SCons / doc / user / nodes.xml

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<!--

  __COPYRIGHT__

  Permission is hereby granted, free of charge, to any person obtaining
  a copy of this software and associated documentation files (the
  "Software"), to deal in the Software without restriction, including
  without limitation the rights to use, copy, modify, merge, publish,
  distribute, sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:

  The above copyright notice and this permission notice shall be included
  in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
  KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
  WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

-->

  <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
       embedManifestExeCheck(target, source, env)
    </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
      embedManifestExeCheck(target, source, env)
    </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>&GetBuildPath;: Getting the Path From a &Node; or String</title>

    <para>

    <function>env.GetBuildPath(file_or_list)</function>
    returns the path of a &Node; or a string representing a
    path.  It can also take a list of &Node;s and/or strings, and
    returns the list of paths.  If passed a single &Node;, the result
    is the same as calling <literal>str(node)</literal> (see above).
    The string(s) can have embedded construction variables, which are
    expanded as usual, using the calling environment's set of
    variables.  The paths can be files or directories, and do not have
    to exist.

    </para>

    <programlisting>
      env=Environment(VAR="value")
      n=File("foo.c")
      print env.GetBuildPath([n, "sub/dir/$VAR"])
    </programlisting>

    <para>

    Would print the following file names:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      ['foo.c', 'sub/dir/value']
      scons: `.' is up to date.
    </screen>

    <para>

    There is also a function version of &GetBuildPath; which can
    be called without an &Environment;; that uses the default SCons
    &Environment; to do substitution on any string arguments.

    </para>

  </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>

  -->
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