path: root/doc/user/depends.sgml

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  Copyright (c) 2001, 2002, 2003 Steven Knight

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<!--

=head2 The C<Salt> method

The C<Salt> method adds a constant value to the signature calculation
for every derived file.  It is invoked as follows:

  Salt $string;

Changing the Salt value will force a complete rebuild of every derived
file.  This can be used to force rebuilds in certain desired
circumstances.  For example,

  Salt `uname -s`;

Would force a complete rebuild of every derived file whenever the
operating system on which the build is performed (as reported by C<uname
-s>) changes.

-->

  <para>

  So far we've seen how &SCons; handles one-time builds.
  But the real point of a build tool like &SCons;
  is to rebuild the necessary things,
  and only the necessary thing, when source files change.
  Put another way,
  &SCons; should <emphasis>not</emphasis>
  waste time rebuilding things that have already been built.
  You can see this at work simply be re-invoking &SCons;
  after building our simple &hello; example:

  </para>

  <literallayout>
     % <userinput>scons</userinput>
     cc -c hello.c -o hello.o
     cc -o hello hello.o
     % <userinput>scons</userinput>
     %
  </literallayout>

  <para>

  The second time it is executed,
  &SCons; realizes that the &hello; program
  is up-to-date with respect to the current &hello_c; source file,
  and avoids rebuilding it.
  You can see this more clearly by naming
  the &hello; program explicitly on the command line:

  </para>

  <literallayout>
     % <userinput>scons hello</userinput>
     cc -c hello.c -o hello.o
     cc -o hello hello.o
     % <userinput>scons hello</userinput>
     scons: `hello' is up to date.
     %
  </literallayout>

  <para>

  Note that &SCons; only reports "...is up to date"
  for target files named explicitly on the command line,
  to avoid cluttering the output.

  </para>

  <section>
  <title>Source File Signatures</title>

    <para>

    The other side of avoiding unnecessary rebuilds
    is the fundamental build tool behavior
    of <emphasis>rebuilding</emphasis>
    things when a source file changes.
    &SCons; keeps track of this through a
    &signature; for each source file,
    and allows you to configure
    whether you want to use the source
    file contents or the modification time (timestamp)
    as the signature.

    </para>

    <section>
    <title>MD5 Signatures</title>

      <para>

      By default,
      &SCons; keeps track of whether a source file has changed
      based on the file's contents,
      not the modification time.
      This means that you may be surprised by the
      default &SCons; behavior if you are used to the
      &Make; convention of forcing
      a rebuild by updating the file's modification time
      (using the &touch; command, for example):

      </para>

      <literallayout>
         % <userinput>scons hello</userinput>
         cc -c hello.c -o hello.o
         cc -o hello hello.o
         % <userinput>touch hello.c</userinput>
         % <userinput>scons hello</userinput>
         scons: `hello' is up to date.
         %
      </literallayout>

      <para>

      Even though the file's modification time has changed,
      &SCons; realizes that the contents of the
      &hello_c; file have <emphasis>not</emphasis> changed,
      and therefore that the &hello; program
      need not be rebuilt.
      This avoids unnecessary rebuilds when,
      for example, someone rewrites the
      contents of a file without making a change.
      But if the contents of the file really do change,
      then &SCons; detects the change
      and rebuilds the program as required:

      </para>

      <literallayout>
         % <userinput>scons hello</userinput>
         cc -c hello.c -o hello.o
         cc -o hello hello.o
         % <userinput>edit hello.c</userinput>
             [CHANGE THE CONTENTS OF hello.c]
         % <userinput>scons hello</userinput>
         cc -c hello.c -o hello.o
         cc -o hello hello.o
         %
      </literallayout>

      <para>

      Note that you can, if you wish,
      specify this default behavior
      (MD5 signatures) explicitly
      using the &SourceSignatures; function as follows:

      </para>

      <programlisting>
        env = Environment()
        env.Program('hello.c')
        SourceSignatures('MD5')
      </programlisting>

    </section>

    <section>
    <title>Time Stamps</title>

      <para>

      If you prefer, you can
      configure &SCons; to use the modification time
      of source files,
      not the file contents,
      when deciding if something needs to be rebuilt.
      To do this, call the &SourceSignatures;
      function as follows:

      </para>

      <programlisting>
        env = Environment()
        env.Program('hello.c')
        SourceSignatures('timestamp')
      </programlisting>

      <para>

      This makes &SCons; act like &Make;
      when a file's modification time is updated
      (using the &touch; command, for example):

      </para>

      <literallayout>
         % <userinput>scons hello</userinput>
         cc -c hello.c -o hello.o
         cc -o hello hello.o
         % <userinput>touch hello.c</userinput>
         % <userinput>scons hello</userinput>
         cc -c hello.c -o hello.o
         cc -o hello hello.o
         %
      </literallayout>

    </section>

  </section>

  <section>
  <title>Target File Signatures</title>

    <section>
    <title>Build Signatures</title>

      <para>

      We've already seen how modifying a source file
      will cause not only its direct target file to be rebuilt,
      but also the target file(s)
      that depend on that direct target file.
      In our example,
      changing the contents of &hello_c; cause
      the &hello_o; file to be rebuilt,
      which in turn causes the
      &hello; program to be rebuilt:

      </para>

      <literallayout>
         % <userinput>scons hello</userinput>
         cc -c hello.c -o hello.o
         cc -o hello hello.o
         % <userinput>edit hello.c</userinput>
           [CHANGE THE CONTENTS OF hello.c]
         % <userinput>scons hello</userinput>
         cc -c hello.c -o hello.o
         cc -o hello hello.o
         %
      </literallayout>

      <para>

      What's not obvious, though,
      is that &SCons; internally handles the signature of
      the target file(s)
      (&hello_o; in the above example)
      differently from the signature of the source file
      (&hello_c;).
      By default,
      &SCons; tracks whether a target file must be rebuilt
      by using a &buildsignature;
      that consists of the combined
      signatures of all the files
      that go into making the target file.
      This is efficient because
      the accumulated signatures
      actually give &SCons; all of the
      information it needs
      to decide if the target file is out of date.

      </para>

      <para>

      If you wish, you can
      specify this default behavior
      (build signatures) explicitly
      using the &TargetSignatures; function:

      </para>

      <programlisting>
        env = Environment()
        env.Program('hello.c')
        TargetSignatures('build')
      </programlisting>

    </section>

    <section>
    <title>File Contents</title>

      <para>

      Sometimes, however,
      a source file can be changed
      in such a way that the
      target file(s) built from it
      will be rebuilt
      exactly the same as the last time.
      If so, then any target files
      that depend on built-but-not-changed target
      file actually need not be rebuilt.
      You can have &SCons;
      realize that a dependent target file
      need not be rebuilt in this situation
      using the &TargetSignatures; function as follows:

      </para>

      <programlisting>
        env = Environment()
        env.Program('hello.c')
        TargetSignatures('content')
      </programlisting>

      <para>

      So if, for example,
      a user were to only change a comment in a C file,
      then the rebuilt &hello_o; file
      would be exactly the same as the one previously built
      (assuming the compiler doesn't put any build-specific
      information in the object file).
      &SCons; would then realize that it would not
      need to rebuild the &hello; program as follows:

      </para>

      <literallayout>
         % <userinput>scons hello</userinput>
         cc -c hello.c -o hello.o
         cc -o hello hello.o
         % <userinput>edit hello.c</userinput>
           [CHANGE A COMMENT IN hello.c]
         % <userinput>scons hello</userinput>
         cc -c hello.c -o hello.o
         %
      </literallayout>

      <para>

      In essence, &SCons; has
      "short-circuited" any dependent builds
      when it realizes that a target file
      has been rebuilt to exactly the same file as the last build.
      So configured,
      &SCons; does take some extra processing time
      to scan the contents of the target (&hello_o;) file,
      but this may save time
      if the rebuild that was avoided
      would have been very time-consuming and expensive.

      </para>

    </section>

  </section>

  <section>
  <title>Implicit Dependencies:  The &CPPPATH; Construction Variable</title>

    <para>

    Now suppose that our "Hello, World!" program
    actually has a <literal>#include</literal> line
    to include &hello_h; file in the compilation:

    </para>

    <programlisting>
       #include "hello.h" 
       int
       main()
       {
           printf("Hello, %s!\n", string);
       }
    </programlisting>

    <para>

    And, for completeness, the &hello_h; file looks like this:

    </para>

    <programlisting>
       #define string    "world"
    </programlisting>

    <para>

    In this case, we want &SCons; to recognize that,
    if the contents of the &hello_h; file change,
    the &hello; program must be recompiled.
    To do this, we need to modify the
    &SConstruct; file like so:

    </para>

    <programlisting>
       env = Environment(CPPPATH = '.')
       hello = env.Program('hello.c')
    </programlisting>

    <para>

    The &CPPPATH; assignment in the &Environment; call
    tells &SCons; to look in the current directory
    (<literal>'.'</literal>)
    for any files included by C source files
    (<filename>.c</filename> or <filename>.h</filename> files).
    With this assignment in the &SConstruct; file:

    </para>

    <literallayout>
       % <userinput>scons hello</userinput>
       cc -I. -c hello.c -o hello.o
       cc -o hello hello.o
       % <userinput>scons hello</userinput>
       scons: `hello' is up to date.
       % <userinput>edit hello.h</userinput>
           [CHANGE THE CONTENTS OF hello.h]
       % <userinput>scons hello</userinput>
       cc -I. -c hello.c -o hello.o
       cc -o hello hello.o
       %
    </literallayout>

    <para>

    First, notice that &SCons;
    added the <literal>-I.</literal> argument
    from the &CPPPATH; variable
    so that the compilation would find the
    &hello_h; file in the local directory.

    </para>

    <para>

    Second, realize that &SCons; knows that the &hello;
    program must be rebuilt
    because it scans the contents of
    the &hello_c; file
    for the <literal>#include</literal> lines that indicate
    another file is being included in the compilation.
    &SCons; records these as
    <emphasis>implicit dependencies</emphasis>
    of the target file,
    Consequently,
    when the &hello_h; file changes,
    &SCons; realizes that the &hello_c; file includes it,
    and rebuilds the resulting &hello; program
    that depends on both the &hello_c; and &hello_h; files.

    </para>

    <para>

    Like the &LIBPATH; variable,
    the &CPPPATH; variable
    may be a list of directories,
    or a string separated by
    the system-specific path separate character
    (':' on POSIX/Linux, ';' on Windows).
    Either way, &SCons; creates the
    right command-line options
    so that the followin example:

    </para>

    <programlisting>
       env = Environment(CPPPATH = ['include', '/home/project/inc'])
       hello = env.Program('hello.c')
    </programlisting>

    <para>

    Will look like this on POSIX or Linux:

    </para>

    <literallayout>
       % <userinput>scons hello</userinput>
       cc -Iinclude -I/home/project/inc -c hello.c -o hello.o
       cc -o hello hello.o
    </literallayout>

    <para>

    And like this on Windows:

    </para>

    <literallayout>
      % <userinput>scons hello</userinput>
      cl /Iinclude /I\home\project\inc /Fohello.obj hello.c
      link /OUT:hello.exe hello.obj
    </literallayout>

  </section>

  <section>
  <title>Caching Implicit Dependencies</title>

    <para>

    Scanning each file for <literal>#include</literal> lines
    does take some extra processing time.
    When you're doing a full build of a large system,
    the scanning time is usually a very small percentage
    of the overall time spent on the build.
    You're most likely to notice the scanning time,
    however, when you rebuild all or part of a large system:
    &SCons; will take some extra time to "think about"
    what must be built before it issues the
    first build command,
    or decides that nothing must be rebuilt.

 <!--
 Isn't this expensive? The answer is, it depends. If you do a full build of a
 large system, the scanning time is insignificant. If you do a rebuild of a
 large system, then Cons will spend a fair amount of time thinking about it
 before it decides that nothing has to be done (although not necessarily more
 time than make!). The good news is that Cons makes it very easy to
 intelligently subset your build, when you are working on localized changes.
 -->

    </para>

    <para>

    In practice, having &SCons; scan files saves time
    relative to the amount of potential time
    lost to tracking down subtle problems
    introduced by incorrect dependencies.
    Nevertheless, the "waiting time"
    while &SCons; scans files can annoy
    individual developers waiting for their builds.
    Consequently, &SCons; lets you cache
    the implicit dependencies

    </para>

    <literallayout>
       % <userinput>scons --implicit-cache hello</userinput>
       cc -c hello.c -o hello.o
       cc -o hello hello.o
       % <userinput>scons hello</userinput>
       scons: `hello' is up to date.
    </literallayout>

    <para>

    &SCons; does not cache implicit dependencies like this by default
    because XXX

    </para>

  </section>

  <section>
  <title>The &Ignore; Method</title>

    <para>

    Sometimes it makes sense 
    to not rebuild a program,
    even if a dependency file changes.
    In this case,
    you would tell &SCons; specifically
    to ignore a dependency as follows:

    </para>

    <programlisting>
       env = Environment()
       hello = env.Program('hello.c')
       env.Ignore(hello, 'hello.h')
    </programlisting>

    <!-- XXX mention that you can use arrays for target and source? -->

    <literallayout>
       % <userinput>scons hello</userinput>
       cc -c hello.c -o hello.o
       cc -o hello hello.o
       % <userinput>scons hello</userinput>
       scons: `hello' is up to date.
       % <userinput>edit hello.h</userinput>
           [CHANGE THE CONTENTS OF hello.h]
       % <userinput>scons hello</userinput>
       scons: `hello' is up to date.
    </literallayout>

    <para>

    Now, the above example is a little contrived,
    because it's hard to imagine a real-world situation
    where you wouldn't to rebuild &hello;
    if the &hello_h; file changed.
    A more realistic example
    might be if the &hello;
    program is being built in a
    directory that is shared between multiple systems
    that have different copies of the
    &stdio_h; include file.
    In that case,
    &SCons; would notice the differences between
    the different systems' copies of &stdio_h;
    and would rebuild &hello;
    each time you change systems.
    You could avoid these rebuilds as follows:

    </para>

    <programlisting>
       env = Environment()
       hello = env.Program('hello.c')
       env.Ignore(hello, '/usr/include/stdio.h')
    </programlisting>

  </section>

  <section>
  <title>The &Depends; Method</title>

    <para>

    On the other hand,
    sometimes a file depends on another
    file that the scanner(s) in &SCons; will not detect.
    For this situation,
    &SCons; allows you to specific that one file explicitly
    depends on another file,
    and must be rebuilt whenever that file changes,
    using the &Depends; method:

    </para>

    <programlisting>
       env = Environment()
       hello = env.Program('hello.c')
       env.Depends(hello, 'other_file')
    </programlisting>

    <!-- XXX mention that you can use arrays for target and source? -->

    <literallayout>
       % <userinput>scons hello</userinput>
       cc -c hello.c -o hello.o
       cc -o hello hello.o
       % <userinput>scons hello</userinput>
       scons: `hello' is up to date.
       % <userinput>edit other_file</userinput>
           [CHANGE THE CONTENTS OF other_file]
       % <userinput>scons hello</userinput>
       cc -c hello.c -o hello.o
       cc -o hello hello.o
    </literallayout>

  </section>

  <!-->

  <section>
  <title>The &Salt; Method</title>

    <para>

    XXX

    </para>

  </section>

  -->