path: root/doc/user/simple.in

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 <para>

 In this chapter,
 you will see several examples of
 very simple build configurations using &SCons;,
 which will demonstrate how easy
 it is to use &SCons; to
 build programs from several different programming languages
 on different types of systems.

 </para>

 <section>
 <title>Building Simple C / C++ Programs</title>

   <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">
      Program('hello.c')
      </file>
      <file name="hello.c">
      int main() { printf("Hello, world!\n"); }
      </file>
   </scons_example>

   <para>

   This minimal configuration file gives
   &SCons; two pieces of information:
   what you want to build
   (an executable program),
   and the input file from
   which you want it built
   (the <filename>hello.c</filename> file).
   &b-link-Program; is a <firstterm>builder_method</firstterm>,
   a Python call that tells &SCons; that you want to build an
   executable program.

   </para>

   <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">
      <scons_output_command>scons</scons_output_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">
      <scons_output_command>scons</scons_output_command>
   </scons_output>

   <para>

   First, notice that you only need
   to specify the name of the source file,
   and that &SCons; correctly deduces the names of
   the object and executable files to be built
   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>

 <section>
 <title>Building Object Files</title>

   <para>

   The &b-link-Program; builder method is only one of
   many builder methods that &SCons; provides
   to build different types of files.
   Another is the &b-link-Object; builder method,
   which tells &SCons; to build an object file
   from the specified source file:

   </para>

   <scons_example name="Object">
      <file name="SConstruct" printme="1">
      Object('hello.c')
      </file>
      <file name="hello.c">
      int main() { printf("Hello, world!\n"); }
      </file>
   </scons_example>

   <para>

   Now when you run the &scons; command to build the program,
   it will build just the &hello_o; object file on a POSIX system:

   </para>

   <scons_output example="Object" os="posix">
      <scons_output_command>scons</scons_output_command>
   </scons_output>

   <para>

   And just the &hello_obj; object file
   on a Windows system (with the Microsoft Visual C++ compiler):

   </para>

   <scons_output example="Object" os="win32">
      <scons_output_command>scons</scons_output_command>
   </scons_output>

 </section>

 <section>
 <title>Simple Java Builds</title>

   <para>

   &SCons; also makes building with Java extremely easy.
   Unlike the &b-link-Program; and &b-link-Object; builder methods,
   however, the &b-link-Java; builder method
   requires that you specify
   the name of a destination directory in which
   you want the class files placed,
   followed by the source directory
   in which the <filename>.java</filename> files live:

   </para>

   <scons_example name="java">
     <file name="SConstruct" printme="1">
     Java('classes', 'src')
     </file>
     <file name="src/hello.java">
     public class Example1
     {
       public static void main(String[] args)
       {
         System.out.println("Hello Java world!\n");
       }
     }
     </file>
   </scons_example>

   <para>

   If the <filename>src</filename> directory
   contains a single <filename>hello.java</filename> file,
   then the output from running the &scons; command
   would look something like this
   (on a POSIX system):

   </para>

   <scons_output example="java" os="posix">
      <scons_output_command>scons</scons_output_command>
   </scons_output>

   <para>

   We'll cover Java builds in more detail,
   including building Java archive (<filename>.jar</filename>)
   and other types of file,
   in <xref linkend="chap-java"></xref>.

   </para>

 </section>

 <section>
 <title>Cleaning Up After a Build</title>

   <para>

   When using &SCons;, it is unnecessary to add special
   commands or target names to clean up after a build.
   Instead, you simply use the
   <literal>-c</literal> or <literal>--clean</literal>
   option when you invoke &SCons;,
   and &SCons; removes the appropriate built files.
   So if we build our example above
   and then invoke <literal>scons -c</literal>
   afterwards, the output on POSIX looks like:

   </para>

   <scons_example name="clean">
      <file name="SConstruct">
      Program('hello.c')
      </file>
      <file name="hello.c">
      int main() { printf("Hello, world!\n"); }
      </file>
   </scons_example>

   <scons_output example="clean" os="posix">
      <scons_output_command>scons</scons_output_command>
      <scons_output_command>scons -c</scons_output_command>
   </scons_output>

   <para>

   And the output on Windows looks like:

   </para>

   <scons_output example="clean" os="win32">
      <scons_output_command>scons</scons_output_command>
      <scons_output_command>scons -c</scons_output_command>
   </scons_output>

   <para>

   Notice that &SCons; changes its output to tell you that it
   is <literal>Cleaning targets ...</literal> and
   <literal>done cleaning targets.</literal>

   </para>

 </section>

 <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>

   <section>
   <title>&SConstruct; Files Are Python Scripts</title>

     <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>
        # Arrange to build the "hello" program.
        Program('hello.c')    # "hello.c" is the source file.
     </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>&SCons; Functions Are Order-Independent</title>

     <para>

     One important way in which the &SConstruct;
     file is not exactly like a normal Python script,
     and is more like a &Makefile,
     is that the order in which
     the &SCons; functions are called in
     the &SConstruct; file
     does <emphasis>not</emphasis>
     affect the order in which &SCons;
     actually builds the programs and object files
     you want it to build.<footnote>
     <para>In programming parlance,
     the &SConstruct; file is
     <emphasis>declarative</emphasis>,
     meaning you tell &SCons; what you want done
     and let it figure out the order in which to do it,
     rather than strictly <emphasis>imperative</emphasis>,
     where you specify explicitly the order in
     which to do things.
     </para>
     </footnote>
     In other words, when you call the &b-link-Program; builder
     (or any other builder method),
     you're not telling &SCons; to build
     the program at the instant the builder method is called.
     Instead, you're telling &SCons; to build the program
     that you want, for example,
     a program built from a file named &hello_c;,
     and it's up to &SCons; to build that program
     (and any other files) whenever it's necessary.
     (We'll learn more about how
     &SCons; decides when building or rebuilding a file
     is necessary in <xref linkend="chap-depends"></xref>, below.)
 
     </para>
 
     <para>

     &SCons; reflects this distinction between
     <emphasis>calling a builder method like</emphasis> &b-Program;
     and <emphasis>actually building the program</emphasis>
     by printing the status messages that indicate
     when it's "just reading" the &SConstruct; file,
     and when it's actually building the target files.
     This is to make it clear when &SCons; is
     executing the Python statements that make up the &SConstruct; file,
     and when &SCons; is actually executing the
     commands or other actions to
     build the necessary files.

     </para>

     <para>

     Let's clarify this with an example.
     Python has a <literal>print</literal> statement that
     prints a string of characters to the screen.
     If we put <literal>print</literal> statements around
     our calls to the &b-Program; builder method:

     </para>

     <scons_example name="declarative">
       <file name="SConstruct" printme="1">
       print "Calling Program('hello.c')"
       Program('hello.c')
       print "Calling Program('goodbye.c')"
       Program('goodbye.c')
       print "Finished calling Program()"
       </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>

     Then when we execute &SCons;,
     we see the output from the <literal>print</literal>
     statements in between the messages about
     reading the &SConscript; files,
     indicating that that is when the
     Python statements are being executed:

     </para>

     <scons_output example="declarative" os="posix">
       <scons_output_command>scons</scons_output_command>
     </scons_output>

     <para>

     Notice also that &SCons; built the &goodbye; program first,
     even though the "reading &SConscript" output
     shows that we called <literal>Program('hello.c')</literal>
     first in the &SConstruct; file.

     </para>

   </section>

 </section>

 <section>
 <title>Making the &SCons; Output Less Verbose</title>

   <para>

   You've already seen how &SCons; prints
   some messages about what it's doing,
   surrounding the actual commands used to build the software:

   </para>

   <scons_output example="ex1" os="win32">
      <scons_output_command>scons</scons_output_command>
   </scons_output>

   <para>

   These messages emphasize the
   order in which &SCons; does its work:
   all of the configuration files
   (generically referred to as &SConscript; files)
   are read and executed first,
   and only then are the target files built.
   Among other benefits, these messages help to distinguish between
   errors that occur while the configuration files are read,
   and errors that occur while targets are being built.

   </para>

   <para>

   One drawback, of course, is that these messages clutter the output.
   Fortunately, they're easily disabled by using
   the &Q; option when invoking &SCons;:

   </para>

   <scons_output example="ex1" os="win32">
      <scons_output_command>scons -Q</scons_output_command>
   </scons_output>

   <para>

   Because we want this User's Guide to focus
   on what &SCons; is actually doing,
   we're going to use the &Q; option
   to remove these messages from the
   output of all the remaining examples in this Guide.

   </para>

 </section>