons_output_command>
<scons_output_command>ls build</scons_output_command>
</scons_output>
<para>
You can specify the same <literal>duplicate=0</literal> argument
that you can specify for an &SConscript; call:
</para>
<scons_example name="ex_duplicate_0">
<file name="SConstruct" printme="1">
VariantDir('build', 'src', duplicate=0)
env = Environment()
env.Program('build/hello.c')
</file>
<file name="src/hello.c">
int main() { printf("Hello, world!\n"); }
</file>
</scons_example>
<para>
In which case &SCons;
will disable duplication of the source files:
</para>
<scons_output example="ex_duplicate_0">
<scons_output_command>ls src</scons_output_command>
<scons_output_command>scons -Q</scons_output_command>
<scons_output_command>ls build</scons_output_command>
</scons_output>
</section>
<section>
<title>Using &VariantDir; With an &SConscript; File</title>
<para>
Even when using the &VariantDir; function,
it's much more natural to use it with
a subsidiary &SConscript; file.
For example, if the
<filename>src/SConscript</filename>
looks like this:
</para>
<scons_example name="example_builddir_sconscript">
<file name="SConstruct">
VariantDir('build', 'src')
SConscript('build/SConscript')
</file>
<file name="src/SConscript" printme="1">
env = Environment()
env.Program('hello.c')
</file>
<file name="src/hello.c">
int main() { printf("Hello, world!\n"); }
</file>
</scons_example>
<para>
Then our &SConstruct; file could look like:
</para>
<scons_example_file example="example_builddir_sconscript" name="SConstruct">
</scons_example_file>
<para>
Yielding the following output:
</para>
<scons_output example="example_builddir_sconscript">
<scons_output_command>ls src</scons_output_command>
<scons_output_command>scons -Q</scons_output_command>
<scons_output_command>ls build</scons_output_command>
</scons_output>
<para>
Notice that this is completely equivalent
to the use of &SConscript; that we
learned about in the previous section.
</para>
</section>
<section>
<title>Using &Glob; with &VariantDir;</title>
<para>
The &Glob; file name pattern matching function
works just as usual when using &VariantDir;.
For example, if the
<filename>src/SConscript</filename>
looks like this:
</para>
<scons_example name="example_glob_builddir_sconscript">
<file name="SConstruct">
VariantDir('build', 'src')
SConscript('build/SConscript')
</file>
<file name="src/SConscript" printme="1">
env = Environment()
env.Program('hello', Glob('*.c'))
</file>
<file name="src/f1.c">
#include "f2.h"
int main() { printf(f2()); }
</file>
<file name="src/f2.c">
const char * f2() { return("Hello, world!\n"); }
</file>
<file name="src/f2.h">
const char * f2();
</file>
</scons_example>
<para>
Then with the same &SConstruct; file as in the previous section,
and source files <filename>f1.c</filename>
and <filename>f2.c</filename> in src,
we would see the following output:
</para>
<scons_output example="example_glob_builddir_sconscript">
<scons_output_command>ls src</scons_output_command>
<scons_output_command>scons -Q</scons_output_command>
<scons_output_command>ls build</scons_output_command>
</scons_output>
<para>
The &Glob; function returns Nodes in the
<filename>build/</filename> tree, as you'd expect.
</para>
</section>
<!--
<section>
<title>Why You'd Want to Call &VariantDir; Instead of &SConscript;</title>
<para>
XXX why call VariantDir() instead of SConscript(variant_dir=)
</para>
</section>
-->
</chapter>
|