SCons 2.3.1

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.

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.

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.

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')
      

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.

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.

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.

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)

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'))
    

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.

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

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'])
    

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.)

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.

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.

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.

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.

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.

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

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.

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.

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.

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 = '.')
      

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:

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
    

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.

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.

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)
      

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.

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
     

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.)

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.

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)

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.
 

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.

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
    

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.)

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'.

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

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.)

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"

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"

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"

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")

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")

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")

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

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)

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)
    

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.

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.

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

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.

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.

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

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

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.

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.

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.

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
    

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

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.

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.

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.

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"])
    

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.

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.

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:

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"])

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:

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.

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

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 file that gets read before any SConstruct or SConscript file, allowing you to change SCons's default behavior.

Each system type (Windows, Mac, Linux, etc.) searches a canonical set of directories for site_scons; see the man page for details. The top-level SConstruct's site_scons dir is always searched last, and its dir is placed first in the tool path so it overrides all others.

If you get a tool from somewhere (the SCons wiki or a third party, for instance) and you'd like to use it in your project, a site_scons dir is the simplest place to put it. Tools come in two flavors; either a Python function that operates on an Environment or a Python module or package containing two functions, exists() and generate().

A single-function Tool can just be included in your site_scons/site_init.py file where it will be parsed and made available for use. For instance, you could have a site_scons/site_init.py file like this:

def TOOL_ADD_HEADER(env):
   """A Tool to add a header from $HEADER to the source file"""
   add_header = Builder(action=['echo "$HEADER" > $TARGET',
                                'cat $SOURCE >> $TARGET'])
   env.Append(BUILDERS = {'AddHeader' : add_header})
   env['HEADER'] = '' # set default value
      

and a SConstruct like this:

# Use TOOL_ADD_HEADER from site_scons/site_init.py
env=Environment(tools=['default', TOOL_ADD_HEADER], HEADER="=====")
env.AddHeader('tgt', 'src')
    

The TOOL_ADD_HEADER tool method will be called to add the AddHeader tool to the environment.

A more full-fledged tool with exists() and generate() methods can be installed either as a module in the file site_scons/site_tools/toolname.py or as a package in the directory site_scons/site_tools/toolname. In the case of using a package, the exists() and generate() are in the file site_scons/site_tools/toolname/__init__.py. (In all the above case toolname is replaced by the name of the tool.) Since site_scons/site_tools is automatically added to the head of the tool search path, any tool found there will be available to all environments. Furthermore, a tool found there will override a built-in tool of the same name, so if you need to change the behavior of a built-in tool, site_scons gives you the hook you need.

Many people have a library of utility Python functions they'd like to include in SConscripts; just put that module in site_scons/my_utils.py or any valid Python module name of your choice. For instance you can do something like this in site_scons/my_utils.py to add build_id and MakeWorkDir functions:

from SCons.Script import *   # for Execute and Mkdir
def build_id():
   """Return a build ID (stub version)"""
   return "100"
def MakeWorkDir(workdir):
   """Create the specified dir immediately"""
   Execute(Mkdir(workdir))
      

And then in your SConscript or any sub-SConscript anywhere in your build, you can import my_utils and use it:

import my_utils
print "build_id=" + my_utils.build_id()
my_utils.MakeWorkDir('/tmp/work')
    

Note that although you can put this library in site_scons/site_init.py, it is no better there than site_scons/my_utils.py since you still have to import that module into your SConscript. Also note that in order to refer to objects in the SCons namespace such as Environment or Mkdir or Execute in any file other than a SConstruct or SConscript you always need to do

from SCons.Script import *
    

This is true in modules in site_scons such as site_scons/site_init.py as well.

You can use any of the user- or machine-wide site dirs such as ~/.scons/site_scons instead of ./site_scons, or use the --site-dir option to point to your own dir. site_init.py and site_tools will be located under that dir. To avoid using a site_scons dir at all, even if it exists, use the --no-site-dir option.

Suppose, for example, that we want to create a simple scanner for .foo files. A .foo file contains some text that will be processed, and can include other files on lines that begin with include followed by a file name:

include filename.foo
    

Scanning a file will be handled by a Python function that you must supply. Here is a function that will use the Python re module to scan for the include lines in our example:

import re

include_re = re.compile(r'^include\s+(\S+)$', re.M)

def kfile_scan(node, env, path, arg):
    contents = node.get_text_contents()
    return env.File(include_re.findall(contents))
    

It is important to note that you have to return a list of File nodes from the scanner function, simple strings for the file names won't do. As in the examples we are showing here, you can use the File function of your current Environment in order to create nodes on the fly from a sequence of file names with relative paths.

The scanner function must accept the four specified arguments and return a list of implicit dependencies. Presumably, these would be dependencies found from examining the contents of the file, although the function can perform any manipulation at all to generate the list of dependencies.

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.) 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.

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. 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/repository2/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 -Q
cc -o hello.o -c hello.c
cc -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 -Q
cc -o hello.o -c /usr/repository1/hello.c
cc -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 -Q
cc -o hello.o -c /usr/repository2/hello.c
cc -o hello hello.o

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 -Q -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.

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 run SCons in a separate build directory and copy the resulting tree to the desired repository:

% cd /usr/repository1
% scons -Q
cc -o file1.o -c file1.c
cc -o file2.o -c file2.c
cc -o hello.o -c hello.c
cc -o hello hello.o file1.o file2.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 -Q -Y /usr/repository1
cc -c -o hello.o hello.c
cc -o hello hello.o /usr/repository1/file1.o /usr/repository1/file2.o
    

Notice that SCons realizes that it does not need to rebuild local copies file1.o and file2.o files, but instead uses the already-compiled files from the repository.

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 -Q -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.
    

(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.)

The basic framework for multi-platform build configuration in SCons is to attach a configure context to a construction environment by calling the Configure function, perform a number of checks for libraries, functions, header files, etc., and to then call the configure context's Finish method to finish off the configuration:

env = Environment()
conf = Configure(env)
# Checks for libraries, header files, etc. go here!
env = conf.Finish()
    

SCons provides a number of basic checks, as well as a mechanism for adding your own custom checks.

Note that SCons uses its own dependency mechanism to determine when a check needs to be run--that is, SCons does not run the checks every time it is invoked, but caches the values returned by previous checks and uses the cached values unless something has changed. This saves a tremendous amount of developer time while working on cross-platform build issues.

The next sections describe the basic checks that SCons supports, as well as how to add your own custom checks.

Testing the existence of a header file requires knowing what language the header file is. A configure context has a CheckCHeader method that checks for the existence of a C header file:

env = Environment()
conf = Configure(env)
if not conf.CheckCHeader('math.h'):
    print 'Math.h must be installed!'
    Exit(1)
if conf.CheckCHeader('foo.h'):
    conf.env.Append('-DHAS_FOO_H')
env = conf.Finish()
    

Note that you can choose to terminate the build if a given header file doesn't exist, or you can modify the construction environment based on the existence of a header file.

If you need to check for the existence a C++ header file, use the CheckCXXHeader method:

env = Environment()
conf = Configure(env)
if not conf.CheckCXXHeader('vector.h'):
    print 'vector.h must be installed!'
    Exit(1)
env = conf.Finish()
    

Check for the availability of a specific function using the CheckFunc method:

env = Environment()
conf = Configure(env)
if not conf.CheckFunc('strcpy'):
    print 'Did not find strcpy(), using local version'
    conf.env.Append(CPPDEFINES = '-Dstrcpy=my_local_strcpy')
env = conf.Finish()
    

Check for the availability of a library using the CheckLib method. You only specify the basename of the library, you don't need to add a lib prefix or a .a or .lib suffix:

env = Environment()
conf = Configure(env)
if not conf.CheckLib('m'):
    print 'Did not find libm.a or m.lib, exiting!'
    Exit(1)
env = conf.Finish()
    

Because the ability to use a library successfully often depends on having access to a header file that describes the library's interface, you can check for a library and a header file at the same time by using the CheckLibWithHeader method:

env = Environment()
conf = Configure(env)
if not conf.CheckLibWithHeader('m', 'math.h', 'c'):
    print 'Did not find libm.a or m.lib, exiting!'
    Exit(1)
env = conf.Finish()
    

This is essentially shorthand for separate calls to the CheckHeader and CheckLib functions.

Check for the availability of a typedef by using the CheckType method:

env = Environment()
conf = Configure(env)
if not conf.CheckType('off_t'):
    print 'Did not find off_t typedef, assuming int'
    conf.env.Append(CCFLAGS = '-Doff_t=int')
env = conf.Finish()
    

You can also add a string that will be placed at the beginning of the test file that will be used to check for the typedef. This provide a way to specify files that must be included to find the typedef:

env = Environment()
conf = Configure(env)
if not conf.CheckType('off_t', '#include <sys/types.h>\n'):
    print 'Did not find off_t typedef, assuming int'
    conf.env.Append(CCFLAGS = '-Doff_t=int')
env = conf.Finish()
    

A custom check is a Python function that checks for a certain condition to exist on the running system, usually using methods that SCons supplies to take care of the details of checking whether a compilation succeeds, a link succeeds, a program is runnable, etc. A simple custom check for the existence of a specific library might look as follows:

mylib_test_source_file = """
#include <mylib.h>
int main(int argc, char **argv)
{
    MyLibrary mylib(argc, argv);
    return 0;
}
"""

def CheckMyLibrary(context):
    context.Message('Checking for MyLibrary...')
    result = context.TryLink(mylib_test_source_file, '.c')
    context.Result(result)
    return result
    

The Message and Result methods should typically begin and end a custom check to let the user know what's going on: the Message call prints the specified message (with no trailing newline) and the Result call prints yes if the check succeeds and no if it doesn't. The TryLink method actually tests for whether the specified program text will successfully link.

(Note that a custom check can modify its check based on any arguments you choose to pass it, or by using or modifying the configure context environment in the context.env attribute.)

This custom check function is then attached to the configure context by passing a dictionary to the Configure call that maps a name of the check to the underlying function:

env = Environment()
conf = Configure(env, custom_tests = {'CheckMyLibrary' : CheckMyLibrary})
    

You'll typically want to make the check and the function name the same, as we've done here, to avoid potential confusion.

We can then put these pieces together and actually call the CheckMyLibrary check as follows:

mylib_test_source_file = """
#include <mylib.h>
int main(int argc, char **argv)
{
    MyLibrary mylib(argc, argv);
    return 0;
}
"""

def CheckMyLibrary(context):
    context.Message('Checking for MyLibrary... ')
    result = context.TryLink(mylib_test_source_file, '.c')
    context.Result(result)
    return result

env = Environment()
conf = Configure(env, custom_tests = {'CheckMyLibrary' : CheckMyLibrary})
if not conf.CheckMyLibrary():
    print 'MyLibrary is not installed!'
    Exit(1)
env = conf.Finish()

# We would then add actual calls like Program() to build
# something using the "env" construction environment.
    

If MyLibrary is not installed on the system, the output will look like:

% scons
scons: Reading SConscript file ...
Checking for MyLibrary... failed
MyLibrary is not installed!
    

If MyLibrary is installed, the output will look like:

% scons
scons: Reading SConscript file ...
Checking for MyLibrary... failed
scons: done reading SConscript
scons: Building targets ...
    .
    .
    .
    

Using multi-platform configuration as described in the previous sections will run the configuration commands even when invoking scons -c to clean targets:

% scons -Q -c
Checking for MyLibrary... yes
Removed foo.o
Removed foo
    

Although running the platform checks when removing targets doesn't hurt anything, it's usually unnecessary. You can avoid this by using the GetOption method to check whether the -c (clean) option has been invoked on the command line:

env = Environment()
if not env.GetOption('clean'):
    conf = Configure(env, custom_tests = {'CheckMyLibrary' : CheckMyLibrary})
    if not conf.CheckMyLibrary():
        print 'MyLibrary is not installed!'
        Exit(1)
    env = conf.Finish()
    

% scons -Q -c
Removed foo.o
Removed foo