3 Copyright (c) 2001, 2002, 2003 Steven Knight
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28 =head1 More on construction environments
30 As previously mentioned, a B<construction environment> is an object that
31 has a set of keyword/value pairs and a set of methods, and which is used
32 to tell Cons how target files should be built. This section describes
33 how Cons uses and expands construction environment values to control its
36 =head2 Construction variable expansion
38 Construction variables from a construction environment are expanded
39 by preceding the keyword with a C<%> (percent sign):
41 Construction variables:
42 XYZZY => 'abracadabra',
44 The string: "The magic word is: %XYZZY!"
45 expands to: "The magic word is: abracadabra!"
47 A construction variable name may be surrounded by C<{> and C<}> (curly
48 braces), which are stripped as part of the expansion. This can
49 sometimes be necessary to separate a variable expansion from trailing
50 alphanumeric characters:
52 Construction variables:
56 The string: "%OPT %{OPT}ION %OPTION %{OPTION}"
57 expands to: "value1 value1ION value2 value2"
59 Construction variable expansion is recursive, that is, a string
60 containing C<%->expansions after substitution will be re-expanded until
61 no further substitutions can be made:
63 Construction variables:
64 STRING => 'The result is: %FOO',
68 The string: "The string says: %STRING"
69 expands to: "The string says: The result is: final value"
71 If a construction variable is not defined in an environment, then the
72 null string is substituted:
74 Construction variables:
78 The string: "%FOO <%NO_VARIABLE> %BAR"
79 expands to: "value1 <> value2"
81 A doubled C<%%> will be replaced by a single C<%>:
83 The string: "Here is a percent sign: %%"
84 expands to: "Here is a percent sign: %"
86 =head2 Default construction variables
88 When you specify no arguments when creating a new construction
93 Cons creates a reference to a new, default construction
94 environment. This contains a number of construction variables and some
95 methods. At the present writing, the default construction variables on a
100 CCCOM => '%CC %CFLAGS %_IFLAGS -c %< -o %>',
102 CXXFLAGS => '%CFLAGS',
103 CXXCOM => '%CXX %CXXFLAGS %_IFLAGS -c %< -o %>',
104 INCDIRPREFIX => '-I',
107 LINKCOM => '%LINK %LDFLAGS -o %> %< %_LDIRS %LIBS',
108 LINKMODULECOM => '%LD -r -o %> %<',
109 LIBDIRPREFIX => '-L',
113 ARCOM => ['%AR %ARFLAGS %> %<', '%RANLIB %>'],
117 ASCOM => '%AS %ASFLAGS %< -o %>',
124 SIGNATURE => [ '*' => 'build' ],
125 ENV => { 'PATH' => '/bin:/usr/bin' },
128 And on a Win32 system (Windows NT), the default construction variables
129 are (unless the default rule style is set using the B<DefaultRules>
134 CCCOM => '%CC %CFLAGS %_IFLAGS /c %< /Fo%>',
135 CXXCOM => '%CXX %CXXFLAGS %_IFLAGS /c %< /Fo%>',
136 INCDIRPREFIX => '/I',
139 LINKCOM => '%LINK %LDFLAGS /out:%> %< %_LDIRS %LIBS',
140 LINKMODULECOM => '%LD /r /o %> %<',
141 LIBDIRPREFIX => '/LIBPATH:',
144 ARFLAGS => '/nologo ',
145 ARCOM => "%AR %ARFLAGS /out:%> %<",
148 LDFLAGS => '/nologo ',
152 SUFLIBS => '.dll:.lib',
154 SIGNATURE => [ '*' => 'build' ],
156 These variables are used by the various methods associated with the
157 environment. In particular, any method that ultimately invokes an external
158 command will substitute these variables into the final command, as
159 appropriate. For example, the C<Objects> method takes a number of source
160 files and arranges to derive, if necessary, the corresponding object
163 Objects $env 'foo.c', 'bar.c';
165 This will arrange to produce, if necessary, F<foo.o> and F<bar.o>. The
166 command invoked is simply C<%CCCOM>, which expands, through substitution,
167 to the appropriate external command required to build each object. The
168 substitution rules will be discussed in detail in the next section.
170 The construction variables are also used for other purposes. For example,
171 C<CPPPATH> is used to specify a colon-separated path of include
172 directories. These are intended to be passed to the C preprocessor and are
173 also used by the C-file scanning machinery to determine the dependencies
174 involved in a C Compilation.
176 Variables beginning with underscore are created by various methods,
177 and should normally be considered ``internal'' variables. For example,
178 when a method is called which calls for the creation of an object from
179 a C source, the variable C<_IFLAGS> is created: this corresponds to the
180 C<-I> switches required by the C compiler to represent the directories
181 specified by C<CPPPATH>.
183 Note that, for any particular environment, the value of a variable is set
184 once, and then never reset (to change a variable, you must create a new
185 environment. Methods are provided for copying existing environments for this
186 purpose). Some internal variables, such as C<_IFLAGS> are created on demand,
187 but once set, they remain fixed for the life of the environment.
189 The C<CFLAGS>, C<LDFLAGS>, and C<ARFLAGS> variables all supply a place
190 for passing options to the compiler, loader, and archiver, respectively.
192 The C<INCDIRPREFIX> and C<INCDIRSUFFIX> variables specify option
193 strings to be appended to the beginning and end, respectively, of each
194 include directory so that the compiler knows where to find F<.h> files.
195 Similarly, the C<LIBDIRPREFIX> and C<LIBDIRSUFFIX> variables specify the
196 option string to be appended to the beginning of and end, respectively,
197 of each directory that the linker should search for libraries.
199 Another variable, C<ENV>, is used to determine the system environment during
200 the execution of an external command. By default, the only environment
201 variable that is set is C<PATH>, which is the execution path for a UNIX
202 command. For the utmost reproducibility, you should really arrange to set
203 your own execution path, in your top-level F<Construct> file (or perhaps by
204 importing an appropriate construction package with the Perl C<use>
205 command). The default variables are intended to get you off the ground.
207 =head2 Expanding variables in construction commands
209 Within a construction command, construction variables will be expanded
210 according to the rules described above. In addition to normal variable
211 expansion from the construction environment, construction commands also
212 expand the following pseudo-variables to insert the specific input and
213 output files in the command line that will be executed:
219 The target file name. In a multi-target command, this expands to the
220 first target mentioned.)
226 =item %1, %2, ..., %9
228 These refer to the first through ninth input file, respectively.
232 The full set of input file names. If any of these have been used
233 anywhere else in the current command line (via C<%1>, C<%2>, etc.), then
234 those will be deleted from the list provided by C<%E<lt>>. Consider the
235 following command found in a F<Conscript> file in the F<test> directory:
237 Command $env 'tgt', qw(foo bar baz), qq(
243 If F<tgt> needed to be updated, then this would result in the execution of
244 the following commands, assuming that no remapping has been established for
245 the F<test> directory:
247 echo test/bar test/baz -i test/foo > test/tgt
248 echo test/foo test/baz -i test/bar >> test/tgt
249 echo test/foo test/bar -i test/baz >> test/tgt
253 Any of the above pseudo-variables may be followed immediately by one of
254 the following suffixes to select a portion of the expanded path name:
256 :a the absolute path to the file name
257 :b the directory plus the file name stripped of any suffix
260 :s the file name suffix
261 :F the file name stripped of any suffix
262 :S the absolute path path to a Linked source file
264 Continuing with the above example, C<%E<lt>:f> would expand to C<foo bar baz>,
265 and C<%E<gt>:d> would expand to C<test>.
267 There are additional C<%> elements which affect the command line(s):
273 It is possible to programmatically rewrite part of the command by
274 enclosing part of it between C<%[> and C<%]>. This will call the
275 construction variable named as the first word enclosed in the brackets
276 as a Perl code reference; the results of this call will be used to
277 replace the contents of the brackets in the command line. For example,
278 given an existing input file named F<tgt.in>:
280 @keywords = qw(foo bar baz);
281 $env = new cons(X_COMMA => sub { join(",", @_) });
282 Command $env 'tgt', 'tgt.in', qq(
283 echo '# Keywords: %[X_COMMA @keywords %]' > %>
289 echo '# Keywords: foo,bar,baz' > tgt
294 Cons includes the text of the command line in the MD5 signature for a
295 build, so that targets get rebuilt if you change the command line (to
296 add or remove an option, for example). Command-line text in between
297 C<%(> and C<%)>, however, will be ignored for MD5 signature calculation.
299 Internally, Cons uses C<%(> and C<%)> around include and library
300 directory options (C<-I> and C<-L> on UNIX systems, C</I> and
301 C</LIBPATH> on Windows NT) to avoid rebuilds just because the directory
302 list changes. Rebuilds occur only if the changed directory list causes
303 any included I<files> to change, and a changed include file is detected
304 by the MD5 signature calculation on the actual file contents.
310 DESCRIBE THE Literal() FUNCTION, TOO
314 =head2 Expanding construction variables in file names
316 Cons expands construction variables in the source and target file names
317 passed to the various construction methods according to the expansion
318 rules described above:
321 DESTDIR => 'programs',
324 Program $env '%DESTDIR/hello', '%SRCDIR/hello.c';
326 This allows for flexible configuration, through the construction
327 environment, of directory names, suffixes, etc.
329 =head1 Default construction methods
331 The list of default construction methods includes the following:
334 =head2 The C<new> constructor
336 The C<new> method is a Perl object constructor. That is, it is not invoked
337 via a reference to an existing construction environment B<reference>, but,
338 rather statically, using the name of the Perl B<package> where the
339 constructor is defined. The method is invoked like this:
341 $env = new cons(<overrides>);
343 The environment you get back is blessed into the package C<cons>, which
344 means that it will have associated with it the default methods described
345 below. Individual construction variables can be overridden by providing
346 name/value pairs in an override list. Note that to override any command
347 environment variable (i.e. anything under C<ENV>), you will have to override
348 all of them. You can get around this difficulty by using the C<copy> method
349 on an existing construction environment.
352 =head2 The C<clone> method
354 The C<clone> method creates a clone of an existing construction environment,
355 and can be called as in the following example:
357 $env2 = $env1->clone(<overrides>);
359 You can provide overrides in the usual manner to create a different
360 environment from the original. If you just want a new name for the same
361 environment (which may be helpful when exporting environments to existing
362 components), you can just use simple assignment.
365 =head2 The C<copy> method
367 The C<copy> method extracts the externally defined construction variables
368 from an environment and returns them as a list of name/value
369 pairs. Overrides can also be provided, in which case, the overridden values
370 will be returned, as appropriate. The returned list can be assigned to a
371 hash, as shown in the prototype, below, but it can also be manipulated in
374 %env = $env1->copy(<overrides>);
376 The value of C<ENV>, which is itself a hash, is also copied to a new hash,
377 so this may be changed without fear of affecting the original
378 environment. So, for example, if you really want to override just the
379 C<PATH> variable in the default environment, you could do the following:
381 %cons = new cons()->copy();
382 $cons{ENV}{PATH} = "<your path here>";
383 $cons = new cons(%cons);
385 This will leave anything else that might be in the default execution
386 environment undisturbed.
392 It is rare that all of the software in a large,
393 complicated system needs to be built the same way.
394 For example, different source files may need different options
395 enabled on the command line,
396 or different executable programs need to be linked
397 with different libraries.
398 &SCons; accomodates these different build
399 requirements by allowing you to create and
400 configure multiple &consenvs;
401 that control how the software is built.
402 Technically, a &consenv; is an object
403 that has a number of associated
404 &consvars;, each with a name and a value.
405 (A &consenv; also has an attached
406 set of &Builder; methods,
407 about which we'll learn more later.)
413 A &consenv; is created by the &Environment;
414 method which you have already seen.
415 What you haven't seen, though,
416 is that when you initialize a &consenv;,
417 you can set the values of the
418 environment's &consvars;
419 to control how a program is built.
425 env = Environment(CC = 'gcc',
433 This example, rather than using the default,
434 explicitly specifies use of the
435 GNU C compiler &gcc;,
436 and further specifies that the <literal>-O2</literal>
437 (optimization level two)
438 flag should be used when compiling the object file.
439 So a run from this example would look like:
444 % <userinput>scons</userinput>
445 gcc -c -O2 foo.c -o foo.o
450 <title>Multiple &ConsEnvs;</title>
455 all of our examples have
456 created a single &consenv; named
457 <literal>env</literal>.
458 <literal>env</literal>, however,
459 is simply a Python variable name,
460 and you can use any other variable name that you like.
466 my_env = Environment(CC = 'gcc',
469 my_env.Program('foo.c')
474 This opens up the possibility of
475 using multiple &consenvs;,
476 each with a separate variable name.
477 We can then use these separate &consenvs;
478 to build different programs in different ways:
483 opt = Environment(CCFLAGS = '-O2')
484 dbg = Environment(CCFLAGS = '-g')
486 opt.Program('foo', 'foo.c')
488 dbg.Program('bar', 'bar.c')
492 % <userinput>scons</userinput>
493 cc -c -O2 bar.c -o bar.o
495 cc -c -g foo.c -o foo.o
501 We can even use multiple &consenvs; to build
502 multiple versions of a single program.
503 If you do this by simply trying to use the
504 &Program; builder with both environments, though,
510 opt = Environment(CCFLAGS = '-O2')
511 dbg = Environment(CCFLAGS = '-g')
513 opt.Program('foo', 'foo.c')
515 dbg.Program('foo', 'foo.c')
520 Then &SCons; generates the following error:
525 % <userinput>scons</userinput>
526 scons: *** Two different environments were specified for the same target: foo.o
527 File "SConstruct", line 6, in ?
532 This is because the two &Program; calls have
533 each implicitly told &SCons; to generate an object file named
534 <filename>foo.o</filename>,
535 one with a &CCFLAGS; value of
536 <literal>-O2</literal>
537 and one with a &CCFLAGS; value of
538 <literal>-g</literal>.
539 To avoid this problem,
540 we must explicitly specify
541 that each environment compile
542 <filename>foo.c</filename>
543 to a separately-named object file
544 using the &Object; call, like so:
549 opt = Environment(CCFLAGS = '-O2')
550 dbg = Environment(CCFLAGS = '-g')
552 o = opt.Object('foo-opt', 'foo.c')
555 d = dbg.Object('foo-dbg', 'foo.c')
561 Notice that each call to the &Object; builder
563 an internal &SCons; object that
564 represents the file that will be built.
565 We then use that object
566 as input to the &Program; builder.
567 This avoids having to specify explicitly
568 the object file name in multiple places,
569 and makes for a compact, readable
571 Our &SCons; output then looks like:
576 % <userinput>scons</userinput>
577 cc -c -g foo.c -o foo-dbg.o
578 cc -o foo-dbg foo-dbg.o
579 cc -c -O2 foo.c -o foo-opt.o
580 cc -o foo-opt foo-opt.o
586 <title>Copying &ConsEnvs;</title>
590 Sometimes you want more than one &consenv;
591 to share the same values for one or more variables.
592 Rather than always having to repeat all of the common
593 variables when you create each &consenv;,
594 you can use the &Copy; method
595 to create a copy of a &consenv;.
601 Like the &Environment; call that creates a &consenv;,
602 the &Copy; method takes &consvar; assignments,
603 which will override the values in the copied &consenv;.
604 For example, suppose we want to use &gcc;
605 to create three versions of a program,
606 one optimized, one debug, and one with neither.
607 We could do this by creating a "base" &consenv;
608 that sets &CC; to &gcc;,
609 and then creating two copies,
610 one which sets &CCFLAGS; for optimization
611 and the other with sets &CCFLAGS; for debugging:
616 env = Environment(CC = 'gcc')
617 opt = env.Copy(CCFLAGS = '-O2')
618 dbg = env.Copy(CCFLAGS = '-g')
620 e = opt.Object('foo', 'foo.c')
622 o = opt.Object('foo-opt', 'foo.c')
625 d = dbg.Object('foo-dbg', 'foo.c')
631 Then our output would look like:
636 % <userinput>scons</userinput>
637 gcc -c foo.c -o foo.o
639 gcc -c -g foo.c -o foo-dbg.o
640 gcc -o foo-dbg foo-dbg.o
641 gcc -c -O2 foo.c -o foo-opt.o
642 gcc -o foo-opt foo-opt.o
648 <title>Fetching Values From a &ConsEnv;</title>
652 You can fetch individual construction variables
653 using the normal syntax
654 for accessing individual named items in a Python dictionary:
660 print "CC is:", env['CC']
665 This example &SConstruct; file doesn't build anything,
666 but because it's actually a Python script,
667 it will print the value of &CC; for us:
672 % <userinput>scons</userinput>
678 A &consenv;, however,
679 is actually a Python object with
680 associated methods, etc.
681 If you want to have direct access to only the
682 dictionary of construction variables,
683 you can fetch this using the &Dictionary; method:
688 env = Environment(FOO = 'foo', BAR = 'bar')
689 dict = env.Dictionary()
690 for key, value in dict.items():
691 print "key = %s, value = %s % (key, value)
696 This &SConstruct; file
697 will print the dictionary items for us as follows:
702 % <userinput>scons</userinput>
703 key = FOO, value = foo
704 key = BAR, value = bar
710 <title>Modifying a &ConsEnv;</title>
714 &SCons; provides various methods that
715 support modifying existing values in a &consenv;.
720 <title>Replacing Values in a &ConsEnv;</title>
724 You can replace existing construction variable values
725 using the &Replace; method:
730 env = Environment(CCFLAGS = '-DDEFINE1)
732 env.Replace(CCFLAGS = '-DDEFINE2')
738 The replaced value completely overwrites
743 % <userinput>scons</userinput>
744 gcc -DDEFINE2 -c bar.c -o bar.o
746 gcc -DDEFINE2 -c foo.c -o foo.o
753 <title>Appending to the End of Values in a &ConsEnv;</title>
757 You can append a value to
758 an existing construction variable
759 using the &Append; method:
764 env = Environment(CCFLAGS = '-DMY_VALUE')
765 env.Append(CCFLAGS = ' -DLAST')
770 % <userinput>scons</userinput>
771 gcc -DMY_VALUE -DLAST -c foo.c -o foo.o
778 <title>Appending to the Beginning of Values in a &ConsEnv;</title>
782 You can append a value to the beginning
783 an existing construction variable
784 using the &Prepend; method:
789 env = Environment(CCFLAGS = '-DMY_VALUE')
790 env.Prepend(CCFLAGS = '-DFIRST ')
795 % <userinput>scons</userinput>
796 gcc -DFIRST -DMY_VALUE -c foo.c -o foo.o