SCons is a general-purpose build system that does not rely on Makefiles to build software. SCons is written in Python, and handles all building and linking itself.

As far as build systems go, SCons is very non-uniform. It provides a common framework for developers to write build scripts, but the build scripts themselves can vary drastically. Some developers add subcommands like:

$ scons clean
$ scons build
$ scons test
$ scons install

Others don't add any subcommands. Some have configuration options that can be specified through variables on the command line. Others don't.


As previously mentioned, SCons allows developers to add subcommands like build and install, but by default, installation usually looks like:

$ scons
$ scons install

To facilitate this, the SConsPackage base class provides the following phases:

  1. build - build the package

  2. install - install the package

Package developers often add unit tests that can be invoked with scons test or scons check. Spack provides a test method to handle this. Since we don't know which one the package developer chose, the test method does nothing by default, but can be easily overridden like so:

def test(self):

Important files

SCons packages can be identified by their SConstruct files. These files handle everything from setting up subcommands and command-line options to linking and compiling.

One thing to look for is the EnsureSConsVersion function:

EnsureSConsVersion(2, 3, 0)

This means that SCons 2.3.0 is the earliest release that will work. You should specify this in a depends_on statement.

Build system dependencies

At the bare minimum, packages that use the SCons build system need a scons dependency. Since this is always the case, the SConsPackage base class already contains:

depends_on('scons', type='build')

If you want to specify a particular version requirement, you can override this in your package:

depends_on('scons@2.3.0:', type='build')

Finding available options

The first place to start when looking for a list of valid options to build a package is scons --help. Some packages like kahip don't bother overwriting the default SCons help message, so this isn't very useful, but other packages like serf print a list of valid command-line variables:

$ scons --help
scons: Reading SConscript files ...
Checking for GNU-compatible C compiler...yes
scons: done reading SConscript files.

PREFIX: Directory to install under ( /path/to/PREFIX )
    default: /usr/local
    actual: /usr/local

LIBDIR: Directory to install architecture dependent libraries under ( /path/to/LIBDIR )
    default: $PREFIX/lib
    actual: /usr/local/lib

APR: Path to apr-1-config, or to APR's install area ( /path/to/APR )
    default: /usr
    actual: /usr

APU: Path to apu-1-config, or to APR's install area ( /path/to/APU )
    default: /usr
    actual: /usr

OPENSSL: Path to OpenSSL's install area ( /path/to/OPENSSL )
    default: /usr
    actual: /usr

ZLIB: Path to zlib's install area ( /path/to/ZLIB )
    default: /usr
    actual: /usr

GSSAPI: Path to GSSAPI's install area ( /path/to/GSSAPI )
    default: None
    actual: None

DEBUG: Enable debugging info and strict compile warnings (yes|no)
    default: False
    actual: False

APR_STATIC: Enable using a static compiled APR (yes|no)
    default: False
    actual: False

CC: Command name or path of the C compiler
    default: None
    actual: gcc

CFLAGS: Extra flags for the C compiler (space-separated)
    default: None

LIBS: Extra libraries passed to the linker, e.g. "-l<library1> -l<library2>" (space separated)
    default: None
    actual: None

LINKFLAGS: Extra flags for the linker (space-separated)
    default: None

CPPFLAGS: Extra flags for the C preprocessor (space separated)
    default: None
    actual: None

Use scons -H for help about command-line options.

More advanced packages like cantera use scons --help to print a list of subcommands:

$ scons --help
scons: Reading SConscript files ...

SCons build script for Cantera

Basic usage:
    'scons help' - print a description of user-specifiable options.

    'scons build' - Compile Cantera and the language interfaces using
                    default options.

    'scons clean' - Delete files created while building Cantera.

    '[sudo] scons install' - Install Cantera.

    '[sudo] scons uninstall' - Uninstall Cantera.

    'scons test' - Run all tests which did not previously pass or for which the
                   results may have changed.

    'scons test-reset' - Reset the passing status of all tests.

    'scons test-clean' - Delete files created while running the tests.

    'scons test-help' - List available tests.

    'scons test-NAME' - Run the test named "NAME".

    'scons <command> dump' - Dump the state of the SCons environment to the
                             screen instead of doing <command>, e.g.
                             'scons build dump'. For debugging purposes.

    'scons samples' - Compile the C++ and Fortran samples.

    'scons msi' - Build a Windows installer (.msi) for Cantera.

    'scons sphinx' - Build the Sphinx documentation

    'scons doxygen' - Build the Doxygen documentation

You'll notice that cantera provides a scons help subcommand. Running scons help prints a list of valid command-line variables.

Passing arguments to scons

Now that you know what arguments the project accepts, you can add them to the package build phase. This is done by overriding build_args like so:

def build_args(self, spec, prefix):
    args = [

    if '+debug' in spec:

    return args

SConsPackage also provides an install_args function that you can override to pass additional arguments to scons install.

Compiler wrappers

By default, SCons builds all packages in a separate execution environment, and doesn't pass any environment variables from the user environment. Even changes to PATH are not propagated unless the package developer does so.

This is particularly troublesome for Spack's compiler wrappers, which depend on environment variables to manage dependencies and linking flags. In many cases, SCons packages are not compatible with Spack's compiler wrappers, and linking must be done manually.

First of all, check the list of valid options for anything relating to environment variables. For example, cantera has the following option:

* env_vars: [ string ]
    Environment variables to propagate through to SCons. Either the
    string "all" or a comma separated list of variable names, e.g.

In the case of cantera, using env_vars=all allows us to use Spack's compiler wrappers. If you don't see an option related to environment variables, try using Spack's compiler wrappers by passing spack_cc, spack_cxx, and spack_fc via the CC, CXX, and FC arguments, respectively. If you pass them to the build and you see an error message like:

Spack compiler must be run from Spack! Input 'SPACK_PREFIX' is missing.

you'll know that the package isn't compatible with Spack's compiler wrappers. In this case, you'll have to use the path to the actual compilers, which are stored in and friends. Note that this may involve passing additional flags to the build to locate dependencies, a task normally done by the compiler wrappers. serf is an example of a package with this limitation.

External documentation

For more information on the SCons build system, see: