An environment is a collection of values that can affect how a program executes. &SCons; distinguishes between three different types of environments that can affect the behavior of &SCons; itself (subject to the configuration in the &SConscript; files), as well as the compilers and other tools it executes: External Environment The external environment is the set of variables in the user's environment at the time the user runs &SCons. These variables are available within the &SConscript; files through the Python os.environ dictionary. See , below. &ConsEnv; A &consenv; is a distinct object creating within a &SConscript; file and and which contains values that affect how &SCons; decides what action to use to build a target, and even to define which targets should be built from which sources. One of the most powerful features of &SCons; is the ability to create multiple &consenvs;, including the ability to clone a new, customized &consenv; from an existing &consenv;. See , below. Execution Environment An execution environment is the values that &SCons; sets when executing an external command (such as a compiler or linker) to build one or more targets. Note that this is not the same as the external environment (see above). See , below. Unlike &Make;, &SCons; does not automatically copy or import values between different environments (with the exception of explicit clones of &consenvs, which inherit values from their parent). This is a deliberate design choice to make sure that builds are, by default, repeatable regardless of the values in the user's external environment. This avoids a whole class of problems with builds where a developer's local build works because a custom variable setting causes a different compiler or build option to be used, but the checked-in change breaks the official build because it uses different environment variable settings. Note that the &SConscript; writer can easily arrange for variables to be copied or imported between environments, and this is often very useful (or even downright necessary) to make it easy for developers to customize the build in appropriate ways. The point is not that copying variables between different environments is evil and must always be avoided. Instead, it should be up to the implementer of the build system to make conscious choices about how and when to import a variable from one environment to another, making informed decisions about striking the right balance between making the build repeatable on the one hand and convenient to use on the other.

The external environment variable settings that the user has in force when executing &SCons; are available through the normal Python os.environ dictionary. This means that you must add an import os statement to any &SConscript; file in which you want to use values from the user's external environment. import os int main() { } More usefully, you can use the os.environ dictionary in your &SConscript; files to initialize &consenvs; with values from the user's external environment. See the next section, , for information on how to do this.

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. 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

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 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() for item in sorted(env.Dictionary().items()): print "construction variable = '%s', value = '%s'" % item

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') One 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 construction variables prefixed with $ (dollar signs), 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.

If a problem occurs when expanding a construction variable, by default it is expanded to '' (a null string), and will not cause scons to fail. env = Environment() print "value is:", env.subst( '->$MISSING<-' ) scons -Q This default behaviour can be changed using the &AllowSubstExceptions; function. When a problem occurs with a variable expansion it generates an exception, and the &AllowSubstExceptions; function controls which of these exceptions are actually fatal and which are allowed to occur safely. By default, &NameError; and &IndexError; are the two exceptions that are allowed to occur: so instead of causing scons to fail, these are caught, the variable expanded to '' and scons execution continues. To require that all construction variable names exist, and that indexes out of range are not allowed, call &AllowSubstExceptions; with no extra arguments. AllowSubstExceptions() env = Environment() print "value is:", env.subst( '->$MISSING<-' ) scons -Q This can also be used to allow other exceptions that might occur, most usefully with the ${...} construction variable syntax. For example, this would allow zero-division to occur in a variable expansion in addition to the default exceptions allowed AllowSubstExceptions(IndexError, NameError, ZeroDivisionError) env = Environment() print "value is:", env.subst( '->${1 / 0}<-' ) scons -Q If &AllowSubstExceptions; is called multiple times, each call completely overwrites the previous list of allowed exceptions.

All of the &Builder; functions that we've introduced so far, like &Program; and &Library;, actually use a default &consenv; that contains settings for the various compilers and other tools that &SCons; configures by default, or otherwise knows about and has discovered on your system. The goal of the default construction environment is to make many configurations to "just work" to build software using readily available tools with a minimum of configuration changes. You can, however, control the settings in the default contstruction environment by using the &DefaultEnvironment; function to initialize various settings: DefaultEnvironment(CC = '/usr/local/bin/gcc') When configured as above, all calls to the &Program; or &Object; Builder will build object files with the /usr/local/bin/gcc compiler. Note that the &DefaultEnvironment; function returns the initialized default construction environment object, which can then be manipulated like any other construction environment. So the following would be equivalent to the previous example, setting the &cv-CC; variable to /usr/local/bin/gcc but as a separate step after the default construction environment has been initialized: env = DefaultEnvironment() env['CC'] = '/usr/local/bin/gcc' One very common use of the &DefaultEnvironment; function is to speed up &SCons; initialization. As part of trying to make most default configurations "just work," &SCons; will actually search the local system for installed compilers and other utilities. This search can take time, especially on systems with slow or networked file systems. If you know which compiler(s) and/or other utilities you want to configure, you can control the search that &SCons; performs by specifying some specific tool modules with which to initialize the default construction environment: env = DefaultEnvironment(tools = ['gcc', 'gnulink'], CC = '/usr/local/bin/gcc') So the above example would tell &SCons; to explicitly configure the default environment to use its normal GNU Compiler and GNU Linker settings (without having to search for them, or any other utilities for that matter), and specifically to use the compiler found at /usr/local/bin/gcc.

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 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.

Sometimes it's useful to be able to specify that a construction variable should be set to a value only if the construction environment does not already have that variable defined You can do this with the &SetDefault; method, which behaves similarly to the set_default method of Python dictionary objects: env.SetDefault(SPECIAL_FLAG = '-extra-option') This is especially useful when writing your own Tool modules to apply variables to construction environments.

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 Note that the &Append; function tries to be "smart" about how the new value is appended to the old value. If both are strings, the previous and new strings are simply concatenated. Similarly, if both are lists, the lists are concatenated. If, however, one is a string and the other is a list, the string is added as a new element to the list.

Some times it's useful to add a new value only if the existing construction variable doesn't already contain the value. This can be done using the &AppendUnique; method: env.AppendUnique(CCFLAGS=['-g']) In the above example, the -g would be added only if the &cv-CCFLAGS; variable does not already contain a -g value.

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 Like the &Append; function, the &Prepend; function tries to be "smart" about how the new value is appended to the old value. If both are strings, the previous and new strings are simply concatenated. Similarly, if both are lists, the lists are concatenated. If, however, one is a string and the other is a list, the string is added as a new element to the list.

Some times it's useful to add a new value to the beginning of a construction variable only if the existing value doesn't already contain the to-be-added value. This can be done using the &PrependUnique; method: env.PrependUnique(CCFLAGS=['-g']) In the above example, the -g would be added only if the &cv-CCFLAGS; variable does not already contain a -g value.

When &SCons; builds a target file, it does not execute the commands with the same external environment that you used to execute &SCons;. Instead, it uses the dictionary stored in the &cv-link-ENV; construction variable as the external environment for executing commands. The most important ramification of this behavior is that the &PATH; environment variable, which controls where the operating system will look for commands and utilities, is not the same as in the external environment from which you called &SCons;. This means that &SCons; will not, by default, necessarily find all of the tools that you can execute from the command line. The default value of the &PATH; environment variable on a POSIX system is /usr/local/bin:/bin:/usr/bin. The default value of the &PATH; environment variable on a Windows system comes from the Windows registry value for the command interpreter. If you want to execute any commands--compilers, linkers, etc.--that are not in these default locations, you need to set the &PATH; value in the &cv-ENV; dictionary in your construction environment. The simplest way to do this is to initialize explicitly the value when you create the construction environment; this is one way to do that: path = ['/usr/local/bin', '/bin', '/usr/bin'] env = Environment(ENV = {'PATH' : path}) Assign a dictionary to the &cv-ENV; construction variable in this way completely resets the external environment so that the only variable that will be set when external commands are executed will be the &PATH; value. If you want to use the rest of the values in &cv-ENV; and only set the value of &PATH;, the most straightforward way is probably: env['ENV']['PATH'] = ['/usr/local/bin', '/bin', '/usr/bin'] Note that &SCons; does allow you to define the directories in the &PATH; in a string, separated by the pathname-separator character for your system (':' on POSIX systems, ';' on Windows): env['ENV']['PATH'] = '/usr/local/bin:/bin:/usr/bin' But doing so makes your &SConscript; file less portable, (although in this case that may not be a huge concern since the directories you list are likley system-specific, anyway).

You may want to propagate the external &PATH; to the execution environment for commands. You do this by initializing the &PATH; variable with the &PATH; value from the os.environ dictionary, which is Python's way of letting you get at the external environment: import os env = Environment(ENV = {'PATH' : os.environ['PATH']}) Alternatively, you may find it easier to just propagate the entire external environment to the execution environment for commands. This is simpler to code than explicity selecting the &PATH; value: import os env = Environment(ENV = os.environ) Either of these will guarantee that &SCons; will be able to execute any command that you can execute from the command line. The drawback is that the build can behave differently if it's run by people with different &PATH; values in their environment--for example, if both the /bin and /usr/local/bin directories have different &cc; commands, then which one will be used to compile programs will depend on which directory is listed first in the user's &PATH; variable.

One of the most common requirements for manipulating a variable in the execution environment is to add one or more custom directories to a search like the $PATH variable on Linux or POSIX systems, or the %PATH% variable on Windows, so that a locally-installed compiler or other utility can be found when &SCons; tries to execute it to update a target. &SCons; provides &PrependENVPath; and &AppendENVPath; functions to make adding things to execution variables convenient. You call these functions by specifying the variable to which you want the value added, and then value itself. So to add some /usr/local directories to the $PATH and $LIB variables, you might: env = Environment(ENV = os.environ) env.PrependENVPath('PATH', '/usr/local/bin') env.AppendENVPath('LIB', '/usr/local/lib') Note that the added values are strings, and if you want to add multiple directories to a variable like $PATH, you must include the path separate character (: on Linux or POSIX, ; on Windows) in the string.