It is rare that all of the software in a large, complicated system needs to be built the same way. For example, different source files may need different options enabled on the command line, or different executable programs need to be linked with different libraries. &SCons; accommodates these different build requirements by allowing you to create and configure multiple &consenvs; that control how the software is built. Technically, a &consenv; is an object that has a number of associated &consvars;, each with a name and a value. (A construction environment also has an attached set of &Builder; methods, about which we'll learn more later.) A &consenv; is created by the &Environment; method: env = Environment() By default, &SCons; initializes every new construction environment with a set of &consvars; based on the tools that it finds on your system, plus the default set of builder methods necessary for using those tools. The construction variables are initialized with values describing the C compiler, the Fortran compiler, the linker, etc., as well as the command lines to invoke them. When you initialize a construction environment you can set the values of the environment's &consvars; to control how a program is built. For example: env = Environment(CC = 'gcc', CCFLAGS = '-O2') env.Program('foo.c') int main() { } The construction environment in this example is still initialized with the same default construction variable values, except that the user has explicitly specified use of the GNU C compiler &gcc;, and further specifies that the -O2 (optimization level two) flag should be used when compiling the object file. In other words, the explicit initializations of &cv-link-CC; and &cv-link-CCFLAGS; override the default values in the newly-created construction environment. So a run from this example would look like: scons -Q

The real advantage of construction environments is that you can create as many different construction environments as you need, each tailored to a different way to build some piece of software or other file. If, for example, we need to build one program with the -O2 flag and another with the -g (debug) flag, we would do this like so: opt = Environment(CCFLAGS = '-O2') dbg = Environment(CCFLAGS = '-g') opt.Program('foo', 'foo.c') dbg.Program('bar', 'bar.c') int main() { } int main() { } scons -Q We can even use multiple construction environments to build multiple versions of a single program. If you do this by simply trying to use the &b-link-Program; builder with both environments, though, like this: opt = Environment(CCFLAGS = '-O2') dbg = Environment(CCFLAGS = '-g') opt.Program('foo', 'foo.c') dbg.Program('foo', 'foo.c') int main() { } Then &SCons; generates the following error: scons -Q This is because the two &b-Program; calls have each implicitly told &SCons; to generate an object file named foo.o, one with a &cv-link-CCFLAGS; value of -O2 and one with a &cv-link-CCFLAGS; value of -g. &SCons; can't just decide that one of them should take precedence over the other, so it generates the error. To avoid this problem, we must explicitly specify that each environment compile foo.c to a separately-named object file using the &b-link-Object; builder, like so: opt = Environment(CCFLAGS = '-O2') dbg = Environment(CCFLAGS = '-g') o = opt.Object('foo-opt', 'foo.c') opt.Program(o) d = dbg.Object('foo-dbg', 'foo.c') dbg.Program(d) int main() { } Notice that each call to the &b-Object; builder returns a value, an internal &SCons; object that represents the object file that will be built. We then use that object as input to the &b-Program; builder. This avoids having to specify explicitly the object file name in multiple places, and makes for a compact, readable &SConstruct; file. Our &SCons; output then looks like: scons -Q

Sometimes you want more than one construction environment to share the same values for one or more variables. Rather than always having to repeat all of the common variables when you create each construction environment, you can use the &Clone; method to create a copy of a construction environment. Like the &Environment; call that creates a construction environment, the &Clone; method takes &consvar; assignments, which will override the values in the copied construction environment. For example, suppose we want to use &gcc; to create three versions of a program, one optimized, one debug, and one with neither. We could do this by creating a "base" construction environment that sets &cv-link-CC; to &gcc;, and then creating two copies, one which sets &cv-link-CCFLAGS; for optimization and the other which sets &cv-CCFLAGS; for debugging: env = Environment(CC = 'gcc') opt = env.Clone(CCFLAGS = '-O2') dbg = env.Clone(CCFLAGS = '-g') env.Program('foo', 'foo.c') o = opt.Object('foo-opt', 'foo.c') opt.Program(o) d = dbg.Object('foo-dbg', 'foo.c') dbg.Program(d) int main() { } Then our output would look like: scons -Q

You can fetch individual construction variables using the normal syntax for accessing individual named items in a Python dictionary: env = Environment() print "CC is:", env['CC'] This example &SConstruct; file doesn't build anything, but because it's actually a Python script, it will print the value of &cv-link-CC; for us: scons -Q A construction environment, however, is actually an object with associated methods, etc. If you want to have direct access to only the dictionary of construction variables, you can fetch this using the &Dictionary; method: env = Environment(FOO = 'foo', BAR = 'bar') dict = env.Dictionary() for key in ['OBJSUFFIX', 'LIBSUFFIX', 'PROGSUFFIX']: print "key = %s, value = %s" % (key, dict[key]) This &SConstruct; file will print the specified dictionary items for us on POSIX systems as follows: scons -Q And on Windows: scons -Q If you want to loop through and print the values of all of the construction variables in a construction environment, the Python code to do that in sorted order might look something like: env = Environment() dict = env.Dictionary() keys = dict.keys() keys.sort() for key in keys: print "construction variable = '%s', value = '%s'" % (key, dict[key])

Another way to get information from a construction environment. is to use the &subst; method on a string containing $-expansions of construction variable names. As a simple example, the example from the previous section that used env['CC'] to fetch the value of &cv-link-CC; could also be written as: env = Environment() print "CC is:", env.subst('$CC') The real advantage of using &subst; to expand strings is that construction variables in the result get re-expanded until there are no expansions left in the string. So a simple fetch of a value like &cv-link-CCCOM;: env = Environment(CCFLAGS = '-DFOO') print "CCCOM is:", env['CCCOM'] Will print the unexpanded value of &cv-CCCOM;, showing us the construction variables that still need to be expanded: % scons -Q CCCOM is: $CC $CCFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS -c -o $TARGET $SOURCES scons: `.' is up to date. Calling the &subst; method on $CCOM, however: env = Environment(CCFLAGS = '-DFOO') print "CCCOM is:", env.subst('$CCCOM') Will recursively expand all of the $-prefixed construction variables, showing us the final output: % scons -Q CCCOM is: gcc -DFOO -c -o scons: `.' is up to date. (Note that because we're not expanding this in the context of building something there are no target or source files for &cv-link-TARGET; and &cv-link-SOURCES; to expand.)

&SCons; provides various methods that support modifying existing values in a construction environment.

You can replace existing construction variable values using the &Replace; method: env = Environment(CCFLAGS = '-DDEFINE1') env.Replace(CCFLAGS = '-DDEFINE2') env.Program('foo.c') int main() { } The replacing value (-DDEFINE2 in the above example) completely replaces the value in the construction environment: scons -Q You can safely call &Replace; for construction variables that don't exist in the construction environment: env = Environment() env.Replace(NEW_VARIABLE = 'xyzzy') print "NEW_VARIABLE =", env['NEW_VARIABLE'] In this case, the construction variable simply gets added to the construction environment: scons -Q Because the variables aren't expanded until the construction environment is actually used to build the targets, and because &SCons; function and method calls are order-independent, the last replacement "wins" and is used to build all targets, regardless of the order in which the calls to Replace() are interspersed with calls to builder methods: env = Environment(CCFLAGS = '-DDEFINE1') print "CCFLAGS =", env['CCFLAGS'] env.Program('foo.c') env.Replace(CCFLAGS = '-DDEFINE2') print "CCFLAGS =", env['CCFLAGS'] env.Program('bar.c') int main() { } int main() { } The timing of when the replacement actually occurs relative to when the targets get built becomes apparent if we run &scons; without the -Q option: scons Because the replacement occurs while the &SConscript; files are being read, the &cv-link-CCFLAGS; variable has already been set to -DDEFINE2 by the time the &foo_o; target is built, even though the call to the &Replace; method does not occur until later in the &SConscript; file.

You can append a value to an existing construction variable using the &Append; method: env = Environment(CCFLAGS = '-DMY_VALUE') env.Append(CCFLAGS = ' -DLAST') env.Program('foo.c') int main() { } &SCons; then supplies both the -DMY_VALUE and -DLAST flags when compiling the object file: scons -Q If the construction variable doesn't already exist, the &Append; method will create it: env = Environment() env.Append(NEW_VARIABLE = 'added') print "NEW_VARIABLE =", env['NEW_VARIABLE'] Which yields: scons -Q

You can append a value to the beginning of an existing construction variable using the &Prepend; method: env = Environment(CCFLAGS = '-DMY_VALUE') env.Prepend(CCFLAGS = '-DFIRST ') env.Program('foo.c') int main() { } &SCons; then supplies both the -DFIRST and -DMY_VALUE flags when compiling the object file: scons -Q If the construction variable doesn't already exist, the &Prepend; method will create it: env = Environment() env.Prepend(NEW_VARIABLE = 'added') print "NEW_VARIABLE =", env['NEW_VARIABLE'] Which yields: scons -Q