14.2. Path Names Are Relative to the SConscript Directory

14.2. Path Names Are Relative to the `SConscript` Directory
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14.2. Path Names Are Relative to the `SConscript` Directory

Subsidiary SConscript files make it easy to create a build hierarchy because all of the file and directory names in a subsidiary SConscript files are interpreted relative to the directory in which that SConscript file lives. Typically, this allows the SConscript file containing the instructions to build a target file to live in the same directory as the source files from which the target will be built, making it easy to update how the software is built whenever files are added or deleted (or other changes are made). It also tends to keep scripts more readable as they don't need to be filled with complex paths.

For example, suppose we want to build two programs prog1 and prog2 in two separate directories with the same names as the programs. One typical way to do this would be with a top-level SConstruct file like this:

SConscript(['prog1/SConscript', 'prog2/SConscript'])

And subsidiary SConscript files that look like this:

env = Environment()
env.Program('prog1', ['main.c', 'foo1.c', 'foo2.c'])

And this:

env = Environment()
env.Program('prog2', ['main.c', 'bar1.c', 'bar2.c'])

Then, when we run SCons in the top-level directory, our build looks like:

% scons -Q
cc -o prog1/foo1.o -c prog1/foo1.c
cc -o prog1/foo2.o -c prog1/foo2.c
cc -o prog1/main.o -c prog1/main.c
cc -o prog1/prog1 prog1/main.o prog1/foo1.o prog1/foo2.o
cc -o prog2/bar1.o -c prog2/bar1.c
cc -o prog2/bar2.o -c prog2/bar2.c
cc -o prog2/main.o -c prog2/main.c
cc -o prog2/prog2 prog2/main.o prog2/bar1.o prog2/bar2.o

Notice the following: First, you can have files with the same names in multiple directories, like main.c in the above example. Second, when building, SCons stays in the top-level directory (where the SConstruct file lives) and issues commands that use the path names from the top-level directory to the target and source files within the hierarchy. This works because SCons reads all the SConscript files in one pass, interpreting each in its local context, building up a tree of information, before starting to execute the needed builds in a second pass. This is quite different than some other build tools which implement a heirarcical build by recursing.