Why SCons?

SCons is a response to a perennial problem: building software is harder than it should be. In a nutshell: the old, reliable model of the venerable and ubiquitous Make program has had a hard time keeping up with how complicated building software has become. The fact that Make has kept up as well as it has is impressive, and a testament to how the simplicity. But anyone who has wrestled with Automake and Autoconf to try to guarantee that a bit of software will build correctly on multiple platforms can tell you that it takes a lot of work to get right.

SCons Principles

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

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

History

SCons originated with a design that was submitted to the Software Carpentry design competition in 2000.

SCons is the direct descendant of a Perl utility called Cons. Cons in turn based some of its ideas on Jam, a build tool from Perforce Systems.

XXX

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, Charles Crain, Steve Leblanc, 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 "Autocons" 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.

SCons has received contributions from many other people, of course: Matt Balvin (extending long command-line support on Win32), Allen Bierbaum (extensions and fixes to Options), Steve Christensen (help text sorting and function action signature fixes), Michael Cook (avoiding losing signal bits from executed commands), Derrick 'dman' Hudson (), Alex Jacques (work on the Win32 scons.bat file), Stephen Kennedy (performance enhancements), Lachlan O'Dea (SharedObject() support for masm and normalized paths for the WhereIs() function), Damyan Pepper (keeping output like Make), Jeff Petkau (significant fixes for CacheDir and other areas), Stefan Reichor (Ghostscript support), Zed Shaw (Append() and Replace() environment methods), Terrel Shumway (build and test fixes, as well as the SCons Wiki), sam th (dynamic checks for utilities) and Moshe Zadke (Debian packaging).

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.

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@lists.sourceforge.net.

If you want to contact the SCons development community directly, send email to scons-devel@lists.sourceforge.net.

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

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.

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:

      env = Environment()    # Create an environment.
      # Arrange to build the "hello" program.
      env.Program('hello.c')
   

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.

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:

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

A build of the above example would look like:

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

      env = Environment()
      env.Program('program', ['main.c', 'file1.c', 'file2.'])
   

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

      % scons
      cc -c file1.c -o file1.o
      cc -c file2.c -o file2.o
      cc -c main.c -o main.o
      cc -o program main.o file1.o file2.o
   

Or on Windows:

      C:\>scons
      cl /Fofile1.obj file1.c
      cl /Fofile2.obj file2.c
      cl /Fomain.obj main.c
      link /Foprogram.exe main.obj file1.obj file2.obj
   

Keeping SConstruct Files Easy 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:

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

Putting the call to the Split function inside the env.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 env.Program function:

      env = Environment()
      list = Split('main.c file1.c file2.')
      env.Program('program', list)
   

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:

      env = Environment()
      list = Split('main.c
                    file1.c
                    file2.c')
      env.Program('program', list)
   

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:

      env = Environment()
      list = Split('main.c file1.c file2.')
      env.Program(target = 'program', source = list)
   

Whether or not you choose to use keyword arguments to identify the target and source files is purely a personal choice; SCons functions the same either way.

Compiling Multiple Programs

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

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

SCons would then build the programs as follows:

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

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 section XXX, 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:

      env = Environment()
      env.Program(Split('foo.c common1.c common2.c'))
      env.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 only need to be built once, even though the files are listed multiple times:

      % scons
      cc -c bar1.c -o bar1.o
      cc -c bar2.c -o bar2.o
      cc -c common1.c -o common1.o
      cc -c common2.c -o common2.o
      cc -o bar bar1.o bar2.o common1.o common2.o
      cc -c foo.c -o foo.o
      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
      env = Environment()
      env.Program('foo', foo_files)
      env.Program('bar', bar_files)
   

This is functionally equivalent to the previous example.

Multiple Construction Environments

So far, all of our examples have created a single construction environment named env. env, however, is simply a Python variable name, and you can use any other variable name that you like. For example:

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

      my_env.Program('foo.c')
   

This opens up the possibility of using multiple construction environments, each with a separate variable name. We can then use these separate construction environments to build different programs in different ways:

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

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

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

      % scons
      cc -c -O2 bar.c -o bar.o
      cc -o bar bar.o
      cc -c -g foo.c -o foo.o
      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
      scons: *** Two different environments were specified for the same target:  foo.o
      File "SConstruct", line 6, in ?
   

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. To avoid this problem, we must explicitly specify that each environment compile foo.c to a separately-named object file using the Object call, 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 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
      cc -c -g foo.c -o foo-dbg.o
      cc -o foo-dbg foo-dbg.o
      cc -c -O2 foo.c -o foo-opt.o
      cc -o foo-opt foo-opt.o
   

Copying Construction Environments

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 Copy method to create a copy of a construction environment.

Like the Environment call that creates a construction environment, the Copy 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 with sets CCFLAGS for debugging:

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

      e = opt.Object('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
      gcc -c foo.c -o foo.o
      gcc -o foo foo.o
      gcc -c -g foo.c -o foo-dbg.o
      gcc -o foo-dbg foo-dbg.o
      gcc -c -O2 foo.c -o foo-opt.o
      gcc -o foo-opt foo-opt.o
   

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
      CC is: cc
   

A construction environment, however, is actually a Python 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, value in dict.items():
          print "key = %s, value = %s % (key, value)
   

This SConstruct file will print the dictionary items for us as follows:

      % scons
      key = FOO, value = foo
      key = BAR, value = bar
   

Modifying a Construction Environment

SCons provides various methods that support modifying existing values in a construction environment.

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

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

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

Building Libraries

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

      env = Environment()
      env.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
      cc -c f1.c -o f1.o
      cc -c f2.c -o f2.o
      cc -c f3.c -o f3.o
      ar r 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
      cl /Fof1.obj f1.c
      cl /Fof2.obj f2.c
      cl /Fof3.obj f3.c
      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.

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:

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

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
      cc -c f1.c -o f1.o
      cc -c f2.c -o f2.o
      cc -c f3.c -o f3.o
      ar r libfoo.a f1.o f2.o f3.o
      ranlib libfoo.a
      cc -c prog.c -o prog.o
      cc -o prog -L. -lfoo prog.o
    

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

      C:\>scons
      cl /Fof1.obj f1.c
      cl /Fof2.obj f2.c
      cl /Fof3.obj f3.c
      lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj
      cl /Foprog.obj prog.c
      link /OUT:prog.exe /LIBPATH:. foo.lib prog.obj
    

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

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:

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

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'
    

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

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

      C:\>scons
      cl /Foprog.obj prog.c
      link /nologo /OUT:program.exe /LIBPATH:\usr\lib;\usr\local\lib m.lib prog.obj
    

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

Source File Signatures

The other side of avoiding unnecessary rebuilds is the fundamental build tool behavior of rebuilding things when a source file changes, so that the built software is up to date. SCons keeps track of this through a signature for each source file, and allows you to configure whether you want to use the source file contents or the modification time (timestamp) as the signature.

        env = Environment()
        env.Program('hello.c')
        SourceSignatures('MD5')
      

        env = Environment()
        env.Program('hello.c')
        SourceSignatures('timestamp')
      

Target File Signatures

As you've just seen, SCons uses signatures to decide whether a target file is up to date or must be rebuilt. When a target file depends on another target file, SCons allows you to separately configure how the signatures of an "intermediate" target file is used when deciding if a dependent target file must be rebuilt.

        env = Environment()
        env.Program('hello.c')
        TargetSignatures('build')
      

        env = Environment()
        env.Program('hello.c')
        TargetSignatures('content')
      

Implicit Dependencies: The CPPPATH Construction Variable

Now suppose that our "Hello, World!" program actually has a #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:

       env = Environment(CPPPATH = '.')
       hello = env.Program('hello.c')
    

The CPPPATH assignment in the Environment call 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 hello
       cc -I. -c hello.c -o hello.o
       cc -o hello hello.o
       % scons hello
       scons: `hello' is up to date.
       % edit hello.h
           [CHANGE THE CONTENTS OF hello.h]
       % scons hello
       cc -I. -c hello.c -o hello.o
       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 separate character (':' on POSIX/Linux, ';' on Windows). Either way, SCons creates the right command-line options so that the followin example:

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

Will look like this on POSIX or Linux:

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

And like this on Windows:

      % scons hello
      cl /Iinclude /I\home\project\inc /Fohello.obj hello.c
      link /OUT:hello.exe hello.obj
    

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 do this either by specifying the --implicit-cache option on the command line:

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

Or 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 XXX

XXX

The Ignore Method

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:

       env = Environment()
       hello = env.Program('hello.c')
       env.Ignore(hello, 'hello.h')
    

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

       env = Environment()
       hello = env.Program('hello.c')
       env.Ignore(hello, '/usr/include/stdio.h')
    

The Depends Method

On the other hand, sometimes a file depends on another file that has no SCons scanner will detect. 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:

       env = Environment()
       hello = env.Program('hello.c')
       env.Depends(hello, 'other_file')
    

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

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 install
       cc -c goodbye.c -o goodbye.o
       cc -o goodbye goodbye.o
       cc -c hello.c -o hello.o
       cc -o hello hello.o
       Install file: "goodbye" as "/usr/bin/goodbye"
       Install file: "hello" as "/usr/bin/hello"
    

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 install
       cc -c hello.c -o hello.o
       cc -o hello hello.o
       Install file: "hello" as "/usr/bin/hello-new"
    

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 the 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])
    

In this case, the InstallAs function loops through both lists simultaneously, and copies each source file into its corresponding target file name:

       % scons install
       cc -c goodbye.c -o goodbye.o
       cc -o goodbye goodbye.o
       cc -c hello.c -o hello.o
       cc -o hello hello.o
       Install file: "goodbye" as "/usr/bin/goodbye-new"
       Install file: "hello" as "/usr/bin/hello-new"
    

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.

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
       cc -c prog1/foo1.c -o prog1/foo1.o
       cc -c prog1/foo2.c -o prog1/foo2.o
       cc -c prog1/main.c -o prog1/main.o
       cc -o prog1/prog1 prog1/main.o prog1/foo1.o prog1/foo2.o
       cc -c prog2/bar1.c -o prog2/bar1.o
       cc -c prog2/bar2.c -o prog2/bar2.o
       cc -c prog2/main.c -o prog2/main.o
       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 and issues commands

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
       cc -c lib/foo1.c -o lib/foo1.o
       cc -c src/prog/foo2.c -o src/prog/foo2.o
       cc -c src/prog/main.c -o src/prog/main.o
       cc -o src/prog/prog prog/main.o lib/foo1.o prog/foo2.o
    

(Notice that the lib/foo1.o object file is built in the same directory as its source file. See section XXX, below, for information about how to build the object file in a different subdirectory.)

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
       cc -c /usr/joe/lib/foo1.c -o /usr/joe/lib/foo1.o
       cc -c src/prog/foo2.c -o src/prog/foo2.o
       cc -c src/prog/main.c -o src/prog/main.o
       cc -o src/prog/prog prog/main.o /usr/joe/lib/foo1.o 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 section XXX, below, for information about how to build the object file in a different subdirectory.)

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.

        env = Environment()
        Export('env')
      

        env = Environment()
        debug = ARGUMENTS['debug']
        Export('env', 'debug')
      

        Export('env debug')
      

        SConscript('src/SConscript', 'env')
      

        SConscript('src/SConscript', exports='env')
      

        SConscript(['src1/SConscript',
                    'src2/SConscript'], exports='env')
      

        Import('env')
        env.Program('prog', ['prog.c'])
      

        Import('env', 'debug')
        env = env.Copy(DEBUG = debug)
        env.Program('prog', ['prog.c'])
      

        Import('env debug')
        env = env.Copy(DEBUG = debug)
        env.Program('prog', ['prog.c'])
      

        obj = env.Object('foo.c')
        Return('obj')
      

        objs = []
        for subdir in ['foo', 'bar']:
            o = SConscript('%s/SConscript' % subdir)
            objs.append(o)
        env.Library('prog', objs)
      

Specifying a Build Directory as Part of an SConscript Call

The most straightforward way to establish a build directory uses the fact that the usual way to set up a build hierarcy is to have an SConscript file in the source subdirectory. If you then pass a build_dir argument to the SConscript function call:

      SConscript('src/SConscript', build_dir='build')
    

SCons will then build all of the files in the build subdirectory:

      % ls -1 src
      SConscript
      hello.c
      % scons
      cc -c build/hello.c -o build/hello.o
      cc -o build/hello build/hello.o
      % ls -1 build
      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.

Why SCons Duplicates Source Files in a Build Directory

SCons duplicates source files in build directories because it's the most straightforward way to guarantee a correct build regardless of include-file directory paths, and the SCons philosophy is to, by default, guarantee a correct build in all cases. Here is an example of an end case where duplicating source files in a build directory is necessary for a correct build:

XXX

      env = Environmnet()
    

XXX

      % scons
      cc -c build/hello.c -o build/hello.o
      cc -o build/hello build/hello.o
    

Telling SCons to Not Duplicate Source Files in the Build Directory

In most cases, however, having SCons place its target files in a build subdirectory without duplicating the source files works just fine. You can disable the default SCons behavior by specifying duplicate=0 when you call the SConscript function:

      SConscript('src/SConscript', build_dir='build', duplicate=0)
    

When this flag is specified, SCons uses the build directory like most people expect--that is, the output files are placed in the build directory while the source files stay in the source directory:

      % ls -1 src
      SConscript
      hello.c
      % scons
      cc -c src/hello.c -o build/hello.o
      cc -o build/hello build/hello.o
      % ls -1 build
      hello
      hello.o
    

The BuildDir Function

Use the BuildDir function to establish that target files should be built in a separate directory from the source files:

      BuildDir('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.

XXX

When using the BuildDir function directly, SCons still duplicates the source files in the build directory by default:

      % ls src
      hello.c
      % scons
      cc -c build/hello.c -o build/hello.o
      cc -o build/hello build/hello.o
      % ls -1 build
      hello
      hello.c
      hello.o
    

You can specify the same duplicate=0 argument that you can specify for an SConscript call:

      BuildDir('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
      cc -c src/hello.c -o build/hello.o
      cc -o build/hello build/hello.o
      % ls -1 build
      hello
      hello.o
    

Using BuildDir With an SConscript File

Even when using the BuildDir 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 SConscript file could look like:

      BuildDir('build', 'src')
      SConscript('build/SConscript')
    

Yielding the following output:

      % ls -1 src
      SConscript
      hello.c
      % scons
      cc -c build/hello.c -o build/hello.o
      cc -o build/hello build/hello.o
      % ls -1 build
      hello
      hello.c
      hello.o
    

Notice that this is completely equivalent to the use of SConscript that we learned about in the previous section.

Why You'd Want to Call BuildDir Instead of SConscript

XXX

Programs: the Program Builder

As we've seen, the Program Builder is used to build an executable program. The source argument is one or more source-code files or object files, and the target argument is the name of the executable program name to be created. For example:

      env = Environment()
      env.Program('prog', 'file1.o')
    

Will create the prog executable on a POSIX system, the prog.exe executable on a Windows system.

The target file's prefix and suffix may be omitted, and the values from the $PROGPREFIX and $PROGSUFFIX construction variables will be appended appropriately. For example:

      env = Environment(PROGPREFIX='my', PROGSUFFIX='.xxx')
      env.Program('prog', ['file1.o', 'file2.o'])
    

Will create a program named myprog.xxx regardless of the system on which it is run.

If you omit the target, the base of the first input file name specified because the base of the target program created. For example:

      env = Environment()
      env.Program(['hello.c', 'goodbye.c'])
    

Will create the hello executable on a POSIX system, the hello.exe executable on a Windows system.

Object-File Builders

SCons provides separate Builder objects to create both static and shared object files.

        XXX
      

        XXX
      

        XXX
      

Library Builders

SCons provides separate Builder objects to create both static and shared libraries.

        XXX
      

        XXX
      

Pre-Compiled Headers: the PCH Builder

XXX

Microsoft Visual C++ Resource Files: the RES Builder

XXX

Source Files

By default SCons supports two Builder objects that know how to build source files from other input files.

        XXX
      

        XXX
      

Documents

SCons provides a number of Builder objects for creating different types of documents.

        XXX
      

        XXX
      

Archives

SCons provides Builder objects for creating two different types of archive files.

        env = Environment()
        env.Tar('out1.tar', ['file1', 'file2'])
        env.Tar('out2', 'directory')
      

        env = Environment(TARFLAGS = '-c -z')
        env.Tar('out.tar.gz', 'directory')
      

        env = Environment(TARFLAGS = '-c -z',
                          TARSUFFIX = '.tgz')
        env.Tar('out', 'directory')
      

        env = Environment()
        env.Zip('out', ['file1', 'file2'])
      

Java

SCons provides Builder objects for creating various types of Java output files.

        env = Environment()
        env.Java(target = 'classes', source = 'src')
      

        env = Environment()
        env.Java(target = 'classes', source = 'src')
        env.Jar(target = '', source = 'classes')
      

        XXX
      

        XXX
      

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 < $TARGET > $SOURCE')
    

All the above line does is create a free-standing Builder object. The next section will show us how to actually use it.

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 < $TARGET > $SOURCE')
       env = Environment(BUILDERS = {'Foo' : bld})
    

With the Builder so attached to our construction environment we can now actually call it like so:

       env.Foo('file.foo', 'file.input')
    

Then when we run SCons it looks like:

      % scons
      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 < $TARGET > $SOURCE')
       env = Environment(BUILDERS = {'Foo' : bld})
       env.Foo('file.foo', 'file.input')
       env.Program('hello.c')
    

      % scons
      scons: Reading SConscript files ...
      other errors
      Traceback (most recent call last):
        File "/usr/lib/scons/SCons/Script/__init__.py", line 901, in main
          _main()
        File "/usr/lib/scons/SCons/Script/__init__.py", line 762, in _main
          SCons.Script.SConscript.SConscript(script)
        File "/usr/lib/scons/SCons/Script/SConscript.py", line 207, in SConscript
          exec _file_ in stack[-1].globals
        File "SConstruct", line 4, in ?
          env.Program('hello.c')
      scons: Environment instance has no attribute 'Program'
    

To be able 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 < $TARGET > $SOURCE')
       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 < $TARGET > $SOURCE')
       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
      foobuild < file.input > file.foo
      cc -c hello.c -o hello.o
      cc -o hello hello.o
    

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 < $TARGET > $SOURCE',
                     suffix = '.foo',
                     src_suffix = '.input')
       env = Environment(BUILDERS = {'Foo' : bld})
       env.Foo('file1')
       env.Foo('file2')
    

      % scons
      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.

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):
           # XXX
           return None
    

The arguments of a builder function are:

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):
           # XXX
           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
      build_function("file.foo", "file.input")
    

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 XXX A generator looks like:

       def generate_actions(source, target, env, for_signature):
           return XXX
    

The arguments of a generator are:

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.

       bld = Builder(generator = generate_actions,
                     suffix = '.foo',
                     src_suffix = '.input')
       env = Environment(BUILDERS = {'Foo' : bld})
       env.Foo('file')
    

      % scons
      XXX
    

Note that it's illegal to specify both an action and a generator for a Builder.

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).

       def modify_targets(XXX):
           return XXX
       bld = Builder(action = 'XXX',
                     suffix = '.foo',
                     src_suffix = '.input',
                     emitter = modify_targets)
       env = Environment(BUILDERS = {'Foo' : bld})
       env.Foo('file')
    

      % scons
      XXX
    

       bld = Builder(action = 'XXX',
                     suffix = '.foo',
                     src_suffix = '.input',
                     emitter = 'MY_EMITTER')
       def modify1(XXX):
           return XXX
       def modify2(XXX):
           return XXX
       env1 = Environment(BUILDERS = {'Foo' : bld},
                          MY_EMITTER = modify1)
       env2 = Environment(BUILDERS = {'Foo' : bld},
                          MY_EMITTER = modify2)
       env1.Foo('file1')
       env2.Foo('file2')
    

Builders That Use Other Builders

XXX

       env = Environment()
       env.SourceCode('.', env.BitKeeper('XXX')
       env.Program('hello.c')
    

      % scons
      XXX
    

XXX

XXX

XXX

XXX

The Repository Method

It's often useful to allow multiple programmers working on a project to build software from source files and/or derived files that are stored in a centrally-accessible repository, a directory copy of the source code tree. (Note that this is not the sort of repository maintained by a source code management system like BitKeeper, CVS, or Subversion. For information about using SCons with these systems, see the section, "Fetching Files From Source Code Management Systems," below.) You use the Repository method to tell SCons to search one or more central code repositories (in order) for any source files and derived files that are not present in the local build tree:

       env = Environment()
       env.Program('hello.c')
       Repository('/usr/repository1', '/usr/repository2')
    

Multiple calls to the Repository method will simply add repositories to the global list that SCons maintains, with the exception that SCons will automatically eliminate the current directory and any non-existent directories from the list.

Finding source files in repositories

The above example specifies that SCons will first search for files under the /usr/repository1 tree and next under the /usr/repository2 tree. SCons expects that any files it searches for will be found in the same position relative to the top-level directory XXX In the above example, if the hello.c file is not found in the local build tree, SCons will search first for a /usr/repository1/hello.c file and then for a /usr/repository1/hello.c file to use in its place.

So given the SConstruct file above, if the hello.c file exists in the local build directory, SCons will rebuild the hello program as normal:

      % scons
      gcc -c hello.c -o hello.o
      gcc -o hello hello.o
    

If, however, there is no local hello.c file, but one exists in /usr/repository1, SCons will recompile the hello program from the source file it finds in the repository:

      % scons
      gcc -c /usr/repository1/hello.c -o hello.o
      gcc -o hello hello.o
    

And similarly, if there is no local hello.c file and no /usr/repository1/hello.c, but one exists in /usr/repository2:

      % scons
      gcc -c /usr/repository2/hello.c -o hello.o
      gcc -o hello hello.o
    

Finding the SConstruct file in repositories

SCons will also search in repositories for the SConstruct file and any specified SConscript files. This poses a problem, though: how can SCons search a repository tree for an SConstruct file if the SConstruct file itself contains the information about the pathname of the repository? To solve this problem, SCons allows you to specify repository directories on the command line using the -Y option:

      % scons -Y /usr/repository1 -Y /usr/repository2
    

When looking for source or derived files, SCons will first search the repositories specified on the command line, and then search the repositories specified in the SConstruct or SConscript files.

Finding derived files in repositories

If a repository contains not only source files, but also derived files (such as object files, libraries, or executables), SCons will perform its normal MD5 signature calculation to decide if a derived file in a repository is up-to-date, or the derived file must be rebuilt in the local build directory. For the SCons signature calculation to work correctly, a repository tree must contain the .sconsign files that SCons uses to keep track of signature information.

Usually, this would be done by a build integrator who would run SCons in the repository to create all of its derived files and .sconsign files, or who would SCons in a separate build directory and copying the resulting tree to the desired repository:

      % cd /usr/repository1
      % scons
      gcc -c hello.c -o hello.o
      gcc -o hello hello.o
    

(Note that this is safe even if the SConstruct file lists /usr/repository1 as a repository, because SCons will remove the current build directory from its repository list for that invocation.)

Now, with the repository populated, we only need to create the one local source file we're interested in working with at the moment, and use the -Y option to tell SCons to fetch any other files it needs from the repository:

      % cd $HOME/build
      % edit hello.c
      % scons -Y /usr/repository1
      gcc -c hello.c -o hello.o
      gcc -o hello hello.o
      XXXXXXX
    

Notice that SCons realizes that it does not need to rebuild a local XXX.o file, but instead uses the already-compiled XXX.o file from the repository.

Guaranteeing local copies of files

If the repository tree contains the complete results of a build, and we try to build from the repository without any files in our local tree, something moderately surprising happens:

      % mkdir $HOME/build2
      % cd $HOME/build2
      % scons -Y /usr/all/repository hello
      scons: `hello' is up-to-date.
    

Why does SCons say that the hello program is up-to-date when there is no hello program in the local build directory? Because the repository (not the local directory) contains the up-to-date hello program, and SCons correctly determines that nothing needs to be done to rebuild that up-to-date copy of the file.

There are, however, many times when you want to ensure that a local copy of a file always exists. A packaging or testing script, for example, may assume that certain generated files exist locally. To tell SCons to make a copy of any up-to-date repository file in the local build directory, use the Local function:

       env = Environment()
       hello = env.Program('hello.c')
       Local(hello)
    

If we then run the same command, SCons will make a local copy of the program from the repository copy, and tell you that it is doing so:

      % scons -Y /usr/all/repository hello
      Local copy of hello from /usr/all/repository/hello
      scons: `hello' is up-to-date.
      XXXXXX DO WE REALLY REPORT up-to-date, TOO?
    

(Notice that, because the act of making the local copy is not considered a "build" of the hello file, SCons still reports that it is up-to-date.) XXXXXX DO WE REALLY REPORT up-to-date, TOO?

Fetching Source Code From BitKeeper

X

      env = Environment()
      env.SourceCode('.', env.BitKeeper('XXX'))
      env.Program('hello.c')
   

     % scons
     XXX
   

Fetching Source Code From CVS

X

      env = Environment()
      env.SourceCode('.', env.CVS('XXX'))
      env.Program('hello.c')
   

     % scons
     XXX
   

Fetching Source Code From RCS

X

      env = Environment()
      env.SourceCode('.', env.RCS())
      env.Program('hello.c')
   

     % scons
     XXX
   

Fetching Source Code From SCCS

X

      env = Environment()
      env.SourceCode('.', env.SCCS())
      env.Program('hello.c')
   

     % scons
     XXX
   

Fetching Source Code From Subversion

X

      env = Environment()
      env.SourceCode('.', env.Subversion('XXX'))
      env.Program('hello.c')
   

     % scons
     XXX
   

Specifying the Shared Cache Directory

To enable sharing of derived files, use the CacheDir function in any SConscript file:

       CacheDir('/usr/local/build_cache')
    

Note that the directory you specify must already exist and be readable and writable by all developers who will be sharing derived files. It should also be in some central location that all builds will be able to access. In environments where developers are using separate systems (like individual workstations) for builds, this directory would typically be on a shared or NFS-mounted file system.

Here's what happens: When a build has a CacheDir specified, every time a file is built, it is stored in the shared cache directory along with its MD5 build signature. On subsequent builds, before an action is invoked to build a file, SCons will check the shared cache directory to see if a file with the exact same build signature already exists. If so, the derived file will not be built locally, but will be copied into the local build directory from the shared cache directory, like so:

      % scons
      cc -c hello.c -o hello.o
      cc -o hello hello.o
      % scons -c
      Removed hello.o
      Removed hello
      % scons
      Retrieved `hello.o' from cache
      Retrieved `hello' from cache
      %
    

Keeping Build Output Consistent

One potential drawback to using a shared cache is that your build output can be inconsistent from invocation to invocation, because any given file may be rebuilt one time and retrieved from the shared cache the next time. This can make analyzing build output more difficult, especially for automated scripts that expect consistent output each time.

If, however, you use the --cache-show option, SCons will print the command line that it would have executed to build the file, even when it is retrieving the file from the shared cache. This makes the build output consistent every time the build is run:

      % scons
      cc -c hello.c -o hello.o
      cc -o hello hello.o
      % scons -c
      Removed hello.o
      Removed hello
      % scons --cache-show
      cc -c hello.c -o hello.o
      cc -o hello hello.o
      %
    

The trade-off, of course, is that you no longer know whether or not SCons has retrieved a derived file from cache or has rebuilt it locally.

Not Retrieving Files From a Shared Cache

Retrieving an already-built file from the shared cache is usually a significant time-savings over rebuilding the file, but how much of a savings (or even whether it saves time at all) can depend a great deal on your system or network configuration. For example, retrieving cached files from a busy server over a busy network might end up being slower than rebuilding the files locally.

In these cases, you can specify the --cache-disable command-line option to tell SCons to not retrieve already-built files from the shared cache directory:

      % scons
      cc -c hello.c -o hello.o
      cc -o hello hello.o
      % scons -c
      Removed hello.o
      Removed hello
      % scons
      Retrieved `hello.o' from cache
      Retrieved `hello' from cache
      % scons -c
      Removed hello.o
      Removed hello
      % scons --cache-disable
      cc -c hello.c -o hello.o
      cc -o hello hello.o
      %
    

Populating a Shared Cache With Already-Built Files

Sometimes, you may have one or more derived files already built in your local build tree that you wish to make available to other people doing builds. For example, you may find it more effective to perform integration builds with the cache disabled (per the previous section) and only populate the shared cache directory with the built files after the integration build has completed successfully. This way, the cache will only get filled up with derived files that are part of a complete, successful build not with files that might be later overwritten while you debug integration problems.

In this case, you can use the the --cache-force option to tell SCons to put all derived files in the cache, even if the files had already been built by a previous invocation:

      % scons --cache-disable
      cc -c hello.c -o hello.o
      cc -o hello hello.o
      % scons -c
      Removed hello.o
      Removed hello
      % scons --cache-disable
      cc -c hello.c -o hello.o
      cc -o hello hello.o
      % scons --cache-force
      % scons -c
      Removed hello.o
      Removed hello
      % scons
      Retrieved `hello.o' from cache
      Retrieved `hello' from cache
      %
    

Notice how the above sample run demonstrates that the --cache-disable option avoids putting the built hello.o and hello files in the cache, but after using the --cache-force option, the files have been put in the cache for the next invocation to retrieve.

Selective Builds

XXX

Overriding Construction Variables

XXX

The SCONSFLAGS Environment Variable

XXX

XXX

XXX