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 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 Windows 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.
308 XXX DESCRIBE THE Literal() FUNCTION, TOO XXX
310 =head2 Expanding construction variables in file names
312 Cons expands construction variables in the source and target file names
313 passed to the various construction methods according to the expansion
314 rules described above:
317 DESTDIR => 'programs',
320 Program $env '%DESTDIR/hello', '%SRCDIR/hello.c';
322 This allows for flexible configuration, through the construction
323 environment, of directory names, suffixes, etc.
329 An <literal>environment</literal>
330 is a collection of values that
331 can affect how a program executes.
332 &SCons; distinguishes between three
333 different types of environments
334 that can affect the behavior of &SCons; itself
335 (subject to the configuration in the &SConscript; files),
336 as well as the compilers and other tools it executes:
343 <term>External Environment</term>
348 The <literal>external environment</literal>
349 is the set of variables in the user's environment
350 at the time the user runs &SCons;.
351 These variables are available within the &SConscript; files
352 through the Python <literal>os.environ</literal> dictionary.
353 See <xref linkend="sect-external-environments"></xref>, below.
360 <term>&ConsEnv;</term>
366 is a distinct object creating within
367 a &SConscript; file and
368 and which contains values that
369 affect how &SCons; decides
370 what action to use to build a target,
371 and even to define which targets
372 should be built from which sources.
373 One of the most powerful features of &SCons;
374 is the ability to create multiple &consenvs;,
375 including the ability to clone a new, customized
376 &consenv; from an existing &consenv;.
377 See <xref linkend="sect-construction-environments"></xref>, below.
384 <term>Execution Environment</term>
389 An <literal>execution environment</literal>
390 is the values that &SCons; sets
391 when executing an external
392 command (such as a compiler or linker)
393 to build one or more targets.
394 Note that this is not the same as
395 the <literal>external environment</literal>
397 See <xref linkend="sect-execution-environments"></xref>, below.
407 Unlike &Make;, &SCons; does not automatically
408 copy or import values between different environments
409 (with the exception of explicit clones of &consenvs;,
410 which inherit values from their parent).
411 This is a deliberate design choice
412 to make sure that builds are,
413 by default, repeatable regardless of
414 the values in the user's external environment.
415 This avoids a whole class of problems with builds
416 where a developer's local build works
417 because a custom variable setting
418 causes a different compiler or build option to be used,
419 but the checked-in change breaks the official build
420 because it uses different environment variable settings.
426 Note that the &SConscript; writer can
427 easily arrange for variables to be
428 copied or imported between environments,
429 and this is often very useful
430 (or even downright necessary)
431 to make it easy for developers
432 to customize the build in appropriate ways.
433 The point is <emphasis>not</emphasis>
434 that copying variables between different environments
435 is evil and must always be avoided.
436 Instead, it should be up to the
437 implementer of the build system
438 to make conscious choices
439 about how and when to import
440 a variable from one environment to another,
441 making informed decisions about
442 striking the right balance
443 between making the build
444 repeatable on the one hand
445 and convenient to use on the other.
449 <section id="sect-external-environments">
450 <title>Using Values From the External Environment</title>
454 The <literal>external environment</literal>
455 variable settings that
456 the user has in force
457 when executing &SCons;
458 are available through the normal Python
459 <envar>os.environ</envar>
461 This means that you must add an
462 <literal>import os</literal> statement
463 to any &SConscript; file
464 in which you want to use
465 values from the user's external environment.
475 More usefully, you can use the
476 <envar>os.environ</envar>
477 dictionary in your &SConscript;
478 files to initialize &consenvs;
479 with values from the user's external environment.
480 See the next section,
481 <xref linkend="sect-construction-environments"></xref>,
482 for information on how to do this.
488 <section id="sect-construction-environments">
489 <title>Construction Environments</title>
493 It is rare that all of the software in a large,
494 complicated system needs to be built the same way.
495 For example, different source files may need different options
496 enabled on the command line,
497 or different executable programs need to be linked
498 with different libraries.
499 &SCons; accommodates these different build
500 requirements by allowing you to create and
501 configure multiple &consenvs;
502 that control how the software is built.
503 A &consenv; is an object
504 that has a number of associated
505 &consvars;, each with a name and a value.
506 (A construction environment also has an attached
507 set of &Builder; methods,
508 about which we'll learn more later.)
513 <title>Creating a &ConsEnv;: the &Environment; Function</title>
517 A &consenv; is created by the &Environment; method:
527 By default, &SCons; initializes every
528 new construction environment
529 with a set of &consvars;
530 based on the tools that it finds on your system,
531 plus the default set of builder methods
532 necessary for using those tools.
533 The construction variables
534 are initialized with values describing
536 the Fortran compiler,
539 as well as the command lines to invoke them.
545 When you initialize a construction environment
546 you can set the values of the
547 environment's &consvars;
548 to control how a program is built.
556 env = Environment(CC = 'gcc',
564 The construction environment in this example
565 is still initialized with the same default
566 construction variable values,
567 except that the user has explicitly specified use of the
568 GNU C compiler &gcc;,
569 and further specifies that the <literal>-O2</literal>
570 (optimization level two)
571 flag should be used when compiling the object file.
572 In other words, the explicit initializations of
573 &cv-link-CC; and &cv-link-CCFLAGS;
574 override the default values in the newly-created
575 construction environment.
576 So a run from this example would look like:
581 % <userinput>scons -Q</userinput>
582 gcc -o foo.o -c -O2 foo.c
589 <title>Fetching Values From a &ConsEnv;</title>
593 You can fetch individual construction variables
594 using the normal syntax
595 for accessing individual named items in a Python dictionary:
601 print "CC is:", env['CC']
606 This example &SConstruct; file doesn't build anything,
607 but because it's actually a Python script,
608 it will print the value of &cv-link-CC; for us:
613 % <userinput>scons -Q</userinput>
615 scons: `.' is up to date.
620 A construction environment, however,
621 is actually an object with associated methods, etc.
622 If you want to have direct access to only the
623 dictionary of construction variables,
624 you can fetch this using the &Dictionary; method:
629 env = Environment(FOO = 'foo', BAR = 'bar')
630 dict = env.Dictionary()
631 for key in ['OBJSUFFIX', 'LIBSUFFIX', 'PROGSUFFIX']:
632 print "key = %s, value = %s" % (key, dict[key])
637 This &SConstruct; file
638 will print the specified dictionary items for us on POSIX
644 % <userinput>scons -Q</userinput>
645 key = OBJSUFFIX, value = .o
646 key = LIBSUFFIX, value = .a
647 key = PROGSUFFIX, value =
648 scons: `.' is up to date.
658 C:\><userinput>scons -Q</userinput>
659 key = OBJSUFFIX, value = .obj
660 key = LIBSUFFIX, value = .lib
661 key = PROGSUFFIX, value = .exe
662 scons: `.' is up to date.
667 If you want to loop and print the values of
668 all of the construction variables in a construction environment,
669 the Python code to do that in sorted order might look something like:
675 dict = env.Dictionary()
679 print "construction variable = '%s', value = '%s'" % (key, dict[key])
685 <title>Expanding Values From a &ConsEnv;: the &subst; Method</title>
689 Another way to get information from
690 a construction environment.
691 is to use the &subst; method
692 on a string containing <literal>$</literal> expansions
693 of construction variable names.
695 the example from the previous
697 <literal>env['CC']</literal>
698 to fetch the value of &cv-link-CC;
699 could also be written as:
705 print "CC is:", env.subst('$CC')
710 One advantage of using
711 &subst; to expand strings is
712 that construction variables
713 in the result get re-expanded until
714 there are no expansions left in the string.
715 So a simple fetch of a value like
721 env = Environment(CCFLAGS = '-DFOO')
722 print "CCCOM is:", env['CCCOM']
727 Will print the unexpanded value of &cv-CCCOM;,
728 showing us the construction
729 variables that still need to be expanded:
734 % <userinput>scons -Q</userinput>
735 CCCOM is: $CC $CCFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS -c -o $TARGET $SOURCES
736 scons: `.' is up to date.
741 Calling the &subst; method on <varname>$CCOM</varname>,
747 env = Environment(CCFLAGS = '-DFOO')
748 print "CCCOM is:", env.subst('$CCCOM')
753 Will recursively expand all of
754 the construction variables prefixed
755 with <literal>$</literal> (dollar signs),
756 showing us the final output:
761 % <userinput>scons -Q</userinput>
762 CCCOM is: gcc -DFOO -c -o
763 scons: `.' is up to date.
768 Note that because we're not expanding this
769 in the context of building something
770 there are no target or source files
771 for &cv-link-TARGET; and &cv-link-SOURCES; to expand.
778 <title>Controlling the Default &ConsEnv;: the &DefaultEnvironment; Function</title>
782 All of the &Builder; functions that we've introduced so far,
783 like &Program; and &Library;,
784 actually use a default &consenv;
785 that contains settings
786 for the various compilers
788 &SCons; configures by default,
789 or otherwise knows about
790 and has discovered on your system.
791 The goal of the default construction environment
792 is to make many configurations to "just work"
793 to build software using
794 readily available tools
795 with a minimum of configuration changes.
801 You can, however, control the settings
802 in the default contstruction environment
803 by using the &DefaultEnvironment; function
804 to initialize various settings:
810 DefaultEnvironment(CC = '/usr/local/bin/gcc')
816 When configured as above,
817 all calls to the &Program;
819 will build object files with the
820 <filename>/usr/local/bin/gcc</filename>
827 Note that the &DefaultEnvironment; function
828 returns the initialized
829 default construction environment object,
830 which can then be manipulated like any
831 other construction environment.
833 would be equivalent to the
836 variable to <filename>/usr/local/bin/gcc</filename>
837 but as a separate step after
838 the default construction environment has been initialized:
844 env = DefaultEnvironment()
845 env['CC'] = '/usr/local/bin/gcc'
851 One very common use of the &DefaultEnvironment; function
852 is to speed up &SCons; initialization.
853 As part of trying to make most default
854 configurations "just work,"
855 &SCons; will actually
856 search the local system for installed
857 compilers and other utilities.
858 This search can take time,
859 especially on systems with
860 slow or networked file systems.
861 If you know which compiler(s) and/or
862 other utilities you want to configure,
863 you can control the search
864 that &SCons; performs
865 by specifying some specific
866 tool modules with which to
867 initialize the default construction environment:
873 env = DefaultEnvironment(tools = ['gcc', 'gnulink'],
874 CC = '/usr/local/bin/gcc')
880 So the above example would tell &SCons;
881 to explicitly configure the default environment
882 to use its normal GNU Compiler and GNU Linker settings
883 (without having to search for them,
884 or any other utilities for that matter),
885 and specifically to use the compiler found at
886 <filename>/usr/local/bin/gcc</filename>.
893 <title>Multiple &ConsEnvs;</title>
897 The real advantage of construction environments
898 is that you can create as many different construction
899 environments as you need,
900 each tailored to a different way to build
901 some piece of software or other file.
902 If, for example, we need to build
903 one program with the <literal>-O2</literal> flag
904 and another with the <literal>-g</literal> (debug) flag,
905 we would do this like so:
910 opt = Environment(CCFLAGS = '-O2')
911 dbg = Environment(CCFLAGS = '-g')
913 opt.Program('foo', 'foo.c')
915 dbg.Program('bar', 'bar.c')
919 % <userinput>scons -Q</userinput>
920 cc -o bar.o -c -g bar.c
922 cc -o foo.o -c -O2 foo.c
928 We can even use multiple construction environments to build
929 multiple versions of a single program.
930 If you do this by simply trying to use the
931 &b-link-Program; builder with both environments, though,
937 opt = Environment(CCFLAGS = '-O2')
938 dbg = Environment(CCFLAGS = '-g')
940 opt.Program('foo', 'foo.c')
942 dbg.Program('foo', 'foo.c')
947 Then &SCons; generates the following error:
952 % <userinput>scons -Q</userinput>
954 scons: *** Two environments with different actions were specified for the same target: foo.o
955 File "/home/my/project/SConstruct", line 6, in <module>
960 This is because the two &b-Program; calls have
961 each implicitly told &SCons; to generate an object file named
962 <filename>foo.o</filename>,
963 one with a &cv-link-CCFLAGS; value of
964 <literal>-O2</literal>
965 and one with a &cv-link-CCFLAGS; value of
966 <literal>-g</literal>.
967 &SCons; can't just decide that one of them
968 should take precedence over the other,
969 so it generates the error.
970 To avoid this problem,
971 we must explicitly specify
972 that each environment compile
973 <filename>foo.c</filename>
974 to a separately-named object file
975 using the &b-link-Object; builder, like so:
980 opt = Environment(CCFLAGS = '-O2')
981 dbg = Environment(CCFLAGS = '-g')
983 o = opt.Object('foo-opt', 'foo.c')
986 d = dbg.Object('foo-dbg', 'foo.c')
992 Notice that each call to the &b-Object; builder
994 an internal &SCons; object that
995 represents the object file that will be built.
996 We then use that object
997 as input to the &b-Program; builder.
998 This avoids having to specify explicitly
999 the object file name in multiple places,
1000 and makes for a compact, readable
1002 Our &SCons; output then looks like:
1007 % <userinput>scons -Q</userinput>
1008 cc -o foo-dbg.o -c -g foo.c
1009 cc -o foo-dbg foo-dbg.o
1010 cc -o foo-opt.o -c -O2 foo.c
1011 cc -o foo-opt foo-opt.o
1017 <title>Making Copies of &ConsEnvs;: the &Clone; Method</title>
1021 Sometimes you want more than one construction environment
1022 to share the same values for one or more variables.
1023 Rather than always having to repeat all of the common
1024 variables when you create each construction environment,
1025 you can use the &Clone; method
1026 to create a copy of a construction environment.
1032 Like the &Environment; call that creates a construction environment,
1033 the &Clone; method takes &consvar; assignments,
1034 which will override the values in the copied construction environment.
1035 For example, suppose we want to use &gcc;
1036 to create three versions of a program,
1037 one optimized, one debug, and one with neither.
1038 We could do this by creating a "base" construction environment
1039 that sets &cv-link-CC; to &gcc;,
1040 and then creating two copies,
1041 one which sets &cv-link-CCFLAGS; for optimization
1042 and the other which sets &cv-CCFLAGS; for debugging:
1047 env = Environment(CC = 'gcc')
1048 opt = env.Clone(CCFLAGS = '-O2')
1049 dbg = env.Clone(CCFLAGS = '-g')
1051 env.Program('foo', 'foo.c')
1053 o = opt.Object('foo-opt', 'foo.c')
1056 d = dbg.Object('foo-dbg', 'foo.c')
1062 Then our output would look like:
1067 % <userinput>scons -Q</userinput>
1068 gcc -o foo.o -c foo.c
1070 gcc -o foo-dbg.o -c -g foo.c
1071 gcc -o foo-dbg foo-dbg.o
1072 gcc -o foo-opt.o -c -O2 foo.c
1073 gcc -o foo-opt foo-opt.o
1079 <title>Replacing Values: the &Replace; Method</title>
1083 You can replace existing construction variable values
1084 using the &Replace; method:
1089 env = Environment(CCFLAGS = '-DDEFINE1')
1090 env.Replace(CCFLAGS = '-DDEFINE2')
1091 env.Program('foo.c')
1097 (<literal>-DDEFINE2</literal> in the above example)
1098 completely replaces the value in the
1099 construction environment:
1104 % <userinput>scons -Q</userinput>
1105 cc -o foo.o -c -DDEFINE2 foo.c
1111 You can safely call &Replace;
1112 for construction variables that
1113 don't exist in the construction environment:
1119 env.Replace(NEW_VARIABLE = 'xyzzy')
1120 print "NEW_VARIABLE =", env['NEW_VARIABLE']
1126 the construction variable simply
1127 gets added to the construction environment:
1132 % <userinput>scons -Q</userinput>
1133 NEW_VARIABLE = xyzzy
1134 scons: `.' is up to date.
1139 Because the variables
1140 aren't expanded until the construction environment
1141 is actually used to build the targets,
1142 and because &SCons; function and method calls
1143 are order-independent,
1144 the last replacement "wins"
1145 and is used to build all targets,
1146 regardless of the order in which
1147 the calls to Replace() are
1148 interspersed with calls to
1154 env = Environment(CCFLAGS = '-DDEFINE1')
1155 print "CCFLAGS =", env['CCFLAGS']
1156 env.Program('foo.c')
1158 env.Replace(CCFLAGS = '-DDEFINE2')
1159 print "CCFLAGS =", env['CCFLAGS']
1160 env.Program('bar.c')
1165 The timing of when the replacement
1166 actually occurs relative
1167 to when the targets get built
1169 if we run &scons; without the <literal>-Q</literal>
1175 % <userinput>scons</userinput>
1176 scons: Reading SConscript files ...
1179 scons: done reading SConscript files.
1180 scons: Building targets ...
1181 cc -o bar.o -c -DDEFINE2 bar.c
1183 cc -o foo.o -c -DDEFINE2 foo.c
1185 scons: done building targets.
1190 Because the replacement occurs while
1191 the &SConscript; files are being read,
1192 the &cv-link-CCFLAGS;
1193 variable has already been set to
1194 <literal>-DDEFINE2</literal>
1195 by the time the &foo_o; target is built,
1196 even though the call to the &Replace;
1197 method does not occur until later in
1198 the &SConscript; file.
1205 <title>Setting Values Only If They're Not Already Defined: the &SetDefault; Method</title>
1209 Sometimes it's useful to be able to specify
1210 that a construction variable should be
1211 set to a value only if the construction environment
1212 does not already have that variable defined
1213 You can do this with the &SetDefault; method,
1214 which behaves similarly to the <function>set_default</function>
1215 method of Python dictionary objects:
1220 env.SetDefault(SPECIAL_FLAG = '-extra-option')
1225 This is especially useful
1226 when writing your own <literal>Tool</literal> modules
1227 to apply variables to construction environments.
1229 See <xref linkend="chap-tool-modules"></xref>
1230 for more information about writing
1239 <title>Appending to the End of Values: the &Append; Method</title>
1243 You can append a value to
1244 an existing construction variable
1245 using the &Append; method:
1250 env = Environment(CCFLAGS = ['-DMY_VALUE'])
1251 env.Append(CCFLAGS = ['-DLAST'])
1252 env.Program('foo.c')
1257 &SCons; then supplies both the <literal>-DMY_VALUE</literal> and
1258 <literal>-DLAST</literal> flags when compiling the object file:
1263 % <userinput>scons -Q</userinput>
1264 cc -o foo.o -c -DMY_VALUE -DLAST foo.c
1270 If the construction variable doesn't already exist,
1271 the &Append; method will create it:
1277 env.Append(NEW_VARIABLE = 'added')
1278 print "NEW_VARIABLE =", env['NEW_VARIABLE']
1288 % <userinput>scons -Q</userinput>
1289 NEW_VARIABLE = added
1290 scons: `.' is up to date.
1295 Note that the &Append; function tries to be "smart"
1296 about how the new value is appended to the old value.
1297 If both are strings, the previous and new strings
1298 are simply concatenated.
1299 Similarly, if both are lists,
1300 the lists are concatenated.
1301 If, however, one is a string and the other is a list,
1302 the string is added as a new element to the list.
1309 <title>Appending Unique Values: the &AppendUnique; Method</title>
1313 Some times it's useful to add a new value
1314 only if the existing construction variable
1315 doesn't already contain the value.
1316 This can be done using the &AppendUnique; method:
1321 env.AppendUnique(CCFLAGS=['-g'])
1326 In the above example,
1327 the <literal>-g</literal> would be added
1328 only if the &cv-CCFLAGS; variable
1329 does not already contain a <literal>-g</literal> value.
1336 <title>Appending to the Beginning of Values: the &Prepend; Method</title>
1340 You can append a value to the beginning of
1341 an existing construction variable
1342 using the &Prepend; method:
1347 env = Environment(CCFLAGS = ['-DMY_VALUE'])
1348 env.Prepend(CCFLAGS = ['-DFIRST'])
1349 env.Program('foo.c')
1354 &SCons; then supplies both the <literal>-DFIRST</literal> and
1355 <literal>-DMY_VALUE</literal> flags when compiling the object file:
1360 % <userinput>scons -Q</userinput>
1361 cc -o foo.o -c -DFIRST -DMY_VALUE foo.c
1367 If the construction variable doesn't already exist,
1368 the &Prepend; method will create it:
1374 env.Prepend(NEW_VARIABLE = 'added')
1375 print "NEW_VARIABLE =", env['NEW_VARIABLE']
1385 % <userinput>scons -Q</userinput>
1386 NEW_VARIABLE = added
1387 scons: `.' is up to date.
1392 Like the &Append; function,
1393 the &Prepend; function tries to be "smart"
1394 about how the new value is appended to the old value.
1395 If both are strings, the previous and new strings
1396 are simply concatenated.
1397 Similarly, if both are lists,
1398 the lists are concatenated.
1399 If, however, one is a string and the other is a list,
1400 the string is added as a new element to the list.
1407 <title>Prepending Unique Values: the &PrependUnique; Method</title>
1411 Some times it's useful to add a new value
1412 to the beginning of a construction variable
1413 only if the existing value
1414 doesn't already contain the to-be-added value.
1415 This can be done using the &PrependUnique; method:
1420 env.PrependUnique(CCFLAGS=['-g'])
1425 In the above example,
1426 the <literal>-g</literal> would be added
1427 only if the &cv-CCFLAGS; variable
1428 does not already contain a <literal>-g</literal> value.
1436 <section id="sect-execution-environments">
1437 <title>Controlling the Execution Environment for Issued Commands</title>
1441 When &SCons; builds a target file,
1442 it does not execute the commands with
1443 the same external environment
1444 that you used to execute &SCons;.
1445 Instead, it uses the dictionary
1446 stored in the &cv-link-ENV; construction variable
1447 as the external environment
1448 for executing commands.
1454 The most important ramification of this behavior
1455 is that the &PATH; environment variable,
1456 which controls where the operating system
1457 will look for commands and utilities,
1458 is not the same as in the external environment
1459 from which you called &SCons;.
1460 This means that &SCons; will not, by default,
1461 necessarily find all of the tools
1462 that you can execute from the command line.
1468 The default value of the &PATH; environment variable
1470 is <literal>/usr/local/bin:/bin:/usr/bin</literal>.
1471 The default value of the &PATH; environment variable
1472 on a Windows system comes from the Windows registry
1473 value for the command interpreter.
1474 If you want to execute any commands--compilers, linkers, etc.--that
1475 are not in these default locations,
1476 you need to set the &PATH; value
1477 in the &cv-ENV; dictionary
1478 in your construction environment.
1484 The simplest way to do this is to initialize explicitly
1485 the value when you create the construction environment;
1486 this is one way to do that:
1491 path = ['/usr/local/bin', '/bin', '/usr/bin']
1492 env = Environment(ENV = {'PATH' : path})
1497 Assign a dictionary to the &cv-ENV;
1498 construction variable in this way
1499 completely resets the external environment
1500 so that the only variable that will be
1501 set when external commands are executed
1502 will be the &PATH; value.
1503 If you want to use the rest of
1504 the values in &cv-ENV; and only
1505 set the value of &PATH;,
1506 the most straightforward way is probably:
1511 env['ENV']['PATH'] = ['/usr/local/bin', '/bin', '/usr/bin']
1516 Note that &SCons; does allow you to define
1517 the directories in the &PATH; in a string,
1518 separated by the pathname-separator character
1519 for your system (':' on POSIX systems, ';' on Windows):
1524 env['ENV']['PATH'] = '/usr/local/bin:/bin:/usr/bin'
1529 But doing so makes your &SConscript; file less portable,
1530 (although in this case that may not be a huge concern
1531 since the directories you list are likley system-specific, anyway).
1537 <scons_example name="ex1">
1538 <file name="SConstruct" printme="1">
1540 env.Command('foo', [], '__ROOT__/usr/bin/printenv.py')
1542 <file name="__ROOT__/usr/bin/printenv.py" chmod="0755">
1543 #!/usr/bin/env python
1546 if len(sys.argv) > 1:
1549 keys = os.environ.keys()
1552 print " " + key + "=" + os.environ[key]
1560 <scons_output example="ex1">
1561 <scons_output_command>scons -Q</scons_output_command>
1567 <title>Propagating &PATH; From the External Environment</title>
1571 You may want to propagate the external &PATH;
1572 to the execution environment for commands.
1573 You do this by initializing the &PATH;
1574 variable with the &PATH; value from
1575 the <literal>os.environ</literal>
1577 which is Python's way of letting you
1578 get at the external environment:
1584 env = Environment(ENV = {'PATH' : os.environ['PATH']})
1589 Alternatively, you may find it easier
1590 to just propagate the entire external
1591 environment to the execution environment
1593 This is simpler to code than explicity
1594 selecting the &PATH; value:
1600 env = Environment(ENV = os.environ)
1605 Either of these will guarantee that
1606 &SCons; will be able to execute
1607 any command that you can execute from the command line.
1608 The drawback is that the build can behave
1609 differently if it's run by people with
1610 different &PATH; values in their environment--for example,
1611 if both the <literal>/bin</literal> and
1612 <literal>/usr/local/bin</literal> directories
1613 have different &cc; commands,
1614 then which one will be used to compile programs
1615 will depend on which directory is listed
1616 first in the user's &PATH; variable.
1623 <title>Adding to <varname>PATH</varname> Values in the Execution Environment</title>
1627 One of the most common requirements
1628 for manipulating a variable in the execution environment
1629 is to add one or more custom directories to a search
1630 like the <envar>$PATH</envar> variable on Linux or POSIX systems,
1631 or the <envar>%PATH%</envar> variable on Windows,
1632 so that a locally-installed compiler or other utility
1633 can be found when &SCons; tries to execute it to update a target.
1634 &SCons; provides &PrependENVPath; and &AppendENVPath; functions
1635 to make adding things to execution variables convenient.
1636 You call these functions by specifying the variable
1637 to which you want the value added,
1638 and then value itself.
1639 So to add some <filename>/usr/local</filename> directories
1640 to the <envar>$PATH</envar> and <envar>$LIB</envar> variables,
1646 env = Environment(ENV = os.environ)
1647 env.PrependENVPath('PATH', '/usr/local/bin')
1648 env.AppendENVPath('LIB', '/usr/local/lib')
1653 Note that the added values are strings,
1654 and if you want to add multiple directories to
1655 a variable like <envar>$PATH</envar>,
1656 you must include the path separate character
1657 (<literal>:</literal> on Linux or POSIX,
1658 <literal>;</literal> on Windows)