path: root/doc/user/less-simple.sgml

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  <para>

  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.

  </para>

  <section>
  <title>Specifying the Name of the Target (Output) File</title>

    <para>

    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:

    </para>

    <programlisting>
       Program('hello.c')
    </programlisting>

    <para>

    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:

    </para>

    <programlisting>
       Program('new_hello', 'hello.c')
    </programlisting>

    <para>

    (&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:
    <literal>"program = source files"</literal>.)

    </para>

    <para>

    Now &SCons; will build an executable program
    named &new_hello; when run on a POSIX system:

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       cc -o hello.o -c hello.c
       cc -o new_hello hello.o
    </screen>

    <para>

    And &SCons; will build an executable program
    named &new_hello_exe; when run on a Windows system:

    </para>

    <screen>
       C:\><userinput>scons -Q</userinput>
       cl /nologo /c hello.c /Fohello.obj
       link /nologo /OUT:new_hello.exe hello.obj
    </screen>

  </section>

  <section>
  <title>Compiling Multiple Source Files</title>

    <para>

    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:

    </para>

    <programlisting>
       Program(['main.c', 'file1.c', 'file2.c'])
    </programlisting>

    <para>

    A build of the above example would look like:

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       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
    </screen>

    <para>

    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:

    </para>

    <programlisting>
       Program('program', ['main.c', 'file1.c', 'file2.c'])
    </programlisting>

    <para>

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

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       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
    </screen>

    <para>

    Or on Windows:

    </para>

    <screen>
       C:\><userinput>scons -Q</userinput>
       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
    </screen>

  </section>

  <section>
  <title>Specifying Single Files Vs. Lists of Files</title>

    <para>

    We've now shown you two ways to specify
    the source for a program,
    one with a list of files:

    </para>

    <programlisting>
       Program('hello', ['file1.c', 'file2.c'])
    </programlisting>

    <para>

    And one with a single file:

    </para>

    <programlisting>
       Program('hello', 'hello.c')
    </programlisting>

    <para>

    You could actually put a single file name in a list, too,
    which you might prefer just for the sake of consistency:

    </para>

    <programlisting>
       Program('hello', ['hello.c'])
    </programlisting>

    <para>

    &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.

    </para>

    <important>
    
    <para>

    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:

    </para>

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

    <para>

    Trying to do "Python things" that mix strings and
    lists will cause errors or lead to incorrect results:

    </para>

    <programlisting>
       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'])
    </programlisting>

    </important>

  </section>

  <section>
  <title>Making Lists of Files Easier to Read</title>

    <para>

    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.

    </para>

    <para>

    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:

    </para>

    <programlisting>
       Program('program', Split('main.c file1.c file2.c'))
    </programlisting>

    <para>

    (If you're already familiar with Python,
    you'll have realized that this is similar to the
    <function>split()</function> method
    in the Python standard <function>string</function> module.
    Unlike the <function>string.split()</function> 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.)

    </para>

    <para>

    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:

    </para>

    <programlisting>
       list = Split('main.c file1.c file2.c')
       Program('program', list)
    </programlisting>

    <para>

    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:

    </para>

    <programlisting>
       list = Split("""main.c
                       file1.c
                       file2.c""")
       Program('program', list)
    </programlisting>

    <para>

    (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.)

    </para>

  </section>

  <section>
  <title>Keyword Arguments</title>

    <para>

    &SCons; also allows you to identify
    the output file and input source files
    using Python keyword arguments.
    The output file is known as the
    <emphasis>target</emphasis>,
    and the source file(s) are known (logically enough) as the
    <emphasis>source</emphasis>.
    The Python syntax for this is:

    </para>

    <programlisting>
       list = Split('main.c file1.c file2.c')
       Program(target = 'program', source = list)
    </programlisting>

    <para>

    Because the keywords explicitly identify
    what each argument is,
    you can actually reverse the order if you prefer:

    </para>

    <programlisting>
       list = Split('main.c file1.c file2.c')
       Program(source = list, target = 'program')
    </programlisting>

    <para>

    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.

    </para>

  </section>

  <section>
  <title>Compiling Multiple Programs</title>

    <para>

    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:

    </para>

    <programlisting>
       Program('foo.c')
       Program('bar', ['bar1.c', 'bar2.c'])
    </programlisting>

    <para>

    &SCons; would then build the programs as follows:

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       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
    </screen>

    <para>

    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.

    </para>

  </section>

  <section>
  <title>Sharing Source Files Between Multiple Programs</title>

    <para>

    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
    <xref linkend="chap-libraries">, below.)

    </para>

    <para>

    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:

    </para>

    <programlisting>
       Program(Split('foo.c common1.c common2.c'))
       Program('bar', Split('bar1.c bar2.c common1.c common2.c'))
    </programlisting>

    <para>

    &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:

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       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
    </screen>

    <para>

    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 &plus; operator:

    </para>

    <programlisting>
       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)
    </programlisting>

    <para>

    This is functionally equivalent to the previous example.

    </para>

  </section>