path: root/doc/user/nodes.xml

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  <para>

  Internally, &SCons; represents all of the files
  and directories it knows about as &Nodes;.
  These internal objects
  (not object <emphasis>files</emphasis>)
  can be used in a variety of ways
  to make your &SConscript;
  files portable and easy to read.

  </para>

  <section>
  <title>Builder Methods Return Lists of Target Nodes</title>

    <para>

    All builder methods return a list of
    &Node; objects that identify the
    target file or files that will be built.
    These returned &Nodes; can be passed
    as arguments to other builder methods.

    </para>

    <para>

    For example, suppose that we want to build
    the two object files that make up a program with different options.
    This would mean calling the &b-link-Object;
    builder once for each object file,
    specifying the desired options:

    </para>

    <programlisting>
    Object('hello.c', CCFLAGS='-DHELLO')
    Object('goodbye.c', CCFLAGS='-DGOODBYE')
    </programlisting>

    <para>

    One way to combine these object files
    into the resulting program
    would be to call the &b-link-Program;
    builder with the names of the object files
    listed as sources:

    </para>

    <programlisting>
    Object('hello.c', CCFLAGS='-DHELLO')
    Object('goodbye.c', CCFLAGS='-DGOODBYE')
    Program(['hello.o', 'goodbye.o'])
    </programlisting>

    <para>

    The problem with specifying the names as strings
    is that our &SConstruct; file is no longer portable
    across operating systems.
    It won't, for example, work on Windows
    because the object files there would be
    named &hello_obj; and &goodbye_obj;,
    not &hello_o; and &goodbye_o;.

    </para>

    <para>

    A better solution is to assign the lists of targets
    returned by the calls to the &b-Object; builder to variables,
    which we can then concatenate in our
    call to the &b-Program; builder:

    </para>

    <programlisting>
      hello_list = Object('hello.c', CCFLAGS='-DHELLO')
      goodbye_list = Object('goodbye.c', CCFLAGS='-DGOODBYE')
      Program(hello_list + goodbye_list)
    </programlisting>

    <para>

    This makes our &SConstruct; file portable again,
    the build output on Linux looking like:

    </para>

    <screen>
       % <userinput>scons -Q</userinput>
       cc -o goodbye.o -c -DGOODBYE goodbye.c
       cc -o hello.o -c -DHELLO hello.c
       cc -o hello hello.o goodbye.o
    </screen>

    <para>

    And on Windows:

    </para>

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

    <para>

    We'll see examples of using the list of nodes
    returned by builder methods throughout
    the rest of this guide.

    </para>

  </section>

  <section>
  <title>Explicitly Creating File and Directory Nodes</title>

    <para>

    It's worth mentioning here that
    &SCons; maintains a clear distinction
    between Nodes that represent files
    and Nodes that represent directories.
    &SCons; supports &File; and &Dir;
    functions that, respectively,
    return a file or directory Node:

    </para>

    <programlisting>
      hello_c = File('hello.c')
      Program(hello_c)

      classes = Dir('classes')
      Java(classes, 'src')
    </programlisting>

    <para>

    Normally, you don't need to call
    &File; or &Dir; directly,
    because calling a builder method automatically
    treats strings as the names of files or directories,
    and translates them into
    the Node objects for you.
    The &File; and &Dir; functions can come in handy
    in situations where you need to explicitly
    instruct &SCons; about the type of Node being
    passed to a builder or other function,
    or unambiguously refer to a specific
    file in a directory tree.
    <!--
    (For an example of when you might
    need to use &File; or &Dir; to
    prevent ambiguous interpretation of a string
    naming a file or directory, see
    <xref linkend="chap-hierarchy">.)
    -->

    </para>

    <para>

    There are also times when you may need to
    refer to an entry in a file system
    without knowing in advance
    whether it's a file or a directory.
    For those situations,
    &SCons; also supports an &Entry; function,
    which returns a Node
    that can represent either a file or a directory.

    </para>

    <programlisting>
    xyzzy = Entry('xyzzy')
    </programlisting>

    <para>

    The returned <literal>xyzzy</literal> Node
    will be turned into a file or directory Node
    the first time it is used by a builder method
    or other function that
    requires one vs. the other.

    </para>

  </section>

  <section>
  <title>Printing &Node; File Names</title>

    <para>

    One of the most common things you can do
    with a Node is use it to print the
    file name that the node represents.
    Keep in mind, though, that because the object
    returned by a builder call
    is a <emphasis>list</emphasis> of Nodes,
    you must use Python subscripts
    to fetch individual Nodes from the list.
    For example, the following &SConstruct; file:

    </para>

    <programlisting>
      hello_c = File('hello.c')
      Program(hello_c)

      classes = Dir('classes')
      Java(classes, 'src')

      object_list = Object('hello.c')
      program_list = Program(object_list)
      print "The object file is:", object_list[0]
      print "The program file is:", program_list[0]
    </programlisting>

    <para>

    Would print the following file names on a POSIX system:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      The object file is: hello.o
      The program file is: hello
      cc -o hello.o -c hello.c
      cc -o hello hello.o
    </screen>

    <para>

    And the following file names on a Windows system:

    </para>

    <screen>
      C:\><userinput>scons -Q</userinput>
      The object file is: hello.obj
      The program file is: hello.exe
      cl /Fohello.obj /c hello.c /nologo
      link /nologo /OUT:hello.exe hello.obj
    </screen>

    <para>

    Note that in the above example,
    the <literal>object_list[0]</literal>
    extracts an actual Node <emphasis>object</emphasis>
    from the list,
    and the Python <literal>print</literal> statement
    converts the object to a string for printing.

    </para>

  </section>

  <section>
  <title>Using a &Node;'s File Name as a String</title>

    <para>

    Printing a &Node;'s name
    as described in the previous section
    works because the string representation of a &Node; object
    is the name of the file.
    If you want to do something other than
    print the name of the file,
    you can fetch it by using the builtin Python
    &str; function.
    For example, if you want to use the Python
    <function>os.path.exists</function>
    to figure out whether a file
    exists while the &SConstruct; file
    is being read and executed,
    you can fetch the string as follows:

    </para>

    <programlisting>
      import os.path
      program_list = Program('hello.c')
      program_name = str(program_list[0])
      if not os.path.exists(program_name):
          print program_name, "does not exist!"
    </programlisting>

    <para>

    Which executes as follows on a POSIX system:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      hello does not exist!
      cc -o hello.o -c hello.c
      cc -o hello hello.o
    </screen>

  </section>

  <!--

  <section>
  <title>Fetching the Contents of a &Node;</title>

    <para>

    XXX Describe using read() and readlines()
    when we add that as a public interface.

    </para>

    <scons_example name="read">
      <file name="SConstruct" printme="1">
      hello_c = File('hello.c')
      contents = hello_c.read()
      print "contents are:"
      print contents
      </file>
      <file name="hello.c">
      int main() { printf("Hello, world!\n"); }
      </file>
    </scons_example>

    <para>

    Which executes as follows on a POSIX system:

    </para>

    <scons_output example="read" os="posix">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

  </section>

  -->

  <!--

  <section>
  <title>Python Value &Node;</title>

    <para>

    XXX Value()

    </para>

  </section>

  -->