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<!--
Copyright (c) 2001, 2002, 2003 Steven Knight
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
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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>
Here's the famous "Hello, World!" program in C:
</para>
<programlisting>
int
main()
{
printf("Hello, world!\n");
}
</programlisting>
<para>
And here's how to build it using &SCons;.
Enter the following into a file named &SConstruct;:
</para>
<scons_example name="ex1">
<file name="SConstruct" printme="1">
env = Environment()
env.Program('hello.c')
</file>
<file name="hello.c">
int main() { printf("Hello, world!\n"); }
</file>
</scons_example>
<para>
That's it. Now run the &scons; command to build the program.
On a POSIX-compliant system like Linux or UNIX,
you'll see something like:
</para>
<scons_output example="ex1" os="posix">
<command>scons</command>
</scons_output>
<para>
On a Windows system with the Microsoft Visual C++ compiler,
you'll see something like:
</para>
<scons_output example="ex1" os="win32">
<command>scons</command>
</scons_output>
<para>
First, notice that you only need
to specify the name of the source file,
and that &SCons; deduces the names of
the object and executable files
correctly from the base of the source file name.
</para>
<para>
Second, notice that the same input &SConstruct; file,
without any changes,
generates the correct output file names on both systems:
<filename>hello.o</filename> and <filename>hello</filename>
on POSIX systems,
<filename>hello.obj</filename> and <filename>hello.exe</filename>
on Windows systems.
This is a simple example of how &SCons;
makes it extremely easy to
write portable software builds.
</para>
<para>
(Note that we won't provide duplicate side-by-side
POSIX and Windows output for all of the examples in this guide;
just keep in mind that, unless otherwise specified,
any of the examples should work equally well on both types of systems.)
</para>
<section>
<title>The &SConstruct; File</title>
<para>
If you're used to build systems like &Make;
you've already figured out that the &SConstruct; file
is the &SCons; equivalent of a &Makefile;.
That is, the &SConstruct; file is the input file
that &SCons; reads to control the build.
</para>
<para>
There is, however, an important difference between
an &SConstruct; file and a &Makefile;:
the &SConstruct; file is actually a Python script.
If you're not already familiar with Python, don't worry.
This User's Guide will introduce you step-by-step
to the relatively small amount of Python you'll
need to know to be able to use &SCons; effectively.
And Python is very easy to learn.
</para>
<para>
One aspect of using Python as the
scripting language is that you can put comments
in your &SConstruct; file using Python's commenting convention;
that is, everything between a '#' and the end of the line
will be ignored:
</para>
<programlisting>
env = Environment() # Create an environment.
# Arrange to build the "hello" program.
env.Program('hello.c')
</programlisting>
<para>
You'll see throughout the remainder of this Guide
that being able to use the power of a
real scripting language
can greatly simplify the solutions
to complex requirements of real-world builds.
</para>
</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>
<scons_example name="ex2">
<file name="SConstruct" printme="1">
env = Environment()
env.Program(['prog.c', 'file1.c', 'file2.c'])
</file>
<file name="prog.c">
int main() { printf("prog.c\n"); }
</file>
<file name="file1.c">
void file1() { printf("file1.c\n"); }
</file>
<file name="file2.c">
void file2() { printf("file2.c\n"); }
</file>
</scons_example>
<para>
A build of the above example would look like:
</para>
<scons_output example="ex2">
<command>scons</command>
</scons_output>
<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 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>
<scons_example name="ex3">
<file name="SConstruct" printme="1">
env = Environment()
env.Program('program', ['main.c', 'file1.c', 'file2.c'])
</file>
<file name="main.c">
int main() { printf("prog.c\n"); }
</file>
<file name="file1.c">
void file1() { printf("file1.c\n"); }
</file>
<file name="file2.c">
void file2() { printf("file2.c\n"); }
</file>
</scons_example>
<para>
On Linux, a build of this example would look like:
</para>
<scons_output example="ex3" os="posix">
<command>scons</command>
</scons_output>
<para>
Or on Windows:
</para>
<scons_output example="ex3" os="win32">
<command>scons</command>
</scons_output>
</section>
<section>
<title>Keeping &SConstruct; Files Easy 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>
env = Environment()
env.Program('program', Split('main.c file1.c file2.'))
</programlisting>
<para>
Putting the call to the &Split; function
inside the <function>env.Program</function> 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
<function>env.Program</function> function:
</para>
<programlisting>
env = Environment()
list = Split('main.c file1.c file2.')
env.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>
env = Environment()
list = Split('main.c
file1.c
file2.c')
env.Program('program', list)
</programlisting>
</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>
env = Environment()
list = Split('main.c file1.c file2.')
env.Program(target = 'program', source = list)
</programlisting>
<para>
Whether or not you choose to use keyword arguments
to identify the target and source files
is purely a personal choice;
&SCons; functions the same either way.
</para>
</section>
<section>
<title>Compiling Multiple Programs</title>
<para>
In order to compile multiple programs
within the same &SConstruct; file,
simply call the <function>env.Program</function> method
multiple times,
once for each program you need to build:
</para>
<scons_example name="ex4">
<file name="SConstruct" printme="1">
env = Environment()
env.Program('foo.c')
env.Program('bar', ['bar1.c', 'bar2.c'])
</file>
<file name="foo.c">
int main() { printf("foo.c\n"); }
</file>
<file name="bar1.c">
int main() { printf("bar1.c\n"); }
</file>
<file name="bar2.c">
void bar2() { printf("bar2.c\n"); }
</file>
</scons_example>
<para>
&SCons; would then build the programs as follows:
</para>
<scons_output example="ex4">
<command>scons</command>
</scons_output>
<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
section XXX, 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>
<scons_example name="ex5">
<file name="SConstruct" printme="1">
env = Environment()
env.Program(Split('foo.c common1.c common2.c'))
env.Program('bar', Split('bar1.c bar2.c common1.c common2.c'))
</file>
<file name="foo.c">
int main() { printf("foo.c\n"); }
</file>
<file name="bar1.c">
int main() { printf("bar1.c\n"); }
</file>
<file name="bar2.c">
int bar2() { printf("bar2.c\n"); }
</file>
<file name="common1.c">
void common1() { printf("common1.c\n"); }
</file>
<file name="common2.c">
void common22() { printf("common2.c\n"); }
</file>
</scons_example>
<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 files are listed multiple times:
</para>
<scons_output example="ex5">
<command>scons</command>
</scons_output>
<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 + operator:
</para>
<programlisting>
common = ['common1.c', 'common2.c']
foo_files = ['foo.c'] + common
bar_files = ['bar1.c', 'bar2.c'] + common
env = Environment()
env.Program('foo', foo_files)
env.Program('bar', bar_files)
</programlisting>
<para>
This is functionally equivalent to the previous example.
</para>
</section>
|