It's often useful to keep any built files completely separate from the source files. In &SCons;, this is usually done by creating one or more separate variant directory trees that are used to hold the built objects files, libraries, and executable programs, etc. for a specific flavor, or variant, of build. &SCons; provides two ways to do this, one through the &SConscript; function that we've already seen, and the second through a more flexible &VariantDir; function. One historical note: the &VariantDir; function used to be called &BuildDir;. That name is still supported but has been deprecated because the &SCons; functionality differs from the model of a "build directory" implemented by other build systems like the GNU Autotools.

The most straightforward way to establish a variant directory tree uses the fact that the usual way to set up a build hierarchy is to have an &SConscript; file in the source subdirectory. If you then pass a &variant_dir; argument to the &SConscript; function call: SConscript('src/SConscript', variant_dir='build') &SCons; will then build all of the files in the &build; subdirectory: % ls src SConscript hello.c % scons -Q cc -o build/hello.o -c build/hello.c cc -o build/hello build/hello.o % ls build SConscript hello hello.c hello.o But wait a minute--what's going on here? &SCons; created the object file build/hello.o in the &build; subdirectory, as expected. But even though our &hello_c; file lives in the &src; subdirectory, &SCons; has actually compiled a build/hello.c file to create the object file. What's happened is that &SCons; has duplicated the &hello_c; file from the &src; subdirectory to the &build; subdirectory, and built the program from there. The next section explains why &SCons; does this.

&SCons; duplicates source files in variant directory trees because it's the most straightforward way to guarantee a correct build regardless of include-file directory paths, relative references between files, or tool support for putting files in different locations, and the &SCons; philosophy is to, by default, guarantee a correct build in all cases. The most direct reason to duplicate source files in variant directories is simply that some tools (mostly older versions) are written to only build their output files in the same directory as the source files. In this case, the choices are either to build the output file in the source directory and move it to the variant directory, or to duplicate the source files in the variant directory. Additionally, relative references between files can cause problems if we don't just duplicate the hierarchy of source files in the variant directory. You can see this at work in use of the C preprocessor #include mechanism with double quotes, not angle brackets: #include "file.h" The de facto standard behavior for most C compilers in this case is to first look in the same directory as the source file that contains the #include line, then to look in the directories in the preprocessor search path. Add to this that the &SCons; implementation of support for code repositories (described below) means not all of the files will be found in the same directory hierarchy, and the simplest way to make sure that the right include file is found is to duplicate the source files into the variant directory, which provides a correct build regardless of the original location(s) of the source files. Although source-file duplication guarantees a correct build even in these end-cases, it can usually be safely disabled. The next section describes how you can disable the duplication of source files in the variant directory.

In most cases and with most tool sets, &SCons; can place its target files in a build subdirectory without duplicating the source files and everything will work just fine. You can disable the default &SCons; behavior by specifying duplicate=0 when you call the &SConscript; function: SConscript('src/SConscript', variant_dir='build', duplicate=0) When this flag is specified, &SCons; uses the variant directory like most people expect--that is, the output files are placed in the variant directory while the source files stay in the source directory: % ls src SConscript hello.c % scons -Q cc -c src/hello.c -o build/hello.o cc -o build/hello build/hello.o % ls build hello hello.o

Use the &VariantDir; function to establish that target files should be built in a separate directory from the source files: VariantDir('build', 'src') env = Environment() env.Program('build/hello.c') Note that when you're not using an &SConscript; file in the &src; subdirectory, you must actually specify that the program must be built from the build/hello.c file that &SCons; will duplicate in the &build; subdirectory. When using the &VariantDir; function directly, &SCons; still duplicates the source files in the variant directory by default: % ls src hello.c % scons -Q cc -o build/hello.o -c build/hello.c cc -o build/hello build/hello.o % ls build hello hello.c hello.o You can specify the same duplicate=0 argument that you can specify for an &SConscript; call: VariantDir('build', 'src', duplicate=0) env = Environment() env.Program('build/hello.c') In which case &SCons; will disable duplication of the source files: % ls src hello.c % scons -Q cc -o build/hello.o -c src/hello.c cc -o build/hello build/hello.o % ls build hello hello.o

Even when using the &VariantDir; function, it's much more natural to use it with a subsidiary &SConscript; file. For example, if the src/SConscript looks like this: env = Environment() env.Program('hello.c') Then our &SConstruct; file could look like: VariantDir('build', 'src') SConscript('build/SConscript') Yielding the following output: % ls src SConscript hello.c % scons -Q cc -o build/hello.o -c build/hello.c cc -o build/hello build/hello.o % ls build SConscript hello hello.c hello.o Notice that this is completely equivalent to the use of &SConscript; that we learned about in the previous section.

The &Glob; file name pattern matching function works just as usual when using &VariantDir;. For example, if the src/SConscript looks like this: env = Environment() env.Program('hello', Glob('*.c')) Then with the same &SConstruct; file as in the previous section, and source files f1.c and f2.c in src, we would see the following output: % ls src SConscript f1.c f2.c f2.h % scons -Q cc -o build/f1.o -c build/f1.c cc -o build/f2.o -c build/f2.c cc -o build/hello build/f1.o build/f2.o % ls build SConscript f1.c f1.o f2.c f2.h f2.o hello The &Glob; function returns Nodes in the build/ tree, as you'd expect.