SCons 2.3.4

User Guide

Steven Knight and the SCons Development Team

Steven Knight

version 2.3.4

SCons User's Guide Copyright (c) 2004, 2005, 2006, 2007 Steven Knight

2004 - 2014


Table of Contents

Preface
1. SCons Principles
2. A Caveat About This Guide's Completeness
3. Acknowledgements
4. Contact
1. Building and Installing SCons
1.1. Installing Python
1.2. Installing SCons From Pre-Built Packages
1.2.1. Installing SCons on Red Hat (and Other RPM-based) Linux Systems
1.2.2. Installing SCons on Debian Linux Systems
1.2.3. Installing SCons on Windows Systems
1.3. Building and Installing SCons on Any System
1.3.1. Building and Installing Multiple Versions of SCons Side-by-Side
1.3.2. Installing SCons in Other Locations
1.3.3. Building and Installing SCons Without Administrative Privileges
2. Simple Builds
2.1. Building Simple C / C++ Programs
2.2. Building Object Files
2.3. Simple Java Builds
2.4. Cleaning Up After a Build
2.5. The SConstruct File
2.5.1. SConstruct Files Are Python Scripts
2.5.2. SCons Functions Are Order-Independent
2.6. Making the SCons Output Less Verbose
3. Less Simple Things to Do With Builds
3.1. Specifying the Name of the Target (Output) File
3.2. Compiling Multiple Source Files
3.3. Making a list of files with Glob
3.4. Specifying Single Files Vs. Lists of Files
3.5. Making Lists of Files Easier to Read
3.6. Keyword Arguments
3.7. Compiling Multiple Programs
3.8. Sharing Source Files Between Multiple Programs
4. Building and Linking with Libraries
4.1. Building Libraries
4.1.1. Building Libraries From Source Code or Object Files
4.1.2. Building Static Libraries Explicitly: the StaticLibrary Builder
4.1.3. Building Shared (DLL) Libraries: the SharedLibrary Builder
4.2. Linking with Libraries
4.3. Finding Libraries: the $LIBPATH Construction Variable
5. Node Objects
5.1. Builder Methods Return Lists of Target Nodes
5.2. Explicitly Creating File and Directory Nodes
5.3. Printing Node File Names
5.4. Using a Node's File Name as a String
5.5. GetBuildPath: Getting the Path From a Node or String
6. Dependencies
6.1. Deciding When an Input File Has Changed: the Decider Function
6.1.1. Using MD5 Signatures to Decide if a File Has Changed
6.1.2. Using Time Stamps to Decide If a File Has Changed
6.1.3. Deciding If a File Has Changed Using Both MD Signatures and Time Stamps
6.1.4. Writing Your Own Custom Decider Function
6.1.5. Mixing Different Ways of Deciding If a File Has Changed
6.2. Older Functions for Deciding When an Input File Has Changed
6.2.1. The SourceSignatures Function
6.2.2. The TargetSignatures Function
6.3. Implicit Dependencies: The $CPPPATH Construction Variable
6.4. Caching Implicit Dependencies
6.4.1. The --implicit-deps-changed Option
6.4.2. The --implicit-deps-unchanged Option
6.5. Explicit Dependencies: the Depends Function
6.6. Dependencies From External Files: the ParseDepends Function
6.7. Ignoring Dependencies: the Ignore Function
6.8. Order-Only Dependencies: the Requires Function
6.9. The AlwaysBuild Function
7. Environments
7.1. Using Values From the External Environment
7.2. Construction Environments
7.2.1. Creating a Construction Environment: the Environment Function
7.2.2. Fetching Values From a Construction Environment
7.2.3. Expanding Values From a Construction Environment: the subst Method
7.2.4. Handling Problems With Value Expansion
7.2.5. Controlling the Default Construction Environment: the DefaultEnvironment Function
7.2.6. Multiple Construction Environments
7.2.7. Making Copies of Construction Environments: the Clone Method
7.2.8. Replacing Values: the Replace Method
7.2.9. Setting Values Only If They're Not Already Defined: the SetDefault Method
7.2.10. Appending to the End of Values: the Append Method
7.2.11. Appending Unique Values: the AppendUnique Method
7.2.12. Appending to the Beginning of Values: the Prepend Method
7.2.13. Prepending Unique Values: the PrependUnique Method
7.3. Controlling the Execution Environment for Issued Commands
7.3.1. Propagating PATH From the External Environment
7.3.2. Adding to PATH Values in the Execution Environment
8. Automatically Putting Command-line Options into their Construction Variables
8.1. Merging Options into the Environment: the MergeFlags Function
8.2. Separating Compile Arguments into their Variables: the ParseFlags Function
8.3. Finding Installed Library Information: the ParseConfig Function
9. Controlling Build Output
9.1. Providing Build Help: the Help Function
9.2. Controlling How SCons Prints Build Commands: the $*COMSTR Variables
9.3. Providing Build Progress Output: the Progress Function
9.4. Printing Detailed Build Status: the GetBuildFailures Function
10. Controlling a Build From the Command Line
10.1. Command-Line Options
10.1.1. Not Having to Specify Command-Line Options Each Time: the SCONSFLAGS Environment Variable
10.1.2. Getting Values Set by Command-Line Options: the GetOption Function
10.1.3. Setting Values of Command-Line Options: the SetOption Function
10.1.4. Strings for Getting or Setting Values of SCons Command-Line Options
10.1.5. Adding Custom Command-Line Options: the AddOption Function
10.2. Command-Line variable=value Build Variables
10.2.1. Controlling Command-Line Build Variables
10.2.2. Providing Help for Command-Line Build Variables
10.2.3. Reading Build Variables From a File
10.2.4. Pre-Defined Build Variable Functions
10.2.5. Adding Multiple Command-Line Build Variables at Once
10.2.6. Handling Unknown Command-Line Build Variables: the UnknownVariables Function
10.3. Command-Line Targets
10.3.1. Fetching Command-Line Targets: the COMMAND_LINE_TARGETS Variable
10.3.2. Controlling the Default Targets: the Default Function
10.3.3. Fetching the List of Build Targets, Regardless of Origin: the BUILD_TARGETS Variable
11. Installing Files in Other Directories: the Install Builder
11.1. Installing Multiple Files in a Directory
11.2. Installing a File Under a Different Name
11.3. Installing Multiple Files Under Different Names
12. Platform-Independent File System Manipulation
12.1. Copying Files or Directories: The Copy Factory
12.2. Deleting Files or Directories: The Delete Factory
12.3. Moving (Renaming) Files or Directories: The Move Factory
12.4. Updating the Modification Time of a File: The Touch Factory
12.5. Creating a Directory: The Mkdir Factory
12.6. Changing File or Directory Permissions: The Chmod Factory
12.7. Executing an action immediately: the Execute Function
13. Controlling Removal of Targets
13.1. Preventing target removal during build: the Precious Function
13.2. Preventing target removal during clean: the NoClean Function
13.3. Removing additional files during clean: the Clean Function
14. Hierarchical Builds
14.1. SConscript Files
14.2. Path Names Are Relative to the SConscript Directory
14.3. Top-Level Path Names in Subsidiary SConscript Files
14.4. Absolute Path Names
14.5. Sharing Environments (and Other Variables) Between SConscript Files
14.5.1. Exporting Variables
14.5.2. Importing Variables
14.5.3. Returning Values From an SConscript File
15. Separating Source and Build Directories
15.1. Specifying a Variant Directory Tree as Part of an SConscript Call
15.2. Why SCons Duplicates Source Files in a Variant Directory Tree
15.3. Telling SCons to Not Duplicate Source Files in the Variant Directory Tree
15.4. The VariantDir Function
15.5. Using VariantDir With an SConscript File
15.6. Using Glob with VariantDir
16. Variant Builds
17. Internationalization and localization with gettext
17.1. Prerequisites
17.2. Simple project
18. Writing Your Own Builders
18.1. Writing Builders That Execute External Commands
18.2. Attaching a Builder to a Construction Environment
18.3. Letting SCons Handle The File Suffixes
18.4. Builders That Execute Python Functions
18.5. Builders That Create Actions Using a Generator
18.6. Builders That Modify the Target or Source Lists Using an Emitter
18.7. Where To Put Your Custom Builders and Tools
19. Not Writing a Builder: the Command Builder
20. Pseudo-Builders: the AddMethod function
21. Writing Scanners
21.1. A Simple Scanner Example
21.2. Adding a search path to a scanner: FindPathDirs
22. Building From Code Repositories
22.1. The Repository Method
22.2. Finding source files in repositories
22.3. Finding #include files in repositories
22.3.1. Limitations on #include files in repositories
22.4. Finding the SConstruct file in repositories
22.5. Finding derived files in repositories
22.6. Guaranteeing local copies of files
23. Multi-Platform Configuration (Autoconf Functionality)
23.1. Configure Contexts
23.2. Checking for the Existence of Header Files
23.3. Checking for the Availability of a Function
23.4. Checking for the Availability of a Library
23.5. Checking for the Availability of a typedef
23.6. Checking the size of a datatype
23.7. Adding Your Own Custom Checks
23.8. Not Configuring When Cleaning Targets
24. Caching Built Files
24.1. Specifying the Shared Cache Directory
24.2. Keeping Build Output Consistent
24.3. Not Using the Shared Cache for Specific Files
24.4. Disabling the Shared Cache
24.5. Populating a Shared Cache With Already-Built Files
24.6. Minimizing Cache Contention: the --random Option
25. Alias Targets
26. Java Builds
26.1. Building Java Class Files: the Java Builder
26.2. How SCons Handles Java Dependencies
26.3. Building Java Archive (.jar) Files: the Jar Builder
26.4. Building C Header and Stub Files: the JavaH Builder
26.5. Building RMI Stub and Skeleton Class Files: the RMIC Builder
27. Miscellaneous Functionality
27.1. Verifying the Python Version: the EnsurePythonVersion Function
27.2. Verifying the SCons Version: the EnsureSConsVersion Function
27.3. Explicitly Terminating SCons While Reading SConscript Files: the Exit Function
27.4. Searching for Files: the FindFile Function
27.5. Handling Nested Lists: the Flatten Function
27.6. Finding the Invocation Directory: the GetLaunchDir Function
28. Troubleshooting
28.1. Why is That Target Being Rebuilt? the --debug=explain Option
28.2. What's in That Construction Environment? the Dump Method
28.3. What Dependencies Does SCons Know About? the --tree Option
28.4. How is SCons Constructing the Command Lines It Executes? the --debug=presub Option
28.5. Where is SCons Searching for Libraries? the --debug=findlibs Option
28.6. Where is SCons Blowing Up? the --debug=stacktrace Option
28.7. How is SCons Making Its Decisions? the --taskmastertrace Option
28.8. Watch SCons prepare targets for building: the --debug=prepare Option
28.9. Why is a file disappearing? the --debug=duplicate Option
A. Construction Variables
B. Builders
C. Tools
D. Functions and Environment Methods
E. Handling Common Tasks

List of Examples

E.1. Wildcard globbing to create a list of filenames
E.2. Filename extension substitution
E.3. Appending a path prefix to a list of filenames
E.4. Substituting a path prefix with another one
E.5. Filtering a filename list to exclude/retain only a specific set of extensions
E.6. The "backtick function": run a shell command and capture the output
E.7. Generating source code: how code can be generated and used by SCons

Preface

Thank you for taking the time to read about SCons. SCons is a next-generation software construction tool, or make tool--that is, a software utility for building software (or other files) and keeping built software up-to-date whenever the underlying input files change.

The most distinctive thing about SCons is that its configuration files are actually scripts, written in the Python programming language. This is in contrast to most alternative build tools, which typically invent a new language to configure the build. SCons still has a learning curve, of course, because you have to know what functions to call to set up your build properly, but the underlying syntax used should be familiar to anyone who has ever looked at a Python script.

Paradoxically, using Python as the configuration file format makes SCons easier for non-programmers to learn than the cryptic languages of other build tools, which are usually invented by programmers for other programmers. This is in no small part due to the consistency and readability that are hallmarks of Python. It just so happens that making a real, live scripting language the basis for the configuration files makes it a snap for more accomplished programmers to do more complicated things with builds, as necessary.

1. SCons Principles

There are a few overriding principles we try to live up to in designing and implementing SCons:

Correctness

First and foremost, by default, SCons guarantees a correct build even if it means sacrificing performance a little. We strive to guarantee the build is correct regardless of how the software being built is structured, how it may have been written, or how unusual the tools are that build it.

Performance

Given that the build is correct, we try to make SCons build software as quickly as possible. In particular, wherever we may have needed to slow down the default SCons behavior to guarantee a correct build, we also try to make it easy to speed up SCons through optimization options that let you trade off guaranteed correctness in all end cases for a speedier build in the usual cases.

Convenience

SCons tries to do as much for you out of the box as reasonable, including detecting the right tools on your system and using them correctly to build the software.

In a nutshell, we try hard to make SCons just "do the right thing" and build software correctly, with a minimum of hassles.

2. A Caveat About This Guide's Completeness

One word of warning as you read through this Guide: Like too much Open Source software out there, the SCons documentation isn't always kept up-to-date with the available features. In other words, there's a lot that SCons can do that isn't yet covered in this User's Guide. (Come to think of it, that also describes a lot of proprietary software, doesn't it?)

Although this User's Guide isn't as complete as we'd like it to be, our development process does emphasize making sure that the SCons man page is kept up-to-date with new features. So if you're trying to figure out how to do something that SCons supports but can't find enough (or any) information here, it would be worth your while to look at the man page to see if the information is covered there. And if you do, maybe you'd even consider contributing a section to the User's Guide so the next person looking for that information won't have to go through the same thing...?

3. Acknowledgements

SCons would not exist without a lot of help from a lot of people, many of whom may not even be aware that they helped or served as inspiration. So in no particular order, and at the risk of leaving out someone:

First and foremost, SCons owes a tremendous debt to Bob Sidebotham, the original author of the classic Perl-based Cons tool which Bob first released to the world back around 1996. Bob's work on Cons classic provided the underlying architecture and model of specifying a build configuration using a real scripting language. My real-world experience working on Cons informed many of the design decisions in SCons, including the improved parallel build support, making Builder objects easily definable by users, and separating the build engine from the wrapping interface.

Greg Wilson was instrumental in getting SCons started as a real project when he initiated the Software Carpentry design competition in February 2000. Without that nudge, marrying the advantages of the Cons classic architecture with the readability of Python might have just stayed no more than a nice idea.

The entire SCons team have been absolutely wonderful to work with, and SCons would be nowhere near as useful a tool without the energy, enthusiasm and time people have contributed over the past few years. The "core team" of Chad Austin, Anthony Roach, Bill Deegan, Charles Crain, Steve Leblanc, Greg Noel, Gary Oberbrunner, Greg Spencer and Christoph Wiedemann have been great about reviewing my (and other) changes and catching problems before they get in the code base. Of particular technical note: Anthony's outstanding and innovative work on the tasking engine has given SCons a vastly superior parallel build model; Charles has been the master of the crucial Node infrastructure; Christoph's work on the Configure infrastructure has added crucial Autoconf-like functionality; and Greg has provided excellent support for Microsoft Visual Studio.

Special thanks to David Snopek for contributing his underlying "Autoscons" code that formed the basis of Christoph's work with the Configure functionality. David was extremely generous in making this code available to SCons, given that he initially released it under the GPL and SCons is released under a less-restrictive MIT-style license.

Thanks to Peter Miller for his splendid change management system, Aegis, which has provided the SCons project with a robust development methodology from day one, and which showed me how you could integrate incremental regression tests into a practical development cycle (years before eXtreme Programming arrived on the scene).

And last, thanks to Guido van Rossum for his elegant scripting language, which is the basis not only for the SCons implementation, but for the interface itself.

4. Contact

The best way to contact people involved with SCons, including the author, is through the SCons mailing lists.

If you want to ask general questions about how to use SCons send email to scons-users@scons.org.

If you want to contact the SCons development community directly, send email to scons-dev@scons.org.

If you want to receive announcements about SCons, join the low-volume announce@scons.tigris.org mailing list.

Chapter 1. Building and Installing SCons

This chapter will take you through the basic steps of installing SCons on your system, and building SCons if you don't have a pre-built package available (or simply prefer the flexibility of building it yourself). Before that, however, this chapter will also describe the basic steps involved in installing Python on your system, in case that is necessary. Fortunately, both SCons and Python are very easy to install on almost any system, and Python already comes installed on many systems.

1.1. Installing Python

Because SCons is written in Python, you must obviously have Python installed on your system to use SCons. Before you try to install Python, you should check to see if Python is already available on your system by typing python -V (capital 'V') or python --version at your system's command-line prompt.

$ python -V
Python 2.5.1
    

And on a Windows system with Python installed:

C:\>python -V
Python 2.5.1
    

If Python is not installed on your system, you will see an error message stating something like "command not found" (on UNIX or Linux) or "'python' is not recognized as an internal or external command, operable progam or batch file" (on Windows). In that case, you need to install Python before you can install SCons.

The standard location for information about downloading and installing Python is http://www.python.org/download/. See that page for information about how to download and install Python on your system.

SCons will work with any 2.x version of Python from 2.7 on; 3.0 and later are not yet supported. If you need to install Python and have a choice, we recommend using the most recent 2.x Python version available. Newer Pythons have significant improvements that help speed up the performance of SCons.

1.2. Installing SCons From Pre-Built Packages

SCons comes pre-packaged for installation on a number of systems, including Linux and Windows systems. You do not need to read this entire section, you should need to read only the section appropriate to the type of system you're running on.

1.2.1. Installing SCons on Red Hat (and Other RPM-based) Linux Systems

SCons comes in RPM (Red Hat Package Manager) format, pre-built and ready to install on Red Hat Linux, Fedora, or any other Linux distribution that uses RPM. Your distribution may already have an SCons RPM built specifically for it; many do, including SUSE, Mandrake and Fedora. You can check for the availability of an SCons RPM on your distribution's download servers, or by consulting an RPM search site like http://www.rpmfind.net/ or http://rpm.pbone.net/.

If your distribution supports installation via yum, you should be able to install SCons by running:

# yum install scons
      

If your Linux distribution does not already have a specific SCons RPM file, you can download and install from the generic RPM provided by the SCons project. This will install the SCons script(s) in /usr/bin, and the SCons library modules in /usr/lib/scons.

To install from the command line, simply download the appropriate .rpm file, and then run:

# rpm -Uvh scons-2.3.4-1.noarch.rpm
      

Or, you can use a graphical RPM package manager. See your package manager application's documention for specific instructions about how to use it to install a downloaded RPM.

1.2.2. Installing SCons on Debian Linux Systems

Debian Linux systems use a different package management format that also makes it very easy to install SCons.

If your system is connected to the Internet, you can install the latest official Debian package by running:

# apt-get install scons
      

1.2.3. Installing SCons on Windows Systems

SCons provides a Windows installer that makes installation extremely easy. Download the scons-2.3.4.win32.exe file from the SCons download page at http://www.scons.org/download.php. Then all you need to do is execute the file (usually by clicking on its icon in Windows Explorer). These will take you through a small sequence of windows that will install SCons on your system.

1.3. Building and Installing SCons on Any System

If a pre-built SCons package is not available for your system, then you can still easily build and install SCons using the native Python distutils package.

The first step is to download either the scons-2.3.4.tar.gz or scons-2.3.4.zip, which are available from the SCons download page at http://www.scons.org/download.html.

Unpack the archive you downloaded, using a utility like tar on Linux or UNIX, or WinZip on Windows. This will create a directory called scons-2.3.4, usually in your local directory. Then change your working directory to that directory and install SCons by executing the following commands:

# cd scons-2.3.4
# python setup.py install
    

This will build SCons, install the scons script in the python which is used to run the setup.py's scripts directory (/usr/local/bin or C:\Python25\Scripts), and will install the SCons build engine in the corresponding library directory for the python used (/usr/local/lib/scons or C:\Python25\scons). Because these are system directories, you may need root (on Linux or UNIX) or Administrator (on Windows) privileges to install SCons like this.

1.3.1. Building and Installing Multiple Versions of SCons Side-by-Side

The SCons setup.py script has some extensions that support easy installation of multiple versions of SCons in side-by-side locations. This makes it easier to download and experiment with different versions of SCons before moving your official build process to a new version, for example.

To install SCons in a version-specific location, add the --version-lib option when you call setup.py:

# python setup.py install --version-lib
      

This will install the SCons build engine in the /usr/lib/scons-2.3.4 or C:\Python25\scons-2.3.4 directory, for example.

If you use the --version-lib option the first time you install SCons, you do not need to specify it each time you install a new version. The SCons setup.py script will detect the version-specific directory name(s) and assume you want to install all versions in version-specific directories. You can override that assumption in the future by explicitly specifying the --standalone-lib option.

1.3.2. Installing SCons in Other Locations

You can install SCons in locations other than the default by specifying the --prefix= option:

# python setup.py install --prefix=/opt/scons
      

This would install the scons script in /opt/scons/bin and the build engine in /opt/scons/lib/scons,

Note that you can specify both the --prefix= and the --version-lib options at the same type, in which case setup.py will install the build engine in a version-specific directory relative to the specified prefix. Adding --version-lib to the above example would install the build engine in /opt/scons/lib/scons-2.3.4.

1.3.3. Building and Installing SCons Without Administrative Privileges

If you don't have the right privileges to install SCons in a system location, simply use the --prefix= option to install it in a location of your choosing. For example, to install SCons in appropriate locations relative to the user's $HOME directory, the scons script in $HOME/bin and the build engine in $HOME/lib/scons, simply type:

$ python setup.py install --prefix=$HOME
      

You may, of course, specify any other location you prefer, and may use the --version-lib option if you would like to install version-specific directories relative to the specified prefix.

This can also be used to experiment with a newer version of SCons than the one installed in your system locations. Of course, the location in which you install the newer version of the scons script ($HOME/bin in the above example) must be configured in your PATH variable before the directory containing the system-installed version of the scons script.

Chapter 2. Simple Builds

In this chapter, you will see several examples of very simple build configurations using SCons, which will demonstrate how easy it is to use SCons to build programs from several different programming languages on different types of systems.

2.1. Building Simple C / C++ Programs

Here's the famous "Hello, World!" program in C:

int
main()
{
    printf("Hello, world!\n");
}
   

And here's how to build it using SCons. Enter the following into a file named SConstruct:

Program('hello.c')
      

This minimal configuration file gives SCons two pieces of information: what you want to build (an executable program), and the input file from which you want it built (the hello.c file). Program is a builder_method, a Python call that tells SCons that you want to build an executable program.

That's it. Now run the scons command to build the program. On a POSIX-compliant system like Linux or UNIX, you'll see something like:

% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cc -o hello.o -c hello.c
cc -o hello hello.o
scons: done building targets.

On a Windows system with the Microsoft Visual C++ compiler, you'll see something like:

C:\>scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cl /Fohello.obj /c hello.c /nologo
link /nologo /OUT:hello.exe hello.obj
embedManifestExeCheck(target, source, env)
scons: done building targets.

First, notice that you only need to specify the name of the source file, and that SCons correctly deduces the names of the object and executable files to be built from the base of the source file name.

Second, notice that the same input SConstruct file, without any changes, generates the correct output file names on both systems: hello.o and hello on POSIX systems, hello.obj and hello.exe on Windows systems. This is a simple example of how SCons makes it extremely easy to write portable software builds.

(Note that we won't provide duplicate side-by-side POSIX and Windows output for all of the examples in this guide; just keep in mind that, unless otherwise specified, any of the examples should work equally well on both types of systems.)

2.2. Building Object Files

The Program builder method is only one of many builder methods that SCons provides to build different types of files. Another is the Object builder method, which tells SCons to build an object file from the specified source file:

Object('hello.c')
      

Now when you run the scons command to build the program, it will build just the hello.o object file on a POSIX system:

% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cc -o hello.o -c hello.c
scons: done building targets.

And just the hello.obj object file on a Windows system (with the Microsoft Visual C++ compiler):

C:\>scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cl /Fohello.obj /c hello.c /nologo
scons: done building targets.

2.3. Simple Java Builds

SCons also makes building with Java extremely easy. Unlike the Program and Object builder methods, however, the Java builder method requires that you specify the name of a destination directory in which you want the class files placed, followed by the source directory in which the .java files live:

Java('classes', 'src')
     

If the src directory contains a single hello.java file, then the output from running the scons command would look something like this (on a POSIX system):

% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
javac -d classes -sourcepath src src/hello.java
scons: done building targets.

We'll cover Java builds in more detail, including building Java archive (.jar) and other types of file, in Chapter 26, Java Builds.

2.4. Cleaning Up After a Build

When using SCons, it is unnecessary to add special commands or target names to clean up after a build. Instead, you simply use the -c or --clean option when you invoke SCons, and SCons removes the appropriate built files. So if we build our example above and then invoke scons -c afterwards, the output on POSIX looks like:

% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cc -o hello.o -c hello.c
cc -o hello hello.o
scons: done building targets.
% scons -c
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Cleaning targets ...
Removed hello.o
Removed hello
scons: done cleaning targets.

And the output on Windows looks like:

C:\>scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cl /Fohello.obj /c hello.c /nologo
link /nologo /OUT:hello.exe hello.obj
embedManifestExeCheck(target, source, env)
scons: done building targets.
C:\>scons -c
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Cleaning targets ...
Removed hello.obj
Removed hello.exe
scons: done cleaning targets.

Notice that SCons changes its output to tell you that it is Cleaning targets ... and done cleaning targets.

2.5. The SConstruct File

If you're used to build systems like Make you've already figured out that the SConstruct file is the SCons equivalent of a Makefile. That is, the SConstruct file is the input file that SCons reads to control the build.

2.5.1. SConstruct Files Are Python Scripts

There is, however, an important difference between an SConstruct file and a Makefile: the SConstruct file is actually a Python script. If you're not already familiar with Python, don't worry. This User's Guide will introduce you step-by-step to the relatively small amount of Python you'll need to know to be able to use SCons effectively. And Python is very easy to learn.

One aspect of using Python as the scripting language is that you can put comments in your SConstruct file using Python's commenting convention; that is, everything between a '#' and the end of the line will be ignored:

# Arrange to build the "hello" program.
Program('hello.c')    # "hello.c" is the source file.
     

You'll see throughout the remainder of this Guide that being able to use the power of a real scripting language can greatly simplify the solutions to complex requirements of real-world builds.

2.5.2. SCons Functions Are Order-Independent

One important way in which the SConstruct file is not exactly like a normal Python script, and is more like a Makefile, is that the order in which the SCons functions are called in the SConstruct file does not affect the order in which SCons actually builds the programs and object files you want it to build.[1] In other words, when you call the Program builder (or any other builder method), you're not telling SCons to build the program at the instant the builder method is called. Instead, you're telling SCons to build the program that you want, for example, a program built from a file named hello.c, and it's up to SCons to build that program (and any other files) whenever it's necessary. (We'll learn more about how SCons decides when building or rebuilding a file is necessary in Chapter 6, Dependencies, below.)

SCons reflects this distinction between calling a builder method like Program and actually building the program by printing the status messages that indicate when it's "just reading" the SConstruct file, and when it's actually building the target files. This is to make it clear when SCons is executing the Python statements that make up the SConstruct file, and when SCons is actually executing the commands or other actions to build the necessary files.

Let's clarify this with an example. Python has a print statement that prints a string of characters to the screen. If we put print statements around our calls to the Program builder method:

print "Calling Program('hello.c')"
Program('hello.c')
print "Calling Program('goodbye.c')"
Program('goodbye.c')
print "Finished calling Program()"
       

Then when we execute SCons, we see the output from the print statements in between the messages about reading the SConscript files, indicating that that is when the Python statements are being executed:

% scons
scons: Reading SConscript files ...
Calling Program('hello.c')
Calling Program('goodbye.c')
Finished calling Program()
scons: done reading SConscript files.
scons: Building targets ...
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o
cc -o hello.o -c hello.c
cc -o hello hello.o
scons: done building targets.

Notice also that SCons built the goodbye program first, even though the "reading SConscript" output shows that we called Program('hello.c') first in the SConstruct file.

2.6. Making the SCons Output Less Verbose

You've already seen how SCons prints some messages about what it's doing, surrounding the actual commands used to build the software:

C:\>scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cl /Fohello.obj /c hello.c /nologo
link /nologo /OUT:hello.exe hello.obj
embedManifestExeCheck(target, source, env)
scons: done building targets.

These messages emphasize the order in which SCons does its work: all of the configuration files (generically referred to as SConscript files) are read and executed first, and only then are the target files built. Among other benefits, these messages help to distinguish between errors that occur while the configuration files are read, and errors that occur while targets are being built.

One drawback, of course, is that these messages clutter the output. Fortunately, they're easily disabled by using the -Q option when invoking SCons:

C:\>scons -Q
cl /Fohello.obj /c hello.c /nologo
link /nologo /OUT:hello.exe hello.obj
embedManifestExeCheck(target, source, env)

Because we want this User's Guide to focus on what SCons is actually doing, we're going to use the -Q option to remove these messages from the output of all the remaining examples in this Guide.



[1] In programming parlance, the SConstruct file is declarative, meaning you tell SCons what you want done and let it figure out the order in which to do it, rather than strictly imperative, where you specify explicitly the order in which to do things.

Chapter 3. Less Simple Things to Do With Builds

In this chapter, you will see several examples of very simple build configurations using SCons, which will demonstrate how easy it is to use SCons to build programs from several different programming languages on different types of systems.

3.1. Specifying the Name of the Target (Output) File

You've seen that when you call the Program builder method, it builds the resulting program with the same base name as the source file. That is, the following call to build an executable program from the hello.c source file will build an executable program named hello on POSIX systems, and an executable program named hello.exe on Windows systems:

Program('hello.c')
    

If you want to build a program with a different name than the base of the source file name, you simply put the target file name to the left of the source file name:

Program('new_hello', 'hello.c')
       

(SCons requires the target file name first, followed by the source file name, so that the order mimics that of an assignment statement in most programming languages, including Python: "program = source files".)

Now SCons will build an executable program named new_hello when run on a POSIX system:

% scons -Q
cc -o hello.o -c hello.c
cc -o new_hello hello.o

And SCons will build an executable program named new_hello.exe when run on a Windows system:

C:\>scons -Q
cl /Fohello.obj /c hello.c /nologo
link /nologo /OUT:new_hello.exe hello.obj
embedManifestExeCheck(target, source, env)

3.2. Compiling Multiple Source Files

You've just seen how to configure SCons to compile a program from a single source file. It's more common, of course, that you'll need to build a program from many input source files, not just one. To do this, you need to put the source files in a Python list (enclosed in square brackets), like so:

Program(['prog.c', 'file1.c', 'file2.c'])
       

A build of the above example would look like:

% scons -Q
cc -o file1.o -c file1.c
cc -o file2.o -c file2.c
cc -o prog.o -c prog.c
cc -o prog prog.o file1.o file2.o

Notice that SCons deduces the output program name from the first source file specified in the list--that is, because the first source file was prog.c, SCons will name the resulting program prog (or prog.exe on a Windows system). If you want to specify a different program name, then (as we've seen in the previous section) you slide the list of source files over to the right to make room for the output program file name. (SCons puts the output file name to the left of the source file names so that the order mimics that of an assignment statement: "program = source files".) This makes our example:

Program('program', ['prog.c', 'file1.c', 'file2.c'])
       

On Linux, a build of this example would look like:

% scons -Q
cc -o file1.o -c file1.c
cc -o file2.o -c file2.c
cc -o prog.o -c prog.c
cc -o program prog.o file1.o file2.o

Or on Windows:

C:\>scons -Q
cl /Fofile1.obj /c file1.c /nologo
cl /Fofile2.obj /c file2.c /nologo
cl /Foprog.obj /c prog.c /nologo
link /nologo /OUT:program.exe prog.obj file1.obj file2.obj
embedManifestExeCheck(target, source, env)

3.3. Making a list of files with Glob

You can also use the Glob function to find all files matching a certain template, using the standard shell pattern matching characters *, ? and [abc] to match any of a, b or c. [!abc] is also supported, to match any character except a, b or c. This makes many multi-source-file builds quite easy:

Program('program', Glob('*.c'))
    

The SCons man page has more details on using Glob with variant directories (see Chapter 16, Variant Builds, below) and repositories (see Chapter 22, Building From Code Repositories, below), and returning strings rather than Nodes.

3.4. Specifying Single Files Vs. Lists of Files

We've now shown you two ways to specify the source for a program, one with a list of files:

Program('hello', ['file1.c', 'file2.c'])
    

And one with a single file:

Program('hello', 'hello.c')
    

You could actually put a single file name in a list, too, which you might prefer just for the sake of consistency:

Program('hello', ['hello.c'])
    

SCons functions will accept a single file name in either form. In fact, internally, SCons treats all input as lists of files, but allows you to omit the square brackets to cut down a little on the typing when there's only a single file name.

Important

Although SCons functions are forgiving about whether or not you use a string vs. a list for a single file name, Python itself is more strict about treating lists and strings differently. So where SCons allows either a string or list:

# The following two calls both work correctly:
Program('program1', 'program1.c')
Program('program2', ['program2.c'])
    

Trying to do "Python things" that mix strings and lists will cause errors or lead to incorrect results:

common_sources = ['file1.c', 'file2.c']

# THE FOLLOWING IS INCORRECT AND GENERATES A PYTHON ERROR
# BECAUSE IT TRIES TO ADD A STRING TO A LIST:
Program('program1', common_sources + 'program1.c')

# The following works correctly, because it's adding two
# lists together to make another list.
Program('program2', common_sources + ['program2.c'])
    

3.5. Making Lists of Files Easier to Read

One drawback to the use of a Python list for source files is that each file name must be enclosed in quotes (either single quotes or double quotes). This can get cumbersome and difficult to read when the list of file names is long. Fortunately, SCons and Python provide a number of ways to make sure that the SConstruct file stays easy to read.

To make long lists of file names easier to deal with, SCons provides a Split function that takes a quoted list of file names, with the names separated by spaces or other white-space characters, and turns it into a list of separate file names. Using the Split function turns the previous example into:

Program('program', Split('main.c file1.c file2.c'))
    

(If you're already familiar with Python, you'll have realized that this is similar to the split() method in the Python standard string module. Unlike the split() member function of strings, however, the Split function does not require a string as input and will wrap up a single non-string object in a list, or return its argument untouched if it's already a list. This comes in handy as a way to make sure arbitrary values can be passed to SCons functions without having to check the type of the variable by hand.)

Putting the call to the Split function inside the Program call can also be a little unwieldy. A more readable alternative is to assign the output from the Split call to a variable name, and then use the variable when calling the Program function:

src_files = Split('main.c file1.c file2.c')
Program('program', src_files)
    

Lastly, the Split function doesn't care how much white space separates the file names in the quoted string. This allows you to create lists of file names that span multiple lines, which often makes for easier editing:

src_files = Split("""main.c
                     file1.c
                     file2.c""")
Program('program', src_files)
    

(Note in this example that we used the Python "triple-quote" syntax, which allows a string to contain multiple lines. The three quotes can be either single or double quotes.)

3.6. Keyword Arguments

SCons also allows you to identify the output file and input source files using Python keyword arguments. The output file is known as the target, and the source file(s) are known (logically enough) as the source. The Python syntax for this is:

src_files = Split('main.c file1.c file2.c')
Program(target = 'program', source = src_files)
    

Because the keywords explicitly identify what each argument is, you can actually reverse the order if you prefer:

src_files = Split('main.c file1.c file2.c')
Program(source = src_files, target = 'program')
    

Whether or not you choose to use keyword arguments to identify the target and source files, and the order in which you specify them when using keywords, are purely personal choices; SCons functions the same regardless.

3.7. Compiling Multiple Programs

In order to compile multiple programs within the same SConstruct file, simply call the Program method multiple times, once for each program you need to build:

Program('foo.c')
Program('bar', ['bar1.c', 'bar2.c'])
       

SCons would then build the programs as follows:

% scons -Q
cc -o bar1.o -c bar1.c
cc -o bar2.o -c bar2.c
cc -o bar bar1.o bar2.o
cc -o foo.o -c foo.c
cc -o foo foo.o

Notice that SCons does not necessarily build the programs in the same order in which you specify them in the SConstruct file. SCons does, however, recognize that the individual object files must be built before the resulting program can be built. We'll discuss this in greater detail in the "Dependencies" section, below.

3.8. Sharing Source Files Between Multiple Programs

It's common to re-use code by sharing source files between multiple programs. One way to do this is to create a library from the common source files, which can then be linked into resulting programs. (Creating libraries is discussed in Chapter 4, Building and Linking with Libraries, below.)

A more straightforward, but perhaps less convenient, way to share source files between multiple programs is simply to include the common files in the lists of source files for each program:

Program(Split('foo.c common1.c common2.c'))
Program('bar', Split('bar1.c bar2.c common1.c common2.c'))
       

SCons recognizes that the object files for the common1.c and common2.c source files each need to be built only once, even though the resulting object files are each linked in to both of the resulting executable programs:

% scons -Q
cc -o bar1.o -c bar1.c
cc -o bar2.o -c bar2.c
cc -o common1.o -c common1.c
cc -o common2.o -c common2.c
cc -o bar bar1.o bar2.o common1.o common2.o
cc -o foo.o -c foo.c
cc -o foo foo.o common1.o common2.o

If two or more programs share a lot of common source files, repeating the common files in the list for each program can be a maintenance problem when you need to change the list of common files. You can simplify this by creating a separate Python list to hold the common file names, and concatenating it with other lists using the Python + operator:

common = ['common1.c', 'common2.c']
foo_files = ['foo.c'] + common
bar_files = ['bar1.c', 'bar2.c'] + common
Program('foo', foo_files)
Program('bar', bar_files)
    

This is functionally equivalent to the previous example.

Chapter 4. Building and Linking with Libraries

It's often useful to organize large software projects by collecting parts of the software into one or more libraries. SCons makes it easy to create libraries and to use them in the programs.

4.1. Building Libraries

You build your own libraries by specifying Library instead of Program:

Library('foo', ['f1.c', 'f2.c', 'f3.c'])
      

SCons uses the appropriate library prefix and suffix for your system. So on POSIX or Linux systems, the above example would build as follows (although ranlib may not be called on all systems):

% scons -Q
cc -o f1.o -c f1.c
cc -o f2.o -c f2.c
cc -o f3.o -c f3.c
ar rc libfoo.a f1.o f2.o f3.o
ranlib libfoo.a

On a Windows system, a build of the above example would look like:

C:\>scons -Q
cl /Fof1.obj /c f1.c /nologo
cl /Fof2.obj /c f2.c /nologo
cl /Fof3.obj /c f3.c /nologo
lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj

The rules for the target name of the library are similar to those for programs: if you don't explicitly specify a target library name, SCons will deduce one from the name of the first source file specified, and SCons will add an appropriate file prefix and suffix if you leave them off.

4.1.1. Building Libraries From Source Code or Object Files

The previous example shows building a library from a list of source files. You can, however, also give the Library call object files, and it will correctly realize In fact, you can arbitrarily mix source code files and object files in the source list:

Library('foo', ['f1.c', 'f2.o', 'f3.c', 'f4.o'])
        

And SCons realizes that only the source code files must be compiled into object files before creating the final library:

% scons -Q
cc -o f1.o -c f1.c
cc -o f3.o -c f3.c
ar rc libfoo.a f1.o f2.o f3.o f4.o
ranlib libfoo.a

Of course, in this example, the object files must already exist for the build to succeed. See Chapter 5, Node Objects, below, for information about how you can build object files explicitly and include the built files in a library.

4.1.2. Building Static Libraries Explicitly: the StaticLibrary Builder

The Library function builds a traditional static library. If you want to be explicit about the type of library being built, you can use the synonym StaticLibrary function instead of Library:

StaticLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])
        

There is no functional difference between the StaticLibrary and Library functions.

4.1.3. Building Shared (DLL) Libraries: the SharedLibrary Builder

If you want to build a shared library (on POSIX systems) or a DLL file (on Windows systems), you use the SharedLibrary function:

SharedLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])
        

The output on POSIX:

% scons -Q
cc -o f1.os -c f1.c
cc -o f2.os -c f2.c
cc -o f3.os -c f3.c
cc -o libfoo.so -shared f1.os f2.os f3.os

And the output on Windows:

C:\>scons -Q
cl /Fof1.obj /c f1.c /nologo
cl /Fof2.obj /c f2.c /nologo
cl /Fof3.obj /c f3.c /nologo
link /nologo /dll /out:foo.dll /implib:foo.lib f1.obj f2.obj f3.obj
RegServerFunc(target, source, env)
embedManifestDllCheck(target, source, env)

Notice again that SCons takes care of building the output file correctly, adding the -shared option for a POSIX compilation, and the /dll option on Windows.

4.2. Linking with Libraries

Usually, you build a library because you want to link it with one or more programs. You link libraries with a program by specifying the libraries in the $LIBS construction variable, and by specifying the directory in which the library will be found in the $LIBPATH construction variable:

Library('foo', ['f1.c', 'f2.c', 'f3.c'])
Program('prog.c', LIBS=['foo', 'bar'], LIBPATH='.')
      

Notice, of course, that you don't need to specify a library prefix (like lib) or suffix (like .a or .lib). SCons uses the correct prefix or suffix for the current system.

On a POSIX or Linux system, a build of the above example would look like:

% scons -Q
cc -o f1.o -c f1.c
cc -o f2.o -c f2.c
cc -o f3.o -c f3.c
ar rc libfoo.a f1.o f2.o f3.o
ranlib libfoo.a
cc -o prog.o -c prog.c
cc -o prog prog.o -L. -lfoo -lbar

On a Windows system, a build of the above example would look like:

C:\>scons -Q
cl /Fof1.obj /c f1.c /nologo
cl /Fof2.obj /c f2.c /nologo
cl /Fof3.obj /c f3.c /nologo
lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj
cl /Foprog.obj /c prog.c /nologo
link /nologo /OUT:prog.exe /LIBPATH:. foo.lib bar.lib prog.obj
embedManifestExeCheck(target, source, env)

As usual, notice that SCons has taken care of constructing the correct command lines to link with the specified library on each system.

Note also that, if you only have a single library to link with, you can specify the library name in single string, instead of a Python list, so that:

Program('prog.c', LIBS='foo', LIBPATH='.')
    

is equivalent to:

Program('prog.c', LIBS=['foo'], LIBPATH='.')
    

This is similar to the way that SCons handles either a string or a list to specify a single source file.

4.3. Finding Libraries: the $LIBPATH Construction Variable

By default, the linker will only look in certain system-defined directories for libraries. SCons knows how to look for libraries in directories that you specify with the $LIBPATH construction variable. $LIBPATH consists of a list of directory names, like so:

Program('prog.c', LIBS = 'm',
                  LIBPATH = ['/usr/lib', '/usr/local/lib'])
      

Using a Python list is preferred because it's portable across systems. Alternatively, you could put all of the directory names in a single string, separated by the system-specific path separator character: a colon on POSIX systems:

LIBPATH = '/usr/lib:/usr/local/lib'
    

or a semi-colon on Windows systems:

LIBPATH = 'C:\\lib;D:\\lib'
    

(Note that Python requires that the backslash separators in a Windows path name be escaped within strings.)

When the linker is executed, SCons will create appropriate flags so that the linker will look for libraries in the same directories as SCons. So on a POSIX or Linux system, a build of the above example would look like:

% scons -Q
cc -o prog.o -c prog.c
cc -o prog prog.o -L/usr/lib -L/usr/local/lib -lm

On a Windows system, a build of the above example would look like:

C:\>scons -Q
cl /Foprog.obj /c prog.c /nologo
link /nologo /OUT:prog.exe /LIBPATH:\usr\lib /LIBPATH:\usr\local\lib m.lib prog.obj
embedManifestExeCheck(target, source, env)

Note again that SCons has taken care of the system-specific details of creating the right command-line options.

Chapter 5. Node Objects

Internally, SCons represents all of the files and directories it knows about as Nodes. These internal objects (not object files) can be used in a variety of ways to make your SConscript files portable and easy to read.

5.1. Builder Methods Return Lists of Target Nodes

All builder methods return a list of Node objects that identify the target file or files that will be built. These returned Nodes can be passed as arguments to other builder methods.

For example, suppose that we want to build the two object files that make up a program with different options. This would mean calling the Object builder once for each object file, specifying the desired options:

Object('hello.c', CCFLAGS='-DHELLO')
Object('goodbye.c', CCFLAGS='-DGOODBYE')
    

One way to combine these object files into the resulting program would be to call the Program builder with the names of the object files listed as sources:

Object('hello.c', CCFLAGS='-DHELLO')
Object('goodbye.c', CCFLAGS='-DGOODBYE')
Program(['hello.o', 'goodbye.o'])
    

The problem with specifying the names as strings is that our SConstruct file is no longer portable across operating systems. It won't, for example, work on Windows because the object files there would be named hello.obj and goodbye.obj, not hello.o and goodbye.o.

A better solution is to assign the lists of targets returned by the calls to the Object builder to variables, which we can then concatenate in our call to the Program builder:

hello_list = Object('hello.c', CCFLAGS='-DHELLO')
goodbye_list = Object('goodbye.c', CCFLAGS='-DGOODBYE')
Program(hello_list + goodbye_list)
      

This makes our SConstruct file portable again, the build output on Linux looking like:

% scons -Q
cc -o goodbye.o -c -DGOODBYE goodbye.c
cc -o hello.o -c -DHELLO hello.c
cc -o hello hello.o goodbye.o

And on Windows:

C:\>scons -Q
cl /Fogoodbye.obj /c goodbye.c -DGOODBYE
cl /Fohello.obj /c hello.c -DHELLO
link /nologo /OUT:hello.exe hello.obj goodbye.obj
embedManifestExeCheck(target, source, env)

We'll see examples of using the list of nodes returned by builder methods throughout the rest of this guide.

5.2. Explicitly Creating File and Directory Nodes

It's worth mentioning here that SCons maintains a clear distinction between Nodes that represent files and Nodes that represent directories. SCons supports File and Dir functions that, respectively, return a file or directory Node:

hello_c = File('hello.c')
Program(hello_c)

classes = Dir('classes')
Java(classes, 'src')
      

Normally, you don't need to call File or Dir directly, because calling a builder method automatically treats strings as the names of files or directories, and translates them into the Node objects for you. The File and Dir functions can come in handy in situations where you need to explicitly instruct SCons about the type of Node being passed to a builder or other function, or unambiguously refer to a specific file in a directory tree.

There are also times when you may need to refer to an entry in a file system without knowing in advance whether it's a file or a directory. For those situations, SCons also supports an Entry function, which returns a Node that can represent either a file or a directory.

xyzzy = Entry('xyzzy')
    

The returned xyzzy Node will be turned into a file or directory Node the first time it is used by a builder method or other function that requires one vs. the other.

5.3. Printing Node File Names

One of the most common things you can do with a Node is use it to print the file name that the node represents. Keep in mind, though, that because the object returned by a builder call is a list of Nodes, you must use Python subscripts to fetch individual Nodes from the list. For example, the following SConstruct file:

object_list = Object('hello.c')
program_list = Program(object_list)
print "The object file is:", object_list[0]
print "The program file is:", program_list[0]
      

Would print the following file names on a POSIX system:

% scons -Q
The object file is: hello.o
The program file is: hello
cc -o hello.o -c hello.c
cc -o hello hello.o

And the following file names on a Windows system:

C:\>scons -Q
The object file is: hello.obj
The program file is: hello.exe
cl /Fohello.obj /c hello.c /nologo
link /nologo /OUT:hello.exe hello.obj
embedManifestExeCheck(target, source, env)

Note that in the above example, the object_list[0] extracts an actual Node object from the list, and the Python print statement converts the object to a string for printing.

5.4. Using a Node's File Name as a String

Printing a Node's name as described in the previous section works because the string representation of a Node object is the name of the file. If you want to do something other than print the name of the file, you can fetch it by using the builtin Python str function. For example, if you want to use the Python os.path.exists to figure out whether a file exists while the SConstruct file is being read and executed, you can fetch the string as follows:

import os.path
program_list = Program('hello.c')
program_name = str(program_list[0])
if not os.path.exists(program_name):
    print program_name, "does not exist!"
      

Which executes as follows on a POSIX system:

% scons -Q
hello does not exist!
cc -o hello.o -c hello.c
cc -o hello hello.o

5.5. GetBuildPath: Getting the Path From a Node or String

env.GetBuildPath(file_or_list) returns the path of a Node or a string representing a path. It can also take a list of Nodes and/or strings, and returns the list of paths. If passed a single Node, the result is the same as calling str(node) (see above). The string(s) can have embedded construction variables, which are expanded as usual, using the calling environment's set of variables. The paths can be files or directories, and do not have to exist.

env=Environment(VAR="value")
n=File("foo.c")
print env.GetBuildPath([n, "sub/dir/$VAR"])
      

Would print the following file names:

% scons -Q
['foo.c', 'sub/dir/value']
scons: `.' is up to date.

There is also a function version of GetBuildPath which can be called without an Environment; that uses the default SCons Environment to do substitution on any string arguments.

Chapter 6. Dependencies

So far we've seen how SCons handles one-time builds. But one of the main functions of a build tool like SCons is to rebuild only what is necessary when source files change--or, put another way, SCons should not waste time rebuilding things that don't need to be rebuilt. You can see this at work simply by re-invoking SCons after building our simple hello example:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q
scons: `.' is up to date.

The second time it is executed, SCons realizes that the hello program is up-to-date with respect to the current hello.c source file, and avoids rebuilding it. You can see this more clearly by naming the hello program explicitly on the command line:

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.

Note that SCons reports "...is up to date" only for target files named explicitly on the command line, to avoid cluttering the output.

6.1. Deciding When an Input File Has Changed: the Decider Function

Another aspect of avoiding unnecessary rebuilds is the fundamental build tool behavior of rebuilding things when an input file changes, so that the built software is up to date. By default, SCons keeps track of this through an MD5 signature, or checksum, of the contents of each file, although you can easily configure SCons to use the modification times (or time stamps) instead. You can even specify your own Python function for deciding if an input file has changed.

6.1.1. Using MD5 Signatures to Decide if a File Has Changed

By default, SCons keeps track of whether a file has changed based on an MD5 checksum of the file's contents, not the file's modification time. This means that you may be surprised by the default SCons behavior if you are used to the Make convention of forcing a rebuild by updating the file's modification time (using the touch command, for example):

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% touch hello.c
% scons -Q hello
scons: `hello' is up to date.

Even though the file's modification time has changed, SCons realizes that the contents of the hello.c file have not changed, and therefore that the hello program need not be rebuilt. This avoids unnecessary rebuilds when, for example, someone rewrites the contents of a file without making a change. But if the contents of the file really do change, then SCons detects the change and rebuilds the program as required:

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
%     [CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o

Note that you can, if you wish, specify this default behavior (MD5 signatures) explicitly using the Decider function as follows:

Program('hello.c')
Decider('MD5')
      

You can also use the string 'content' as a synonym for 'MD5' when calling the Decider function.

6.1.1.1. Ramifications of Using MD5 Signatures

Using MD5 signatures to decide if an input file has changed has one surprising benefit: if a source file has been changed in such a way that the contents of the rebuilt target file(s) will be exactly the same as the last time the file was built, then any "downstream" target files that depend on the rebuilt-but-not-changed target file actually need not be rebuilt.

So if, for example, a user were to only change a comment in a hello.c file, then the rebuilt hello.o file would be exactly the same as the one previously built (assuming the compiler doesn't put any build-specific information in the object file). SCons would then realize that it would not need to rebuild the hello program as follows:

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
%   [CHANGE A COMMENT IN hello.c]
% scons -Q hello
cc -o hello.o -c hello.c
scons: `hello' is up to date.

In essence, SCons "short-circuits" any dependent builds when it realizes that a target file has been rebuilt to exactly the same file as the last build. This does take some extra processing time to read the contents of the target (hello.o) file, but often saves time when the rebuild that was avoided would have been time-consuming and expensive.

6.1.2. Using Time Stamps to Decide If a File Has Changed

If you prefer, you can configure SCons to use the modification time of a file, not the file contents, when deciding if a target needs to be rebuilt. SCons gives you two ways to use time stamps to decide if an input file has changed since the last time a target has been built.

The most familiar way to use time stamps is the way Make does: that is, have SCons decide that a target must be rebuilt if a source file's modification time is newer than the target file. To do this, call the Decider function as follows:

Object('hello.c')
Decider('timestamp-newer')
        

This makes SCons act like Make when a file's modification time is updated (using the touch command, for example):

% scons -Q hello.o
cc -o hello.o -c hello.c
% touch hello.c
% scons -Q hello.o
cc -o hello.o -c hello.c

And, in fact, because this behavior is the same as the behavior of Make, you can also use the string 'make' as a synonym for 'timestamp-newer' when calling the Decider function:

Object('hello.c')
Decider('make')
      

One drawback to using times stamps exactly like Make is that if an input file's modification time suddenly becomes older than a target file, the target file will not be rebuilt. This can happen if an old copy of a source file is restored from a backup archive, for example. The contents of the restored file will likely be different than they were the last time a dependent target was built, but the target won't be rebuilt because the modification time of the source file is not newer than the target.

Because SCons actually stores information about the source files' time stamps whenever a target is built, it can handle this situation by checking for an exact match of the source file time stamp, instead of just whether or not the source file is newer than the target file. To do this, specify the argument 'timestamp-match' when calling the Decider function:

Object('hello.c')
Decider('timestamp-match')
        

When configured this way, SCons will rebuild a target whenever a source file's modification time has changed. So if we use the touch -t option to change the modification time of hello.c to an old date (January 1, 1989), SCons will still rebuild the target file:

% scons -Q hello.o
cc -o hello.o -c hello.c
% touch -t 198901010000 hello.c
% scons -Q hello.o
cc -o hello.o -c hello.c

In general, the only reason to prefer timestamp-newer instead of timestamp-match, would be if you have some specific reason to require this Make-like behavior of not rebuilding a target when an otherwise-modified source file is older.

6.1.3. Deciding If a File Has Changed Using Both MD Signatures and Time Stamps

As a performance enhancement, SCons provides a way to use MD5 checksums of file contents but to read those contents only when the file's timestamp has changed. To do this, call the Decider function with 'MD5-timestamp' argument as follows:

Program('hello.c')
Decider('MD5-timestamp')
        

So configured, SCons will still behave like it does when using Decider('MD5'):

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% touch hello.c
% scons -Q hello
scons: `hello' is up to date.
% edit hello.c
    [CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
      

However, the second call to SCons in the above output, when the build is up-to-date, will have been performed by simply looking at the modification time of the hello.c file, not by opening it and performing an MD5 checksum calcuation on its contents. This can significantly speed up many up-to-date builds.

The only drawback to using Decider('MD5-timestamp') is that SCons will not rebuild a target file if a source file was modified within one second of the last time SCons built the file. While most developers are programming, this isn't a problem in practice, since it's unlikely that someone will have built and then thought quickly enough to make a substantive change to a source file within one second. Certain build scripts or continuous integration tools may, however, rely on the ability to apply changes to files automatically and then rebuild as quickly as possible, in which case use of Decider('MD5-timestamp') may not be appropriate.

6.1.4. Writing Your Own Custom Decider Function

The different string values that we've passed to the Decider function are essentially used by SCons to pick one of several specific internal functions that implement various ways of deciding if a dependency (usually a source file) has changed since a target file has been built. As it turns out, you can also supply your own function to decide if a dependency has changed.

For example, suppose we have an input file that contains a lot of data, in some specific regular format, that is used to rebuild a lot of different target files, but each target file really only depends on one particular section of the input file. We'd like to have each target file depend on only its section of the input file. However, since the input file may contain a lot of data, we want to open the input file only if its timestamp has changed. This could be done with a custom Decider function that might look something like this:

Program('hello.c')
def decide_if_changed(dependency, target, prev_ni):
    if self.get_timestamp() != prev_ni.timestamp:
        dep = str(dependency)
        tgt = str(target)
        if specific_part_of_file_has_changed(dep, tgt):
            return True
    return False
Decider(decide_if_changed)
        

Note that in the function definition, the dependency (input file) is the first argument, and then the target. Both of these are passed to the functions as SCons Node objects, which we convert to strings using the Python str().

The third argument, prev_ni, is an object that holds the signature or timestamp information that was recorded about the dependency the last time the target was built. A prev_ni object can hold different information, depending on the type of thing that the dependency argument represents. For normal files, the prev_ni object has the following attributes:

.csig

The content signature, or MD5 checksum, of the contents of the dependency file the list time the target was built.

.size

The size in bytes of the dependency file the list time the target was built.

.timestamp

The modification time of the dependency file the list time the target was built.

Note that ignoring some of the arguments in your custom Decider function is a perfectly normal thing to do, if they don't impact the way you want to decide if the dependency file has changed.

Another thing to look out for is the fact that the three attributes above may not be present at the time of the first run. Without any prior build, no targets have been created and no .sconsign DB file exists yet. So, you should always check whether the prev_ni attribute in question is available.

We finally present a small example for a csig-based decider function. Note how the signature information for the dependency file has to get initialized via get_csig during each function call (this is mandatory!).

env = Environment()

def config_file_decider(dependency, target, prev_ni):
    import os.path

    # We always have to init the .csig value...
    dep_csig = dependency.get_csig()
    # .csig may not exist, because no target was built yet...
    if 'csig' not in dir(prev_ni):
        return True
    # Target file may not exist yet
    if not os.path.exists(str(target.abspath)):
        return True
    if dep_csig != prev_ni.csig:
        # Some change on source file => update installed one
        return True
    return False

def update_file():
    f = open("test.txt","a")
    f.write("some line\n")
    f.close()

update_file()

# Activate our own decider function
env.Decider(config_file_decider)

env.Install("install","test.txt")
      

6.1.5. Mixing Different Ways of Deciding If a File Has Changed

The previous examples have all demonstrated calling the global Decider function to configure all dependency decisions that SCons makes. Sometimes, however, you want to be able to configure different decision-making for different targets. When that's necessary, you can use the env.Decider method to affect only the configuration decisions for targets built with a specific construction environment.

For example, if we arbitrarily want to build one program using MD5 checkums and another using file modification times from the same source we might configure it this way:

env1 = Environment(CPPPATH = ['.'])
env2 = env1.Clone()
env2.Decider('timestamp-match')
env1.Program('prog-MD5', 'program1.c')
env2.Program('prog-timestamp', 'program2.c')
        

If both of the programs include the same inc.h file, then updating the modification time of inc.h (using the touch command) will cause only prog-timestamp to be rebuilt:

% scons -Q
cc -o program1.o -c -I. program1.c
cc -o prog-MD5 program1.o
cc -o program2.o -c -I. program2.c
cc -o prog-timestamp program2.o
% touch inc.h
% scons -Q
cc -o program2.o -c -I. program2.c
cc -o prog-timestamp program2.o

6.2. Older Functions for Deciding When an Input File Has Changed

SCons still supports two functions that used to be the primary methods for configuring the decision about whether or not an input file has changed. These functions have been officially deprecated as SCons version 2.0, and their use is discouraged, mainly because they rely on a somewhat confusing distinction between how source files and target files are handled. These functions are documented here mainly in case you encounter them in older SConscript files.

6.2.1. The SourceSignatures Function

The SourceSignatures function is fairly straightforward, and supports two different argument values to configure whether source file changes should be decided using MD5 signatures:

Program('hello.c')
SourceSignatures('MD5')
      

Or using time stamps:

Program('hello.c')
SourceSignatures('timestamp')
      

These are roughly equivalent to specifying Decider('MD5') or Decider('timestamp-match'), respectively, although it only affects how SCons makes decisions about dependencies on source files--that is, files that are not built from any other files.

6.2.2. The TargetSignatures Function

The TargetSignatures function specifies how SCons decides when a target file has changed when it is used as a dependency of (input to) another target--that is, the TargetSignatures function configures how the signatures of "intermediate" target files are used when deciding if a "downstream" target file must be rebuilt. [2]

The TargetSignatures function supports the same 'MD5' and 'timestamp' argument values that are supported by the SourceSignatures, with the same meanings, but applied to target files. That is, in the example:

Program('hello.c')
TargetSignatures('MD5')
      

The MD5 checksum of the hello.o target file will be used to decide if it has changed since the last time the "downstream" hello target file was built. And in the example:

Program('hello.c')
TargetSignatures('timestamp')
      

The modification time of the hello.o target file will be used to decide if it has changed since the last time the "downstream" hello target file was built.

The TargetSignatures function supports two additional argument values: 'source' and 'build'. The 'source' argument specifies that decisions involving whether target files have changed since a previous build should use the same behavior for the decisions configured for source files (using the SourceSignatures function). So in the example:

Program('hello.c')
TargetSignatures('source')
SourceSignatures('timestamp')
      

All files, both targets and sources, will use modification times when deciding if an input file has changed since the last time a target was built.

Lastly, the 'build' argument specifies that SCons should examine the build status of a target file and always rebuild a "downstream" target if the target file was itself rebuilt, without re-examining the contents or timestamp of the newly-built target file. If the target file was not rebuilt during this scons invocation, then the target file will be examined the same way as configured by the SourceSignature call to decide if it has changed.

This mimics the behavior of build signatures in earlier versions of SCons. A build signature re-combined signatures of all the input files that went into making the target file, so that the target file itself did not need to have its contents read to compute an MD5 signature. This can improve performance for some configurations, but is generally not as effective as using Decider('MD5-timestamp').

6.3. Implicit Dependencies: The $CPPPATH Construction Variable

Now suppose that our "Hello, World!" program actually has an #include line to include the hello.h file in the compilation:

#include <hello.h>
int
main()
{
    printf("Hello, %s!\n", string);
}
      

And, for completeness, the hello.h file looks like this:

#define string    "world"
      

In this case, we want SCons to recognize that, if the contents of the hello.h file change, the hello program must be recompiled. To do this, we need to modify the SConstruct file like so:

Program('hello.c', CPPPATH = '.')
      

The $CPPPATH value tells SCons to look in the current directory ('.') for any files included by C source files (.c or .h files). With this assignment in the SConstruct file:

% scons -Q hello
cc -o hello.o -c -I. hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
%     [CHANGE THE CONTENTS OF hello.h]
% scons -Q hello
cc -o hello.o -c -I. hello.c
cc -o hello hello.o

First, notice that SCons added the -I. argument from the $CPPPATH variable so that the compilation would find the hello.h file in the local directory.

Second, realize that SCons knows that the hello program must be rebuilt because it scans the contents of the hello.c file for the #include lines that indicate another file is being included in the compilation. SCons records these as implicit dependencies of the target file, Consequently, when the hello.h file changes, SCons realizes that the hello.c file includes it, and rebuilds the resulting hello program that depends on both the hello.c and hello.h files.

Like the $LIBPATH variable, the $CPPPATH variable may be a list of directories, or a string separated by the system-specific path separation character (':' on POSIX/Linux, ';' on Windows). Either way, SCons creates the right command-line options so that the following example:

Program('hello.c', CPPPATH = ['include', '/home/project/inc'])
      

Will look like this on POSIX or Linux:

% scons -Q hello
cc -o hello.o -c -Iinclude -I/home/project/inc hello.c
cc -o hello hello.o

And like this on Windows:

C:\>scons -Q hello.exe
cl /Fohello.obj /c hello.c /nologo /Iinclude /I\home\project\inc
link /nologo /OUT:hello.exe hello.obj
embedManifestExeCheck(target, source, env)

6.4. Caching Implicit Dependencies

Scanning each file for #include lines does take some extra processing time. When you're doing a full build of a large system, the scanning time is usually a very small percentage of the overall time spent on the build. You're most likely to notice the scanning time, however, when you rebuild all or part of a large system: SCons will likely take some extra time to "think about" what must be built before it issues the first build command (or decides that everything is up to date and nothing must be rebuilt).

In practice, having SCons scan files saves time relative to the amount of potential time lost to tracking down subtle problems introduced by incorrect dependencies. Nevertheless, the "waiting time" while SCons scans files can annoy individual developers waiting for their builds to finish. Consequently, SCons lets you cache the implicit dependencies that its scanners find, for use by later builds. You can do this by specifying the --implicit-cache option on the command line:

% scons -Q --implicit-cache hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.

If you don't want to specify --implicit-cache on the command line each time, you can make it the default behavior for your build by setting the implicit_cache option in an SConscript file:

SetOption('implicit_cache', 1)
    

SCons does not cache implicit dependencies like this by default because the --implicit-cache causes SCons to simply use the implicit dependencies stored during the last run, without any checking for whether or not those dependencies are still correct. Specifically, this means --implicit-cache instructs SCons to not rebuild "correctly" in the following cases:

  • When --implicit-cache is used, SCons will ignore any changes that may have been made to search paths (like $CPPPATH or $LIBPATH,). This can lead to SCons not rebuilding a file if a change to $CPPPATH would normally cause a different, same-named file from a different directory to be used.

  • When --implicit-cache is used, SCons will not detect if a same-named file has been added to a directory that is earlier in the search path than the directory in which the file was found last time.

6.4.1. The --implicit-deps-changed Option

When using cached implicit dependencies, sometimes you want to "start fresh" and have SCons re-scan the files for which it previously cached the dependencies. For example, if you have recently installed a new version of external code that you use for compilation, the external header files will have changed and the previously-cached implicit dependencies will be out of date. You can update them by running SCons with the --implicit-deps-changed option:

% scons -Q --implicit-deps-changed hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.

In this case, SCons will re-scan all of the implicit dependencies and cache updated copies of the information.

6.4.2. The --implicit-deps-unchanged Option

By default when caching dependencies, SCons notices when a file has been modified and re-scans the file for any updated implicit dependency information. Sometimes, however, you may want to force SCons to use the cached implicit dependencies, even if the source files changed. This can speed up a build for example, when you have changed your source files but know that you haven't changed any #include lines. In this case, you can use the --implicit-deps-unchanged option:

% scons -Q --implicit-deps-unchanged hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.

In this case, SCons will assume that the cached implicit dependencies are correct and will not bother to re-scan changed files. For typical builds after small, incremental changes to source files, the savings may not be very big, but sometimes every bit of improved performance counts.

6.5. Explicit Dependencies: the Depends Function

Sometimes a file depends on another file that is not detected by an SCons scanner. For this situation, SCons allows you to specific explicitly that one file depends on another file, and must be rebuilt whenever that file changes. This is specified using the Depends method:

hello = Program('hello.c')
Depends(hello, 'other_file')
    
% scons -Q hello
cc -c hello.c -o hello.o
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
% edit other_file
    [CHANGE THE CONTENTS OF other_file]
% scons -Q hello
cc -c hello.c -o hello.o
cc -o hello hello.o
    

Note that the dependency (the second argument to Depends) may also be a list of Node objects (for example, as returned by a call to a Builder):

hello = Program('hello.c')
goodbye = Program('goodbye.c')
Depends(hello, goodbye)
    

in which case the dependency or dependencies will be built before the target(s):

% scons -Q hello
cc -c goodbye.c -o goodbye.o
cc -o goodbye goodbye.o
cc -c hello.c -o hello.o
cc -o hello hello.o
    

6.6. Dependencies From External Files: the ParseDepends Function

SCons has built-in scanners for a number of languages. Sometimes these scanners fail to extract certain implicit dependencies due to limitations of the scanner implementation.

The following example illustrates a case where the built-in C scanner is unable to extract the implicit dependency on a header file.

#define FOO_HEADER <foo.h>
#include FOO_HEADER

int main() {
    return FOO;
}
      
% scons -Q
cc -o hello.o -c -I. hello.c
cc -o hello hello.o
%    [CHANGE CONTENTS OF foo.h]
% scons -Q
scons: `.' is up to date.

Apparently, the scanner does not know about the header dependency. Being not a full-fledged C preprocessor, the scanner does not expand the macro.

In these cases, you may also use the compiler to extract the implicit dependencies. ParseDepends can parse the contents of the compiler output in the style of Make, and explicitly establish all of the listed dependencies.

The following example uses ParseDepends to process a compiler generated dependency file which is generated as a side effect during compilation of the object file:

obj = Object('hello.c', CCFLAGS='-MD -MF hello.d', CPPPATH='.')
SideEffect('hello.d', obj)
ParseDepends('hello.d')
Program('hello', obj)
      
% scons -Q
cc -o hello.o -c -MD -MF hello.d -I. hello.c
cc -o hello hello.o
%    [CHANGE CONTENTS OF foo.h]
% scons -Q
cc -o hello.o -c -MD -MF hello.d -I. hello.c

Parsing dependencies from a compiler-generated .d file has a chicken-and-egg problem, that causes unnecessary rebuilds:

% scons -Q
cc -o hello.o -c -MD -MF hello.d -I. hello.c
cc -o hello hello.o
% scons -Q --debug=explain
scons: rebuilding `hello.o' because `foo.h' is a new dependency
cc -o hello.o -c -MD -MF hello.d -I. hello.c
% scons -Q
scons: `.' is up to date.
    

In the first pass, the dependency file is generated while the object file is compiled. At that time, SCons does not know about the dependency on foo.h. In the second pass, the object file is regenerated because foo.h is detected as a new dependency.

ParseDepends immediately reads the specified file at invocation time and just returns if the file does not exist. A dependency file generated during the build process is not automatically parsed again. Hence, the compiler-extracted dependencies are not stored in the signature database during the same build pass. This limitation of ParseDepends leads to unnecessary recompilations. Therefore, ParseDepends should only be used if scanners are not available for the employed language or not powerful enough for the specific task.

6.7. Ignoring Dependencies: the Ignore Function

Sometimes it makes sense to not rebuild a program, even if a dependency file changes. In this case, you would tell SCons specifically to ignore a dependency as follows:

hello_obj=Object('hello.c')
hello = Program(hello_obj)
Ignore(hello_obj, 'hello.h')
      
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
% edit hello.h
  [CHANGE THE CONTENTS OF hello.h]
% scons -Q hello
scons: `hello' is up to date.
    

Now, the above example is a little contrived, because it's hard to imagine a real-world situation where you wouldn't want to rebuild hello if the hello.h file changed. A more realistic example might be if the hello program is being built in a directory that is shared between multiple systems that have different copies of the stdio.h include file. In that case, SCons would notice the differences between the different systems' copies of stdio.h and would rebuild hello each time you change systems. You could avoid these rebuilds as follows:

hello = Program('hello.c', CPPPATH=['/usr/include'])
Ignore(hello, '/usr/include/stdio.h')
    

Ignore can also be used to prevent a generated file from being built by default. This is due to the fact that directories depend on their contents. So to ignore a generated file from the default build, you specify that the directory should ignore the generated file. Note that the file will still be built if the user specifically requests the target on scons command line, or if the file is a dependency of another file which is requested and/or is built by default.

hello_obj=Object('hello.c')
hello = Program(hello_obj)
Ignore('.',[hello,hello_obj])
      
% scons -Q
scons: `.' is up to date.
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.

6.8. Order-Only Dependencies: the Requires Function

Occasionally, it may be useful to specify that a certain file or directory must, if necessary, be built or created before some other target is built, but that changes to that file or directory do not require that the target itself be rebuilt. Such a relationship is called an order-only dependency because it only affects the order in which things must be built--the dependency before the target--but it is not a strict dependency relationship because the target should not change in response to changes in the dependent file.

For example, suppose that you want to create a file every time you run a build that identifies the time the build was performed, the version number, etc., and which is included in every program that you build. The version file's contents will change every build. If you specify a normal dependency relationship, then every program that depends on that file would be rebuilt every time you ran SCons. For example, we could use some Python code in a SConstruct file to create a new version.c file with a string containing the current date every time we run SCons, and then link a program with the resulting object file by listing version.c in the sources:

import time

version_c_text = """
char *date = "%s";
""" % time.ctime(time.time())
open('version.c', 'w').write(version_c_text)

hello = Program(['hello.c', 'version.c'])
      

If we list version.c as an actual source file, though, then the version.o file will get rebuilt every time we run SCons (because the SConstruct file itself changes the contents of version.c) and the hello executable will get re-linked every time (because the version.o file changes):

% scons -Q hello
cc -o hello.o -c hello.c
cc -o version.o -c version.c
cc -o hello hello.o version.o
% sleep 1
% scons -Q hello
cc -o version.o -c version.c
cc -o hello hello.o version.o
% sleep 1
% scons -Q hello
cc -o version.o -c version.c
cc -o hello hello.o version.o

(Note that for the above example to work, we sleep for one second in between each run, so that the SConstruct file will create a version.c file with a time string that's one second later than the previous run.)

One solution is to use the Requires function to specify that the version.o must be rebuilt before it is used by the link step, but that changes to version.o should not actually cause the hello executable to be re-linked:

import time

version_c_text = """
char *date = "%s";
""" % time.ctime(time.time())
open('version.c', 'w').write(version_c_text)

version_obj = Object('version.c')

hello = Program('hello.c',
                LINKFLAGS = str(version_obj[0]))

Requires(hello, version_obj)
      

Notice that because we can no longer list version.c as one of the sources for the hello program, we have to find some other way to get it into the link command line. For this example, we're cheating a bit and stuffing the object file name (extracted from version_obj list returned by the Object call) into the $LINKFLAGS variable, because $LINKFLAGS is already included in the $LINKCOM command line.

With these changes, we get the desired behavior of only re-linking the hello executable when the hello.c has changed, even though the version.o is rebuilt (because the SConstruct file still changes the version.c contents directly each run):

% scons -Q hello
cc -o version.o -c version.c
cc -o hello.o -c hello.c
cc -o hello version.o hello.o
% sleep 1
% scons -Q hello
cc -o version.o -c version.c
scons: `hello' is up to date.
% sleep 1
%     [CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
cc -o version.o -c version.c
cc -o hello.o -c hello.c
cc -o hello version.o hello.o
% sleep 1
% scons -Q hello
cc -o version.o -c version.c
scons: `hello' is up to date.

6.9. The AlwaysBuild Function

How SCons handles dependencies can also be affected by the AlwaysBuild method. When a file is passed to the AlwaysBuild method, like so:

hello = Program('hello.c')
AlwaysBuild(hello)
      

Then the specified target file (hello in our example) will always be considered out-of-date and rebuilt whenever that target file is evaluated while walking the dependency graph:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q
cc -o hello hello.o

The AlwaysBuild function has a somewhat misleading name, because it does not actually mean the target file will be rebuilt every single time SCons is invoked. Instead, it means that the target will, in fact, be rebuilt whenever the target file is encountered while evaluating the targets specified on the command line (and their dependencies). So specifying some other target on the command line, a target that does not itself depend on the AlwaysBuild target, will still be rebuilt only if it's out-of-date with respect to its dependencies:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello.o
scons: `hello.o' is up to date.


[2] This easily-overlooked distinction between how SCons decides if the target itself must be rebuilt and how the target is then used to decide if a different target must be rebuilt is one of the confusing things that has led to the TargetSignatures and SourceSignatures functions being replaced by the simpler Decider function.

Chapter 7. Environments

An environment is a collection of values that can affect how a program executes. SCons distinguishes between three different types of environments that can affect the behavior of SCons itself (subject to the configuration in the SConscript files), as well as the compilers and other tools it executes:

External Environment

The external environment is the set of variables in the user's environment at the time the user runs SCons. These variables are available within the SConscript files through the Python os.environ dictionary. See Section 7.1, “Using Values From the External Environment”, below.

Construction Environment

A construction environment is a distinct object creating within a SConscript file and and which contains values that affect how SCons decides what action to use to build a target, and even to define which targets should be built from which sources. One of the most powerful features of SCons is the ability to create multiple construction environments, including the ability to clone a new, customized construction environment from an existing construction environment. See Section 7.2, “Construction Environments”, below.

Execution Environment

An execution environment is the values that SCons sets when executing an external command (such as a compiler or linker) to build one or more targets. Note that this is not the same as the external environment (see above). See Section 7.3, “Controlling the Execution Environment for Issued Commands”, below.

Unlike Make, SCons does not automatically copy or import values between different environments (with the exception of explicit clones of construction environments, which inherit values from their parent). This is a deliberate design choice to make sure that builds are, by default, repeatable regardless of the values in the user's external environment. This avoids a whole class of problems with builds where a developer's local build works because a custom variable setting causes a different compiler or build option to be used, but the checked-in change breaks the official build because it uses different environment variable settings.

Note that the SConscript writer can easily arrange for variables to be copied or imported between environments, and this is often very useful (or even downright necessary) to make it easy for developers to customize the build in appropriate ways. The point is not that copying variables between different environments is evil and must always be avoided. Instead, it should be up to the implementer of the build system to make conscious choices about how and when to import a variable from one environment to another, making informed decisions about striking the right balance between making the build repeatable on the one hand and convenient to use on the other.

7.1. Using Values From the External Environment

The external environment variable settings that the user has in force when executing SCons are available through the normal Python os.environ dictionary. This means that you must add an import os statement to any SConscript file in which you want to use values from the user's external environment.

import os
     

More usefully, you can use the os.environ dictionary in your SConscript files to initialize construction environments with values from the user's external environment. See the next section, Section 7.2, “Construction Environments”, for information on how to do this.

7.2. Construction Environments

It is rare that all of the software in a large, complicated system needs to be built the same way. For example, different source files may need different options enabled on the command line, or different executable programs need to be linked with different libraries. SCons accommodates these different build requirements by allowing you to create and configure multiple construction environments that control how the software is built. A construction environment is an object that has a number of associated construction variables, each with a name and a value. (A construction environment also has an attached set of Builder methods, about which we'll learn more later.)

7.2.1. Creating a Construction Environment: the Environment Function

A construction environment is created by the Environment method:

env = Environment()
       

By default, SCons initializes every new construction environment with a set of construction variables based on the tools that it finds on your system, plus the default set of builder methods necessary for using those tools. The construction variables are initialized with values describing the C compiler, the Fortran compiler, the linker, etc., as well as the command lines to invoke them.

When you initialize a construction environment you can set the values of the environment's construction variables to control how a program is built. For example:

 env = Environment(CC = 'gcc',
                   CCFLAGS = '-O2')

 env.Program('foo.c')
         

The construction environment in this example is still initialized with the same default construction variable values, except that the user has explicitly specified use of the GNU C compiler gcc, and further specifies that the -O2 (optimization level two) flag should be used when compiling the object file. In other words, the explicit initializations of $CC and $CCFLAGS override the default values in the newly-created construction environment. So a run from this example would look like:

% scons -Q
gcc -o foo.o -c -O2 foo.c
gcc -o foo foo.o

7.2.2. Fetching Values From a Construction Environment

You can fetch individual construction variables using the normal syntax for accessing individual named items in a Python dictionary:

env = Environment()
print "CC is:", env['CC']
        

This example SConstruct file doesn't build anything, but because it's actually a Python script, it will print the value of $CC for us:

% scons -Q
CC is: cc
scons: `.' is up to date.

A construction environment, however, is actually an object with associated methods, etc. If you want to have direct access to only the dictionary of construction variables, you can fetch this using the Dictionary method:

env = Environment(FOO = 'foo', BAR = 'bar')
dict = env.Dictionary()
for key in ['OBJSUFFIX', 'LIBSUFFIX', 'PROGSUFFIX']:
    print "key = %s, value = %s" % (key, dict[key])
         

This SConstruct file will print the specified dictionary items for us on POSIX systems as follows:

% scons -Q
key = OBJSUFFIX, value = .o
key = LIBSUFFIX, value = .a
key = PROGSUFFIX, value = 
scons: `.' is up to date.

And on Windows:

C:\>scons -Q
key = OBJSUFFIX, value = .obj
key = LIBSUFFIX, value = .lib
key = PROGSUFFIX, value = .exe
scons: `.' is up to date.

If you want to loop and print the values of all of the construction variables in a construction environment, the Python code to do that in sorted order might look something like:

env = Environment()
for item in sorted(env.Dictionary().items()):
    print "construction variable = '%s', value = '%s'" % item
      

7.2.3. Expanding Values From a Construction Environment: the subst Method

Another way to get information from a construction environment is to use the subst method on a string containing $ expansions of construction variable names. As a simple example, the example from the previous section that used env['CC'] to fetch the value of $CC could also be written as:

env = Environment()
print "CC is:", env.subst('$CC')
      

One advantage of using subst to expand strings is that construction variables in the result get re-expanded until there are no expansions left in the string. So a simple fetch of a value like $CCCOM:

env = Environment(CCFLAGS = '-DFOO')
print "CCCOM is:", env['CCCOM']
      

Will print the unexpanded value of $CCCOM, showing us the construction variables that still need to be expanded:

% scons -Q
CCCOM is: $CC $CCFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS -c -o $TARGET $SOURCES
scons: `.' is up to date.
      

Calling the subst method on $CCOM, however:

env = Environment(CCFLAGS = '-DFOO')
print "CCCOM is:", env.subst('$CCCOM')
      

Will recursively expand all of the construction variables prefixed with $ (dollar signs), showing us the final output:

% scons -Q
CCCOM is: gcc -DFOO -c -o
scons: `.' is up to date.
      

Note that because we're not expanding this in the context of building something there are no target or source files for $TARGET and $SOURCES to expand.

7.2.4. Handling Problems With Value Expansion

If a problem occurs when expanding a construction variable, by default it is expanded to '' (a null string), and will not cause scons to fail.

env = Environment()
print "value is:", env.subst( '->$MISSING<-' )
        
% scons -Q
value is: -><-
scons: `.' is up to date.

This default behaviour can be changed using the AllowSubstExceptions function. When a problem occurs with a variable expansion it generates an exception, and the AllowSubstExceptions function controls which of these exceptions are actually fatal and which are allowed to occur safely. By default, NameError and IndexError are the two exceptions that are allowed to occur: so instead of causing scons to fail, these are caught, the variable expanded to '' and scons execution continues. To require that all construction variable names exist, and that indexes out of range are not allowed, call AllowSubstExceptions with no extra arguments.

AllowSubstExceptions()
env = Environment()
print "value is:", env.subst( '->$MISSING<-' )
        
% scons -Q
value is:
scons: *** NameError `MISSING' trying to evaluate `$MISSING'
File "/home/my/project/SConstruct", line 3, in <module>

This can also be used to allow other exceptions that might occur, most usefully with the ${...} construction variable syntax. For example, this would allow zero-division to occur in a variable expansion in addition to the default exceptions allowed

AllowSubstExceptions(IndexError, NameError, ZeroDivisionError)
env = Environment()
print "value is:", env.subst( '->${1 / 0}<-' )
        
% scons -Q
value is: -><-
scons: `.' is up to date.

If AllowSubstExceptions is called multiple times, each call completely overwrites the previous list of allowed exceptions.

7.2.5. Controlling the Default Construction Environment: the DefaultEnvironment Function

All of the Builder functions that we've introduced so far, like Program and Library, actually use a default construction environment that contains settings for the various compilers and other tools that SCons configures by default, or otherwise knows about and has discovered on your system. The goal of the default construction environment is to make many configurations to "just work" to build software using readily available tools with a minimum of configuration changes.

You can, however, control the settings in the default construction environment by using the DefaultEnvironment function to initialize various settings:

DefaultEnvironment(CC = '/usr/local/bin/gcc')
      

When configured as above, all calls to the Program or Object Builder will build object files with the /usr/local/bin/gcc compiler.

Note that the DefaultEnvironment function returns the initialized default construction environment object, which can then be manipulated like any other construction environment. So the following would be equivalent to the previous example, setting the $CC variable to /usr/local/bin/gcc but as a separate step after the default construction environment has been initialized:

env = DefaultEnvironment()
env['CC'] = '/usr/local/bin/gcc'
      

One very common use of the DefaultEnvironment function is to speed up SCons initialization. As part of trying to make most default configurations "just work," SCons will actually search the local system for installed compilers and other utilities. This search can take time, especially on systems with slow or networked file systems. If you know which compiler(s) and/or other utilities you want to configure, you can control the search that SCons performs by specifying some specific tool modules with which to initialize the default construction environment:

env = DefaultEnvironment(tools = ['gcc', 'gnulink'],
                         CC = '/usr/local/bin/gcc')
      

So the above example would tell SCons to explicitly configure the default environment to use its normal GNU Compiler and GNU Linker settings (without having to search for them, or any other utilities for that matter), and specifically to use the compiler found at /usr/local/bin/gcc.

7.2.6. Multiple Construction Environments

The real advantage of construction environments is that you can create as many different construction environments as you need, each tailored to a different way to build some piece of software or other file. If, for example, we need to build one program with the -O2 flag and another with the -g (debug) flag, we would do this like so:

opt = Environment(CCFLAGS = '-O2')
dbg = Environment(CCFLAGS = '-g')

opt.Program('foo', 'foo.c')

dbg.Program('bar', 'bar.c')
        
% scons -Q
cc -o bar.o -c -g bar.c
cc -o bar bar.o
cc -o foo.o -c -O2 foo.c
cc -o foo foo.o

We can even use multiple construction environments to build multiple versions of a single program. If you do this by simply trying to use the Program builder with both environments, though, like this:

opt = Environment(CCFLAGS = '-O2')
dbg = Environment(CCFLAGS = '-g')

opt.Program('foo', 'foo.c')

dbg.Program('foo', 'foo.c')
        

Then SCons generates the following error:

% scons -Q

scons: *** Two environments with different actions were specified for the same target: foo.o
File "/home/my/project/SConstruct", line 6, in <module>

This is because the two Program calls have each implicitly told SCons to generate an object file named foo.o, one with a $CCFLAGS value of -O2 and one with a $CCFLAGS value of -g. SCons can't just decide that one of them should take precedence over the other, so it generates the error. To avoid this problem, we must explicitly specify that each environment compile foo.c to a separately-named object file using the Object builder, like so:

opt = Environment(CCFLAGS = '-O2')
dbg = Environment(CCFLAGS = '-g')

o = opt.Object('foo-opt', 'foo.c')
opt.Program(o)

d = dbg.Object('foo-dbg', 'foo.c')
dbg.Program(d)
        

Notice that each call to the Object builder returns a value, an internal SCons object that represents the object file that will be built. We then use that object as input to the Program builder. This avoids having to specify explicitly the object file name in multiple places, and makes for a compact, readable SConstruct file. Our SCons output then looks like:

% scons -Q
cc -o foo-dbg.o -c -g foo.c
cc -o foo-dbg foo-dbg.o
cc -o foo-opt.o -c -O2 foo.c
cc -o foo-opt foo-opt.o

7.2.7. Making Copies of Construction Environments: the Clone Method

Sometimes you want more than one construction environment to share the same values for one or more variables. Rather than always having to repeat all of the common variables when you create each construction environment, you can use the Clone method to create a copy of a construction environment.

Like the Environment call that creates a construction environment, the Clone method takes construction variable assignments, which will override the values in the copied construction environment. For example, suppose we want to use gcc to create three versions of a program, one optimized, one debug, and one with neither. We could do this by creating a "base" construction environment that sets $CC to gcc, and then creating two copies, one which sets $CCFLAGS for optimization and the other which sets $CCFLAGS for debugging:

env = Environment(CC = 'gcc')
opt = env.Clone(CCFLAGS = '-O2')
dbg = env.Clone(CCFLAGS = '-g')

env.Program('foo', 'foo.c')

o = opt.Object('foo-opt', 'foo.c')
opt.Program(o)

d = dbg.Object('foo-dbg', 'foo.c')
dbg.Program(d)
        

Then our output would look like:

% scons -Q
gcc -o foo.o -c foo.c
gcc -o foo foo.o
gcc -o foo-dbg.o -c -g foo.c
gcc -o foo-dbg foo-dbg.o
gcc -o foo-opt.o -c -O2 foo.c
gcc -o foo-opt foo-opt.o

7.2.8. Replacing Values: the Replace Method

You can replace existing construction variable values using the Replace method:

env = Environment(CCFLAGS = '-DDEFINE1')
env.Replace(CCFLAGS = '-DDEFINE2')
env.Program('foo.c')
        

The replacing value (-DDEFINE2 in the above example) completely replaces the value in the construction environment:

% scons -Q
cc -o foo.o -c -DDEFINE2 foo.c
cc -o foo foo.o

You can safely call Replace for construction variables that don't exist in the construction environment:

env = Environment()
env.Replace(NEW_VARIABLE = 'xyzzy')
print "NEW_VARIABLE =", env['NEW_VARIABLE']
        

In this case, the construction variable simply gets added to the construction environment:

% scons -Q
NEW_VARIABLE = xyzzy
scons: `.' is up to date.

Because the variables aren't expanded until the construction environment is actually used to build the targets, and because SCons function and method calls are order-independent, the last replacement "wins" and is used to build all targets, regardless of the order in which the calls to Replace() are interspersed with calls to builder methods:

env = Environment(CCFLAGS = '-DDEFINE1')
print "CCFLAGS =", env['CCFLAGS']
env.Program('foo.c')

env.Replace(CCFLAGS = '-DDEFINE2')
print "CCFLAGS =", env['CCFLAGS']
env.Program('bar.c')
        

The timing of when the replacement actually occurs relative to when the targets get built becomes apparent if we run scons without the -Q option:

% scons
scons: Reading SConscript files ...
CCFLAGS = -DDEFINE1
CCFLAGS = -DDEFINE2
scons: done reading SConscript files.
scons: Building targets ...
cc -o bar.o -c -DDEFINE2 bar.c
cc -o bar bar.o
cc -o foo.o -c -DDEFINE2 foo.c
cc -o foo foo.o
scons: done building targets.

Because the replacement occurs while the SConscript files are being read, the $CCFLAGS variable has already been set to -DDEFINE2 by the time the foo.o target is built, even though the call to the Replace method does not occur until later in the SConscript file.

7.2.9. Setting Values Only If They're Not Already Defined: the SetDefault Method

Sometimes it's useful to be able to specify that a construction variable should be set to a value only if the construction environment does not already have that variable defined You can do this with the SetDefault method, which behaves similarly to the set_default method of Python dictionary objects:

env.SetDefault(SPECIAL_FLAG = '-extra-option')
      

This is especially useful when writing your own Tool modules to apply variables to construction environments.

7.2.10. Appending to the End of Values: the Append Method

You can append a value to an existing construction variable using the Append method:

env = Environment(CCFLAGS = ['-DMY_VALUE'])
env.Append(CCFLAGS = ['-DLAST'])
env.Program('foo.c')
        

SCons then supplies both the -DMY_VALUE and -DLAST flags when compiling the object file:

% scons -Q
cc -o foo.o -c -DMY_VALUE -DLAST foo.c
cc -o foo foo.o

If the construction variable doesn't already exist, the Append method will create it:

env = Environment()
env.Append(NEW_VARIABLE = 'added')
print "NEW_VARIABLE =", env['NEW_VARIABLE']
        

Which yields:

% scons -Q
NEW_VARIABLE = added
scons: `.' is up to date.

Note that the Append function tries to be "smart" about how the new value is appended to the old value. If both are strings, the previous and new strings are simply concatenated. Similarly, if both are lists, the lists are concatenated. If, however, one is a string and the other is a list, the string is added as a new element to the list.

7.2.11. Appending Unique Values: the AppendUnique Method

Some times it's useful to add a new value only if the existing construction variable doesn't already contain the value. This can be done using the AppendUnique method:

env.AppendUnique(CCFLAGS=['-g'])
      

In the above example, the -g would be added only if the $CCFLAGS variable does not already contain a -g value.

7.2.12. Appending to the Beginning of Values: the Prepend Method

You can append a value to the beginning of an existing construction variable using the Prepend method:

env = Environment(CCFLAGS = ['-DMY_VALUE'])
env.Prepend(CCFLAGS = ['-DFIRST'])
env.Program('foo.c')
        

SCons then supplies both the -DFIRST and -DMY_VALUE flags when compiling the object file:

% scons -Q
cc -o foo.o -c -DFIRST -DMY_VALUE foo.c
cc -o foo foo.o

If the construction variable doesn't already exist, the Prepend method will create it:

env = Environment()
env.Prepend(NEW_VARIABLE = 'added')
print "NEW_VARIABLE =", env['NEW_VARIABLE']
        

Which yields:

% scons -Q
NEW_VARIABLE = added
scons: `.' is up to date.

Like the Append function, the Prepend function tries to be "smart" about how the new value is appended to the old value. If both are strings, the previous and new strings are simply concatenated. Similarly, if both are lists, the lists are concatenated. If, however, one is a string and the other is a list, the string is added as a new element to the list.

7.2.13. Prepending Unique Values: the PrependUnique Method

Some times it's useful to add a new value to the beginning of a construction variable only if the existing value doesn't already contain the to-be-added value. This can be done using the PrependUnique method:

env.PrependUnique(CCFLAGS=['-g'])
      

In the above example, the -g would be added only if the $CCFLAGS variable does not already contain a -g value.

7.3. Controlling the Execution Environment for Issued Commands

When SCons builds a target file, it does not execute the commands with the same external environment that you used to execute SCons. Instead, it uses the dictionary stored in the $ENV construction variable as the external environment for executing commands.

The most important ramification of this behavior is that the PATH environment variable, which controls where the operating system will look for commands and utilities, is not the same as in the external environment from which you called SCons. This means that SCons will not, by default, necessarily find all of the tools that you can execute from the command line.

The default value of the PATH environment variable on a POSIX system is /usr/local/bin:/bin:/usr/bin. The default value of the PATH environment variable on a Windows system comes from the Windows registry value for the command interpreter. If you want to execute any commands--compilers, linkers, etc.--that are not in these default locations, you need to set the PATH value in the $ENV dictionary in your construction environment.

The simplest way to do this is to initialize explicitly the value when you create the construction environment; this is one way to do that:

path = ['/usr/local/bin', '/bin', '/usr/bin']
env = Environment(ENV = {'PATH' : path})
    

Assign a dictionary to the $ENV construction variable in this way completely resets the external environment so that the only variable that will be set when external commands are executed will be the PATH value. If you want to use the rest of the values in $ENV and only set the value of PATH, the most straightforward way is probably:

env['ENV']['PATH'] = ['/usr/local/bin', '/bin', '/usr/bin']
    

Note that SCons does allow you to define the directories in the PATH in a string, separated by the pathname-separator character for your system (':' on POSIX systems, ';' on Windows):

env['ENV']['PATH'] = '/usr/local/bin:/bin:/usr/bin'
    

But doing so makes your SConscript file less portable, (although in this case that may not be a huge concern since the directories you list are likley system-specific, anyway).

7.3.1. Propagating PATH From the External Environment

You may want to propagate the external PATH to the execution environment for commands. You do this by initializing the PATH variable with the PATH value from the os.environ dictionary, which is Python's way of letting you get at the external environment:

import os
env = Environment(ENV = {'PATH' : os.environ['PATH']})
      

Alternatively, you may find it easier to just propagate the entire external environment to the execution environment for commands. This is simpler to code than explicity selecting the PATH value:

import os
env = Environment(ENV = os.environ)
      

Either of these will guarantee that SCons will be able to execute any command that you can execute from the command line. The drawback is that the build can behave differently if it's run by people with different PATH values in their environment--for example, if both the /bin and /usr/local/bin directories have different cc commands, then which one will be used to compile programs will depend on which directory is listed first in the user's PATH variable.

7.3.2. Adding to PATH Values in the Execution Environment

One of the most common requirements for manipulating a variable in the execution environment is to add one or more custom directories to a search like the $PATH variable on Linux or POSIX systems, or the %PATH% variable on Windows, so that a locally-installed compiler or other utility can be found when SCons tries to execute it to update a target. SCons provides PrependENVPath and AppendENVPath functions to make adding things to execution variables convenient. You call these functions by specifying the variable to which you want the value added, and then value itself. So to add some /usr/local directories to the $PATH and $LIB variables, you might:

env = Environment(ENV = os.environ)
env.PrependENVPath('PATH', '/usr/local/bin')
env.AppendENVPath('LIB', '/usr/local/lib')
      

Note that the added values are strings, and if you want to add multiple directories to a variable like $PATH, you must include the path separate character (: on Linux or POSIX, ; on Windows) in the string.

Chapter 8. Automatically Putting Command-line Options into their Construction Variables

This chapter describes the MergeFlags, ParseFlags, and ParseConfig methods of a construction environment.

8.1. Merging Options into the Environment: the MergeFlags Function

SCons construction environments have a MergeFlags method that merges a dictionary of values into the construction environment. MergeFlags treats each value in the dictionary as a list of options such as one might pass to a command (such as a compiler or linker). MergeFlags will not duplicate an option if it already exists in the construction environment variable.

MergeFlags tries to be intelligent about merging options. When merging options to any variable whose name ends in PATH, MergeFlags keeps the leftmost occurrence of the option, because in typical lists of directory paths, the first occurrence "wins." When merging options to any other variable name, MergeFlags keeps the rightmost occurrence of the option, because in a list of typical command-line options, the last occurrence "wins."

env = Environment()
env.Append(CCFLAGS = '-option -O3 -O1')
flags = { 'CCFLAGS' : '-whatever -O3' }
env.MergeFlags(flags)
print env['CCFLAGS']
   
% scons -Q
['-option', '-O1', '-whatever', '-O3']
scons: `.' is up to date.

Note that the default value for $CCFLAGS is an internal SCons object which automatically converts the options we specified as a string into a list.

env = Environment()
env.Append(CPPPATH = ['/include', '/usr/local/include', '/usr/include'])
flags = { 'CPPPATH' : ['/usr/opt/include', '/usr/local/include'] }
env.MergeFlags(flags)
print env['CPPPATH']
   
% scons -Q
['/include', '/usr/local/include', '/usr/include', '/usr/opt/include']
scons: `.' is up to date.

Note that the default value for $CPPPATH is a normal Python list, so we must specify its values as a list in the dictionary we pass to the MergeFlags function.

If MergeFlags is passed anything other than a dictionary, it calls the ParseFlags method to convert it into a dictionary.

env = Environment()
env.Append(CCFLAGS = '-option -O3 -O1')
env.Append(CPPPATH = ['/include', '/usr/local/include', '/usr/include'])
env.MergeFlags('-whatever -I/usr/opt/include -O3 -I/usr/local/include')
print env['CCFLAGS']
print env['CPPPATH']
   
% scons -Q
['-option', '-O1', '-whatever', '-O3']
['/include', '/usr/local/include', '/usr/include', '/usr/opt/include']
scons: `.' is up to date.

In the combined example above, ParseFlags has sorted the options into their corresponding variables and returned a dictionary for MergeFlags to apply to the construction variables in the specified construction environment.

8.2. Separating Compile Arguments into their Variables: the ParseFlags Function

SCons has a bewildering array of construction variables for different types of options when building programs. Sometimes you may not know exactly which variable should be used for a particular option.

SCons construction environments have a ParseFlags method that takes a set of typical command-line options and distrbutes them into the appropriate construction variables. Historically, it was created to support the ParseConfig method, so it focuses on options used by the GNU Compiler Collection (GCC) for the C and C++ toolchains.

ParseFlags returns a dictionary containing the options distributed into their respective construction variables. Normally, this dictionary would be passed to MergeFlags to merge the options into a construction environment, but the dictionary can be edited if desired to provide additional functionality. (Note that if the flags are not going to be edited, calling MergeFlags with the options directly will avoid an additional step.)

env = Environment()
d = env.ParseFlags("-I/opt/include -L/opt/lib -lfoo")
for k,v in sorted(d.items()):
    if v:
        print k, v
env.MergeFlags(d)
env.Program('f1.c')
   
% scons -Q
CPPPATH ['/opt/include']
LIBPATH ['/opt/lib']
LIBS ['foo']
cc -o f1.o -c -I/opt/include f1.c
cc -o f1 f1.o -L/opt/lib -lfoo

Note that if the options are limited to generic types like those above, they will be correctly translated for other platform types:

C:\>scons -Q
CPPPATH ['/opt/include']
LIBPATH ['/opt/lib']
LIBS ['foo']
cl /Fof1.obj /c f1.c /nologo /I\opt\include
link /nologo /OUT:f1.exe /LIBPATH:\opt\lib foo.lib f1.obj
embedManifestExeCheck(target, source, env)

Since the assumption is that the flags are used for the GCC toolchain, unrecognized flags are placed in $CCFLAGS so they will be used for both C and C++ compiles:

env = Environment()
d = env.ParseFlags("-whatever")
for k,v in sorted(d.items()):
    if v:
        print k, v
env.MergeFlags(d)
env.Program('f1.c')
   
% scons -Q
CCFLAGS -whatever
cc -o f1.o -c -whatever f1.c
cc -o f1 f1.o

ParseFlags will also accept a (recursive) list of strings as input; the list is flattened before the strings are processed:

env = Environment()
d = env.ParseFlags(["-I/opt/include", ["-L/opt/lib", "-lfoo"]])
for k,v in sorted(d.items()):
    if v:
        print k, v
env.MergeFlags(d)
env.Program('f1.c')
   
% scons -Q
CPPPATH ['/opt/include']
LIBPATH ['/opt/lib']
LIBS ['foo']
cc -o f1.o -c -I/opt/include f1.c
cc -o f1 f1.o -L/opt/lib -lfoo

If a string begins with a "!" (an exclamation mark, often called a bang), the string is passed to the shell for execution. The output of the command is then parsed:

env = Environment()
d = env.ParseFlags(["!echo -I/opt/include", "!echo -L/opt/lib", "-lfoo"])
for k,v in sorted(d.items()):
    if v:
        print k, v
env.MergeFlags(d)
env.Program('f1.c')
   
% scons -Q
CPPPATH ['/opt/include']
LIBPATH ['/opt/lib']
LIBS ['foo']
cc -o f1.o -c -I/opt/include f1.c
cc -o f1 f1.o -L/opt/lib -lfoo

ParseFlags is regularly updated for new options; consult the man page for details about those currently recognized.

8.3. Finding Installed Library Information: the ParseConfig Function

Configuring the right options to build programs to work with libraries--especially shared libraries--that are available on POSIX systems can be very complicated. To help this situation, various utilies with names that end in config return the command-line options for the GNU Compiler Collection (GCC) that are needed to use these libraries; for example, the command-line options to use a library named lib would be found by calling a utility named lib-config.

A more recent convention is that these options are available from the generic pkg-config program, which has common framework, error handling, and the like, so that all the package creator has to do is provide the set of strings for his particular package.

SCons construction environments have a ParseConfig method that executes a *config utility (either pkg-config or a more specific utility) and configures the appropriate construction variables in the environment based on the command-line options returned by the specified command.

env = Environment()
env['CPPPATH'] = ['/lib/compat']
env.ParseConfig("pkg-config x11 --cflags --libs")
print env['CPPPATH']
   

SCons will execute the specified command string, parse the resultant flags, and add the flags to the appropriate environment variables.

% scons -Q
['/lib/compat', '/usr/X11/include']
scons: `.' is up to date.
 

In the example above, SCons has added the include directory to CPPPATH. (Depending upon what other flags are emitted by the pkg-config command, other variables may have been extended as well.)

Note that the options are merged with existing options using the MergeFlags method, so that each option only occurs once in the construction variable:

env = Environment()
env.ParseConfig("pkg-config x11 --cflags --libs")
env.ParseConfig("pkg-config x11 --cflags --libs")
print env['CPPPATH']
   
% scons -Q
['/usr/X11/include']
scons: `.' is up to date.
 

Chapter 9. Controlling Build Output

A key aspect of creating a usable build configuration is providing good output from the build so its users can readily understand what the build is doing and get information about how to control the build. SCons provides several ways of controlling output from the build configuration to help make the build more useful and understandable.

9.1. Providing Build Help: the Help Function

It's often very useful to be able to give users some help that describes the specific targets, build options, etc., that can be used for your build. SCons provides the Help function to allow you to specify this help text:

Help("""
Type: 'scons program' to build the production program,
      'scons debug' to build the debug version.
""")
       

(Note the above use of the Python triple-quote syntax, which comes in very handy for specifying multi-line strings like help text.)

When the SConstruct or SConscript files contain such a call to the Help function, the specified help text will be displayed in response to the SCons -h option:

% scons -h
scons: Reading SConscript files ...
scons: done reading SConscript files.

Type: 'scons program' to build the production program,
      'scons debug' to build the debug version.

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

The SConscript files may contain multiple calls to the Help function, in which case the specified text(s) will be concatenated when displayed. This allows you to split up the help text across multiple SConscript files. In this situation, the order in which the SConscript files are called will determine the order in which the Help functions are called, which will determine the order in which the various bits of text will get concatenated.

Another use would be to make the help text conditional on some variable. For example, suppose you only want to display a line about building a Windows-only version of a program when actually run on Windows. The following SConstruct file:

env = Environment()

Help("\nType: 'scons program' to build the production program.\n")

if env['PLATFORM'] == 'win32':
    Help("\nType: 'scons windebug' to build the Windows debug version.\n")
       

Will display the complete help text on Windows:

C:\>scons -h
scons: Reading SConscript files ...
scons: done reading SConscript files.

Type: 'scons program' to build the production program.

Type: 'scons windebug' to build the Windows debug version.

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

But only show the relevant option on a Linux or UNIX system:

% scons -h
scons: Reading SConscript files ...
scons: done reading SConscript files.

Type: 'scons program' to build the production program.

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

If there is no Help text in the SConstruct or SConscript files, SCons will revert to displaying its standard list that describes the SCons command-line options. This list is also always displayed whenever the -H option is used.

9.2. Controlling How SCons Prints Build Commands: the $*COMSTR Variables

Sometimes the commands executed to compile object files or link programs (or build other targets) can get very long, long enough to make it difficult for users to distinguish error messages or other important build output from the commands themselves. All of the default $*COM variables that specify the command lines used to build various types of target files have a corresponding $*COMSTR variable that can be set to an alternative string that will be displayed when the target is built.

For example, suppose you want to have SCons display a "Compiling" message whenever it's compiling an object file, and a "Linking" when it's linking an executable. You could write a SConstruct file that looks like:

env = Environment(CCCOMSTR = "Compiling $TARGET",
                  LINKCOMSTR = "Linking $TARGET")
env.Program('foo.c')
       

Which would then yield the output:

% scons -Q
Compiling foo.o
Linking foo
    

SCons performs complete variable substitution on $*COMSTR variables, so they have access to all of the standard variables like $TARGET $SOURCES, etc., as well as any construction variables that happen to be configured in the construction environment used to build a specific target.

Of course, sometimes it's still important to be able to see the exact command that SCons will execute to build a target. For example, you may simply need to verify that SCons is configured to supply the right options to the compiler, or a developer may want to cut-and-paste a compile command to add a few options for a custom test.

One common way to give users control over whether or not SCons should print the actual command line or a short, configured summary is to add support for a VERBOSE command-line variable to your SConstruct file. A simple configuration for this might look like:

env = Environment()
if ARGUMENTS.get('VERBOSE') != "1':
    env['CCCOMSTR'] = "Compiling $TARGET"
    env['LINKCOMSTR'] = "Linking $TARGET"
env.Program('foo.c')
       

By only setting the appropriate $*COMSTR variables if the user specifies VERBOSE=1 on the command line, the user has control over how SCons displays these particular command lines:

% scons -Q
Compiling foo.o
Linking foo
% scons -Q -c
Removed foo.o
Removed foo
% scons -Q VERBOSE=1
cc -o foo.o -c foo.c
cc -o foo foo.o
    

9.3. Providing Build Progress Output: the Progress Function

Another aspect of providing good build output is to give the user feedback about what SCons is doing even when nothing is being built at the moment. This can be especially true for large builds when most of the targets are already up-to-date. Because SCons can take a long time making absolutely sure that every target is, in fact, up-to-date with respect to a lot of dependency files, it can be easy for users to mistakenly conclude that SCons is hung or that there is some other problem with the build.

One way to deal with this perception is to configure SCons to print something to let the user know what it's "thinking about." The Progress function allows you to specify a string that will be printed for every file that SCons is "considering" while it is traversing the dependency graph to decide what targets are or are not up-to-date.

Progress('Evaluating $TARGET\n')
Program('f1.c')
Program('f2.c')
      

Note that the Progress function does not arrange for a newline to be printed automatically at the end of the string (as does the Python print statement), and we must specify the \n that we want printed at the end of the configured string. This configuration, then, will have SCons print that it is Evaluating each file that it encounters in turn as it traverses the dependency graph:

% scons -Q
Evaluating SConstruct
Evaluating f1.c
Evaluating f1.o
cc -o f1.o -c f1.c
Evaluating f1
cc -o f1 f1.o
Evaluating f2.c
Evaluating f2.o
cc -o f2.o -c f2.c
Evaluating f2
cc -o f2 f2.o
Evaluating .

Of course, normally you don't want to add all of these additional lines to your build output, as that can make it difficult for the user to find errors or other important messages. A more useful way to display this progress might be to have the file names printed directly to the user's screen, not to the same standard output stream where build output is printed, and to use a carriage return character (\r) so that each file name gets re-printed on the same line. Such a configuration would look like:

Progress('$TARGET\r',
         file=open('/dev/tty', 'w'),
         overwrite=True)
Program('f1.c')
Program('f2.c')
    

Note that we also specified the overwrite=True argument to the Progress function, which causes SCons to "wipe out" the previous string with space characters before printing the next Progress string. Without the overwrite=True argument, a shorter file name would not overwrite all of the charactes in a longer file name that precedes it, making it difficult to tell what the actual file name is on the output. Also note that we opened up the /dev/tty file for direct access (on POSIX) to the user's screen. On Windows, the equivalent would be to open the con: file name.

Also, it's important to know that although you can use $TARGET to substitute the name of the node in the string, the Progress function does not perform general variable substitution (because there's not necessarily a construction environment involved in evaluating a node like a source file, for example).

You can also specify a list of strings to the Progress function, in which case SCons will display each string in turn. This can be used to implement a "spinner" by having SCons cycle through a sequence of strings:

Progress(['-\r', '\\\r', '|\r', '/\r'], interval=5)
Program('f1.c')
Program('f2.c')
    

Note that here we have also used the interval= keyword argument to have SCons only print a new "spinner" string once every five evaluated nodes. Using an interval= count, even with strings that use $TARGET like our examples above, can be a good way to lessen the work that SCons expends printing Progress strings, while still giving the user feedback that indicates SCons is still working on evaluating the build.

Lastly, you can have direct control over how to print each evaluated node by passing a Python function (or other Python callable) to the Progress function. Your function will be called for each evaluated node, allowing you to implement more sophisticated logic like adding a counter:

screen = open('/dev/tty', 'w')
count = 0
def progress_function(node)
    count += 1
    screen.write('Node %4d: %s\r' % (count, node))

Progress(progress_function)
      

Of course, if you choose, you could completely ignore the node argument to the function, and just print a count, or anything else you wish.

(Note that there's an obvious follow-on question here: how would you find the total number of nodes that will be evaluated so you can tell the user how close the build is to finishing? Unfortunately, in the general case, there isn't a good way to do that, short of having SCons evaluate its dependency graph twice, first to count the total and the second time to actually build the targets. This would be necessary because you can't know in advance which target(s) the user actually requested to be built. The entire build may consist of thousands of Nodes, for example, but maybe the user specifically requested that only a single object file be built.)

9.4. Printing Detailed Build Status: the GetBuildFailures Function

SCons, like most build tools, returns zero status to the shell on success and nonzero status on failure. Sometimes it's useful to give more information about the build status at the end of the run, for instance to print an informative message, send an email, or page the poor slob who broke the build.

SCons provides a GetBuildFailures method that you can use in a python atexit function to get a list of objects describing the actions that failed while attempting to build targets. There can be more than one if you're using -j. Here's a simple example:

import atexit

def print_build_failures():
    from SCons.Script import GetBuildFailures
    for bf in GetBuildFailures():
        print "%s failed: %s" % (bf.node, bf.errstr)
atexit.register(print_build_failures)
      

The atexit.register call registers print_build_failures as an atexit callback, to be called before SCons exits. When that function is called, it calls GetBuildFailures to fetch the list of failed objects. See the man page for the detailed contents of the returned objects; some of the more useful attributes are .node, .errstr, .filename, and .command. The filename is not necessarily the same file as the node; the node is the target that was being built when the error occurred, while the filenameis the file or dir that actually caused the error. Note: only call GetBuildFailures at the end of the build; calling it at any other time is undefined.

Here is a more complete example showing how to turn each element of GetBuildFailures into a string:

# Make the build fail if we pass fail=1 on the command line
if ARGUMENTS.get('fail', 0):
    Command('target', 'source', ['/bin/false'])

def bf_to_str(bf):
    """Convert an element of GetBuildFailures() to a string
    in a useful way."""
    import SCons.Errors
    if bf is None: # unknown targets product None in list
        return '(unknown tgt)'
    elif isinstance(bf, SCons.Errors.StopError):
        return str(bf)
    elif bf.node:
        return str(bf.node) + ': ' + bf.errstr
    elif bf.filename:
        return bf.filename + ': ' + bf.errstr
    return 'unknown failure: ' + bf.errstr
import atexit

def build_status():
    """Convert the build status to a 2-tuple, (status, msg)."""
    from SCons.Script import GetBuildFailures
    bf = GetBuildFailures()
    if bf:
        # bf is normally a list of build failures; if an element is None,
        # it's because of a target that scons doesn't know anything about.
        status = 'failed'
        failures_message = "\n".join(["Failed building %s" % bf_to_str(x)
                           for x in bf if x is not None])
    else:
        # if bf is None, the build completed successfully.
        status = 'ok'
        failures_message = ''
    return (status, failures_message)

def display_build_status():
    """Display the build status.  Called by atexit.
    Here you could do all kinds of complicated things."""
    status, failures_message = build_status()
    if status == 'failed':
       print "FAILED!!!!"  # could display alert, ring bell, etc.
    elif status == 'ok':
       print "Build succeeded."
    print failures_message

atexit.register(display_build_status)
      

When this runs, you'll see the appropriate output:

% scons -Q
scons: `.' is up to date.
Build succeeded.
% scons -Q fail=1
scons: *** [target] Source `source' not found, needed by target `target'.
FAILED!!!!
Failed building target: Source `source' not found, needed by target `target'.

Chapter 10. Controlling a Build From the Command Line

SCons provides a number of ways for the writer of the SConscript files to give the users who will run SCons a great deal of control over the build execution. The arguments that the user can specify on the command line are broken down into three types:

Options

Command-line options always begin with one or two - (hyphen) characters. SCons provides ways for you to examine and set options values from within your SConscript files, as well as the ability to define your own custom options. See Section 10.1, “Command-Line Options”, below.

Variables

Any command-line argument containing an = (equal sign) is considered a variable setting with the form variable=value. SCons provides direct access to all of the command-line variable settings, the ability to apply command-line variable settings to construction environments, and functions for configuring specific types of variables (Boolean values, path names, etc.) with automatic validation of the user's specified values. See Section 10.2, “Command-Line variable=value Build Variables”, below.

Targets

Any command-line argument that is not an option or a variable setting (does not begin with a hyphen and does not contain an equal sign) is considered a target that the user (presumably) wants SCons to build. A list of Node objects representing the target or targets to build. SCons provides access to the list of specified targets, as well as ways to set the default list of targets from within the SConscript files. See Section 10.3, “Command-Line Targets”, below.

10.1. Command-Line Options

SCons has many command-line options that control its behavior. A SCons command-line option always begins with one or two - (hyphen) characters.

10.1.1. Not Having to Specify Command-Line Options Each Time: the SCONSFLAGS Environment Variable

Users may find themselves supplying the same command-line options every time they run SCons. For example, you might find it saves time to specify a value of -j 2 to have SCons run up to two build commands in parallel. To avoid having to type -j 2 by hand every time, you can set the external environment variable SCONSFLAGS to a string containing command-line options that you want SCons to use.

If, for example, you're using a POSIX shell that's compatible with the Bourne shell, and you always want SCons to use the -Q option, you can set the SCONSFLAGS environment as follows:

% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
    ... [build output] ...
scons: done building targets.
% export SCONSFLAGS="-Q"
% scons
    ... [build output] ...

Users of csh-style shells on POSIX systems can set the SCONSFLAGS environment as follows:

$ setenv SCONSFLAGS "-Q"
      

Windows users may typically want to set the SCONSFLAGS in the appropriate tab of the System Properties window.

10.1.2. Getting Values Set by Command-Line Options: the GetOption Function

SCons provides the GetOption function to get the values set by the various command-line options. One common use of this is to check whether or not the -h or --help option has been specified. Normally, SCons does not print its help text until after it has read all of the SConscript files, because it's possible that help text has been added by some subsidiary SConscript file deep in the source tree hierarchy. Of course, reading all of the SConscript files takes extra time.

If you know that your configuration does not define any additional help text in subsidiary SConscript files, you can speed up the command-line help available to users by using the GetOption function to load the subsidiary SConscript files only if the the user has not specified the -h or --help option, like so:

if not GetOption('help'):
    SConscript('src/SConscript', export='env')
      

In general, the string that you pass to the GetOption function to fetch the value of a command-line option setting is the same as the "most common" long option name (beginning with two hyphen characters), although there are some exceptions. The list of SCons command-line options and the GetOption strings for fetching them, are available in the Section 10.1.4, “Strings for Getting or Setting Values of SCons Command-Line Options” section, below.

10.1.3. Setting Values of Command-Line Options: the SetOption Function

You can also set the values of SCons command-line options from within the SConscript files by using the SetOption function. The strings that you use to set the values of SCons command-line options are available in the Section 10.1.4, “Strings for Getting or Setting Values of SCons Command-Line Options” section, below.

One use of the SetOption function is to specify a value for the -j or --jobs option, so that users get the improved performance of a parallel build without having to specify the option by hand. A complicating factor is that a good value for the -j option is somewhat system-dependent. One rough guideline is that the more processors your system has, the higher you want to set the -j value, in order to take advantage of the number of CPUs.

For example, suppose the administrators of your development systems have standardized on setting a NUM_CPU environment variable to the number of processors on each system. A little bit of Python code to access the environment variable and the SetOption function provide the right level of flexibility:

import os
num_cpu = int(os.environ.get('NUM_CPU', 2))
SetOption('num_jobs', num_cpu)
print "running with -j", GetOption('num_jobs')
        

The above snippet of code sets the value of the --jobs option to the value specified in the $NUM_CPU environment variable. (This is one of the exception cases where the string is spelled differently from the from command-line option. The string for fetching or setting the --jobs value is num_jobs for historical reasons.) The code in this example prints the num_jobs value for illustrative purposes. It uses a default value of 2 to provide some minimal parallelism even on single-processor systems:

% scons -Q
running with -j 2
scons: `.' is up to date.

But if the $NUM_CPU environment variable is set, then we use that for the default number of jobs:

% export NUM_CPU="4"
% scons -Q
running with -j 4
scons: `.' is up to date.

But any explicit -j or --jobs value the user specifies an the command line is used first, regardless of whether or not the $NUM_CPU environment variable is set:

% scons -Q -j 7
running with -j 7
scons: `.' is up to date.
% export NUM_CPU="4"
% scons -Q -j 3
running with -j 3
scons: `.' is up to date.

10.1.4. Strings for Getting or Setting Values of SCons Command-Line Options

The strings that you can pass to the GetOption and SetOption functions usually correspond to the first long-form option name (beginning with two hyphen characters: --), after replacing any remaining hyphen characters with underscores.

The full list of strings and the variables they correspond to is as follows:

String for GetOption and SetOptionCommand-Line Option(s)
cache_debug--cache-debug
cache_disable--cache-disable
cache_force--cache-force
cache_show--cache-show
clean-c, --clean, --remove
config--config
directory-C, --directory
diskcheck--diskcheck
duplicate--duplicate
file-f, --file, --makefile , --sconstruct
help-h, --help
ignore_errors--ignore-errors
implicit_cache--implicit-cache
implicit_deps_changed--implicit-deps-changed
implicit_deps_unchanged--implicit-deps-unchanged
interactive--interact, --interactive
keep_going-k, --keep-going
max_drift--max-drift
no_exec-n, --no-exec, --just-print, --dry-run, --recon
no_site_dir--no-site-dir
num_jobs-j, --jobs
profile_file--profile
question-q, --question
random--random
repository-Y, --repository, --srcdir
silent-s, --silent, --quiet
site_dir--site-dir
stack_size--stack-size
taskmastertrace_file--taskmastertrace
warn--warn --warning

10.1.5. Adding Custom Command-Line Options: the AddOption Function

SCons also allows you to define your own command-line options with the AddOption function. The AddOption function takes the same arguments as the optparse.add_option function from the standard Python library. [3] Once you have added a custom command-line option with the AddOption function, the value of the option (if any) is immediately available using the standard GetOption function. (The value can also be set using SetOption, although that's not very useful in practice because a default value can be specified in directly in the AddOption call.)

One useful example of using this functionality is to provide a --prefix for users:

AddOption('--prefix',
          dest='prefix',
          type='string',
          nargs=1,
          action='store',
          metavar='DIR',
          help='installation prefix')

env = Environment(PREFIX = GetOption('prefix'))

installed_foo = env.Install('$PREFIX/usr/bin', 'foo.in')
Default(installed_foo)
        

The above code uses the GetOption function to set the $PREFIX construction variable to any value that the user specifies with a command-line option of --prefix. Because $PREFIX will expand to a null string if it's not initialized, running SCons without the option of --prefix will install the file in the /usr/bin/ directory:

% scons -Q -n
Install file: "foo.in" as "/usr/bin/foo.in"

But specifying --prefix=/tmp/install on the command line causes the file to be installed in the /tmp/install/usr/bin/ directory:

% scons -Q -n --prefix=/tmp/install
Install file: "foo.in" as "/tmp/install/usr/bin/foo.in"

10.2. Command-Line variable=value Build Variables

You may want to control various aspects of your build by allowing the user to specify variable=value values on the command line. For example, suppose you want users to be able to build a debug version of a program by running SCons as follows:

% scons -Q debug=1
    

SCons provides an ARGUMENTS dictionary that stores all of the variable=value assignments from the command line. This allows you to modify aspects of your build in response to specifications on the command line. (Note that unless you want to require that users always specify a variable, you probably want to use the Python ARGUMENTS.get() function, which allows you to specify a default value to be used if there is no specification on the command line.)

The following code sets the $CCFLAGS construction variable in response to the debug flag being set in the ARGUMENTS dictionary:

env = Environment()
debug = ARGUMENTS.get('debug', 0)
if int(debug):
    env.Append(CCFLAGS = '-g')
env.Program('prog.c')
       

This results in the -g compiler option being used when debug=1 is used on the command line:

% scons -Q debug=0
cc -o prog.o -c prog.c
cc -o prog prog.o
% scons -Q debug=0
scons: `.' is up to date.
% scons -Q debug=1
cc -o prog.o -c -g prog.c
cc -o prog prog.o
% scons -Q debug=1
scons: `.' is up to date.

Notice that SCons keeps track of the last values used to build the object files, and as a result correctly rebuilds the object and executable files only when the value of the debug argument has changed.

The ARGUMENTS dictionary has two minor drawbacks. First, because it is a dictionary, it can only store one value for each specified keyword, and thus only "remembers" the last setting for each keyword on the command line. This makes the ARGUMENTS dictionary inappropriate if users should be able to specify multiple values on the command line for a given keyword. Second, it does not preserve the order in which the variable settings were specified, which is a problem if you want the configuration to behave differently in response to the order in which the build variable settings were specified on the command line.

To accomodate these requirements, SCons provides an ARGLIST variable that gives you direct access to variable=value settings on the command line, in the exact order they were specified, and without removing any duplicate settings. Each element in the ARGLIST variable is itself a two-element list containing the keyword and the value of the setting, and you must loop through, or otherwise select from, the elements of ARGLIST to process the specific settings you want in whatever way is appropriate for your configuration. For example, the following code to let the user add to the CPPDEFINES construction variable by specifying multiple define= settings on the command line:

cppdefines = []
for key, value in ARGLIST:
    if key == 'define':
        cppdefines.append(value)
env = Environment(CPPDEFINES = cppdefines)
env.Object('prog.c')
       

Yields the following output:

% scons -Q define=FOO
cc -o prog.o -c -DFOO prog.c
% scons -Q define=FOO define=BAR
cc -o prog.o -c -DFOO -DBAR prog.c

Note that the ARGLIST and ARGUMENTS variables do not interfere with each other, but merely provide slightly different views into how the user specified variable=value settings on the command line. You can use both variables in the same SCons configuration. In general, the ARGUMENTS dictionary is more convenient to use, (since you can just fetch variable settings through a dictionary access), and the ARGLIST list is more flexible (since you can examine the specific order in which the user's command-line variabe settings).

10.2.1. Controlling Command-Line Build Variables

Being able to use a command-line build variable like debug=1 is handy, but it can be a chore to write specific Python code to recognize each such variable, check for errors and provide appropriate messages, and apply the values to a construction variable. To help with this, SCons supports a class to define such build variables easily, and a mechanism to apply the build variables to a construction environment. This allows you to control how the build variables affect construction environments.

For example, suppose that you want users to set a RELEASE construction variable on the command line whenever the time comes to build a program for release, and that the value of this variable should be added to the command line with the appropriate -D option (or other command line option) to pass the value to the C compiler. Here's how you might do that by setting the appropriate value in a dictionary for the $CPPDEFINES construction variable:

vars = Variables(None, ARGUMENTS)
vars.Add('RELEASE', 'Set to 1 to build for release', 0)
env = Environment(variables = vars,
                  CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
env.Program(['foo.c', 'bar.c'])
        

This SConstruct file first creates a Variables object which uses the values from the command-line options dictionary ARGUMENTS (the vars = Variables(None, ARGUMENTS) call). It then uses the object's Add method to indicate that the RELEASE variable can be set on the command line, and that its default value will be 0 (the third argument to the Add method). The second argument is a line of help text; we'll learn how to use it in the next section.

We then pass the created Variables object as a variables keyword argument to the Environment call used to create the construction environment. This then allows a user to set the RELEASE build variable on the command line and have the variable show up in the command line used to build each object from a C source file:

% scons -Q RELEASE=1
cc -o bar.o -c -DRELEASE_BUILD=1 bar.c
cc -o foo.o -c -DRELEASE_BUILD=1 foo.c
cc -o foo foo.o bar.o

NOTE: Before SCons release 0.98.1, these build variables were known as "command-line build options." The class was actually named the Options class, and in the sections below, the various functions were named BoolOption, EnumOption, ListOption, PathOption, PackageOption and AddOptions. These older names still work, and you may encounter them in older SConscript files, but they have been officially deprecated as of SCons version 2.0.

10.2.2. Providing Help for Command-Line Build Variables

To make command-line build variables most useful, you ideally want to provide some help text that will describe the available variables when the user runs scons -h. You could write this text by hand, but SCons provides an easier way. Variables objects support a GenerateHelpText method that will, as its name suggests, generate text that describes the various variables that have been added to it. You then pass the output from this method to the Help function:

vars = Variables(None, ARGUMENTS)
vars.Add('RELEASE', 'Set to 1 to build for release', 0)
env = Environment(variables = vars)
Help(vars.GenerateHelpText(env))
        

SCons will now display some useful text when the -h option is used:

% scons -Q -h

RELEASE: Set to 1 to build for release
    default: 0
    actual: 0

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

Notice that the help output shows the default value, and the current actual value of the build variable.

10.2.3. Reading Build Variables From a File

Giving the user a way to specify the value of a build variable on the command line is useful, but can still be tedious if users must specify the variable every time they run SCons. We can let users provide customized build variable settings in a local file by providing a file name when we create the Variables object:

vars = Variables('custom.py')
vars.Add('RELEASE', 'Set to 1 to build for release', 0)
env = Environment(variables = vars,
                  CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
env.Program(['foo.c', 'bar.c'])
Help(vars.GenerateHelpText(env))
        

This then allows the user to control the RELEASE variable by setting it in the custom.py file:

RELEASE = 1
        

Note that this file is actually executed like a Python script. Now when we run SCons:

% scons -Q
cc -o bar.o -c -DRELEASE_BUILD=1 bar.c
cc -o foo.o -c -DRELEASE_BUILD=1 foo.c
cc -o foo foo.o bar.o

And if we change the contents of custom.py to:

RELEASE = 0
        

The object files are rebuilt appropriately with the new variable:

% scons -Q
cc -o bar.o -c -DRELEASE_BUILD=0 bar.c
cc -o foo.o -c -DRELEASE_BUILD=0 foo.c
cc -o foo foo.o bar.o

Finally, you can combine both methods with:

vars = Variables('custom.py', ARGUMENTS)
      

where values in the option file custom.py get overwritten by the ones specified on the command line.

10.2.4. Pre-Defined Build Variable Functions

SCons provides a number of functions that provide ready-made behaviors for various types of command-line build variables.

10.2.4.1. True/False Values: the BoolVariable Build Variable Function

It's often handy to be able to specify a variable that controls a simple Boolean variable with a true or false value. It would be even more handy to accomodate users who have different preferences for how to represent true or false values. The BoolVariable function makes it easy to accomodate these common representations of true or false.

The BoolVariable function takes three arguments: the name of the build variable, the default value of the build variable, and the help string for the variable. It then returns appropriate information for passing to the Add method of a Variables object, like so:

vars = Variables('custom.py')
vars.Add(BoolVariable('RELEASE', 'Set to build for release', 0))
env = Environment(variables = vars,
                  CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
env.Program('foo.c')
          

With this build variable, the RELEASE variable can now be enabled by setting it to the value yes or t:

% scons -Q RELEASE=yes foo.o
cc -o foo.o -c -DRELEASE_BUILD=True foo.c
% scons -Q RELEASE=t foo.o
cc -o foo.o -c -DRELEASE_BUILD=True foo.c

Other values that equate to true include y, 1, on and all.

Conversely, RELEASE may now be given a false value by setting it to no or f:

% scons -Q RELEASE=no foo.o
cc -o foo.o -c -DRELEASE_BUILD=False foo.c
% scons -Q RELEASE=f foo.o
cc -o foo.o -c -DRELEASE_BUILD=False foo.c

Other values that equate to false include n, 0, off and none.

Lastly, if a user tries to specify any other value, SCons supplies an appropriate error message:

% scons -Q RELEASE=bad_value foo.o

scons: *** Error converting option: RELEASE
Invalid value for boolean option: bad_value
File "/home/my/project/SConstruct", line 4, in <module>

10.2.4.2. Single Value From a List: the EnumVariable Build Variable Function

Suppose that we want a user to be able to set a COLOR variable that selects a background color to be displayed by an application, but that we want to restrict the choices to a specific set of allowed colors. This can be set up quite easily using the EnumVariable, which takes a list of allowed_values in addition to the variable name, default value, and help text arguments:

vars = Variables('custom.py')
vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                    allowed_values=('red', 'green', 'blue')))
env = Environment(variables = vars,
                  CPPDEFINES={'COLOR' : '"${COLOR}"'})
env.Program('foo.c')
          

The user can now explicity set the COLOR build variable to any of the specified allowed values:

% scons -Q COLOR=red foo.o
cc -o foo.o -c -DCOLOR="red" foo.c
% scons -Q COLOR=blue foo.o
cc -o foo.o -c -DCOLOR="blue" foo.c
% scons -Q COLOR=green foo.o
cc -o foo.o -c -DCOLOR="green" foo.c

But, almost more importantly, an attempt to set COLOR to a value that's not in the list generates an error message:

% scons -Q COLOR=magenta foo.o

scons: *** Invalid value for option COLOR: magenta.  Valid values are: ('red', 'green', 'blue')
File "/home/my/project/SConstruct", line 5, in <module>

The EnumVariable function also supports a way to map alternate names to allowed values. Suppose, for example, that we want to allow the user to use the word navy as a synonym for blue. We do this by adding a map dictionary that will map its key values to the desired legal value:

vars = Variables('custom.py')
vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                    allowed_values=('red', 'green', 'blue'),
                    map={'navy':'blue'}))
env = Environment(variables = vars,
                  CPPDEFINES={'COLOR' : '"${COLOR}"'})
env.Program('foo.c')
          

As desired, the user can then use navy on the command line, and SCons will translate it into blue when it comes time to use the COLOR variable to build a target:

% scons -Q COLOR=navy foo.o
cc -o foo.o -c -DCOLOR="blue" foo.c

By default, when using the EnumVariable function, arguments that differ from the legal values only in case are treated as illegal values:

% scons -Q COLOR=Red foo.o

scons: *** Invalid value for option COLOR: Red.  Valid values are: ('red', 'green', 'blue')
File "/home/my/project/SConstruct", line 5, in <module>
% scons -Q COLOR=BLUE foo.o

scons: *** Invalid value for option COLOR: BLUE.  Valid values are: ('red', 'green', 'blue')
File "/home/my/project/SConstruct", line 5, in <module>
% scons -Q COLOR=nAvY foo.o

scons: *** Invalid value for option COLOR: nAvY.  Valid values are: ('red', 'green', 'blue')
File "/home/my/project/SConstruct", line 5, in <module>

The EnumVariable function can take an additional ignorecase keyword argument that, when set to 1, tells SCons to allow case differences when the values are specified:

vars = Variables('custom.py')
vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                    allowed_values=('red', 'green', 'blue'),
                    map={'navy':'blue'},
                    ignorecase=1))
env = Environment(variables = vars,
                  CPPDEFINES={'COLOR' : '"${COLOR}"'})
env.Program('foo.c')
          

Which yields the output:

% scons -Q COLOR=Red foo.o
cc -o foo.o -c -DCOLOR="Red" foo.c
% scons -Q COLOR=BLUE foo.o
cc -o foo.o -c -DCOLOR="BLUE" foo.c
% scons -Q COLOR=nAvY foo.o
cc -o foo.o -c -DCOLOR="blue" foo.c
% scons -Q COLOR=green foo.o
cc -o foo.o -c -DCOLOR="green" foo.c

Notice that an ignorecase value of 1 preserves the case-spelling that the user supplied. If you want SCons to translate the names into lower-case, regardless of the case used by the user, specify an ignorecase value of 2:

vars = Variables('custom.py')
vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                    allowed_values=('red', 'green', 'blue'),
                    map={'navy':'blue'},
                    ignorecase=2))
env = Environment(variables = vars,
                  CPPDEFINES={'COLOR' : '"${COLOR}"'})
env.Program('foo.c')
          

Now SCons will use values of red, green or blue regardless of how the user spells those values on the command line:

% scons -Q COLOR=Red foo.o
cc -o foo.o -c -DCOLOR="red" foo.c
% scons -Q COLOR=nAvY foo.o
cc -o foo.o -c -DCOLOR="blue" foo.c
% scons -Q COLOR=GREEN foo.o
cc -o foo.o -c -DCOLOR="green" foo.c

10.2.4.3. Multiple Values From a List: the ListVariable Build Variable Function

Another way in which you might want to allow users to control a build variable is to specify a list of one or more legal values. SCons supports this through the ListVariable function. If, for example, we want a user to be able to set a COLORS variable to one or more of the legal list of values:

vars = Variables('custom.py')
vars.Add(ListVariable('COLORS', 'List of colors', 0,
                    ['red', 'green', 'blue']))
env = Environment(variables = vars,
                  CPPDEFINES={'COLORS' : '"${COLORS}"'})
env.Program('foo.c')
          

A user can now specify a comma-separated list of legal values, which will get translated into a space-separated list for passing to the any build commands:

% scons -Q COLORS=red,blue foo.o
cc -o foo.o -c -DCOLORS="red blue" foo.c
% scons -Q COLORS=blue,green,red foo.o
cc -o foo.o -c -DCOLORS="blue green red" foo.c

In addition, the ListVariable function allows the user to specify explicit keywords of all or none to select all of the legal values, or none of them, respectively:

% scons -Q COLORS=all foo.o
cc -o foo.o -c -DCOLORS="red green blue" foo.c
% scons -Q COLORS=none foo.o
cc -o foo.o -c -DCOLORS="" foo.c

And, of course, an illegal value still generates an error message:

% scons -Q COLORS=magenta foo.o

scons: *** Error converting option: COLORS
Invalid value(s) for option: magenta
File "/home/my/project/SConstruct", line 5, in <module>

10.2.4.4. Path Names: the PathVariable Build Variable Function

SCons supports a PathVariable function to make it easy to create a build variable to control an expected path name. If, for example, you need to define a variable in the preprocessor that controls the location of a configuration file:

vars = Variables('custom.py')
vars.Add(PathVariable('CONFIG',
                    'Path to configuration file',
                    '/etc/my_config'))
env = Environment(variables = vars,
                  CPPDEFINES={'CONFIG_FILE' : '"$CONFIG"'})
env.Program('foo.c')
          

This then allows the user to override the CONFIG build variable on the command line as necessary:

% scons -Q foo.o
cc -o foo.o -c -DCONFIG_FILE="/etc/my_config" foo.c
% scons -Q CONFIG=/usr/local/etc/other_config foo.o
scons: `foo.o' is up to date.

By default, PathVariable checks to make sure that the specified path exists and generates an error if it doesn't:

% scons -Q CONFIG=/does/not/exist foo.o

scons: *** Path for option CONFIG does not exist: /does/not/exist
File "/home/my/project/SConstruct", line 6, in <module>

PathVariable provides a number of methods that you can use to change this behavior. If you want to ensure that any specified paths are, in fact, files and not directories, use the PathVariable.PathIsFile method:

vars = Variables('custom.py')
vars.Add(PathVariable('CONFIG',
                    'Path to configuration file',
                    '/etc/my_config',
                    PathVariable.PathIsFile))
env = Environment(variables = vars,
                  CPPDEFINES={'CONFIG_FILE' : '"$CONFIG"'})
env.Program('foo.c')
          

Conversely, to ensure that any specified paths are directories and not files, use the PathVariable.PathIsDir method:

vars = Variables('custom.py')
vars.Add(PathVariable('DBDIR',
                    'Path to database directory',
                    '/var/my_dbdir',
                    PathVariable.PathIsDir))
env = Environment(variables = vars,
                  CPPDEFINES={'DBDIR' : '"$DBDIR"'})
env.Program('foo.c')
          

If you want to make sure that any specified paths are directories, and you would like the directory created if it doesn't already exist, use the PathVariable.PathIsDirCreate method:

vars = Variables('custom.py')
vars.Add(PathVariable('DBDIR',
                    'Path to database directory',
                    '/var/my_dbdir',
                    PathVariable.PathIsDirCreate))
env = Environment(variables = vars,
                  CPPDEFINES={'DBDIR' : '"$DBDIR"'})
env.Program('foo.c')
          

Lastly, if you don't care whether the path exists, is a file, or a directory, use the PathVariable.PathAccept method to accept any path that the user supplies:

vars = Variables('custom.py')
vars.Add(PathVariable('OUTPUT',
                    'Path to output file or directory',
                    None,
                    PathVariable.PathAccept))
env = Environment(variables = vars,
                  CPPDEFINES={'OUTPUT' : '"$OUTPUT"'})
env.Program('foo.c')
          

10.2.4.5. Enabled/Disabled Path Names: the PackageVariable Build Variable Function

Sometimes you want to give users even more control over a path name variable, allowing them to explicitly enable or disable the path name by using yes or no keywords, in addition to allow them to supply an explicit path name. SCons supports the PackageVariable function to support this:

vars = Variables('custom.py')
vars.Add(PackageVariable('PACKAGE',
                       'Location package',
                       '/opt/location'))
env = Environment(variables = vars,
                  CPPDEFINES={'PACKAGE' : '"$PACKAGE"'})
env.Program('foo.c')
          

When the SConscript file uses the PackageVariable funciton, user can now still use the default or supply an overriding path name, but can now explicitly set the specified variable to a value that indicates the package should be enabled (in which case the default should be used) or disabled:

% scons -Q foo.o
cc -o foo.o -c -DPACKAGE="/opt/location" foo.c
% scons -Q PACKAGE=/usr/local/location foo.o
cc -o foo.o -c -DPACKAGE="/usr/local/location" foo.c
% scons -Q PACKAGE=yes foo.o
cc -o foo.o -c -DPACKAGE="True" foo.c
% scons -Q PACKAGE=no foo.o
cc -o foo.o -c -DPACKAGE="False" foo.c

10.2.5. Adding Multiple Command-Line Build Variables at Once

Lastly, SCons provides a way to add multiple build variables to a Variables object at once. Instead of having to call the Add method multiple times, you can call the AddVariables method with a list of build variables to be added to the object. Each build variable is specified as either a tuple of arguments, just like you'd pass to the Add method itself, or as a call to one of the pre-defined functions for pre-packaged command-line build variables. in any order:

vars = Variables()
vars.AddVariables(
    ('RELEASE', 'Set to 1 to build for release', 0),
    ('CONFIG', 'Configuration file', '/etc/my_config'),
    BoolVariable('warnings', 'compilation with -Wall and similiar', 1),
    EnumVariable('debug', 'debug output and symbols', 'no',
               allowed_values=('yes', 'no', 'full'),
               map={}, ignorecase=0),  # case sensitive
    ListVariable('shared',
               'libraries to build as shared libraries',
               'all',
               names = list_of_libs),
    PackageVariable('x11',
                  'use X11 installed here (yes = search some places)',
                  'yes'),
    PathVariable('qtdir', 'where the root of Qt is installed', qtdir),
)
        

10.2.6. Handling Unknown Command-Line Build Variables: the UnknownVariables Function

Users may, of course, occasionally misspell variable names in their command-line settings. SCons does not generate an error or warning for any unknown variables the users specifies on the command line. (This is in no small part because you may be processing the arguments directly using the ARGUMENTS dictionary, and therefore SCons can't know in the general case whether a given "misspelled" variable is really unknown and a potential problem, or something that your SConscript file will handle directly with some Python code.)

If, however, you're using a Variables object to define a specific set of command-line build variables that you expect users to be able to set, you may want to provide an error message or warning of your own if the user supplies a variable setting that is not among the defined list of variable names known to the Variables object. You can do this by calling the UnknownVariables method of the Variables object:

vars = Variables(None)
vars.Add('RELEASE', 'Set to 1 to build for release', 0)
env = Environment(variables = vars,
                  CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
unknown = vars.UnknownVariables()
if unknown:
    print "Unknown variables:", unknown.keys()
    Exit(1)
env.Program('foo.c')
        

The UnknownVariables method returns a dictionary containing the keywords and values of any variables the user specified on the command line that are not among the variables known to the Variables object (from having been specified using the Variables object'sAdd method). In the examble above, we check for whether the dictionary returned by the UnknownVariables is non-empty, and if so print the Python list containing the names of the unknwown variables and then call the Exit function to terminate SCons:

% scons -Q NOT_KNOWN=foo
Unknown variables: ['NOT_KNOWN']

Of course, you can process the items in the dictionary returned by the UnknownVariables function in any way appropriate to your build configuration, including just printing a warning message but not exiting, logging an error somewhere, etc.

Note that you must delay the call of UnknownVariables until after you have applied the Variables object to a construction environment with the variables= keyword argument of an Environment call.

10.3. Command-Line Targets

10.3.1. Fetching Command-Line Targets: the COMMAND_LINE_TARGETS Variable

SCons supports a COMMAND_LINE_TARGETS variable that lets you fetch the list of targets that the user specified on the command line. You can use the targets to manipulate the build in any way you wish. As a simple example, suppose that you want to print a reminder to the user whenever a specific program is built. You can do this by checking for the target in the COMMAND_LINE_TARGETS list:

if 'bar' in COMMAND_LINE_TARGETS:
    print "Don't forget to copy `bar' to the archive!"
Default(Program('foo.c'))
Program('bar.c')
        

Then, running SCons with the default target works as it always does, but explicity specifying the bar target on the command line generates the warning message:

% scons -Q
cc -o foo.o -c foo.c
cc -o foo foo.o
% scons -Q bar
Don't forget to copy `bar' to the archive!
cc -o bar.o -c bar.c
cc -o bar bar.o

Another practical use for the COMMAND_LINE_TARGETS variable might be to speed up a build by only reading certain subsidiary SConscript files if a specific target is requested.

10.3.2. Controlling the Default Targets: the Default Function

One of the most basic things you can control is which targets SCons will build by default--that is, when there are no targets specified on the command line. As mentioned previously, SCons will normally build every target in or below the current directory by default--that is, when you don't explicitly specify one or more targets on the command line. Sometimes, however, you may want to specify explicitly that only certain programs, or programs in certain directories, should be built by default. You do this with the Default function:

env = Environment()
hello = env.Program('hello.c')
env.Program('goodbye.c')
Default(hello)
         

This SConstruct file knows how to build two programs, hello and goodbye, but only builds the hello program by default:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q
scons: `hello' is up to date.
% scons -Q goodbye
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o

Note that, even when you use the Default function in your SConstruct file, you can still explicitly specify the current directory (.) on the command line to tell SCons to build everything in (or below) the current directory:

% scons -Q .
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o
cc -o hello.o -c hello.c
cc -o hello hello.o

You can also call the Default function more than once, in which case each call adds to the list of targets to be built by default:

env = Environment()
prog1 = env.Program('prog1.c')
Default(prog1)
prog2 = env.Program('prog2.c')
prog3 = env.Program('prog3.c')
Default(prog3)
         

Or you can specify more than one target in a single call to the Default function:

env = Environment()
prog1 = env.Program('prog1.c')
prog2 = env.Program('prog2.c')
prog3 = env.Program('prog3.c')
Default(prog1, prog3)
      

Either of these last two examples will build only the prog1 and prog3 programs by default:

% scons -Q
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
cc -o prog3.o -c prog3.c
cc -o prog3 prog3.o
% scons -Q .
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o

You can list a directory as an argument to Default:

env = Environment()
env.Program(['prog1/main.c', 'prog1/foo.c'])
env.Program(['prog2/main.c', 'prog2/bar.c'])
Default('prog1')
         

In which case only the target(s) in that directory will be built by default:

% scons -Q
cc -o prog1/foo.o -c prog1/foo.c
cc -o prog1/main.o -c prog1/main.c
cc -o prog1/main prog1/main.o prog1/foo.o
% scons -Q
scons: `prog1' is up to date.
% scons -Q .
cc -o prog2/bar.o -c prog2/bar.c
cc -o prog2/main.o -c prog2/main.c
cc -o prog2/main prog2/main.o prog2/bar.o

Lastly, if for some reason you don't want any targets built by default, you can use the Python None variable:

env = Environment()
prog1 = env.Program('prog1.c')
prog2 = env.Program('prog2.c')
Default(None)
         

Which would produce build output like:

% scons -Q
scons: *** No targets specified and no Default() targets found.  Stop.
Found nothing to build
% scons -Q .
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o

10.3.2.1. Fetching the List of Default Targets: the DEFAULT_TARGETS Variable

SCons supports a DEFAULT_TARGETS variable that lets you get at the current list of default targets. The DEFAULT_TARGETS variable has two important differences from the COMMAND_LINE_TARGETS variable. First, the DEFAULT_TARGETS variable is a list of internal SCons nodes, so you need to convert the list elements to strings if you want to print them or look for a specific target name. Fortunately, you can do this easily by using the Python map function to run the list through str:

prog1 = Program('prog1.c')
Default(prog1)
print "DEFAULT_TARGETS is", map(str, DEFAULT_TARGETS)
           

(Keep in mind that all of the manipulation of the DEFAULT_TARGETS list takes place during the first phase when SCons is reading up the SConscript files, which is obvious if we leave off the -Q flag when we run SCons:)

% scons
scons: Reading SConscript files ...
DEFAULT_TARGETS is ['prog1']
scons: done reading SConscript files.
scons: Building targets ...
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
scons: done building targets.

Second, the contents of the DEFAULT_TARGETS list change in response to calls to the Default function, as you can see from the following SConstruct file:

prog1 = Program('prog1.c')
Default(prog1)
print "DEFAULT_TARGETS is now", map(str, DEFAULT_TARGETS)
prog2 = Program('prog2.c')
Default(prog2)
print "DEFAULT_TARGETS is now", map(str, DEFAULT_TARGETS)
           

Which yields the output:

% scons
scons: Reading SConscript files ...
DEFAULT_TARGETS is now ['prog1']
DEFAULT_TARGETS is now ['prog1', 'prog2']
scons: done reading SConscript files.
scons: Building targets ...
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o
scons: done building targets.

In practice, this simply means that you need to pay attention to the order in which you call the Default function and refer to the DEFAULT_TARGETS list, to make sure that you don't examine the list before you've added the default targets you expect to find in it.

10.3.3. Fetching the List of Build Targets, Regardless of Origin: the BUILD_TARGETS Variable

We've already been introduced to the COMMAND_LINE_TARGETS variable, which contains a list of targets specified on the command line, and the DEFAULT_TARGETS variable, which contains a list of targets specified via calls to the Default method or function. Sometimes, however, you want a list of whatever targets SCons will try to build, regardless of whether the targets came from the command line or a Default call. You could code this up by hand, as follows:

if COMMAND_LINE_TARGETS:
    targets = COMMAND_LINE_TARGETS
else:
    targets = DEFAULT_TARGETS
      

SCons, however, provides a convenient BUILD_TARGETS variable that eliminates the need for this by-hand manipulation. Essentially, the BUILD_TARGETS variable contains a list of the command-line targets, if any were specified, and if no command-line targets were specified, it contains a list of the targets specified via the Default method or function.

Because BUILD_TARGETS may contain a list of SCons nodes, you must convert the list elements to strings if you want to print them or look for a specific target name, just like the DEFAULT_TARGETS list:

prog1 = Program('prog1.c')
Program('prog2.c')
Default(prog1)
print "BUILD_TARGETS is", map(str, BUILD_TARGETS)
        

Notice how the value of BUILD_TARGETS changes depending on whether a target is specified on the command line:

% scons -Q
BUILD_TARGETS is ['prog1']
cc -o prog1.o -c prog1.c
cc -o prog1 prog1.o
% scons -Q prog2
BUILD_TARGETS is ['prog2']
cc -o prog2.o -c prog2.c
cc -o prog2 prog2.o
% scons -Q -c .
BUILD_TARGETS is ['.']
Removed prog1.o
Removed prog1
Removed prog2.o
Removed prog2


[3] The AddOption function is, in fact, implemented using a subclass of the optparse.OptionParser.

Chapter 11. Installing Files in Other Directories: the Install Builder

Once a program is built, it is often appropriate to install it in another directory for public use. You use the Install method to arrange for a program, or any other file, to be copied into a destination directory:

env = Environment()
hello = env.Program('hello.c')
env.Install('/usr/bin', hello)
     

Note, however, that installing a file is still considered a type of file "build." This is important when you remember that the default behavior of SCons is to build files in or below the current directory. If, as in the example above, you are installing files in a directory outside of the top-level SConstruct file's directory tree, you must specify that directory (or a higher directory, such as /) for it to install anything there:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q /usr/bin
Install file: "hello" as "/usr/bin/hello"

It can, however, be cumbersome to remember (and type) the specific destination directory in which the program (or any other file) should be installed. This is an area where the Alias function comes in handy, allowing you, for example, to create a pseudo-target named install that can expand to the specified destination directory:

env = Environment()
hello = env.Program('hello.c')
env.Install('/usr/bin', hello)
env.Alias('install', '/usr/bin')
    

This then yields the more natural ability to install the program in its destination as follows:

% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q install
Install file: "hello" as "/usr/bin/hello"

11.1. Installing Multiple Files in a Directory

You can install multiple files into a directory simply by calling the Install function multiple times:

env = Environment()
hello = env.Program('hello.c')
goodbye = env.Program('goodbye.c')
env.Install('/usr/bin', hello)
env.Install('/usr/bin', goodbye)
env.Alias('install', '/usr/bin')
      

Or, more succinctly, listing the multiple input files in a list (just like you can do with any other builder):

env = Environment()
hello = env.Program('hello.c')
goodbye = env.Program('goodbye.c')
env.Install('/usr/bin', [hello, goodbye])
env.Alias('install', '/usr/bin')
    

Either of these two examples yields:

% scons -Q install
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o
Install file: "goodbye" as "/usr/bin/goodbye"
cc -o hello.o -c hello.c
cc -o hello hello.o
Install file: "hello" as "/usr/bin/hello"

11.2. Installing a File Under a Different Name

The Install method preserves the name of the file when it is copied into the destination directory. If you need to change the name of the file when you copy it, use the InstallAs function:

env = Environment()
hello = env.Program('hello.c')
env.InstallAs('/usr/bin/hello-new', hello)
env.Alias('install', '/usr/bin')
      

This installs the hello program with the name hello-new as follows:

% scons -Q install
cc -o hello.o -c hello.c
cc -o hello hello.o
Install file: "hello" as "/usr/bin/hello-new"

11.3. Installing Multiple Files Under Different Names

Lastly, if you have multiple files that all need to be installed with different file names, you can either call the InstallAs function multiple times, or as a shorthand, you can supply same-length lists for both the target and source arguments:

env = Environment()
hello = env.Program('hello.c')
goodbye = env.Program('goodbye.c')
env.InstallAs(['/usr/bin/hello-new',
               '/usr/bin/goodbye-new'],
               [hello, goodbye])
env.Alias('install', '/usr/bin')
      

In this case, the InstallAs function loops through both lists simultaneously, and copies each source file into its corresponding target file name:

% scons -Q install
cc -o goodbye.o -c goodbye.c
cc -o goodbye goodbye.o
Install file: "goodbye" as "/usr/bin/goodbye-new"
cc -o hello.o -c hello.c
cc -o hello hello.o
Install file: "hello" as "/usr/bin/hello-new"

Chapter 12. Platform-Independent File System Manipulation

SCons provides a number of platform-independent functions, called factories, that perform common file system manipulations like copying, moving or deleting files and directories, or making directories. These functions are factories because they don't perform the action at the time they're called, they each return an Action object that can be executed at the appropriate time.

12.1. Copying Files or Directories: The Copy Factory

Suppose you want to arrange to make a copy of a file, and don't have a suitable pre-existing builder. [4] One way would be to use the Copy action factory in conjunction with the Command builder:

Command("file.out", "file.in", Copy("$TARGET", "$SOURCE"))
      

Notice that the action returned by the Copy factory will expand the $TARGET and $SOURCE strings at the time file.out is built, and that the order of the arguments is the same as that of a builder itself--that is, target first, followed by source:

% scons -Q
Copy("file.out", "file.in")

You can, of course, name a file explicitly instead of using $TARGET or $SOURCE:

Command("file.out", [], Copy("$TARGET", "file.in"))
      

Which executes as:

% scons -Q
Copy("file.out", "file.in")

The usefulness of the Copy factory becomes more apparent when you use it in a list of actions passed to the Command builder. For example, suppose you needed to run a file through a utility that only modifies files in-place, and can't "pipe" input to output. One solution is to copy the source file to a temporary file name, run the utility, and then copy the modified temporary file to the target, which the Copy factory makes extremely easy:

Command("file.out", "file.in",
        [
          Copy("tempfile", "$SOURCE"),
          "modify tempfile",
          Copy("$TARGET", "tempfile"),
        ])
      

The output then looks like:

% scons -Q
Copy("tempfile", "file.in")
modify tempfile
Copy("file.out", "tempfile")

The Copy factory has a third optional argument which controls how symlinks are copied.

# Symbolic link shallow copied as a new symbolic link:
Command("LinkIn", "LinkOut", Copy("$TARGET", "$SOURCE"[, True]))

# Symbolic link target copied as a file or directory:
Command("LinkIn", "FileOrDirectoryOut", Copy("$TARGET", "$SOURCE", False))
      

12.2. Deleting Files or Directories: The Delete Factory

If you need to delete a file, then the Delete factory can be used in much the same way as the Copy factory. For example, if we want to make sure that the temporary file in our last example doesn't exist before we copy to it, we could add Delete to the beginning of the command list:

Command("file.out", "file.in",
        [
          Delete("tempfile"),
          Copy("tempfile", "$SOURCE"),
          "modify tempfile",
          Copy("$TARGET", "tempfile"),
        ])
      

Which then executes as follows:

% scons -Q
Delete("tempfile")
Copy("tempfile", "file.in")
modify tempfile
Copy("file.out", "tempfile")

Of course, like all of these Action factories, the Delete factory also expands $TARGET and $SOURCE variables appropriately. For example:

Command("file.out", "file.in",
        [
          Delete("$TARGET"),
          Copy("$TARGET", "$SOURCE")
        ])
      

Executes as:

% scons -Q
Delete("file.out")
Copy("file.out", "file.in")

Note, however, that you typically don't need to call the Delete factory explicitly in this way; by default, SCons deletes its target(s) for you before executing any action.

One word of caution about using the Delete factory: it has the same variable expansions available as any other factory, including the $SOURCE variable. Specifying Delete("$SOURCE") is not something you usually want to do!

12.3. Moving (Renaming) Files or Directories: The Move Factory

The Move factory allows you to rename a file or directory. For example, if we don't want to copy the temporary file, we could use:

Command("file.out", "file.in",
        [
          Copy("tempfile", "$SOURCE"),
          "modify tempfile",
          Move("$TARGET", "tempfile"),
        ])
      

Which would execute as:

% scons -Q
Copy("tempfile", "file.in")
modify tempfile
Move("file.out", "tempfile")

12.4. Updating the Modification Time of a File: The Touch Factory

If you just need to update the recorded modification time for a file, use the Touch factory:

Command("file.out", "file.in",
        [
          Copy("$TARGET", "$SOURCE"),
          Touch("$TARGET"),
        ])
      

Which executes as:

% scons -Q
Copy("file.out", "file.in")
Touch("file.out")

12.5. Creating a Directory: The Mkdir Factory

If you need to create a directory, use the Mkdir factory. For example, if we need to process a file in a temporary directory in which the processing tool will create other files that we don't care about, you could use:

Command("file.out", "file.in",
        [
          Delete("tempdir"),
          Mkdir("tempdir"),
          Copy("tempdir/${SOURCE.file}", "$SOURCE"),
          "process tempdir",
          Move("$TARGET", "tempdir/output_file"),
          Delete("tempdir"),
        ])
      

Which executes as:

% scons -Q
Delete("tempdir")
Mkdir("tempdir")
Copy("tempdir/file.in", "file.in")
process tempdir
Move("file.out", "tempdir/output_file")
scons: *** [file.out] tempdir/output_file: No such file or directory

12.6. Changing File or Directory Permissions: The Chmod Factory

To change permissions on a file or directory, use the Chmod factory. The permission argument uses POSIX-style permission bits and should typically be expressed as an octal, not decimal, number:

Command("file.out", "file.in",
        [
          Copy("$TARGET", "$SOURCE"),
          Chmod("$TARGET", 0755),
        ])
      

Which executes:

% scons -Q
Copy("file.out", "file.in")
Chmod("file.out", 0755)

12.7. Executing an action immediately: the Execute Function

We've been showing you how to use Action factories in the Command function. You can also execute an Action returned by a factory (or actually, any Action) at the time the SConscript file is read by using the Execute function. For example, if we need to make sure that a directory exists before we build any targets,

Execute(Mkdir('/tmp/my_temp_directory'))
      

Notice that this will create the directory while the SConscript file is being read:

% scons
scons: Reading SConscript files ...
Mkdir("/tmp/my_temp_directory")
scons: done reading SConscript files.
scons: Building targets ...
scons: `.' is up to date.
scons: done building targets.

If you're familiar with Python, you may wonder why you would want to use this instead of just calling the native Python os.mkdir() function. The advantage here is that the Mkdir action will behave appropriately if the user specifies the SCons -n or -q options--that is, it will print the action but not actually make the directory when -n is specified, or make the directory but not print the action when -q is specified.

The Execute function returns the exit status or return value of the underlying action being executed. It will also print an error message if the action fails and returns a non-zero value. SCons will not, however, actually stop the build if the action fails. If you want the build to stop in response to a failure in an action called by Execute, you must do so by explicitly checking the return value and calling the Exit function (or a Python equivalent):

if Execute(Mkdir('/tmp/my_temp_directory')):
    # A problem occurred while making the temp directory.
    Exit(1)
    


[4] Unfortunately, in the early days of SCons design, we used the name Copy for the function that returns a copy of the environment, otherwise that would be the logical choice for a Builder that copies a file or directory tree to a target location.

Chapter 13. Controlling Removal of Targets

There are two occasions when SCons will, by default, remove target files. The first is when SCons determines that an target file needs to be rebuilt and removes the existing version of the target before executing The second is when SCons is invoked with the -c option to "clean" a tree of its built targets. These behaviours can be suppressed with the Precious and NoClean functions, respectively.

13.1. Preventing target removal during build: the Precious Function

By default, SCons removes targets before building them. Sometimes, however, this is not what you want. For example, you may want to update a library incrementally, not by having it deleted and then rebuilt from all of the constituent object files. In such cases, you can use the Precious method to prevent SCons from removing the target before it is built:

  env = Environment(RANLIBCOM='')
  lib = env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
  env.Precious(lib)
      

Although the output doesn't look any different, SCons does not, in fact, delete the target library before rebuilding it:

% scons -Q
cc -o f1.o -c f1.c
cc -o f2.o -c f2.c
cc -o f3.o -c f3.c
ar rc libfoo.a f1.o f2.o f3.o

SCons will, however, still delete files marked as Precious when the -c option is used.

13.2. Preventing target removal during clean: the NoClean Function

By default, SCons removes all built targets when invoked with the -c option to clean a source tree of built targets. Sometimes, however, this is not what you want. For example, you may want to remove only intermediate generated files (such as object files), but leave the final targets (the libraries) untouched. In such cases, you can use the NoClean method to prevent SCons from removing a target during a clean:

env = Environment(RANLIBCOM='')
lib = env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
env.NoClean(lib)
      

Notice that the libfoo.a is not listed as a removed file:

% scons -Q
cc -o f1.o -c f1.c
cc -o f2.o -c f2.c
cc -o f3.o -c f3.c
ar rc libfoo.a f1.o f2.o f3.o
% scons -c
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Cleaning targets ...
Removed f1.o
Removed f2.o
Removed f3.o
scons: done cleaning targets.

13.3. Removing additional files during clean: the Clean Function

There may be additional files that you want removed when the -c option is used, but which SCons doesn't know about because they're not normal target files. For example, perhaps a command you invoke creates a log file as part of building the target file you want. You would like the log file cleaned, but you don't want to have to teach SCons that the command "builds" two files.

You can use the Clean function to arrange for additional files to be removed when the -c option is used. Notice, however, that the Clean function takes two arguments, and the second argument is the name of the additional file you want cleaned (foo.log in this example):

t = Command('foo.out', 'foo.in', 'build -o $TARGET $SOURCE')
Clean(t, 'foo.log')
      

The first argument is the target with which you want the cleaning of this additional file associated. In the above example, we've used the return value from the Command function, which represents the foo.out target. Now whenever the foo.out target is cleaned by the -c option, the foo.log file will be removed as well:

% scons -Q
build -o foo.out foo.in
% scons -Q -c
Removed foo.out
Removed foo.log

Chapter 14. Hierarchical Builds

The source code for large software projects rarely stays in a single directory, but is nearly always divided into a hierarchy of directories. Organizing a large software build using SCons involves creating a hierarchy of build scripts using the SConscript function.

14.1. SConscript Files

As we've already seen, the build script at the top of the tree is called SConstruct. The top-level SConstruct file can use the SConscript function to include other subsidiary scripts in the build. These subsidiary scripts can, in turn, use the SConscript function to include still other scripts in the build. By convention, these subsidiary scripts are usually named SConscript. For example, a top-level SConstruct file might arrange for four subsidiary scripts to be included in the build as follows:

SConscript(['drivers/display/SConscript',
            'drivers/mouse/SConscript',
            'parser/SConscript',
            'utilities/SConscript'])
    

In this case, the SConstruct file lists all of the SConscript files in the build explicitly. (Note, however, that not every directory in the tree necessarily has an SConscript file.) Alternatively, the drivers subdirectory might contain an intermediate SConscript file, in which case the SConscript call in the top-level SConstruct file would look like:

SConscript(['drivers/SConscript',
            'parser/SConscript',
            'utilities/SConscript'])
    

And the subsidiary SConscript file in the drivers subdirectory would look like:

SConscript(['display/SConscript',
            'mouse/SConscript'])
    

Whether you list all of the SConscript files in the top-level SConstruct file, or place a subsidiary SConscript file in intervening directories, or use some mix of the two schemes, is up to you and the needs of your software.

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 the 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).

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, unlike standard recursive use of Make, 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.

14.3. Top-Level Path Names in Subsidiary SConscript Files

If you need to use a file from another directory, it's sometimes more convenient to specify the path to a file in another directory from the top-level SConstruct directory, even when you're using that file in a subsidiary SConscript file in a subdirectory. You can tell SCons to interpret a path name as relative to the top-level SConstruct directory, not the local directory of the SConscript file, by appending a # (hash mark) to the beginning of the path name:

env = Environment()
env.Program('prog', ['main.c', '#lib/foo1.c', 'foo2.c'])
       

In this example, the lib directory is directly underneath the top-level SConstruct directory. If the above SConscript file is in a subdirectory named src/prog, the output would look like:

% scons -Q
cc -o lib/foo1.o -c lib/foo1.c
cc -o src/prog/foo2.o -c src/prog/foo2.c
cc -o src/prog/main.o -c src/prog/main.c
cc -o src/prog/prog src/prog/main.o lib/foo1.o src/prog/foo2.o

(Notice that the lib/foo1.o object file is built in the same directory as its source file. See Chapter 15, Separating Source and Build Directories, below, for information about how to build the object file in a different subdirectory.)

14.4. Absolute Path Names

Of course, you can always specify an absolute path name for a file--for example:

env = Environment()
env.Program('prog', ['main.c', '/usr/joe/lib/foo1.c', 'foo2.c'])
       

Which, when executed, would yield:

% scons -Q
cc -o src/prog/foo2.o -c src/prog/foo2.c
cc -o src/prog/main.o -c src/prog/main.c
cc -o /usr/joe/lib/foo1.o -c /usr/joe/lib/foo1.c
cc -o src/prog/prog src/prog/main.o /usr/joe/lib/foo1.o src/prog/foo2.o

(As was the case with top-relative path names, notice that the /usr/joe/lib/foo1.o object file is built in the same directory as its source file. See Chapter 15, Separating Source and Build Directories, below, for information about how to build the object file in a different subdirectory.)

14.5. Sharing Environments (and Other Variables) Between SConscript Files

In the previous example, each of the subsidiary SConscript files created its own construction environment by calling Environment separately. This obviously works fine, but if each program must be built with the same construction variables, it's cumbersome and error-prone to initialize separate construction environments in the same way over and over in each subsidiary SConscript file.

SCons supports the ability to export variables from a parent SConscript file to its subsidiary SConscript files, which allows you to share common initialized values throughout your build hierarchy.

14.5.1. Exporting Variables

There are two ways to export a variable, such as a construction environment, from an SConscript file, so that it may be used by other SConscript files. First, you can call the Export function with a list of variables, or a string of white-space separated variable names. Each call to Export adds one or more variables to a global list of variables that are available for import by other SConscript files.

env = Environment()
Export('env')
      

You may export more than one variable name at a time:

env = Environment()
debug = ARGUMENTS['debug']
Export('env', 'debug')
      

Because white space is not legal in Python variable names, the Export function will even automatically split a string into separate names for you:

Export('env debug')
      

Second, you can specify a list of variables to export as a second argument to the SConscript function call:

SConscript('src/SConscript', 'env')
      

Or as the exports keyword argument:

SConscript('src/SConscript', exports='env')
      

These calls export the specified variables to only the listed SConscript files. You may, however, specify more than one SConscript file in a list:

SConscript(['src1/SConscript',
            'src2/SConscript'], exports='env')
      

This is functionally equivalent to calling the SConscript function multiple times with the same exports argument, one per SConscript file.

14.5.2. Importing Variables

Once a variable has been exported from a calling SConscript file, it may be used in other SConscript files by calling the Import function:

Import('env')
env.Program('prog', ['prog.c'])
      

The Import call makes the env construction environment available to the SConscript file, after which the variable can be used to build programs, libraries, etc.

Like the Export function, the Import function can be used with multiple variable names:

Import('env', 'debug')
env = env.Clone(DEBUG = debug)
env.Program('prog', ['prog.c'])
      

And the Import function will similarly split a string along white-space into separate variable names:

Import('env debug')
env = env.Clone(DEBUG = debug)
env.Program('prog', ['prog.c'])
      

Lastly, as a special case, you may import all of the variables that have been exported by supplying an asterisk to the Import function:

Import('*')
env = env.Clone(DEBUG = debug)
env.Program('prog', ['prog.c'])
      

If you're dealing with a lot of SConscript files, this can be a lot simpler than keeping arbitrary lists of imported variables in each file.

14.5.3. Returning Values From an SConscript File

Sometimes, you would like to be able to use information from a subsidiary SConscript file in some way. For example, suppose that you want to create one library from source files scattered throughout a number of subsidiary SConscript files. You can do this by using the Return function to return values from the subsidiary SConscript files to the calling file.

If, for example, we have two subdirectories foo and bar that should each contribute a source file to a Library, what we'd like to be able to do is collect the object files from the subsidiary SConscript calls like this:

env = Environment()
Export('env')
objs = []
for subdir in ['foo', 'bar']:
    o = SConscript('%s/SConscript' % subdir)
    objs.append(o)
env.Library('prog', objs)
        

We can do this by using the Return function in the foo/SConscript file like this:

Import('env')
obj = env.Object('foo.c')
Return('obj')
        

(The corresponding bar/SConscript file should be pretty obvious.) Then when we run SCons, the object files from the subsidiary subdirectories are all correctly archived in the desired library:

% scons -Q
cc -o bar/bar.o -c bar/bar.c
cc -o foo/foo.o -c foo/foo.c
ar rc libprog.a foo/foo.o bar/bar.o
ranlib libprog.a

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. That name is still supported but has been deprecated 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.

15.2. Why SCons Duplicates Source Files in a Variant Directory Tree

SCons duplicates source files in variant directory trees because it's the most straightforward way to guarantee a correct build regardless of include-file directory paths, relative references between files, or tool support for putting files in different locations, and the SCons philosophy is to, by default, guarantee a correct build in all cases.

The most direct reason to duplicate source files in variant directories is simply that some tools (mostly older versions) are written to only build their output files in the same directory as the source files. In this case, the choices are either to build the output file in the source directory and move it to the variant directory, or to duplicate the source files in the variant directory.

Additionally, relative references between files can cause problems if we don't just duplicate the hierarchy of source files in the variant directory. You can see this at work in use of the C preprocessor #include mechanism with double quotes, not angle brackets:

#include "file.h"
    

The de facto standard behavior for most C compilers in this case is to first look in the same directory as the source file that contains the #include line, then to look in the directories in the preprocessor search path. Add to this that the SCons implementation of support for code repositories (described below) means not all of the files will be found in the same directory hierarchy, and the simplest way to make sure that the right include file is found is to duplicate the source files into the variant directory, which provides a correct build regardless of the original location(s) of the source files.

Although source-file duplication guarantees a correct build even in these end-cases, it can usually be safely disabled. The next section describes how you can disable the duplication of source files in the variant directory.

15.3. Telling SCons to Not Duplicate Source Files in the Variant Directory Tree

In most cases and with most tool sets, SCons can place its target files in a build subdirectory without duplicating the source files and everything will work just fine. You can disable the default SCons behavior by specifying duplicate=0 when you call the SConscript function:

SConscript('src/SConscript', variant_dir='build', duplicate=0)
    

When this flag is specified, SCons uses the variant directory like most people expect--that is, the output files are placed in the variant directory while the source files stay in the source directory:

% ls src
SConscript
hello.c
% scons -Q
cc -c src/hello.c -o build/hello.o
cc -o build/hello build/hello.o
% ls build
hello
hello.o
    

15.4. The VariantDir Function

Use the VariantDir function to establish that target files should be built in a separate directory from the source files:

VariantDir('build', 'src')
env = Environment()
env.Program('build/hello.c')
      

Note that when you're not using an SConscript file in the src subdirectory, you must actually specify that the program must be built from the build/hello.c file that SCons will duplicate in the build subdirectory.

When using the VariantDir function directly, SCons still duplicates the source files in the variant directory by default:

% ls src
hello.c
% scons -Q
cc -o build/hello.o -c build/hello.c
cc -o build/hello build/hello.o
% ls build
hello  hello.c  hello.o

You can specify the same duplicate=0 argument that you can specify for an SConscript call:

VariantDir('build', 'src', duplicate=0)
env = Environment()
env.Program('build/hello.c')
      

In which case SCons will disable duplication of the source files:

% ls src
hello.c
% scons -Q
cc -o build/hello.o -c src/hello.c
cc -o build/hello build/hello.o
% ls build
hello  hello.o

15.5. Using VariantDir With an SConscript File

Even when using the VariantDir function, it's much more natural to use it with a subsidiary SConscript file. For example, if the src/SConscript looks like this:

env = Environment()
env.Program('hello.c')
      

Then our SConstruct file could look like:

VariantDir('build', 'src')
SConscript('build/SConscript')
      

Yielding the following output:

% 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

Notice that this is completely equivalent to the use of SConscript that we learned about in the previous section.

15.6. Using Glob with VariantDir

The Glob file name pattern matching function works just as usual when using VariantDir. For example, if the src/SConscript looks like this:

env = Environment()
env.Program('hello', Glob('*.c'))
      

Then with the same SConstruct file as in the previous section, and source files f1.c and f2.c in src, we would see the following output:

% ls src
SConscript  f1.c  f2.c  f2.h
% scons -Q
cc -o build/f1.o -c build/f1.c
cc -o build/f2.o -c build/f2.c
cc -o build/hello build/f1.o build/f2.o
% ls build
SConscript  f1.c  f1.o  f2.c  f2.h  f2.o  hello

The Glob function returns Nodes in the build/ tree, as you'd expect.

Chapter 16. Variant Builds

The variant_dir keyword argument of the SConscript function provides everything we need to show how easy it is to create variant builds using SCons. Suppose, for example, that we want to build a program for both Windows and Linux platforms, but that we want to build it in a shared directory with separate side-by-side build directories for the Windows and Linux versions of the program.

platform = ARGUMENTS.get('OS', Platform())

include = "#export/$PLATFORM/include"
lib = "#export/$PLATFORM/lib"
bin = "#export/$PLATFORM/bin"

env = Environment(PLATFORM = platform,
                  BINDIR = bin,
                  INCDIR = include,
                  LIBDIR = lib,
                  CPPPATH = [include],
                  LIBPATH = [lib],
                  LIBS = 'world')

Export('env')

env.SConscript('src/SConscript', variant_dir='build/$PLATFORM')
    

This SConstruct file, when run on a Linux system, yields:

% scons -Q OS=linux
Install file: "build/linux/world/world.h" as "export/linux/include/world.h"
cc -o build/linux/hello/hello.o -c -Iexport/linux/include build/linux/hello/hello.c
cc -o build/linux/world/world.o -c -Iexport/linux/include build/linux/world/world.c
ar rc build/linux/world/libworld.a build/linux/world/world.o
ranlib build/linux/world/libworld.a
Install file: "build/linux/world/libworld.a" as "export/linux/lib/libworld.a"
cc -o build/linux/hello/hello build/linux/hello/hello.o -Lexport/linux/lib -lworld
Install file: "build/linux/hello/hello" as "export/linux/bin/hello"

The same SConstruct file on Windows would build:

C:\>scons -Q OS=windows
Install file: "build/windows/world/world.h" as "export/windows/include/world.h"
cl /Fobuild\windows\hello\hello.obj /c build\windows\hello\hello.c /nologo /Iexport\windows\include
cl /Fobuild\windows\world\world.obj /c build\windows\world\world.c /nologo /Iexport\windows\include
lib /nologo /OUT:build\windows\world\world.lib build\windows\world\world.obj
Install file: "build/windows/world/world.lib" as "export/windows/lib/world.lib"
link /nologo /OUT:build\windows\hello\hello.exe /LIBPATH:export\windows\lib world.lib build\windows\hello\hello.obj
embedManifestExeCheck(target, source, env)
Install file: "build/windows/hello/hello.exe" as "export/windows/bin/hello.exe"

Chapter 17. Internationalization and localization with gettext

The gettext toolset supports internationalization and localization of SCons-based projects. Builders provided by gettext automatize generation and updates of translation files. You can manage translations and translation templates similarly to how it's done with autotools.

17.1. Prerequisites

To follow examples provided in this chapter set up your operating system to support two or more languages. In following examples we use locales en_US, de_DE, and pl_PL.

Ensure, that you have GNU gettext utilities installed on your system.

To edit translation files you may wish to install poedit editor.

17.2. Simple project

Let's start with a very simple project, the "Hello world" program for example

/* hello.c */
#include <stdio.h>
int main(int argc, char* argv[])
{
  printf("Hello world\n");
  return 0;
}
    

Prepare a SConstruct to compile the program as usual.

# SConstruct
env = Environment()
hello = Program(["hello.c"])
    

Now we'll convert the project to a multi-lingual one. If you don't already have GNU gettext utilities installed, install them from your preffered package repository, or download from http://ftp.gnu.org/gnu/gettext/. For the purpose of this example, you should have following three locales installed on your system: en_US, de_DE and pl_PL. On debian, for example, you may enable certain locales through dpkg-reconfigure locales.

First prepare the hello.c program for internationalization. Change the previous code so it reads as follows:

/* hello.c */
#include <stdio.h>
#include <libintl.h>
#include <locale.h>
int main(int argc, char* argv[])
{
  bindtextdomain("hello", "locale");
  setlocale(LC_ALL, "");
  textdomain("hello");
  printf(gettext("Hello world\n"));
  return 0;
}
    

Detailed recipes for such conversion can be found at http://www.gnu.org/software/gettext/manual/gettext.html#Sources. The gettext("...") has two purposes. First, it marks messages for the xgettext(1) program, which we will use to extract from the sources the messages for localization. Second, it calls the gettext library internals to translate the message at runtime.

Now we shall instruct SCons how to generate and maintain translation files. For that, use the Translate builder and MOFiles builder. The first one takes source files, extracts internationalized messages from them, creates so-called POT file (translation template), and then creates PO translation files, one for each requested language. Later, during the development lifecycle, the builder keeps all these files up-to date. The MOFiles builder compiles the PO files to binary form. Then install the MO files under directory called locale.

The completed SConstruct is as follows:

# SConstruct
env = Environment( tools = ['default', 'gettext'] )
hello = env.Program(["hello.c"])
env['XGETTEXTFLAGS'] = [
  '--package-name=%s' % 'hello',
  '--package-version=%s' % '1.0',
]
po = env.Translate(["pl","en", "de"], ["hello.c"], POAUTOINIT = 1)
mo = env.MOFiles(po)
InstallAs(["locale/en/LC_MESSAGES/hello.mo"], ["en.mo"])
InstallAs(["locale/pl/LC_MESSAGES/hello.mo"], ["pl.mo"])
InstallAs(["locale/de/LC_MESSAGES/hello.mo"], ["de.mo"])
    

Generate the translation files with scons po-update. You should see the output from SCons simillar to this:

user@host:$ scons po-update
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
Entering '/home/ptomulik/projects/tmp'
xgettext --package-name=hello --package-version=1.0 -o - hello.c
Leaving '/home/ptomulik/projects/tmp'
Writting 'messages.pot' (new file)
msginit --no-translator -l pl -i messages.pot -o pl.po
Created pl.po.
msginit --no-translator -l en -i messages.pot -o en.po
Created en.po.
msginit --no-translator -l de -i messages.pot -o de.po
Created de.po.
scons: done building targets.
    

If everything is right, you should see following new files.

user@host:$ ls *.po*
de.po  en.po  messages.pot  pl.po
    

Open en.po in poedit and provide the English translation to message "Hello world\n". Do the same for de.po (deutsch) and pl.po (polish). Let the translations be, for example:

  • en: "Welcome to beautiful world!\n"

  • de: "Hallo Welt!\n"

  • pl: "Witaj swiecie!\n"

Now compile the project by executing scons. The output should be similar to this:

user@host:$ scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
msgfmt -c -o de.mo de.po
msgfmt -c -o en.mo en.po
gcc -o hello.o -c hello.c
gcc -o hello hello.o
Install file: "de.mo" as "locale/de/LC_MESSAGES/hello.mo"
Install file: "en.mo" as "locale/en/LC_MESSAGES/hello.mo"
msgfmt -c -o pl.mo pl.po
Install file: "pl.mo" as "locale/pl/LC_MESSAGES/hello.mo"
scons: done building targets.
    

SCons automatically compiled the PO files to binary format MO, and the InstallAs lines installed these files under locale folder.

Your program should be now ready. You may try it as follows (linux):

user@host:$ LANG=en_US.UTF-8 ./hello
Welcome to beautiful world
    

user@host:$ LANG=de_DE.UTF-8 ./hello
Hallo Welt
    

user@host:$ LANG=pl_PL.UTF-8 ./hello
Witaj swiecie
    

To demonstrate the further life of translation files, let's change Polish translation (poedit pl.po) to "Witaj drogi swiecie\n". Run scons to see how scons reacts to this

user@host:$scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
msgfmt -c -o pl.mo pl.po
Install file: "pl.mo" as "locale/pl/LC_MESSAGES/hello.mo"
scons: done building targets.
    

Now, open hello.c and add another one printf line with new message.

/* hello.c */
#include <stdio.h>
#include <libintl.h>
#include <locale.h>
int main(int argc, char* argv[])
{
  bindtextdomain("hello", "locale");
  setlocale(LC_ALL, "");
  textdomain("hello");
  printf(gettext("Hello world\n"));
  printf(gettext("and good bye\n"));
  return 0;
}
    

Compile project with scons. This time, the msgmerge(1) program is used by SCons to update PO file. The output from compilation is like:

user@host:$scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
Entering '/home/ptomulik/projects/tmp'
xgettext --package-name=hello --package-version=1.0 -o - hello.c
Leaving '/home/ptomulik/projects/tmp'
Writting 'messages.pot' (messages in file were outdated)
msgmerge --update de.po messages.pot
... done.
msgfmt -c -o de.mo de.po
msgmerge --update en.po messages.pot
... done.
msgfmt -c -o en.mo en.po
gcc -o hello.o -c hello.c
gcc -o hello hello.o
Install file: "de.mo" as "locale/de/LC_MESSAGES/hello.mo"
Install file: "en.mo" as "locale/en/LC_MESSAGES/hello.mo"
msgmerge --update pl.po messages.pot
... done.
msgfmt -c -o pl.mo pl.po
Install file: "pl.mo" as "locale/pl/LC_MESSAGES/hello.mo"
scons: done building targets.
    

The next example demonstrates what happens if we change the source code in such way that the internationalized messages do not change. The answer is that none of translation files (POT, PO) are touched (i.e. no content changes, no creation/modification time changed and so on). Let's append another line to the program (after the last printf), so its code becomes:

/* hello.c */
#include <stdio.h>
#include <libintl.h>
#include <locale.h>
int main(int argc, char* argv[])
{
  bindtextdomain("hello", "locale");
  setlocale(LC_ALL, "");
  textdomain("hello");
  printf(gettext("Hello world\n"));
  printf(gettext("and good bye\n"));
  printf("----------------\n");
  return a;
}
    

Compile the project. You'll see on your screen

user@host:$scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
Entering '/home/ptomulik/projects/tmp'
xgettext --package-name=hello --package-version=1.0 -o - hello.c
Leaving '/home/ptomulik/projects/tmp'
Not writting 'messages.pot' (messages in file found to be up-to-date)
gcc -o hello.o -c hello.c
gcc -o hello hello.o
scons: done building targets.
    

As you see, the internationalized messages ditn't change, so the POT and the rest of translation files have not even been touched.

Chapter 18. Writing Your Own Builders

Although SCons provides many useful methods for building common software products (programs, libraries, documents, etc.), you frequently want to be able to build some other type of file not supported directly by SCons. Fortunately, SCons makes it very easy to define your own Builder objects for any custom file types you want to build. (In fact, the SCons interfaces for creating Builder objects are flexible enough and easy enough to use that all of the the SCons built-in Builder objects are created using the mechanisms described in this section.)

18.1. Writing Builders That Execute External Commands

The simplest Builder to create is one that executes an external command. For example, if we want to build an output file by running the contents of the input file through a command named foobuild, creating that Builder might look like:

bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
    

All the above line does is create a free-standing Builder object. The next section will show us how to actually use it.

18.2. Attaching a Builder to a Construction Environment

A Builder object isn't useful until it's attached to a construction environment so that we can call it to arrange for files to be built. This is done through the $BUILDERS construction variable in an environment. The $BUILDERS variable is a Python dictionary that maps the names by which you want to call various Builder objects to the objects themselves. For example, if we want to call the Builder we just defined by the name Foo, our SConstruct file might look like:

bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
    

With the Builder attached to our construction environment with the name Foo, we can now actually call it like so:

env.Foo('file.foo', 'file.input')
    

Then when we run SCons it looks like:

% scons -Q
foobuild < file.input > file.foo

Note, however, that the default $BUILDERS variable in a construction environment comes with a default set of Builder objects already defined: Program, Library, etc. And when we explicitly set the $BUILDERS variable when we create the construction environment, the default Builders are no longer part of the environment:

bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
       
% scons -Q
AttributeError: 'SConsEnvironment' object has no attribute 'Program':
  File "/home/my/project/SConstruct", line 4:
    env.Program('hello.c')

To be able to use both our own defined Builder objects and the default Builder objects in the same construction environment, you can either add to the $BUILDERS variable using the Append function:

env = Environment()
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env.Append(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
    

Or you can explicitly set the appropriately-named key in the $BUILDERS dictionary:

env = Environment()
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env['BUILDERS']['Foo'] = bld
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
    

Either way, the same construction environment can then use both the newly-defined Foo Builder and the default Program Builder:

% scons -Q
foobuild < file.input > file.foo
cc -o hello.o -c hello.c
cc -o hello hello.o

18.3. Letting SCons Handle The File Suffixes

By supplying additional information when you create a Builder, you can let SCons add appropriate file suffixes to the target and/or the source file. For example, rather than having to specify explicitly that you want the Foo Builder to build the file.foo target file from the file.input source file, you can give the .foo and .input suffixes to the Builder, making for more compact and readable calls to the Foo Builder:

bld = Builder(action = 'foobuild < $SOURCE > $TARGET',
              suffix = '.foo',
              src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file1')
env.Foo('file2')
    
% scons -Q
foobuild < file1.input > file1.foo
foobuild < file2.input > file2.foo

You can also supply a prefix keyword argument if it's appropriate to have SCons append a prefix to the beginning of target file names.

18.4. Builders That Execute Python Functions

In SCons, you don't have to call an external command to build a file. You can, instead, define a Python function that a Builder object can invoke to build your target file (or files). Such a builder function definition looks like:

def build_function(target, source, env):
    # Code to build "target" from "source"
    return None
    

The arguments of a builder function are:

target

A list of Node objects representing the target or targets to be built by this builder function. The file names of these target(s) may be extracted using the Python str function.

source

A list of Node objects representing the sources to be used by this builder function to build the targets. The file names of these source(s) may be extracted using the Python str function.

env

The construction environment used for building the target(s). The builder function may use any of the environment's construction variables in any way to affect how it builds the targets.

The builder function must return a 0 or None value if the target(s) are built successfully. The builder function may raise an exception or return any non-zero value to indicate that the build is unsuccessful,

Once you've defined the Python function that will build your target file, defining a Builder object for it is as simple as specifying the name of the function, instead of an external command, as the Builder's action argument:

def build_function(target, source, env):
    # Code to build "target" from "source"
    return None
bld = Builder(action = build_function,
              suffix = '.foo',
              src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
       

And notice that the output changes slightly, reflecting the fact that a Python function, not an external command, is now called to build the target file:

% scons -Q
build_function(["file.foo"], ["file.input"])

18.5. Builders That Create Actions Using a Generator

SCons Builder objects can create an action "on the fly" by using a function called a generator. This provides a great deal of flexibility to construct just the right list of commands to build your target. A generator looks like:

def generate_actions(source, target, env, for_signature):
    return 'foobuild < %s > %s' % (target[0], source[0])
    

The arguments of a generator are:

source

A list of Node objects representing the sources to be built by the command or other action generated by this function. The file names of these source(s) may be extracted using the Python str function.

target

A list of Node objects representing the target or targets to be built by the command or other action generated by this function. The file names of these target(s) may be extracted using the Python str function.

env

The construction environment used for building the target(s). The generator may use any of the environment's construction variables in any way to determine what command or other action to return.

for_signature

A flag that specifies whether the generator is being called to contribute to a build signature, as opposed to actually executing the command.

The generator must return a command string or other action that will be used to build the specified target(s) from the specified source(s).

Once you've defined a generator, you create a Builder to use it by specifying the generator keyword argument instead of action.

def generate_actions(source, target, env, for_signature):
    return 'foobuild < %s > %s' % (source[0], target[0])
bld = Builder(generator = generate_actions,
              suffix = '.foo',
              src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
    
% scons -Q
foobuild < file.input > file.foo

Note that it's illegal to specify both an action and a generator for a Builder.

18.6. Builders That Modify the Target or Source Lists Using an Emitter

SCons supports the ability for a Builder to modify the lists of target(s) from the specified source(s). You do this by defining an emitter function that takes as its arguments the list of the targets passed to the builder, the list of the sources passed to the builder, and the construction environment. The emitter function should return the modified lists of targets that should be built and sources from which the targets will be built.

For example, suppose you want to define a Builder that always calls a foobuild program, and you want to automatically add a new target file named new_target and a new source file named new_source whenever it's called. The SConstruct file might look like this:

def modify_targets(target, source, env):
    target.append('new_target')
    source.append('new_source')
    return target, source
bld = Builder(action = 'foobuild $TARGETS - $SOURCES',
              suffix = '.foo',
              src_suffix = '.input',
              emitter = modify_targets)
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
    

And would yield the following output:

% scons -Q
foobuild file.foo new_target - file.input new_source

One very flexible thing that you can do is use a construction variable to specify different emitter functions for different construction variable. To do this, specify a string containing a construction variable expansion as the emitter when you call the Builder function, and set that construction variable to the desired emitter function in different construction environments:

bld = Builder(action = 'my_command $SOURCES > $TARGET',
              suffix = '.foo',
              src_suffix = '.input',
              emitter = '$MY_EMITTER')
def modify1(target, source, env):
    return target, source + ['modify1.in']
def modify2(target, source, env):
    return target, source + ['modify2.in']
env1 = Environment(BUILDERS = {'Foo' : bld},
                   MY_EMITTER = modify1)
env2 = Environment(BUILDERS = {'Foo' : bld},
                   MY_EMITTER = modify2)
env1.Foo('file1')
env2.Foo('file2')
import os
env1['ENV']['PATH'] = env2['ENV']['PATH'] + os.pathsep + os.getcwd()
env2['ENV']['PATH'] = env2['ENV']['PATH'] + os.pathsep + os.getcwd()
        
bld = Builder(action = 'my_command $SOURCES > $TARGET',
              suffix = '.foo',
              src_suffix = '.input',
              emitter = '$MY_EMITTER')
def modify1(target, source, env):
    return target, source + ['modify1.in']
def modify2(target, source, env):
    return target, source + ['modify2.in']
env1 = Environment(BUILDERS = {'Foo' : bld},
                   MY_EMITTER = modify1)
env2 = Environment(BUILDERS = {'Foo' : bld},
                   MY_EMITTER = modify2)
env1.Foo('file1')
env2.Foo('file2')
    

In this example, the modify1.in and modify2.in files get added to the source lists of the different commands:

% scons -Q
my_command file1.input modify1.in > file1.foo
my_command file2.input modify2.in > file2.foo

18.7. Where To Put Your Custom Builders and Tools

The site_scons directories give you a place to put Python modules and packages that you can import into your SConscript files (site_scons), add-on tools that can integrate into SCons (site_scons/site_tools), and a site_scons/site_init.py