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<?xml version='1.0'?>
<!DOCTYPE sconsdoc [
<!ENTITY % scons SYSTEM "../scons.mod">
%scons;
<!ENTITY % builders-mod SYSTEM "../generated/builders.mod">
%builders-mod;
<!ENTITY % functions-mod SYSTEM "../generated/functions.mod">
%functions-mod;
<!ENTITY % tools-mod SYSTEM "../generated/tools.mod">
%tools-mod;
<!ENTITY % variables-mod SYSTEM "../generated/variables.mod">
%variables-mod;
]>
<chapter id="chap-nodes"
xmlns="http://www.scons.org/dbxsd/v1.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 scons.xsd">
<title>Node Objects</title>
<!--
__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>
<sconstruct>
Object('hello.c', CCFLAGS='-DHELLO')
Object('goodbye.c', CCFLAGS='-DGOODBYE')
</sconstruct>
<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>
<sconstruct>
Object('hello.c', CCFLAGS='-DHELLO')
Object('goodbye.c', CCFLAGS='-DGOODBYE')
Program(['hello.o', 'goodbye.o'])
</sconstruct>
<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>
<scons_example name="nodes_ex1">
<file name="SConstruct" printme="1">
hello_list = Object('hello.c', CCFLAGS='-DHELLO')
goodbye_list = Object('goodbye.c', CCFLAGS='-DGOODBYE')
Program(hello_list + goodbye_list)
</file>
<file name="hello.c">
int main() { printf("Hello, world!\n"); }
</file>
<file name="goodbye.c">
int main() { printf("Goodbye, world!\n"); }
</file>
</scons_example>
<para>
This makes our &SConstruct; file portable again,
the build output on Linux looking like:
</para>
<scons_output example="nodes_ex1" os="posix">
<scons_output_command suffix="1">scons -Q</scons_output_command>
</scons_output>
<para>
And on Windows:
</para>
<scons_output example="nodes_ex1" os="win32">
<scons_output_command suffix="2">scons -Q</scons_output_command>
</scons_output>
<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>
<scons_example name="nodes_print">
<file name="SConstruct" printme="1">
hello_c = File('hello.c')
Program(hello_c)
classes = Dir('classes')
Java(classes, 'src')
</file>
</scons_example>
<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>
<sconstruct>
xyzzy = Entry('xyzzy')
</sconstruct>
<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>
<scons_example name="nodes_print">
<file name="SConstruct" printme="1">
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]
</file>
<file name="hello.c">
int main() { printf("Hello, world!\n"); }
</file>
</scons_example>
<para>
Would print the following file names on a POSIX system:
</para>
<scons_output example="nodes_print" os="posix">
<scons_output_command suffix="1">scons -Q</scons_output_command>
</scons_output>
<para>
And the following file names on a Windows system:
</para>
<scons_output example="nodes_print" os="win32">
<scons_output_command suffix="2">scons -Q</scons_output_command>
</scons_output>
<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>
<scons_example name="nodes_exists">
<file name="SConstruct" printme="1">
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!"
</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="nodes_exists" os="posix">
<scons_output_command suffix="1">scons -Q</scons_output_command>
</scons_output>
</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>
<scons_example name="nodes_GetBuildPath">
<file name="SConstruct" printme="1">
env=Environment(VAR="value")
n=File("foo.c")
print env.GetBuildPath([n, "sub/dir/$VAR"])
</file>
</scons_example>
<para>
Would print the following file names:
</para>
<scons_output example="nodes_GetBuildPath" os="posix">
<scons_output_command suffix="1">scons -Q</scons_output_command>
</scons_output>
<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="nodes_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="nodes_read" os="posix">
<scons_output_command suffix="1">scons -Q</scons_output_command>
</scons_output>
</section>
-->
<!--
<section>
<title>Python Value &Node;</title>
<para>
XXX Value()
</para>
</section>
-->
</chapter>
|