SamJansen's work in progress.

What?

This document describes moving from the traditional make(1) build system to the Python-based SCons replacement.

Why?

make(1) is not for everyone. I've had to maintain the build system for a reasonably large and complex project and using makefiles has been more and more of a headache as time has passed. I have found almost no redeeming features in make - the only thing I can say for it is that most people who wish to compile software will have it installed.

Who?

Should you use SCons too? Maybe. SCons requires that whoever has to compile the software has both Python and SCons installed. If this is an acceptable requirement, then the answer is probably "yes".

The worst thing about SCons is that it does take some time to learn. It is not hard to learn; being based on the simple but powerful programming language Python helps here, but it can be hard to make the leap from a system you already know to a new one, even if the old system is inferior, simply because of the extra time it takes to learn the new system. However, I recommend learning modern build systems that don't employ make.

The simple stuff

SCons does a few things for free. Dependency tracking is the first one that springs to mind. Consider the following makefile and its SCons equivalent:

MakeFile:

program: main.o sam.o johno.o
        gcc -o $@ $^

SConstruct:

Program(source = ["main.c", "sam.c", "johno.c"], target = "program")

Both of these work the same at the first glance, though I believe the SConstruct file (yes, SConstruct is the name of the file that replaces your MakeFile) to be a lot more readable. However the SCons file has the following extra features:

• Proper dependency tracking. SCons scans the source file for headers so it knows when each file needs to be compiled. Make has no knowledge of this by default, and doing proper dependency tracking in make is quite a nightmare (but possible).
• MD5 checksum based checking of the source files to see which ones need to be recompiled. Make uses timestamps, which is also an option for SCons.
• No explicit rules needed for this simple example. In this particular MakeFile the linking line is still necessary, along with strange syntax: a casual reader has no idea what $@ and $^ mean. To be fair, make has a lot of implicit rules, and the command wouldn't be necessary here if there was a dependency with the same basename as the target, ie something like foo: foo.o bar.o.
• You can automatically clean the build with scons -c. This cleans only the files generated in the build. Clean rules have to be specified by the programmer in MakeFiles.
• Even works in Windows with the MicrosoftVisualStudio C Compiler.

Setting CFLAGS and similar

You generally create an construction environment to set environment variables in SCons like so

env = Environment(CCFLAGS = '-Wall -pipe', LIBS = ['m', 'jpeg'])
env.Program('sam', ["lib.c", "sam.cc"], CPPPATH = ['include', 'common/include'])
env.SharedLibrary('testlib', ["source1.cc", "source2.c"])

This also sets the variable CPPPATH, which specifies which directories are to be searched for include files that are scanned during depency tracking. These are also passed to the compiler with the -I flag in the case of GCC. The flags can also be passed in the Program or Library line, constructing and Environment is usually done so multiple things can be built with the same environment.

A full list of the possible options can be found in the SCons manual.

Intermediate steps

In many cases the above will be enough: often you just want to build a directory of files into a library, shared library, or executable with a specific set of flags and dependencies. Of course, SCons lets you do a lot more, and at little to no extra cost in complexity.

Globbing multiple source files

It is simplicity itself to include multiple files in the build. Remember that SCons is Python-based?

import glob
Library(glob.glob('*.c') + glob.glob('source_dir_2/*.cc'), 'sam')

Python's glob module does all the hard work. Again, full dependency tracking and all other features of SCons work correctly.

Hierarchical builds

SCons supports hierarchical builds with the use of the SConscript directive. These scripts will be read and their information processed. No new SCons is forked for the SConscript directives, unlike the usual recursive make solution.

Including other directories in the build is as simple as

SConscript("dir1/")
SConscript("dir2/")

This will read in the SConscript files in dir1 and dir2. It is possible to export environments to these other scripts if need be.