5 Permission is hereby granted, free of charge, to any person obtaining
6 a copy of this software and associated documentation files (the
7 "Software"), to deal in the Software without restriction, including
8 without limitation the rights to use, copy, modify, merge, publish,
9 distribute, sublicense, and/or sell copies of the Software, and to
10 permit persons to whom the Software is furnished to do so, subject to
11 the following conditions:
13 The above copyright notice and this permission notice shall be included
14 in all copies or substantial portions of the Software.
16 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
17 KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
18 WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
19 NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
20 LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
21 OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
22 WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
28 =head1 Separating source and build trees
30 It's often desirable to keep any derived files from the build completely
31 separate from the source files. This makes it much easier to keep track of
32 just what is a source file, and also makes it simpler to handle B<variant>
33 builds, especially if you want the variant builds to co-exist.
35 =head2 Separating build and source directories using the Link command
37 Cons provides a simple mechanism that handles all of these requirements. The
38 C<Link> command is invoked as in this example:
40 Link 'build' => 'src';
42 The specified directories are ``linked'' to the specified source
43 directory. Let's suppose that you setup a source directory, F<src>, with the
44 sub-directories F<world> and F<hello> below it, as in the previous
45 example. You could then substitute for the original build lines the
53 Notice that you treat the F<Conscript> file as if it existed in the build
54 directory. Now if you type the same command as before, you will get the
58 Install build/world/world.h as export/include/world.h
59 cc -Iexport/include -c build/hello/hello.c -o build/hello/hello.o
60 cc -Iexport/include -c build/world/world.c -o build/world/world.o
61 ar r build/world/libworld.a build/world/world.o
62 ar: creating build/world/libworld.a
63 ranlib build/world/libworld.a
64 Install build/world/libworld.a as export/lib/libworld.a
65 cc -o build/hello/hello build/hello/hello.o -Lexport/lib -lworld
66 Install build/hello/hello as export/bin/hello
68 Again, Cons has taken care of the details for you. In particular, you will
69 notice that all the builds are done using source files and object files from
70 the build directory. For example, F<build/world/world.o> is compiled from
71 F<build/world/world.c>, and F<export/include/world.h> is installed from
72 F<build/world/world.h>. This is accomplished on most systems by the simple
73 expedient of ``hard'' linking the required files from each source directory
74 into the appropriate build directory.
76 The links are maintained correctly by Cons, no matter what you do to the
77 source directory. If you modify a source file, your editor may do this ``in
78 place'' or it may rename it first and create a new file. In the latter case,
79 any hard link will be lost. Cons will detect this condition the next time
80 the source file is needed, and will relink it appropriately.
82 You'll also notice, by the way, that B<no> changes were required to the
83 underlying F<Conscript> files. And we can go further, as we shall see in the
86 =head2 Explicit references to the source directory
88 When using the C<Link> command on some operating systems or with some
89 tool chains, it's sometimes useful to have a command actually use
90 the path name to the source directory, not the build directory. For
91 example, on systems that must copy, not "hard link," the F<src/> and
92 F<build/> copies of C<Linked> files, using the F<src/> path of a file
93 name might make an editor aware that a syntax error must be fixed in the
94 source directory, not the build directory.
96 You can tell Cons that you want to use the "source path" for a file by
97 preceding the file name with a ``!'' (exclamation point). For example,
98 if we add a ``!'' to the beginning of a source file:
100 Program $env "foo", "!foo.c"; # Notice initial ! on foo.c
102 Cons will compile the target as follows:
104 cc -c src/foo.c -o build/foo.o
105 cc -o build/foo build/foo.o
107 Notice that Cons has compiled the program from the the F<src/foo.c>
108 source file. Without the initial ``!'', Cons would have compiled the
109 program using the F<build/foo.c> path name.
115 It's often useful to keep any built files completely
116 separate from the source files.
117 This is usually done by creating one or more separate
118 <emphasis>build directories</emphasis>
119 that are used to hold the built objects files, libraries,
120 and executable programs, etc.
121 for a specific flavor of build.
122 &SCons; provides two ways to do this,
123 one through the &SConscript; function that we've already seen,
124 and the second through a more flexible &BuildDir; function.
129 <title>Specifying a Build Directory as Part of an &SConscript; Call</title>
133 The most straightforward way to establish a build directory
134 uses the fact that the usual way to
135 set up a build hierarchy is to have an
136 &SConscript; file in the source subdirectory.
137 If you then pass a &build_dir; argument to the
138 &SConscript; function call:
142 <scons_example name="ex1">
143 <file name="SConstruct" printme="1">
144 SConscript('src/SConscript', build_dir='build')
146 <file name="src/SConscript">
148 env.Program('hello.c')
150 <file name="src/hello.c">
151 int main() { printf("Hello, world!\n"); }
157 &SCons; will then build all of the files in
158 the &build; subdirectory:
162 <scons_output example="ex1">
163 <command>ls src</command>
164 <command>scons -Q</command>
165 <command>ls build</command>
170 But wait a minute--what's going on here?
171 &SCons; created the object file
172 <filename>build/hello.o</filename>
173 in the &build; subdirectory,
175 But even though our &hello_c; file lives in the &src; subdirectory,
176 &SCons; has actually compiled a
177 <filename>build/hello.c</filename> file
178 to create the object file.
184 What's happened is that &SCons; has <emphasis>duplicated</emphasis>
185 the &hello_c; file from the &src; subdirectory
186 to the &build; subdirectory,
187 and built the program from there.
188 The next section explains why &SCons; does this.
195 <title>Why &SCons; Duplicates Source Files in a Build Directory</title>
199 &SCons; duplicates source files in build directories
200 because it's the most straightforward way to guarantee a correct build
201 <emphasis>regardless of include-file directory paths,
202 relative references between files,
203 or tool support for putting files in different locations</emphasis>,
204 and the &SCons; philosophy is to, by default,
205 guarantee a correct build in all cases.
211 The most direct reason to duplicate source files
213 is simply that some tools (mostly older vesions)
214 are written to only build their output files
215 in the same directory as the source files.
216 In this case, the choices are either
217 to build the output file in the source directory
218 and move it to the build directory,
219 or to duplicate the source files in the build directory.
226 relative references between files
227 can cause problems if we don't
228 just duplicate the hierarchy of source files
229 in the build directory.
230 You can see this at work in
231 use of the C preprocessor <literal>#include</literal>
232 mechanism with double quotes, not angle brackets:
242 The <emphasis>de facto</emphasis> standard behavior
243 for most C compilers in this case
244 is to first look in the same directory
245 as the source file that contains the <literal>#include</literal> line,
246 then to look in the directories in the preprocessor search path.
247 Add to this that the &SCons; implementation of
248 support for code repositories
250 means not all of the files
251 will be found in the same directory hierarchy,
252 and the simplest way to make sure
253 that the right include file is found
254 is to duplicate the source files into the build directory,
255 which provides a correct build
256 regardless of the original location(s) of the source files.
262 Although source-file duplication guarantees a correct build
263 even in these end-cases,
264 it <emphasis>can</emphasis> usually be safely disabled.
265 The next section describes
266 how you can disable the duplication of source files
267 in the build directory.
274 <title>Telling &SCons; to Not Duplicate Source Files in the Build Directory</title>
278 In most cases and with most tool sets,
279 &SCons; can place its target files in a build subdirectory
280 <emphasis>without</emphasis>
281 duplicating the source files
282 and everything will work just fine.
283 You can disable the default &SCons; behavior
284 by specifying <literal>duplicate=0</literal>
285 when you call the &SConscript; function:
290 SConscript('src/SConscript', build_dir='build', duplicate=0)
295 When this flag is specified,
296 &SCons; uses the build directory
297 like most people expect--that is,
298 the output files are placed in the build directory
299 while the source files stay in the source directory:
304 % <userinput>ls src</userinput>
307 % <userinput>scons -Q</userinput>
308 cc -c src/hello.c -o build/hello.o
309 cc -o build/hello build/hello.o
310 % <userinput>ls build</userinput>
318 <title>The &BuildDir; Function</title>
322 Use the &BuildDir; function to establish that target
323 files should be built in a separate directory
324 from the source files:
328 <scons_example name="ex_builddir">
329 <file name="SConstruct" printme="1">
330 BuildDir('build', 'src')
332 env.Program('build/hello.c')
334 <file name="src/hello.c">
335 int main() { printf("Hello, world!\n"); }
341 Note that when you're not using
342 an &SConscript; file in the &src; subdirectory,
343 you must actually specify that
344 the program must be built from
345 the <filename>build/hello.c</filename>
346 file that &SCons; will duplicate in the
347 &build; subdirectory.
353 When using the &BuildDir; function directly,
354 &SCons; still duplicates the source files
355 in the build directory by default:
359 <scons_output example="ex_builddir">
360 <command>ls src</command>
361 <command>scons -Q</command>
362 <command>ls build</command>
367 You can specify the same <literal>duplicate=0</literal> argument
368 that you can specify for an &SConscript; call:
372 <scons_example name="ex_duplicate_0">
373 <file name="SConstruct" printme="1">
374 BuildDir('build', 'src', duplicate=0)
376 env.Program('build/hello.c')
378 <file name="src/hello.c">
379 int main() { printf("Hello, world!\n"); }
385 In which case &SCons;
386 will disable duplication of the source files:
390 <scons_output example="ex_duplicate_0">
391 <command>ls src</command>
392 <command>scons -Q</command>
393 <command>ls build</command>
399 <title>Using &BuildDir; With an &SConscript; File</title>
403 Even when using the &BuildDir; function,
404 it's much more natural to use it with
405 a subsidiary &SConscript; file.
407 <filename>src/SConscript</filename>
412 <scons_example name="example_builddir_sconscript">
413 <file name="SConstruct">
414 BuildDir('build', 'src')
415 SConscript('build/SConscript')
417 <file name="src/SConscript" printme="1">
419 env.Program('hello.c')
421 <file name="src/hello.c">
422 int main() { printf("Hello, world!\n"); }
428 Then our &SConstruct; file could look like:
432 <scons_example_file example="example_builddir_sconscript" name="SConstruct">
433 </scons_example_file>
437 Yielding the following output:
441 <scons_output example="example_builddir_sconscript">
442 <command>ls src</command>
443 <command>scons -Q</command>
444 <command>ls build</command>
449 Notice that this is completely equivalent
450 to the use of &SConscript; that we
451 learned about in the previous section.
460 <title>Why You'd Want to Call &BuildDir; Instead of &SConscript;</title>