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<!--
__COPYRIGHT__
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LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
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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 source files 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 &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 &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 listing 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 &Object; builder to variables,
which we can then concatenate in our
call to the &Program; builder:
</para>
<scons_example name="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="ex1" os="posix">
<command>scons -Q</command>
</scons_output>
<para>
And on Windows:
</para>
<scons_output example="ex1" os="win32">
<command>scons -Q</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, repectively,
return a file or directory Node:
</para>
<scons_example name="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.
For example, the following &SConstruct; file:
</para>
<scons_example name="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="print" os="posix">
<command>scons -Q</command>
</scons_output>
<para>
And the following file names on a Windows system:
</para>
<scons_output example="print" os="win32">
<command>scons -Q</command>
</scons_output>
</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;
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="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="print" os="posix">
<command>scons -Q</command>
</scons_output>
</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="exists">
<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="print" os="posix">
<command>scons -Q</command>
</scons_output>
</section>
-->
|