Thank you for taking the time to read about SCons. SCons is a next-generation software construction tool, or make tool--that is, a software utility for building software (or other files) and keeping built software up-to-date whenever the underlying input files change.
The most distinctive thing about SCons is that its configuration files are actually scripts, written in the Python programming language. This is in contrast to most alternative build tools, which typically invent a new language to configure the build. SCons still has a learning curve, of course, because you have to know what functions to call to set up your build properly, but the underlying syntax used should be familiar to anyone who has ever looked at a Python script.
Paradoxically, using Python as the configuration file format makes SCons easier for non-programmers to learn than the cryptic languages of other build tools, which are usually invented by programmers for other programmers. This is in no small part due to the consistency and readability that are built in to Python. It just so happens that making a real, live scripting language the basis for the configuration files makes it a snap for more accomplished programmers to do more complicated things with builds, as necessary.
There are a few overriding principles we try to live up to in designing and implementing SCons:
First and foremost, by default, SCons guarantees a correct build even if it means sacrificing performance a little. We strive to guarantee the build is correct regardless of how the software being built is structured, how it may have been written, or how unusual the tools are that build it.
Given that the build is correct, we try to make SCons build software as quickly as possible. In particular, wherever we may have needed to slow down the default SCons behavior to guarantee a correct build, we also try to make it easy to speed up SCons through optimization options that let you trade off guaranteed correctness in all end cases for a speedier build in the usual cases.
SCons tries to do as much for you out of the box as reasonable, including detecting the right tools on your system and using them correctly to build the software.
In a nutshell, we try hard to make SCons just "do the right thing" and build software correctly, with a minimum of hassles.
SCons would not exist without a lot of help from a lot of people, many of whom may not even be aware that they helped or served as inspiration. So in no particular order, and at the risk of leaving out someone:
First and foremost, SCons owes a tremendous debt to Bob Sidebotham, the original author of the classic Perl-based Cons tool which Bob first released to the world back around 1996. Bob's work on Cons classic provided the underlying architecture and model of specifying a build configuration using a real scripting language. My real-world experience working on Cons informed many of the design decisions in SCons, including the improved parallel build support, making Builder objects easily definable by users, and separating the build engine from the wrapping interface.
Greg Wilson was instrumental in getting SCons started as a real project when he initiated the Software Carpentry design competition in February 2000. Without that nudge, marrying the advantages of the Cons classic architecture with the readability of Python might have just stayed no more than a nice idea.
The entire SCons team have been absolutely wonderful to work with, and SCons would be nowhere near as useful a tool without the energy, enthusiasm and time people have contributed over the past few years. The "core team" of Chad Austin, Anthony Roach, Charles Crain, Steve Leblanc, Gary Oerbrunner, Greg Spencer and Christoph Wiedemann have been great about reviewing my (and other) changes and catching problems before they get in the code base. Of particular technical note: Anthony's outstanding and innovative work on the tasking engine has given SCons a vastly superior parallel build model; Charles has been the master of the crucial Node infrastructure; Christoph's work on the Configure infrastructure has added crucial Autoconf-like functionality; and Greg has provided excellent support for Microsoft Visual Studio.
Special thanks to David Snopek for contributing his underlying "Autoscons" code that formed the basis of Christoph's work with the Configure functionality. David was extremely generous in making this code available to SCons, given that he initially released it under the GPL and SCons is released under a less-restrictive MIT-style license.
Thanks to Peter Miller for his splendid change management system, Aegis, which has provided the SCons project with a robust development methodology from day one, and which showed me how you could integrate incremental regression tests into a practical development cycle (years before eXtreme Programming arrived on the scene).
And last, thanks to Guido van Rossum for his elegant scripting language, which is the basis not only for the SCons implementation, but for the interface itself.
The best way to contact people involved with SCons, including the author, is through the SCons mailing lists.
If you want to ask general questions about how to use SCons send email to scons-users@lists.sourceforge.net.
If you want to contact the SCons development community directly, send email to scons-devel@lists.sourceforge.net.
If you want to receive announcements about SCons, join the low-volume scons-announce@lists.sourceforge.net mailing list.
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')
|
That's it. Now run the scons command to build the program. On a POSIX-compliant system like Linux or UNIX, you'll see something like:
% scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cc -c -o hello.o hello.c
cc -o hello hello.o
scons: done building targets.
On a Windows system with the Microsoft Visual C++ compiler, you'll see something like:
C:\>scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
cl /nologo /c hello.c /Fohello.obj
link /nologo /OUT:hello.exe hello.obj
scons: done building targets.
First, notice that you only need to specify the name of the source file, and that SCons deduces the names of the object and executable files correctly from the base of the source file name.
Second, notice that the same input SConstruct file, without any changes, generates the correct output file names on both systems: hello.o and hello on POSIX systems, hello.obj and hello.exe on Windows systems. This is a simple example of how SCons makes it extremely easy to write portable software builds.
(Note that we won't provide duplicate side-by-side POSIX and Windows output for all of the examples in this guide; just keep in mind that, unless otherwise specified, any of the examples should work equally well on both types of systems.)
If you're used to build systems like Make you've already figured out that the SConstruct file is the SCons equivalent of a Makefile. That is, the SConstruct file is the input file that SCons reads to control the build.
There is, however, an important difference between an SConstruct file and a Makefile: the SConstruct file is actually a Python script. If you're not already familiar with Python, don't worry. This User's Guide will introduce you step-by-step to the relatively small amount of Python you'll need to know to be able to use SCons effectively. And Python is very easy to learn.
One aspect of using Python as the scripting language is that you can put comments in your SConstruct file using Python's commenting convention; that is, everything between a '#' and the end of the line will be ignored:
# Arrange to build the "hello" program.
Program('hello.c') # "hello.c" is the source file.
|
You'll see throughout the remainder of this Guide that being able to use the power of a real scripting language can greatly simplify the solutions to complex requirements of real-world builds.
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 /nologo /c hello.c /Fohello.obj
link /nologo /OUT:hello.exe hello.obj
scons: done building targets.
These messages emphasize the order in which SCons does its work: 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.
The 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 /nologo /c hello.c /Fohello.obj
link /nologo /OUT:hello.exe hello.obj
Because we want this User's Guide to focus on what SCons is actually doing, we're going 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 -c -o file1.o file1.c
cc -c -o file2.o file2.c
cc -c -o prog.o 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 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', ['main.c', 'file1.c', 'file2.c'])
|
On Linux, a build of this example would look like:
% scons -Q
cc -c -o file1.o file1.c
cc -c -o file2.o file2.c
cc -c -o main.o main.c
cc -o program main.o file1.o file2.o
Or on Windows:
C:\>scons -Q
cl /nologo /c file1.c /Fofile1.obj
cl /nologo /c file2.c /Fofile2.obj
cl /nologo /c main.c /Fomain.obj
link /nologo /OUT:program.exe main.obj file1.obj file2.obj
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.'))
|
(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 string.split() method, 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:
list = Split('main.c file1.c file2.')
Program('program', list)
|
Lastly, the Split function doesn't care how much white space separates the file names in the quoted string. This allows you to create lists of file names that span multiple lines, which often makes for easier editing:
list = Split('main.c
file1.c
file2.c')
Program('program', list)
|
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:
list = Split('main.c file1.c file2.')
Program(target = 'program', source = list)
|
Because the keywords explicitly identify what each argument is, you can actually reverse the order if you prefer:
list = Split('main.c file1.c file2.')
Program(source = list, 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 -c -o bar1.o bar1.c
cc -c -o bar2.o bar2.c
cc -o bar bar1.o bar2.o
cc -c -o foo.o 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.
It's common to re-use code by sharing source files between multiple programs. One way to do this is to create a library from the common source files, which can then be linked into resulting programs. (Creating libraries is discussed in section XXX, below.)
A more straightforward, but perhaps less convenient, way to share source files between multiple programs is simply to include the common files in the lists of source files for each program:
Program(Split('foo.c common1.c common2.c'))
Program('bar', Split('bar1.c bar2.c common1.c common2.c'))
|
SCons recognizes that the object files for the common1.c and common2.c source files each only need to be built once, even though the resulting object files are each linked in to both of the resulting executable programs:
% scons -Q
cc -c -o bar1.o bar1.c
cc -c -o bar2.o bar2.c
cc -c -o common1.o common1.c
cc -c -o common2.o common2.c
cc -o bar bar1.o bar2.o common1.o common2.o
cc -c -o foo.o foo.c
cc -o foo foo.o common1.o common2.o
If two or more programs share a lot of common source files, repeating the common files in the list for each program can be a maintenance problem when you need to change the list of common files. You can simplify this by creating a separate Python list to hold the common file names, and concatenating it with other lists using the Python + operator:
common = ['common1.c', 'common2.c']
foo_files = ['foo.c'] + common
bar_files = ['bar1.c', 'bar2.c'] + common
Program('foo', foo_files)
Program('bar', bar_files)
|
This is functionally equivalent to the previous example.
One of the more useful ways in which you can use multiple construction environments is to link programs with different sets of libraries.
You build your own libraries by specifying Library instead of Program:
Library('foo', ['f1.c', 'f2.c', 'f3.c'])
|
SCons uses the appropriate library prefix and suffix for your system. So on POSIX or Linux systems, the above example would build as follows (although ranlib may not be called on all systems):
% scons -Q
cc -c -o f1.o f1.c
cc -c -o f2.o f2.c
cc -c -o f3.o f3.c
ar r libfoo.a f1.o f2.o f3.o
ranlib libfoo.a
On a Windows system, a build of the above example would look like:
C:\>scons -Q
cl /nologo /c f1.c /Fof1.obj
cl /nologo /c f2.c /Fof2.obj
cl /nologo /c f3.c /Fof3.obj
lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj
The rules for the target name of the library are similar to those for programs: if you don't explicitly specify a target library name, SCons will deduce one from the name of the first source file specified, and SCons will add an appropriate file prefix and suffix if you leave them off.
Usually, you build a library because you want to link it with one or more programs. You link libraries with a program by specifying the libraries in the LIBS construction variable, and by specifying the directory in which the library will be found in the LIBPATH construction variable: env = Environment(LIBS = 'foo', LIBPATH = '.')
Library('foo', ['f1.c', 'f2.c', 'f3.c'])
Program('prog.c', LIBS='foo', LIBPATH='.')
|
Notice, of course, that you don't need to specify a library prefix (like lib) or suffix (like .a or .lib). SCons uses the correct prefix or suffix for the current system.
On a POSIX or Linux system, a build of the above example would look like:
% scons -Q
cc -c -o f1.o f1.c
cc -c -o f2.o f2.c
cc -c -o f3.o f3.c
ar r libfoo.a f1.o f2.o f3.o
ranlib libfoo.a
cc -c -o prog.o prog.c
cc -o prog prog.o -L. -lfoo
On a Windows system, a build of the above example would look like:
C:\>scons -Q
cl /nologo /c f1.c /Fof1.obj
cl /nologo /c f2.c /Fof2.obj
cl /nologo /c f3.c /Fof3.obj
lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj
cl /nologo /c prog.c /Foprog.obj
link /nologo /OUT:prog.exe /LIBPATH:. foo.lib prog.obj
As usual, notice that SCons has taken care of constructing the correct command lines to link with the specified library on each system.
By default, the linker will only look in certain system-defined directories for libraries. SCons knows how to look for libraries in directories that you specify with the LIBPATH construction variable. LIBPATH consists of a list of directory names, like so:
Program('prog.c', LIBS = 'm',
LIBPATH = ['/usr/lib', '/usr/local/lib'])
|
Using a Python list is preferred because it's portable across systems. Alternatively, you could put all of the directory names in a single string, separated by the system-specific path separator character: a colon on POSIX systems:
LIBPATH = '/usr/lib:/usr/local/lib'
|
or a semi-colon on Windows systems:
LIBPATH = 'C:\lib;D:\lib'
|
When the linker is executed, SCons will create appropriate flags so that the linker will look for libraries in the same directories as SCons. So on a POSIX or Linux system, a build of the above example would look like:
% scons -Q
cc -c -o prog.o prog.c
cc -o prog prog.o -L/usr/lib -L/usr/local/lib -lm
On a Windows system, a build of the above example would look like:
C:\>scons -Q
cl /nologo /c prog.c /Foprog.obj
link /nologo /OUT:prog.exe /LIBPATH:\usr\lib /LIBPATH:\usr\local\lib m.lib prog.obj
Note again that SCons has taken care of the system-specific details of creating the right command-line options.
So far we've seen how SCons handles one-time builds. But the real point of a build tool like SCons is to rebuild only the necessary things when source files change--or, put another way, SCons should not waste time rebuilding things that have already been built. You can see this at work simply be re-invoking SCons after building our simple hello example:
% scons -Q
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q
scons: `.' is up to date.
The second time it is executed, SCons realizes that the hello program is up-to-date with respect to the current hello.c source file, and avoids rebuilding it. You can see this more clearly by naming the hello program explicitly on the command line:
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
Note that SCons reports "...is up to date" only for target files named explicitly on the command line, to avoid cluttering the output.
The other side of avoiding unnecessary rebuilds is the fundamental build tool behavior of rebuilding things when a source file changes, so that the built software is up to date. SCons keeps track of this through a signature for each source file, and allows you to configure whether you want to use the source file contents or the modification time (timestamp) as the signature.
By default, SCons keeps track of whether a source file has changed based on the file's contents, not the modification time. This means that you may be surprised by the default SCons behavior if you are used to the Make convention of forcing a rebuild by updating the file's modification time (using the touch command, for example):
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% touch hello.c
% scons -Q hello
scons: `hello' is up to date.
Even though the file's modification time has changed, SCons realizes that the contents of the hello.c file have not changed, and therefore that the hello program need not be rebuilt. This avoids unnecessary rebuilds when, for example, someone rewrites the contents of a file without making a change. But if the contents of the file really do change, then SCons detects the change and rebuilds the program as required:
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% edit hello.c
[CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
Note that you can, if you wish, specify this default behavior (MD5 signatures) explicitly using the SourceSignatures function as follows:
Program('hello.c')
SourceSignatures('MD5')
|
If you prefer, you can configure SCons to use the modification time of source files, not the file contents, when deciding if something needs to be rebuilt. To do this, call the SourceSignatures function as follows:
Program('hello.c')
SourceSignatures('timestamp')
|
This makes SCons act like Make when a file's modification time is updated (using the touch command, for example):
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% touch hello.c
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
As you've just seen, SCons uses signatures to decide whether a target file is up to date or must be rebuilt. When a target file depends on another target file, SCons allows you to separately configure how the signatures of "intermediate" target files are used when deciding if a dependent target file must be rebuilt.
Modifying a source file will cause not only its direct target file to be rebuilt, but also the target file(s) that depend on that direct target file. In our example, changing the contents of the hello.c file causes the hello.o file to be rebuilt, which in turn causes the hello program to be rebuilt:
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% edit hello.c
[CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
What's not obvious, though, is that SCons internally handles the signature of the target file(s) (hello.o in the above example) differently from the signature of the source file (hello.c). By default, SCons tracks whether a target file must be rebuilt by using a build signature that consists of the combined signatures of all the files that go into making the target file. This is efficient because the accumulated signatures actually give SCons all of the information it needs to decide if the target file is out of date.
If you wish, you can specify this default behavior (build signatures) explicitly using the TargetSignatures function:
Program('hello.c')
TargetSignatures('build')
|
Sometimes a source file can be changed in such a way that the contents of the rebuilt target file(s) will be exactly the same as the last time the file was built. If so, then any other target files that depend on such a built-but-not-changed target file actually need not be rebuilt. You can make SCons realize that it does not need to rebuild a dependent target file in this situation using the TargetSignatures function as follows:
Program('hello.c')
TargetSignatures('content')
|
So if, for example, a user were to only change a comment in a C file, then the rebuilt hello.o file would be exactly the same as the one previously built (assuming the compiler doesn't put any build-specific information in the object file). SCons would then realize that it would not need to rebuild the hello program as follows:
% scons -Q hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% edit hello.c
[CHANGE A COMMENT IN hello.c]
% scons -Q hello
cc -c -o hello.o hello.c
scons: `hello' is up to date.
In essence, SCons has "short-circuited" any dependent builds when it realizes that a target file has been rebuilt to exactly the same file as the last build. So configured, SCons does take some extra processing time to scan the contents of the target (hello.o) file, but this may save time if the rebuild that was avoided would have been very time-consuming and expensive.
Now suppose that our "Hello, World!" program actually has a #include line to include the hello.h file in the compilation:
#include "hello.h"
int
main()
{
printf("Hello, %s!\n", string);
}
|
And, for completeness, the hello.h file looks like this:
#define string "world"
|
In this case, we want SCons to recognize that, if the contents of the hello.h file change, the hello program must be recompiled. To do this, we need to modify the SConstruct file like so:
Program('hello.c', CPPPATH = '.')
|
The CPPPATH value tells SCons to look in the current directory ('.') for any files included by C source files (.c or .h files). With this assignment in the SConstruct file:
% scons -Q hello
cc -I. -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
cc -I. -c -o hello.o hello.c
cc -o hello hello.o
First, notice that SCons added the -I. argument from the CPPPATH variable so that the compilation would find the hello.h file in the local directory.
Second, realize that SCons knows that the hello program must be rebuilt because it scans the contents of the hello.c file for the #include lines that indicate another file is being included in the compilation. SCons records these as implicit dependencies of the target file, Consequently, when the hello.h file changes, SCons realizes that the hello.c file includes it, and rebuilds the resulting hello program that depends on both the hello.c and hello.h files.
Like the LIBPATH variable, the CPPPATH variable may be a list of directories, or a string separated by the system-specific path separate character (':' on POSIX/Linux, ';' on Windows). Either way, SCons creates the right command-line options so that the following example:
Program('hello.c', CPPPATH = ['include', '/home/project/inc'])
|
Will look like this on POSIX or Linux:
% scons -Q hello
cc -Iinclude -I/home/project/inc -c -o hello.o hello.c
cc -o hello hello.o
And like this on Windows:
C:\>scons -Q hello.exe
cl /nologo /Iinclude /I\home\project\inc /c hello.c /Fohello.obj
link /nologo /OUT:hello.exe hello.obj
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 -c -o hello.o 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)
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When using cached implicit dependencies, sometimes you want to "start fresh" and have SCons re-scan the files for which it previously cached the dependencies. For example, if you have recently installed a new version of external code that you use for compilation, the external header files will have changed and the previously-cached implicit dependencies will be out of date. You can update them by running SCons with the --implicit-deps-changed option:
% scons -Q --implicit-deps-changed hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
In this case, SCons will re-scan all of the implicit dependencies and cache updated copies of the information.
By default when caching dependencies, SCons notices when a file has been modified and re-scans the file for any updated implicit dependency information. Sometimes, however, you may want to force SCons to use the cached implicit dependencies, even if the source files changed. This can speed up a build for example, when you have changed your source files but know that you haven't changed any #include lines. In this case, you can use the --implicit-deps-unchanged option:
% scons -Q --implicit-deps-unchanged hello
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
In this case, SCons will assume that the cached implicit dependencies are correct and will not bother to re-scan changed files. For typical builds after small, incremental changes to source files, the savings may not be very big, but sometimes every bit of improved performance counts.
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 = Program('hello.c')
Ignore(hello, '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 to rebuild hello if the hello.h file changed. A more realistic example might be if the hello program is being built in a directory that is shared between multiple systems that have different copies of the stdio.h include file. In that case, SCons would notice the differences between the different systems' copies of stdio.h and would rebuild hello each time you change systems. You could avoid these rebuilds as follows:
env = Environment()
hello = env.Program('hello.c')
env.Ignore(hello, '/usr/include/stdio.h')
|
On the other hand, 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:
env = Environment()
hello = env.Program('hello.c')
env.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
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 accomodates these different build requirements by allowing you to create and configure multiple construction environments that control how the software is built. Technically, 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.)
A construction environment is created by the Environment method. When you initialize a construction environment, you can set the values of the environment's construction variables to control how a program is built. For example:
env = Environment(CC = 'gcc',
CCFLAGS = '-O2')
env.Program('foo.c')
|
This example, rather than using the default, explicitly specifies use of the GNU C compiler gcc, and further specifies that the -O2 (optimization level two) flag should be used when compiling the object file. So a run from this example would look like:
% scons -Q
gcc -O2 -c -o foo.o foo.c
gcc -o foo foo.o
The real advantage of construction environments become apparent when you realize that you can create as many different construction environments as you need, each tailored to a different way to build some piece of software or other file. If, for example, we need to build one program with the -O2 flag and another with the -g (debug) flag, we would do this like so:
opt = Environment(CCFLAGS = '-O2')
dbg = Environment(CCFLAGS = '-g')
opt.Program('foo', 'foo.c')
dbg.Program('bar', 'bar.c')
|
% scons -Q
cc -g -c -o bar.o bar.c
cc -o bar bar.o
cc -O2 -c -o foo.o foo.c
cc -o foo foo.o
We can even use multiple construction environments to build multiple versions of a single program. If you do this by simply trying to use the Program builder with both environments, though, like this:
opt = Environment(CCFLAGS = '-O2')
dbg = Environment(CCFLAGS = '-g')
opt.Program('foo', 'foo.c')
dbg.Program('foo', 'foo.c')
|
Then SCons generates the following error:
% scons -Q
scons: *** Two different environments were specified for the same target: foo.o
File "SConstruct", line 6, in ?
This is because the two Program calls have each implicitly told SCons to generate an object file named foo.o, one with a CCFLAGS value of -O2 and one with a CCFLAGS value of -g. SCons can't just decide that one of them should take precedence over the other, so it generates the error. To avoid this problem, we must explicitly specify that each environment compile foo.c to a separately-named object file using the Object call, like so:
opt = Environment(CCFLAGS = '-O2')
dbg = Environment(CCFLAGS = '-g')
o = opt.Object('foo-opt', 'foo.c')
opt.Program(o)
d = dbg.Object('foo-dbg', 'foo.c')
dbg.Program(d)
|
Notice that each call to the Object builder returns a value, an internal SCons object that represents the object file that will be built. We then use that object as input to the Program builder. This avoids having to specify explicitly the object file name in multiple places, and makes for a compact, readable SConstruct file. Our SCons output then looks like:
% scons -Q
cc -g -c -o foo-dbg.o foo.c
cc -o foo-dbg foo-dbg.o
cc -O2 -c -o foo-opt.o foo.c
cc -o foo-opt foo-opt.o
Sometimes you want more than one construction environment to share the same values for one or more variables. Rather than always having to repeat all of the common variables when you create each construction environment, you can use the Copy method to create a copy of a construction environment.
Like the Environment call that creates a construction environment, the Copy method takes construction variable assignments, which will override the values in the copied construction environment. For example, suppose we want to use gcc to create three versions of a program, one optimized, one debug, and one with neither. We could do this by creating a "base" construction environment that sets CC to gcc, and then creating two copies, one which sets CCFLAGS for optimization and the other which sets CCFLAGS for debugging:
env = Environment(CC = 'gcc')
opt = env.Copy(CCFLAGS = '-O2')
dbg = env.Copy(CCFLAGS = '-g')
env.Program('foo', 'foo.c')
o = opt.Object('foo-opt', 'foo.c')
opt.Program(o)
d = dbg.Object('foo-dbg', 'foo.c')
dbg.Program(d)
|
Then our output would look like:
% scons -Q
gcc -c -o foo.o foo.c
gcc -o foo foo.o
gcc -g -c -o foo-dbg.o foo.c
gcc -o foo-dbg foo-dbg.o
gcc -O2 -c -o foo-opt.o foo.c
gcc -o foo-opt foo-opt.o
You can fetch individual construction variables using the normal syntax for accessing individual named items in a Python dictionary:
env = Environment()
print "CC is:", env['CC']
|
This example SConstruct file doesn't build anything, but because it's actually a Python script, it will print the value of CC for us:
% scons -Q
CC is: cc
scons: `.' is up to date.
A construction environment, however, is actually a Python object with associated methods, etc. If you want to have direct access to only the dictionary of construction variables, you can fetch this using the Dictionary method:
env = Environment(FOO = 'foo', BAR = 'bar')
dict = env.Dictionary()
for key in ['OBJSUFFIX', 'LIBSUFFIX', 'PROGSUFFIX']:
print "key = %s, value = %s" % (key, dict[key])
|
This SConstruct file will print the specified dictionary items for us on POSIX systems as follows:
% scons -Q
key = OBJSUFFIX, value = .o
key = LIBSUFFIX, value = .a
key = PROGSUFFIX, value =
scons: `.' is up to date.
And on Win32:
C:\>scons -Q
key = OBJSUFFIX, value = .obj
key = LIBSUFFIX, value = .lib
key = PROGSUFFIX, value = .exe
scons: `.' is up to date.
SCons provides various methods that support modifying existing values in a construction environment.
You can replace existing construction variable values using the Replace method:
env = Environment(CCFLAGS = '-DDEFINE1')
env.Program('foo.c')
env.Replace(CCFLAGS = '-DDEFINE2')
env.Program('bar.c')
|
The replaced value completely overwrites
% scons -Q
cc -DDEFINE2 -c -o bar.o bar.c
cc -o bar bar.o
cc -DDEFINE1 -c -o foo.o foo.c
cc -o foo foo.o
You can append a value to an existing construction variable using the Append method:
env = Environment(CCFLAGS = '-DMY_VALUE')
env.Append(CCFLAGS = ' -DLAST')
env.Program('foo.c')
|
% scons -Q
cc -DMY_VALUE -DLAST -c -o foo.o foo.c
cc -o foo foo.o
You can append a value to the beginning an existing construction variable using the Prepend method:
env = Environment(CCFLAGS = '-DMY_VALUE')
env.Prepend(CCFLAGS = '-DFIRST ')
env.Program('foo.c')
|
% scons -Q
cc -DFIRST -DMY_VALUE -c -o foo.o foo.c
cc -o foo foo.o
As mentioned previously, SCons will build every target in or below the current directory by default--that is, when you don't explicitly specify one or more targets on the command line. Sometimes, however, you may want to specify explicitly that only certain programs should be built by default. You do this with the Default function:
env = Environment()
hello = env.Program('hello.c')
env.Program('goodbye.c')
Default(hello)
|
This SConstruct file knows how to build two programs, hello and goodbye, but only builds the hello program by default:
% scons -Q
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q
scons: `hello' is up to date.
% scons -Q goodbye
cc -c -o goodbye.o goodbye.c
cc -o goodbye goodbye.o
Note that, even when you use the Default function in your SConstruct file, you can still explicitly specify the current directory (.) on the command line to tell SCons to build everything in (or below) the current directory:
% scons -Q .
cc -c -o goodbye.o goodbye.c
cc -o goodbye goodbye.o
cc -c -o hello.o hello.c
cc -o hello hello.o
You can also call the Default function more than once, in which case each call adds to the list of targets to be built by default:
env = Environment()
prog1 = env.Program('prog1.c')
Default(prog1)
prog2 = env.Program('prog2.c')
prog3 = env.Program('prog3.c')
Default(prog3)
|
Or you can specify more than one target in a single call to the Default function:
env = Environment()
prog1 = env.Program('prog1.c')
prog2 = env.Program('prog2.c')
prog3 = env.Program('prog3.c')
Default(prog1, prog3)
|
Either of these last two examples will build only the prog1 and prog3 programs by default:
% scons -Q
cc -c -o prog1.o prog1.c
cc -o prog1 prog1.o
cc -c -o prog3.o prog3.c
cc -o prog3 prog3.o
% scons -Q .
cc -c -o prog2.o prog2.c
cc -o prog2 prog2.o
Lastly, if for some reason you don't want any targets built by default, you can use the Python None variable:
env = Environment()
prog1 = env.Program('prog1.c')
prog2 = env.Program('prog2.c')
Default(None)
|
Which would produce build output like:
% scons -Q
scons: *** No targets specified and no Default() targets found. Stop.
% scons -Q .
cc -c -o prog1.o prog1.c
cc -o prog1 prog1.o
cc -c -o prog2.o prog2.c
cc -o prog2 prog2.o
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.
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.
Once a program is built, it is often appropriate to install it in another directory for public use. You use the Install method to arrange for a program, or any other file, to be copied into a destination directory:
env = Environment()
hello = env.Program('hello.c')
env.Install('/usr/bin', hello)
|
Note, however, that installing a file is still considered a type of file "build." This is important when you remember that the default behavior of SCons is to build files in or below the current directory. If, as in the example above, you are installing files in a directory outside of the top-level SConstruct file's directory tree, you must specify that directory (or a higher directory, such as /) for it to install anything there:
% scons -Q
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q /usr/bin
Install file: "hello" as "/usr/bin/hello"
It can, however, be cumbersome to remember (and type) the specific destination directory in which the program (or any other file) should be installed. This is an area where the Alias function comes in handy, allowing you, for example, to create a pseudo-target named install that can expand to the specified destination directory:
env = Environment()
hello = env.Program('hello.c')
env.Install('/usr/bin', hello)
env.Alias('install', '/usr/bin')
|
This then yields the more natural ability to install the program in its destination as follows:
% scons -Q
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q install
Install file: "hello" as "/usr/bin/hello"
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 -c -o goodbye.o goodbye.c
cc -o goodbye goodbye.o
Install file: "goodbye" as "/usr/bin/goodbye"
cc -c -o hello.o 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 -c -o hello.o 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 the both the target and source arguments:
env = Environment()
hello = env.Program('hello.c')
goodbye = env.Program('goodbye.c')
env.InstallAs(['/usr/bin/hello-new',
'/usr/bin/goodbye-new'],
[hello, goodbye])
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 -c -o goodbye.o goodbye.c
cc -o goodbye goodbye.o
Install file: "goodbye" as "/usr/bin/goodbye-new"
cc -c -o hello.o hello.c
cc -o hello hello.o
Install file: "hello" as "/usr/bin/hello-new"
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()
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 -c -o f1.o f1.c
cc -c -o f2.o f2.c
cc -c -o f3.o f3.c
ar r libfoo.a f1.o f2.o f3.o
ranlib libfoo.a
SCons will, however, still delete files marked as Precious when the -c option is used.
The source code for large software projects rarely stays in a single directory, but is nearly always divided into a hierarchy of directories. Organizing a large software build using SCons involves creating a hierarchy of build scripts using the SConscript function.
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 -c -o prog1/foo1.o prog1/foo1.c
cc -c -o prog1/foo2.o prog1/foo2.c
cc -c -o prog1/main.o prog1/main.c
cc -o prog1/prog1 prog1/main.o prog1/foo1.o prog1/foo2.o
cc -c -o prog2/bar1.o prog2/bar1.c
cc -c -o prog2/bar2.o prog2/bar2.c
cc -c -o prog2/main.o 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 -c -o lib/foo1.o lib/foo1.c
cc -c -o src/prog/foo2.o src/prog/foo2.c
cc -c -o src/prog/main.o src/prog/main.c
cc -o src/prog/prog src/prog/main.o lib/foo1.o src/prog/foo2.o
(Notice that the lib/foo1.o object file is built in the same directory as its source file. See section XXX, below, for information about how to build the object file in a different subdirectory.)
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 -c -o src/prog/foo2.o src/prog/foo2.c
cc -c -o src/prog/main.o src/prog/main.c
cc -c -o /usr/joe/lib/foo1.o /usr/joe/lib/foo1.c
cc -o src/prog/prog src/prog/main.o /usr/joe/lib/foo1.o src/prog/foo2.o
(As was the case with top-relative path names, notice that the /usr/joe/lib/foo1.o object file is built in the same directory as its source file. See section XXX, below, for information about how to build the object file in a different subdirectory.)
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.
There are two ways to export a variable, such as a construction environment, from an SConscript file, so that it may be used by other SConscript files. First, you can call the Export function with a list of variables, or a string white-space separated variable names. Each call to Export adds one or more variables to a global list of variables that are available for import by other SConscript files.
env = Environment()
Export('env')
|
You may export more than one variable name at a time:
env = Environment()
debug = ARGUMENTS['debug']
Export('env', 'debug')
|
Because white space is not legal in Python variable names, the Export function will even automatically split a string into separate names for you:
Export('env debug')
|
Second, you can specify a list of variables to export as a second argument to the SConscript function call:
SConscript('src/SConscript', 'env')
|
Or as the exports keyword argument:
SConscript('src/SConscript', exports='env')
|
These calls export the specified variables to only the listed SConscript files. You may, however, specify more than one SConscript file in a list:
SConscript(['src1/SConscript',
'src2/SConscript'], exports='env')
|
This is functionally equivalent to calling the SConscript function multiple times with the same exports argument, one per SConscript file.
Once a variable has been exported from a calling SConscript file, it may be used in other SConscript files by calling the Import function:
Import('env')
env.Program('prog', ['prog.c'])
|
The Import call makes the env construction environment available to the SConscript file, after which the variable can be used to build programs, libraries, etc.
Like the Export function, the Import function can be used with multiple variable names:
Import('env', 'debug')
env = env.Copy(DEBUG = debug)
env.Program('prog', ['prog.c'])
|
And the Import function will similarly split a string along white-space into separate variable names:
Import('env debug')
env = env.Copy(DEBUG = debug)
env.Program('prog', ['prog.c'])
|
Lastly, as a special case, you may import all of the variables that have been exported by supplying an asterisk to the Import function:
Import('*')
env = env.Copy(DEBUG = debug)
env.Program('prog', ['prog.c'])
|
If you're dealing with a lot of SConscript files, this can be a lot simpler than keeping arbitrary lists of imported variables in each file.
Sometimes, you would like to be able to use information from a subsidiary SConscript file in some way. For example, suppose that you want to create one library from source files scattered throughout a number of subsidiary SConscript files. You can do this by using the Return function to return values from the subsidiary SConscript files to the calling file.
If, for example, we have two subdirectories foo and bar that should each contribute a source file to a Library, what we'd like to be able to do is collect the object files from the subsidiary SConscript calls like this:
env = Environment()
Export('env')
objs = []
for subdir in ['foo', 'bar']:
o = SConscript('%s/SConscript' % subdir)
objs.append(o)
env.Library('prog', objs)
|
We can do this by using the Return function in the foo/SConscript file like this:
Import('env')
obj = env.Object('foo.c')
Return('obj')
|
(The corresponding bar/SConscript file should be pretty obvious.) Then when we run SCons, the object files from the subsidiary subdirectories are all correctly archived in the desired library:
% scons -Q
cc -c -o bar/bar.o bar/bar.c
cc -c -o foo/foo.o foo/foo.c
ar r libprog.a foo/foo.o bar/bar.o
ranlib libprog.a
It's often useful to keep any built files completely separate from the source files. This is usually done by creating one or more separate build directories that are used to hold the built objects files, libraries, and executable programs, etc. for a specific flavor of build. SCons provides two ways to do this, one through the SConscript function that we've already seen, and the second through a more flexible BuildDir function.
The most straightforward way to establish a build directory 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 build_dir argument to the SConscript function call:
SConscript('src/SConscript', build_dir='build')
|
SCons will then build all of the files in the build subdirectory:
% ls src
SConscript hello.c
% scons -Q
cc -c -o build/hello.o 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 build directories 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 build directories is simply that some tools (mostly older vesions) 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 build directory, or to duplicate the source files in the build directory.
Additionally, relative references between files can cause problems if we don't just duplicate the hierarchy of source files in the build directory. You can see this at work in use of the C preprocessor #include mechanism with double quotes, not angle brackets:
#include "file.h"
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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 build 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 build 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', build_dir='build', duplicate=0)
|
When this flag is specified, SCons uses the build directory like most people expect--that is, the output files are placed in the build directory while the source files stay in the source directory:
% ls 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 BuildDir function to establish that target files should be built in a separate directory from the source files:
BuildDir('build', 'src')
env = Environment()
env.Program('build/hello.c')
|
Note that when you're not using an SConscript file in the src subdirectory, you must actually specify that the program must be built from the build/hello.c file that SCons will duplicate in the build subdirectory.
When using the BuildDir function directly, SCons still duplicates the source files in the build directory by default:
% ls src
hello.c
% scons -Q
cc -c -o build/hello.o 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:
BuildDir('build', 'src', duplicate=0)
env = Environment()
env.Program('build/hello.c')
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In which case SCons will disable duplication of the source files:
% ls src
hello.c
% scons -Q
cc -c -o build/hello.o src/hello.c
cc -o build/hello build/hello.o
% ls build
hello hello.o
Even when using the BuildDir function, it's much more natural to use it with a subsidiary SConscript file. For example, if the src/SConscript looks like this:
env = Environment()
env.Program('hello.c')
|
Then our SConstruct file could look like:
BuildDir('build', 'src')
SConscript('build/SConscript')
|
Yielding the following output:
% ls src
SConscript hello.c
% scons -Q
cc -c -o build/hello.o 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 BuildDir function now gives us everything we need to show how easy it is to create variant builds using SCons. Suppose, for example, that we want to build a program for both Windows and Linux platforms, but that we want to build it in a shared directory with separate side-by-side build directories for the Windows and Linux versions of the program.
platform = ARGUMENTS.get('OS', Platform())
include = "#export/$PLATFORM/include"
lib = "#export/$PLATFORM/lib"
bin = "#export/$PLATFORM/bin"
env = Environment(PLATFORM = platform,
BINDIR = bin,
INCDIR = include,
LIBDIR = lib,
CPPPATH = [include],
LIBPATH = [lib],
LIBS = 'world')
Export('env')
env.SConscript('src/SConscript', build_dir='build/$PLATFORM')
#
#BuildDir("#build/$PLATFORM", 'src')
#SConscript("build/$PLATFORM/hello/SConscript")
#SConscript("build/$PLATFORM/world/SConscript")
|
This SConstruct file, when run on a Linux system, yields:
% scons -Q OS=linux
Install file: "build/linux/world/world.h" as "export/linux/include/world.h"
cc -Iexport/linux/include -c -o build/linux/hello/hello.o build/linux/hello/hello.c
cc -Iexport/linux/include -c -o build/linux/world/world.o build/linux/world/world.c
ar r build/linux/world/libworld.a build/linux/world/world.o
ranlib build/linux/world/libworld.a
Install file: "build/linux/world/libworld.a" as "export/linux/lib/libworld.a"
cc -o build/linux/hello/hello build/linux/hello/hello.o -Lexport/linux/lib -lworld
Install file: "build/linux/hello/hello" as "export/linux/bin/hello"
The same SConstruct file on Windows would build:
C:\>scons -Q OS=windows
Install file: "build/windows/world/world.h" as "export/windows/include/world.h"
cl /nologo /Iexport\windows\include /c build\windows\hello\hello.c /Fobuild\windows\hello\hello.obj
cl /nologo /Iexport\windows\include /c build\windows\world\world.c /Fobuild\windows\world\world.obj
lib /nologo /OUT:build\windows\world\world.lib build\windows\world\world.obj
Install file: "build/windows/world/world.lib" as "export/windows/lib/world.lib"
link /nologo /OUT:build\windows\hello\hello.exe /LIBPATH:export\windows\lib world.lib build\windows\hello\hello.obj
Install file: "build/windows/hello/hello.exe" as "export/windows/bin/hello.exe"
Although SCons provides many useful methods for building common software products: programs, libraries, documents. you frequently want to be able to build some other type of file not supported directly by SCons Fortunately, SCons makes it very easy to define your own Builder objects for any custom file types you want to build. (In fact, the SCons interfaces for creating Builder objects are flexible enough and easy enough to use that all of the the SCons built-in Builder objects are created the mechanisms described in this section.)
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.
A Builder object isn't useful until it's attached to a construction environment so that we can call it to arrange for files to be built. This is done through the BUILDERS construction variable in an environment. The BUILDERS variable is a Python dictionary that maps the names by which you want to call various Builder objects to the objects themselves. For example, if we want to call the Builder we just defined by the name Foo, our SConstruct file might look like:
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
|
With the Builder so attached to our construction environment we can now actually call it like so:
env.Foo('file.foo', 'file.input')
|
Then when we run SCons it looks like:
% scons -Q
foobuild < file.input > file.foo
Note, however, that the default BUILDERS variable in a construction environment comes with a default set of Builder objects already defined: Program, Library, etc. And when we explicitly set the BUILDERS variable when we create the construction environment, the default Builders are no longer part of the environment:
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
|
% scons -Q
AttributeError: SConsEnvironment instance has no attribute 'Program':
To be able use both our own defined Builder objects and the default Builder objects in the same construction environment, you can either add to the BUILDERS variable using the Append function:
env = Environment()
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env.Append(BUILDERS = {'Foo' : bld})
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
|
Or you can explicitly set the appropriately-named key in the BUILDERS dictionary:
env = Environment()
bld = Builder(action = 'foobuild < $SOURCE > $TARGET')
env['BUILDERS']['Foo'] = bld
env.Foo('file.foo', 'file.input')
env.Program('hello.c')
|
Either way, the same construction environment can then use both the newly-defined Foo Builder and the default Program Builder:
% scons -Q
foobuild < file.input > file.foo
cc -c -o hello.o hello.c
cc -o hello hello.o
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:
A list of Node objects representing the target or targets to be built by this builder function. The file names of these target(s) may be extracted using the Python str funcion.
A list of Node objects representing the sources to be used by this builder function to build the targets. The file names of these source(s) may be extracted using the Python str funcion.
The construction environment used for building the target(s). The builder function may use any of the environment's construction variables in any way to affect how it builds the targets.
The builder function must return a 0 or None value if the target(s) are built successfully. The builder function may raise an exception or return any non-zero value to indicate that the build is unsuccessful,
Once you've defined the Python function that will build your target file, defining a Builder object for it is as simple as specifying the name of the function, instead of an external command, as the Builder's action argument:
def build_function(target, source, env):
# Code to build "target" from "source"
return None
bld = Builder(action = build_function,
suffix = '.foo',
src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
|
And notice that the output changes slightly, reflecting the fact that a Python function, not an external command, is now called to build the target file:
% scons -Q
build_function("file.foo", "file.input")
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:
A list of Node objects representing the sources to be built by the command or other action generated by this function. The file names of these source(s) may be extracted using the Python str funcion.
A list of Node objects representing the target or targets to be built by the command or other action generated by this function. The file names of these target(s) may be extracted using the Python str funcion.
The construction environment used for building the target(s). The generator may use any of the environment's construction variables in any way to determine what command or other action to return.
A flag that specifies whether the generator is being called to contribute to a build signature, as opposed to actually executing the command.
The generator must return a command string or other action that will be used to build the specified target(s) from the specified source(s).
Once you've defined a generator, you create a Builder to use it by specifying the generator keyword argument instead of action.
def generate_actions(source, target, env, for_signature):
return 'foobuild < %s > %s' % (source[0], target[0])
bld = Builder(generator = generate_actions,
suffix = '.foo',
src_suffix = '.input')
env = Environment(BUILDERS = {'Foo' : bld})
env.Foo('file')
|
% scons -Q
foobuild < file.input > file.foo
Note that it's illegal to specify both an action and a generator for a Builder.
SCons supports the ability for a Builder to modify the lists of target(s) from the specified source(s).
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')
|
% scons -Q
foobuild file.foo new_target - file.input new_source
bld = Builder(action = 'XXX',
suffix = '.foo',
src_suffix = '.input',
emitter = 'MY_EMITTER')
def modify1(target, source, env):
return target, source
def modify2(target, source, env):
return target, source
env1 = Environment(BUILDERS = {'Foo' : bld},
MY_EMITTER = modify1)
env2 = Environment(BUILDERS = {'Foo' : bld},
MY_EMITTER = modify2)
env1.Foo('file1')
env2.Foo('file2')
|
Creating a Builder and attaching it to a construction environment allows for a lot of flexibility when you want to re-use actions to build multiple files of the same type. This can, however, be cumbersome if you only need to execute one specific command to build a single file (or group of files). For these situations, SCons supports a Command Builder that arranges for a specific action to be executed to build a specific file or files. This looks a lot like the other builders (like Program, Object, etc.), but takes as an additional argument the command to be executed to build the file:
env = Environment()
env.Command('foo.out', 'foo.in', "sed 's/x/y/' < $SOURCE > $TARGET")
|
% scons -Q
sed 's/x/y/' < foo.in > foo.out
This is often more convenient than creating a Builder object and adding it to the BUILDERS variable of a construction environment
Note that the action you
env = Environment()
def build(target, source, env):
# Whatever it takes to build
return None
env.Command('foo.out', 'foo.in', build)
|
% scons -Q
build("foo.out", "foo.in")
SCons has built-in scanners that know how to look in C, Fortran and IDL source files for information about other files that targets built from those files depend on--for example, in the case of files that use the C preprocessor, the .h files that are specified using #include lines in the source. You can use the same mechanisms that SCons uses to create its built-in scanners to write scanners of your own for file types that SCons does not know how to scan "out of the box."
Suppose, for example, that we want to create a simple scanner for .foo files. A .foo file contains some text that will be processed, and can include other files on lines that begin with include followed by a file name:
include filename.foo
|
Scanning a file will be handled by a Python function that you must supply. Here is a function that will use the Python re module to scan for the include lines in our example:
import re
include_re = re.compile(r'^include\\s+(\\S+)$', re.M)
def kfile_scan(node, env, path, arg):
contents = node.get_contents()
return include_re.findall(contents)
|
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.
An SCons node object representing the file being scanned. The path name to the file can be used by converting the node to a string using the str() function, or an internal SCons get_contents() object method can be used to fetch the contents.
The construction environment in effect for this scan. The scanner function may choose to use construction variables from this environment to affect its behavior.
A list of directories that form the search path for included files for this scanner. This is how SCons handles the CPPPATH and LIBPATH variables.
An optional argument that you can choose to have passed to this scanner function by various scanner instances.
A Scanner object is created using the Scanner function, which typically takes an skeys argument to associate the type of file suffix with this scanner. The Scanner object must then be associated with the SCANNERS construction variable of a construction environment, typically by using the Append method:
kscan = Scanner(function = kfile_scan,
skeys = ['.k'])
env.Append(SCANNERS = kscan)
|
When we put it all together, it looks like:
import re
include_re = re.compile(r'^include\\s+(\\S+)$', re.M)
def kfile_scan(node, env, path):
contents = node.get_contents()
includes = include_re.findall(contents)
return includes
kscan = Scanner(function = kfile_scan,
skeys = ['.k'])
env = Environment(ENV = {'PATH' : '/usr/local/bin'})
env.Append(SCANNERS = kscan)
env.Command('foo', 'foo.k', 'kprocess < $SOURCES > $TARGET')
|
Often, a software project will have one or more central repositories, directory trees that contain source code, or derived files, or both. You can eliminate additional unnecessary rebuilds of files by having SCons use files from one or more code repositories to build files in your local build tree.
It's often useful to allow multiple programmers working on a project to build software from source files and/or derived files that are stored in a centrally-accessible repository, a directory copy of the source code tree. (Note that this is not the sort of repository maintained by a source code management system like BitKeeper, CVS, or Subversion. For information about using SCons with these systems, see the section, "Fetching Files From Source Code Management Systems," below.) You use the Repository method to tell SCons to search one or more central code repositories (in order) for any source files and derived files that are not present in the local build tree:
env = Environment()
env.Program('hello.c')
Repository('/usr/repository1', '/usr/repository2')
|
Multiple calls to the Repository method will simply add repositories to the global list that SCons maintains, with the exception that SCons will automatically eliminate the current directory and any non-existent directories from the list.
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/repository1/hello.c file to use in its place.
So given the SConstruct file above, if the hello.c file exists in the local build directory, SCons will rebuild the hello program as normal:
% scons -Q
cc -c -o hello.o 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 -c -o hello.o hello.c
cc -o hello hello.o
gcc -c /usr/repository1/hello.c -o hello.o
gcc -o hello hello.o
And similarly, if there is no local hello.c file and no /usr/repository1/hello.c, but one exists in /usr/repository2:
% scons -Q
cc -c -o hello.o 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 SCons in a separate build directory and copying the resulting tree to the desired repository:
% cd /usr/repository1
% scons -Q
cc -c -o file1.o file1.c
cc -c -o file2.o file2.c
cc -c -o hello.o 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.)
On multi-developer software projects, you can sometimes speed up every developer's builds a lot by allowing them to share the derived files that they build. SCons makes this easy, as well as reliable.
To enable sharing of derived files, use the CacheDir function in any SConscript file:
CacheDir('/usr/local/build_cache')
|
Note that the directory you specify must already exist and be readable and writable by all developers who will be sharing derived files. It should also be in some central location that all builds will be able to access. In environments where developers are using separate systems (like individual workstations) for builds, this directory would typically be on a shared or NFS-mounted file system.
Here's what happens: When a build has a CacheDir specified, every time a file is built, it is stored in the shared cache directory along with its MD5 build signature. On subsequent builds, before an action is invoked to build a file, SCons will check the shared cache directory to see if a file with the exact same build signature already exists. If so, the derived file will not be built locally, but will be copied into the local build directory from the shared cache directory, like so:
% scons -Q
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q -c
Removed hello.o
Removed hello
% scons -Q
Retrieved `hello.o' from cache
Retrieved `hello' from cache
One potential drawback to using a shared cache is that your build output can be inconsistent from invocation to invocation, because any given file may be rebuilt one time and retrieved from the shared cache the next time. This can make analyzing build output more difficult, especially for automated scripts that expect consistent output each time.
If, however, you use the --cache-show option, SCons will print the command line that it would have executed to build the file, even when it is retrieving the file from the shared cache. This makes the build output consistent every time the build is run:
% scons -Q
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q -c
Removed hello.o
Removed hello
% scons -Q --cache-show
cc -c -o hello.o hello.c
cc -o hello hello.o
The trade-off, of course, is that you no longer know whether or not SCons has retrieved a derived file from cache or has rebuilt it locally.
Retrieving an already-built file from the shared cache is usually a significant time-savings over rebuilding the file, but how much of a savings (or even whether it saves time at all) can depend a great deal on your system or network configuration. For example, retrieving cached files from a busy server over a busy network might end up being slower than rebuilding the files locally.
In these cases, you can specify the --cache-disable command-line option to tell SCons to not retrieve already-built files from the shared cache directory:
% scons -Q
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q -c
Removed hello.o
Removed hello
% scons -Q
Retrieved `hello.o' from cache
Retrieved `hello' from cache
% scons -Q -c
Removed hello.o
Removed hello
% scons -Q --cache-disable
cc -c -o hello.o hello.c
cc -o hello hello.o
Sometimes, you may have one or more derived files already built in your local build tree that you wish to make available to other people doing builds. For example, you may find it more effective to perform integration builds with the cache disabled (per the previous section) and only populate the shared cache directory with the built files after the integration build has completed successfully. This way, the cache will only get filled up with derived files that are part of a complete, successful build not with files that might be later overwritten while you debug integration problems.
In this case, you can use the the --cache-force option to tell SCons to put all derived files in the cache, even if the files had already been built by a previous invocation:
% scons -Q --cache-disable
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q -c
Removed hello.o
Removed hello
% scons -Q --cache-disable
cc -c -o hello.o hello.c
cc -o hello hello.o
% scons -Q --cache-force
scons: `.' is up to date.
% scons -Q -c
Removed hello.o
Removed hello
% scons -Q
Retrieved `hello.o' from cache
Retrieved `hello' from cache
Notice how the above sample run demonstrates that the --cache-disable option avoids putting the built hello.o and hello files in the cache, but after using the --cache-force option, the files have been put in the cache for the next invocation to retrieve.
We've already seen how you can use the Alias function to create a target named install:
env = Environment()
hello = env.Program('hello.c')
env.Install('/usr/bin', hello)
env.Alias('install', '/usr/bin')
|
You can then use this alias on the command line to tell SCons more naturally that you want to install files:
% scons -Q install
cc -c -o hello.o hello.c
cc -o hello hello.o
Install file: "hello" as "/usr/bin/hello"
Like other Builder methods, though, the Alias method returns an object representing the alias being built. You can then use this object as input to anothother Builder. This is especially useful if you use such an object as input to another call to the Alias Builder, allowing you to create a hierarchy of nested aliases:
env = Environment()
p = env.Program('foo.c')
l = env.Library('bar.c')
env.Install('/usr/bin', p)
env.Install('/usr/lib', l)
ib = env.Alias('install-bin', '/usr/bin')
il = env.Alias('install-lib', '/usr/lib')
env.Alias('install', [ib, il])
|
This example defines separate install, install-bin, and install-lib aliases, allowing you finer control over what gets installed:
% scons -Q install-bin
cc -c -o foo.o foo.c
cc -o foo foo.o
Install file: "foo" as "/usr/bin/foo"
% scons -Q install-lib
cc -c -o bar.o bar.c
ar r libbar.a bar.o
ranlib libbar.a
Install file: "libbar.a" as "/usr/lib/libbar.a"
% scons -Q -c /
Removed foo.o
Removed foo
Removed /usr/bin/foo
Removed bar.o
Removed libbar.a
Removed /usr/lib/libbar.a
% scons -Q install
cc -c -o foo.o foo.c
cc -o foo foo.o
Install file: "foo" as "/usr/bin/foo"
cc -c -o bar.o bar.c
ar r libbar.a bar.o
ranlib libbar.a
Install file: "libbar.a" as "/usr/lib/libbar.a"