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') int main() { printf("Hello, world!\n"); } 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 On a Windows system with the Microsoft Visual C++ compiler, you'll see something like: scons 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: scons 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;: scons -Q 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']) int main() { printf("prog.c\n"); } void file1() { printf("file1.c\n"); } void file2() { printf("file2.c\n"); } A build of the above example would look like: scons -Q 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']) int main() { printf("prog.c\n"); } void file1() { printf("file1.c\n"); } void file2() { printf("file2.c\n"); } On Linux, a build of this example would look like: scons -Q Or on Windows: scons -Q

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']) int main() { printf("foo.c\n"); } int main() { printf("bar1.c\n"); } void bar2() { printf("bar2.c\n"); } &SCons; would then build the programs as follows: scons -Q 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')) int main() { printf("foo.c\n"); } int main() { printf("bar1.c\n"); } int bar2() { printf("bar2.c\n"); } void common1() { printf("common1.c\n"); } void common22() { printf("common2.c\n"); } &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 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.