2009-12-18

# Simulating “make distclean” in SCons

SCons provides an automatic “clean” target out of the box — just run scons -c, and SCons will delete all of the objects that it knows how to build. This is a very useful feature; however, there are two main missing features that I want to add to my build scripts. First, I want to be able to delete all of the temporary files SCons uses, such as its configuration cache and any files I use to store variable values. These aren’t included in the default list of the files to clean up. Second, I want more control over which items are deleted by default, when you specify scons -c without any targets. I’ll describe my solution to the first problem in this post. I’ll write up the second problem in another post.

## Deleting SCons’s temporary files

This feature is akin to the make distclean target that Automake puts into the Makefiles that it generates. This differs from make clean; make clean is intended to delete all of the build products, but leave behind the results of the configure step, whereas make distclean is supposed delete everything, returning the source tree to the same state as when you’d just unpacked the tarball.

The scons -c command is analogous to make clean, and requires no setup. It will automatically delete any of the build products that are created by running scons. There are several cache files that SCons creates, however, and it would be nice to have an equivalent to make distclean. This is especially useful when developing a new Configuration check, for instance — if you make a change to the test, you want to be able to (easily) force SCons to ignore any cached results, and try all of the tests again.

This is actually fairly easy to set up, assuming you know the list of temporary files that SCons will create. You can add the following rule to your top-level SConstruct file:

env.Clean("distclean",
[
".sconsign.dblite",
".sconf_temp",
"config.log",
])


As far as I can tell, these three files are always created by SCons. To delete these files, you simply run scons -c distclean. Because we’ve defined the target using Clean, it will only be run when you pass in the -c option to scons.

Since we’re putting together the file list manually, you should make sure to add any additional cache files that your SCons scripts use. For instance, I’m using some Variable options, which I store into a file called .scons.vars. (This means that the user doesn’t have to type them in with every invocation of scons.) By using these variables, I have to add another entry to the distclean target:

vars = Variables('.scons.vars', ARGUMENTS)
# ...define a bunch of variables
vars.Update(env)
vars.Save(".scons.vars", env)

env.Clean("distclean", [".scons.vars"])


Note how, just like with any SCons target, I can define the distclean target multiple times. SCons will take care of merging them into a single action, deleting all of the specified files when you run scons -c distclean.