Chapter 15. Separating Source and Build Directories

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, a name which was removed because the SCons functionality differs from the model of a "build directory" implemented by other build systems like the GNU Autotools.

15.1. Specifying a Variant Directory Tree as Part of an SConscript Call

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.