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

You've seen that when you call the &b-link-Program; builder method, it builds the resulting program with the same base name as the source file. That is, the following call to build an executable program from the &hello_c; source file will build an executable program named &hello; on POSIX systems, and an executable program named &hello_exe; on Windows systems: Program('hello.c') If you want to build a program with a different name than the base of the source file name, you simply put the target file name to the left of the source file name: Program('new_hello', 'hello.c') (&SCons; requires the target file name first, followed by the source file name, so that the order mimics that of an assignment statement in most programming languages, including Python: "program = source files".) Now &SCons; will build an executable program named &new_hello; when run on a POSIX system: % scons -Q cc -o hello.o -c hello.c cc -o new_hello hello.o And &SCons; will build an executable program named &new_hello_exe; when run on a Windows system: C:\>scons -Q cl /nologo /c hello.c /Fohello.obj link /nologo /OUT:new_hello.exe hello.obj

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(['main.c', 'file1.c', 'file2.c']) A build of the above example would look like: % scons -Q cc -o file1.o -c file1.c cc -o file2.o -c file2.c cc -o main.o -c main.c cc -o main main.o file1.o file2.o Notice that &SCons; deduces the output program name from the first source file specified in the list--that is, because the first source file was &prog_c;, &SCons; will name the resulting program &prog; (or &prog_exe; on a Windows system). If you want to specify a different program name, then (as we've seen in the previous section) you slide the list of source files over to the right to make room for the output program file name. (&SCons; puts the output file name to the left of the source file names so that the order mimics that of an assignment statement: "program = source files".) This makes our example: Program('program', ['main.c', 'file1.c', 'file2.c']) On Linux, a build of this example would look like: % scons -Q cc -o file1.o -c file1.c cc -o file2.o -c file2.c cc -o main.o -c 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

We've now shown you two ways to specify the source for a program, one with a list of files: Program('hello', ['file1.c', 'file2.c']) And one with a single file: Program('hello', 'hello.c') You could actually put a single file name in a list, too, which you might prefer just for the sake of consistency: Program('hello', ['hello.c']) &SCons; functions will accept a single file name in either form. In fact, internally, &SCons; treats all input as lists of files, but allows you to omit the square brackets to cut down a little on the typing when there's only a single file name. Although &SCons; functions are forgiving about whether or not you use a string vs. a list for a single file name, Python itself is more strict about treating lists and strings differently. So where &SCons; allows either a string or list: # The following two calls both work correctly: Program('program1', 'program1.c') Program('program2', ['program2.c']) Trying to do "Python things" that mix strings and lists will cause errors or lead to incorrect results: common_sources = ['file1.c', 'file2.c'] # THE FOLLOWING IS INCORRECT AND GENERATES A PYTHON ERROR # BECAUSE IT TRIES TO ADD A STRING TO A LIST: Program('program1', common_sources + 'program1.c') # The following works correctly, because it's adding two # lists together to make another list. Program('program2', common_sources + ['program2.c'])

One drawback to the use of a Python list for source files is that each file name must be enclosed in quotes (either single quotes or double quotes). This can get cumbersome and difficult to read when the list of file names is long. Fortunately, &SCons; and Python provide a number of ways to make sure that the &SConstruct; file stays easy to read. To make long lists of file names easier to deal with, &SCons; provides a &Split; function that takes a quoted list of file names, with the names separated by spaces or other white-space characters, and turns it into a list of separate file names. Using the &Split; function turns the previous example into: Program('program', Split('main.c file1.c file2.c')) (If you're already familiar with Python, you'll have realized that this is similar to the split() method in the Python standard string module. Unlike the 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 &b-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 &b-Program; function: list = Split('main.c file1.c file2.c') 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) (Note in this example that we used the Python "triple-quote" syntax, which allows a string to contain multiple lines. The three quotes can be either single or double quotes.)

&SCons; also allows you to identify the output file and input source files using Python keyword arguments. The output file is known as the target, and the source file(s) are known (logically enough) as the source. The Python syntax for this is: list = Split('main.c file1.c file2.c') 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.c') 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 -o bar1.o -c bar1.c cc -o bar2.o -c bar2.c cc -o bar bar1.o bar2.o cc -o foo.o -c foo.c cc -o foo foo.o Notice that &SCons; does not necessarily build the programs in the same order in which you specify them in the &SConstruct; file. &SCons; does, however, recognize that the individual object files must be built before the resulting program can be built. We'll discuss this in greater detail in the "Dependencies" section, below.

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 , 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 -o bar1.o -c bar1.c cc -o bar2.o -c bar2.c cc -o common1.o -c common1.c cc -o common2.o -c common2.c cc -o bar bar1.o bar2.o common1.o common2.o cc -o foo.o -c foo.c cc -o foo foo.o common1.o common2.o If two or more programs share a lot of common source files, repeating the common files in the list for each program can be a maintenance problem when you need to change the list of common files. You can simplify this by creating a separate Python list to hold the common file names, and concatenating it with other lists using the Python + operator: common = ['common1.c', 'common2.c'] foo_files = ['foo.c'] + common bar_files = ['bar1.c', 'bar2.c'] + common Program('foo', foo_files) Program('bar', bar_files) This is functionally equivalent to the previous example.