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  <para>

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

  </para>

  <scons_example name="ex1">
    <file name="SConstruct">
    Program('hello.c')
    </file>
    <file name="hello.c">
    int main() { printf("Hello, world!\n"); }
    </file>
  </scons_example>

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

  <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>

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

  <para>

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

  </para>

  <section>
  <title>Deciding When an Input File Has Changed:  the &Decider; Function</title>

    <para>

    Another aspect of avoiding unnecessary rebuilds
    is the fundamental build tool behavior
    of <emphasis>rebuilding</emphasis>
    things when an input file changes,
    so that the built software is up to date.
    By default,
    &SCons; keeps track of this through an
    MD5 &signature;, or checksum, of the contents of each file,
    although you can easily configure
    &SCons; to use the
    modification times (or time stamps)
    instead.
    You can even specify your own Python function
    for deciding if an input file has changed.

    </para>

    <section>
    <title>Using MD5 Signatures to Decide if a File Has Changed</title>

      <para>

      By default,
      &SCons; keeps track of whether a file has changed
      based on an MD5 checksum of the file's contents,
      not the file's 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>

      <scons_output example="ex1" os="posix">
         <scons_output_command>scons -Q hello</scons_output_command>
         <scons_output_command>touch hello.c</scons_output_command>
         <scons_output_command>scons -Q hello</scons_output_command>
      </scons_output>

      <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>

      <scons_output example="ex1" os="posix">
         <scons_output_command>scons -Q hello</scons_output_command>
         <scons_output_command output="    [CHANGE THE CONTENTS OF hello.c]">edit hello.c</scons_output_command>
         <scons_output_command>scons -Q hello</scons_output_command>
      </scons_output>

      <para>

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

      </para>

      <sconstruct>
        Program('hello.c')
        Decider('MD5')
      </sconstruct>

      <para>

      You can also use the string <literal>'content'</literal>
      as a synonym for <literal>'MD5'</literal>
      when calling the &Decider; function.

      </para>

      <section>
      <title>Ramifications of Using MD5 Signatures</title>

        <para>

        Using MD5 Signatures to decide if an input file has changed
        has one surprising benefit:
        if a source file has been changed
        in such a way that the contents of the
        rebuilt target file(s)
        will be exactly the same as the last time
        the file was built,
        then any "downstream" target files
        that depend on the rebuilt-but-not-changed target
        file actually need not be rebuilt.

        </para>

        <para>

        So if, for example,
        a user were to only change a comment in a &hello_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>

        <scons_output example="ex1" os="posix">
           <scons_output_command>scons -Q hello</scons_output_command>
           <scons_output_command output="  [CHANGE A COMMENT IN hello.c]" edit="STRIP CCCOM line">edit hello.c</scons_output_command>
           <scons_output_command>scons -Q hello</scons_output_command>
        </scons_output>

        <para>

        In essence, &SCons;
        "short-circuits" any dependent builds
        when it realizes that a target file
        has been rebuilt to exactly the same file as the last build.
        This does take some extra processing time
        to read the contents of the target (&hello_o;) file,
        but often saves time when the rebuild that was avoided
        would have been time-consuming and expensive.

        </para>

      </section>

    </section>

    <section>
    <title>Using Time Stamps to Decide If a File Has Changed</title>

      <para>

      If you prefer, you can
      configure &SCons; to use the modification time
      of a file, not the file contents,
      when deciding if a target needs to be rebuilt.
      &SCons; gives you two ways to use time stamps
      to decide if an input file has changed
      since the last time a target has been built.

      </para>

      <para>

      The most familiar way to use time stamps
      is the way &Make; does:
      that is, have &SCons; decide
      and target must be rebuilt if
      if a source file's modification time is
      <emphasis>newer</emphasis>
      than the target file.
      To do this, call the &Decider;
      function as follows:

      </para>

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

      <para>

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

      </para>

      <scons_output example="newer" os="posix">
         <scons_output_command>scons -Q hello</scons_output_command>
         <scons_output_command>touch hello.c</scons_output_command>
         <scons_output_command>scons -Q hello</scons_output_command>
      </scons_output>

      <para>

      And, in fact, because this behavior is the same
      as the behavior of &Make;,
      you can also use the string <literal>'make'</literal>
      as a synonym for <literal>'timestamp-newer'</literal>
      when calling the &Decider; function:

      </para>

      <sconstruct>
        Program('hello.c')
        Decider('make')
      </sconstruct>

      <para>

      One drawback to using times stamps exactly like &Make;
      is that if an input file's modification time suddenly
      becomes <emphasis>older</emphasis> than a target file,
      the target file will not be rebuilt.
      This can happen if an old copy of a source file is restored
      from a backup archive, for example.
      The contents of the restored file will likely be different
      than they were the last time a dependent target was built,
      but the target won't be rebuilt
      because the modification time of the source file
      is not newer than the target.

      </para>

      <para>

      Because &SCons; actually stores information
      about the source files' time stamps whenever a target is built,
      it can handle this situation by checking for
      an exact match of the source file time stamp,
      instead of just whether or not the source file
      is newer than the target file.
      To do this, specify the argument
      <literal>'timestamp-match'</literal>
      when calling the &Decider; function:

      </para>

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

      <para>

      When configured this way,
      &SCons; will rebuild a target whenever
      a source file's modification time has changed.
      So if we use the <literal>touch -t</literal>
      option to change the modification time of
      &hello_c; to an old date (January 1, 1989),
      &SCons; will still rebuild the target file:

      </para>

      <scons_output example="match" os="posix">
         <scons_output_command>scons -Q hello</scons_output_command>
         <scons_output_command>touch -t 198901010000 hello.c</scons_output_command>
         <scons_output_command>scons -Q hello</scons_output_command>
      </scons_output>

      <para>

      In general, the only reason to prefer
      <literal>timestamp-newer</literal>
      instead of
      <literal>timestamp-match</literal>,
      would be if you have some specific reason
      to require this &Make;-like behavior of 
      not rebuilding a target when an otherwise-modified
      source file is older.

      </para>

    </section>

    <section>
    <title>Deciding If a File Has Changed Using Both MD Signatures and Time Stamps</title>

      <para>

      As a performance enhancement,
      &SCons; provides a way to use
      MD5 checksums of file contents
      but to only read the contents
      whenever the file's timestamp has changed.
      To do this, call the &Decider;
      function with <literal>'MD5-timestamp'</literal>
      argument as follows:

      </para>

      <scons_example name="MD5-timestamp">
        <file name="SConstruct" printme="1">
        Program('hello.c')
        Decider('MD5-timestamp')
        </file>
        <file name="hello.c">
        int main() { printf("Hello, world!\n"); }
        </file>
      </scons_example>

      <para>

      So configured, &SCons will still behave like
      it does when using <literal>Decider('MD5')</literal>:

      </para>

      <scons_output example="MD5-timestamp" os="posix">
         <scons_output_command>scons -Q hello</scons_output_command>
         <scons_output_command>touch hello.c</scons_output_command>
         <scons_output_command>scons -Q hello</scons_output_command>
       <scons_output_command output="    [CHANGE THE CONTENTS OF hello.c]">edit hello.c</scons_output_command>
         <scons_output_command>scons -Q hello</scons_output_command>
      </scons_output>

      <para>

      However, the second call to &SCons; in the above output,
      when the build is up-to-date,
      will have been performed by simply looking at the
      modification time of the &hello_c; file,
      not by opening it and performing
      an MD5 checksum calcuation on its contents.
      This can significantly speed up many up-to-date builds.

      </para>

      <para>

      The only drawback to using
      <literal>Decider('MD5-timestamp')</literal>
      is that &SCons; will <emphasis>not</emphasis>
      rebuild a target file if a source file was modified
      within one second of the last time &SCons; built the file.
      While most developers are programming,
      this isn't a problem in practice,
      since it's unlikely that someone will have built
      and then thought quickly enought to make a substantive
      change to a source file within one second.
      Certain build scripts or
      continuous integration tools may, however,
      rely on the ability to applying changes to files
      automatically and then rebuild as quickly as possible,
      in which case use of
      <literal>Decider('MD5-timestamp')</literal>
      may not be appropriate.

      </para>

    </section>

    <section>
    <title>Writing Your Own Custom &Decider; Function</title>

      <para>

      The different string values that we've passed to
      the &Decider; function are essentially used by &SCons;
      to pick one of several specific internal functions
      that implement various ways of deciding if a dependency
      (usually a source file)
      has changed since a target file has been built.
      As it turns out,
      you can also supply your own function
      to decide if a dependency has changed.

      </para>

      <para>

      For example, suppose we have an input file
      that contains a lot of data,
      in some specific regular format,
      that is used to rebuild a lot of different target files,
      but each target file really only depends on
      one particular section of the input file.
      We'd like to have each target file depend on
      only its section of the input file.
      However, since the input file may contain a lot of data,
      we only want to open the input file if its timestamp has changed.
      This could done with a custom
      &Decider; function that might look something like this:

      </para>

      <scons_example name="function">
        <file name="SConstruct" printme="1">
        Program('hello.c')
        def decide_if_changed(dependency, target, prev_ni):
            if self.get_timestamp() != prev_ni.timestamp:
                dep = str(dependency)
                tgt = str(target)
                if specific_part_of_file_has_changed(dep, tgt):
                    return True
            return False
        Decider(decide_if_changed)
        </file>
        <file name="hello.c">
        int main() { printf("Hello, world!\n"); }
        </file>
      </scons_example>

      <para>

      Note that in the function definition,
      the <literal>dependency</literal>
      (input file) is the first argument,
      and then the <literal>target</literal>.
      Both of these are passed to the functions as
      SCons &Node; objects,
      which we convert to strings using the Python
      <function>str()</function>.
      The third argument, <literal>prev_ni</literal>,
      is an object that holds the
      signature or timestamp information
      that was recorded about the dependency
      the last time the target was built.

      </para>

      <para>

      Note that ignoring some of the arguments
      in your custom &Decider; function
      is a perfectly normal thing to do,
      if they don't impact the way you want to
      decide if the dependency file has changed.

      </para>

    </section>

    <section>
    <title>Mixing Different Ways of Deciding If a File Has Changed</title>

      <para>

      The previous examples have all demonstrated calling
      the global &Decider; function
      to configure all dependency decisions that &SCons; makes.
      Sometimes, however, you want to be able to configure
      different decision-making for different targets.
      When that's necessary, you can use the
      <function>env.Decider</function>
      method to affect only the configuration
      decisions for targets built with a
      specific construction environment.

      </para>

      <para>

      For example, if we arbitrarily want to build
      one program using MD5 checkums
      and another use file modification times
      from the same source
      we might configure it this way:

      </para>

      <scons_example name="mixing">
        <file name="SConstruct" printme="1">
        env1 = Environment(CPPPATH = ['.'])
        env2 = env1.Clone()
        env2.Decider('timestamp-match')
        env1.Program('prog-MD5', 'program1.c')
        env2.Program('prog-timestamp', 'program2.c')
        </file>
        <file name="program1.c">
        #include "inc.h"
        int main() { printf("Hello, world!\n"); }
        </file>
        <file name="program2.c">
        #include "inc.h"
        int main() { printf("Hello, world!\n"); }
        </file>
        <file name="inc.h">
        #define INC     1
        </file>
      </scons_example>

      <para>

      If both of the programs include the same
      <filename>inc.h</filename> file,
      then updating the modification time of
      <filename>inc.h</filename>
      (using the &touch; command)
      will cause only <filename>prog-timestamp</filename>
      to be rebuilt:

      </para>

      <scons_output example="mixing" os="posix">
         <scons_output_command>scons -Q</scons_output_command>
         <scons_output_command>touch inc.h</scons_output_command>
         <scons_output_command>scons -Q</scons_output_command>
      </scons_output>

    </section>

  </section>

  <section>
  <title>Older Functions for Deciding When an Input File Has Changed</title>

    <para>

    &SCons; still supports two functions that used to be the
    primary methods for configuring the
    decision about whether or not an input file has changed.
    Although they're not officially deprecated yet,
    their use is discouraged,
    mainly because they rely on a somewhat
    confusing distinction between how
    source files and target files are handled.
    These functions are documented here mainly in case you
    encounter them in existing &SConscript; files.

    </para>
  
    <section>
    <title>The &SourceSignatures; Function</title>

      <para>

      The &SourceSignatures; function is fairly straightforward,
      and supports two different argument values
      to configure whether source file changes should be decided
      using MD5 signatures:

      </para>

      <sconstruct>
        Program('hello.c')
        SourceSignatures('MD5')
      </sconstruct>

      <para>

      Or using time stamps:

      </para>

      <sconstruct>
        Program('hello.c')
        SourceSignatures('timestamp')
      </sconstruct>

      <para>

      These are roughly equivalent to specifying
      <function>Decider('MD5')</function>
      or
      <function>Decider('timestamp-match')</function>,
      respectively,
      although it only affects how SCons makes
      decisions about dependencies on
      <emphasis>source</emphasis> files--that is,
      files that are not built from any other files.

      </para>

    </section>

    <section>
    <title>The &TargetSignatures; Function</title>

      <para>

      The &TargetSignatures; function
      specifies how &SCons; decides
      when a target file has changed
      <emphasis>when it is used as a
      dependency of (input to) another target</emphasis>--that is,
      the &TargetSignatures; function configures
      how the signatures of "intermediate" target files
      are used when deciding if a "downstream" target file
      must be rebuilt.
      <footnote><para>
      This easily-overlooked distinction between
      how &SCons; decides if the target itself must be rebuilt
      and how the target is then used to decide if a different
      target must be rebuilt is one of the confusing
      things that has led to the &TargetSignatures;
      and &SourceSignatures; functions being
      replaced by the simpler &Decider; function.
      </para></footnote>

      </para>

      <para>

      The &TargetSignatures; function supports the same
      <literal>'MD5'</literal> and <literal>'timestamp'</literal>
      argument values that are supported by the &SourceSignatures;,
      with the same meanings, but applied to target files.
      That is, in the example:

      </para>

      <sconstruct>
        Program('hello.c')
        TargetSignatures('MD5')
      </sconstruct>

      <para>

      The MD5 checksum of the &hello_o; target file
      will be used to decide if it has changed since the last
      time the "downstream" &hello; target file was built.
      And in the example:
      
      </para>

      <sconstruct>
        Program('hello.c')
        TargetSignatures('timestamp')
      </sconstruct>

      <para>

      The modification time of the &hello_o; target file
      will be used to decide if it has changed since the last
      time the "downstream" &hello; target file was built.

      </para>

      <para>

      The &TargetSignatures; function supports
      two additional argument values:
      <literal>'source'</literal> and <literal>'build'</literal>.
      The <literal>'source'</literal> argument
      specifies that decisions involving
      whether target files have changed
      since a previous build
      should use the same behavior
      for the decisions configured for source files
      (using the &SourceSignatures; function).
      So in the example:

      </para>

      <sconstruct>
        Program('hello.c')
        TargetSignatures('source')
        SourceSignatures('timestamp')
      </sconstruct>

      <para>

      All files, both targets and sources,
      will use modification times
      when deciding if an input file
      has changed since the last
      time a target was built.

      </para>

      <para>

      Lastly, the <literal>'build'</literal> argument
      specifies that &SCons; should examine
      the build status of a target file
      and always rebuild a "downstream" target
      if the target file was itself rebuilt,
      without re-examining the contents or timestamp
      of the newly-built target file.
      If the target file was not rebuilt during
      this &scons; invocation,
      then the target file will be examined
      the same way as configured by
      the &SourceSignature; call
      to decide if it has changed.

      </para>

      <para>

      This mimics the behavior of
      <literal>build signatures</literal>
      in earlier versions of &SCons;.
      A &buildsignature; re-combined
      signatures of all the input files
      that went into making the target file,
      so that the target file itself
      did not need to have its contents read
      to compute an MD5 signature.
      This can improve performance for some configurations,
      but is generally not as effective as using
      <literal>Decider('MD5-timestamp')</literal>.

      </para>

    </section>

  </section>

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

    <para>

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

    </para>

    <scons_example name="include">
      <file name="SConstruct">
       Program('hello.c', CPPPATH = '.')
      </file>
      <file name="hello.c" printme="1">
       #include &lt;hello.h&gt;
       int
       main()
       {
           printf("Hello, %s!\n", string);
       }
      </file>
      <file name="hello.h">
       #define string    "world"
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_example_file example="include"  name="hello.h">
    </scons_example_file>

    <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>

    <scons_example_file example="include"  name="SConstruct">
    </scons_example_file>

    <para>

    The &cv-link-CPPPATH; value
    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>

    <scons_output example="include" os="posix">
       <scons_output_command>scons -Q hello</scons_output_command>
       <scons_output_command>scons -Q hello</scons_output_command>
       <scons_output_command output="    [CHANGE THE CONTENTS OF hello.h]">edit hello.h</scons_output_command>
       <scons_output_command>scons -Q hello</scons_output_command>
    </scons_output>

    <para>

    First, notice that &SCons;
    added the <literal>-I.</literal> argument
    from the &cv-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 &cv-link-LIBPATH; variable,
    the &cv-CPPPATH; variable
    may be a list of directories,
    or a string separated by
    the system-specific path separation character
    (':' on POSIX/Linux, ';' on Windows).
    Either way, &SCons; creates the
    right command-line options
    so that the following example:

    </para>

    <scons_example name="ex5">
      <file name="SConstruct" printme="1">
       Program('hello.c', CPPPATH = ['include', '/home/project/inc'])
      </file>
      <file name="hello.c">
      int main() { printf("Hello, world!\n"); }
      </file>
    </scons_example>

    <para>

    Will look like this on POSIX or Linux:

    </para>

    <scons_output example="ex5" os="posix">
       <scons_output_command>scons -Q hello</scons_output_command>
    </scons_output>

    <para>

    And like this on Windows:

    </para>

    <scons_output example="ex5" os="win32">
       <scons_output_command>scons -Q hello.exe</scons_output_command>
    </scons_output>

  </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 <emphasis>rebuild</emphasis>
    all or part of a large system:
    &SCons; will likely take some extra time to "think about"
    what must be built before it issues the
    first build command
    (or decides that everything is up to date
    and 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 to finish.
    Consequently, &SCons; lets you cache
    the implicit dependencies
    that its scanners find,
    for use by later builds.
    You can do this by specifying the
    &implicit-cache; option on the command line:

    </para>

    <scons_output example="ex1">
       <scons_output_command>scons -Q --implicit-cache hello</scons_output_command>
       <scons_output_command>scons -Q hello</scons_output_command>
    </scons_output>

    <para>

    If you don't want to specify &implicit-cache;
    on the command line each time,
    you can make it the default behavior for your build
    by setting the &implicit_cache; option
    in an &SConscript; file:

    </para>

    <sconstruct>
       SetOption('implicit_cache', 1)
    </sconstruct>

    <para>

    &SCons; does not cache implicit dependencies like this by default
    because the &implicit-cache; causes &SCons; to simply use the implicit
    dependencies stored during the last run, without any checking
    for whether or not those dependencies are still correct.
    Specifically, this means &implicit-cache; instructs &SCons;
    to <emphasis>not</emphasis> rebuild "correctly" in the
    following cases:


    </para>

    <itemizedlist>

      <listitem>
        <para>

        When &implicit-cache; is used, &SCons; will ignore any changes that
        may have been made to search paths
        (like &cv-CPPPATH; or &cv-LIBPATH;,).
        This can lead to &SCons; not rebuilding a file if a change to
        &cv-CPPPATH; would normally cause a different, same-named file from
        a different directory to be used.

        </para>
      </listitem>

      <listitem>
        <para>

        When &implicit-cache; is used, &SCons; will not detect if a
        same-named file has been added to a directory that is earlier in
        the search path than the directory in which the file was found
        last time.

        </para>
      </listitem>

    </itemizedlist>

    <section>
    <title>The &implicit-deps-changed; Option</title>

      <para>

      When using cached implicit dependencies,
      sometimes you want to "start fresh"
      and have &SCons; re-scan the files
      for which it previously cached the dependencies.
      For example,
      if you have recently installed a new version of
      external code that you use for compilation,
      the external header files will have changed
      and the previously-cached implicit dependencies
      will be out of date.
      You can update them by
      running &SCons; with the &implicit-deps-changed; option:

      </para>

      <scons_output example="ex1">
         <scons_output_command>scons -Q --implicit-deps-changed hello</scons_output_command>
         <scons_output_command>scons -Q hello</scons_output_command>
      </scons_output>

      <para>

      In this case, &SCons; will re-scan all of the implicit dependencies
      and cache updated copies of the information.

      </para>

    </section>

    <section>
    <title>The &implicit-deps-unchanged; Option</title>

      <para>

      By default when caching dependencies,
      &SCons; notices when a file has been modified
      and re-scans the file for any updated
      implicit dependency information.
      Sometimes, however, you may want
      to force &SCons; to use the cached implicit dependencies,
      even if the source files changed.
      This can speed up a build for example,
      when you have changed your source files
      but know that you haven't changed
      any <literal>#include</literal> lines.
      In this case,
      you can use the &implicit-deps-unchanged; option:

      </para>

      <scons_output example="ex1">
         <scons_output_command>scons -Q --implicit-deps-unchanged hello</scons_output_command>
         <scons_output_command>scons -Q hello</scons_output_command>
      </scons_output>

      <para>

      In this case,
      &SCons; will assume that the cached implicit
      dependencies are correct and
      will not bother to re-scan changed files.
      For typical builds after small,
      incremental changes to source files,
      the savings may not be very big,
      but sometimes every bit of
      improved performance counts.

      </para>

    </section>

    <!--

    <section>
    <title>XXX max drift</title>

      XXX SetOption('max_drift')

    </section>

    -->

  </section>

  <section>
  <title>Explicit Dependencies:  the &Depends; Function</title>

    <para>

    Sometimes a file depends on another file
    that is not detected by an &SCons; scanner.
    For this situation,
    &SCons; allows you to specific explicitly that one file
    depends on another file,
    and must be rebuilt whenever that file changes.
    This is specified using the &Depends; method:

    </para>

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

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

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

  </section>

  <section>
  <title>Ignoring Dependencies:  the &Ignore; Function</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>

    <scons_example name="ignore">
      <file name="SConstruct" printme="1">
      hello = Program('hello.c')
      Ignore(hello, 'hello.h')
      </file>
      <file name="hello.c">
      #include "hello.h"
      int main() { printf("Hello, %s!\n", string); }
      </file>
      <file name="hello.h">
      #define string    "world"
      </file>
    </scons_example>

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

    <!--
    <scons_output example="ignore">
      <scons_output_command>scons -Q hello</scons_output_command>
      <scons_output_command>scons -Q hello</scons_output_command>
      <scons_output_command output="    [CHANGE THE CONTENTS OF hello.h]">edit hello.h</scons_output_command>
      <scons_output_command>scons -Q hello</scons_output_command>
      XXX THIS EXAMPLE SHOULD BE UP-TO-DATE! XXX
    </scons_output>
    -->

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

    <para>

    Now, the above example is a little contrived,
    because it's hard to imagine a real-world situation
    where you wouldn't want 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>
       hello = Program('hello.c')
       Ignore(hello, '/usr/include/stdio.h')
    </programlisting>

  </section>

  <section>
  <title>The &AlwaysBuild; Function</title>

    <para>

    How &SCons; handles dependencies can also be affected
    by the &AlwaysBuild; method.
    When a file is passed to the &AlwaysBuild; method,
    like so:

    </para>

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

    <para>

    Then the specified target file (&hello; in our example)
    will always be considered out-of-date and
    rebuilt whenever that target file is evaluated
    while walking the dependency graph:

    </para>

    <scons_output example="AlwaysBuild">
      <scons_output_command>scons -Q</scons_output_command>
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    The &AlwaysBuild; function has a somewhat misleading name,
    because it does not actually mean the target file will
    be rebuilt every single time &SCons; is invoked.
    Instead, it means that the target will, in fact,
    be rebuilt whenever the target file is encountered
    while evaluating the targets specified on
    the command line (and their dependencies).
    So specifying some other target on the command line,
    a target that does <emphasis>not</emphasis>
    itself depend on the &AlwaysBuild; target,
    will still be rebuilt only if it's out-of-date
    with respect to its dependencies:

    </para>

    <scons_output example="AlwaysBuild">
      <scons_output_command>scons -Q</scons_output_command>
      <scons_output_command>scons -Q hello.o</scons_output_command>
    </scons_output>

    <!--

      XXX AlwaysBuild() and Alias Nodes

      XXX AlwaysBuild() and Dir Nodes

      XXX AlwaysBuild() with no sources

    -->

  </section>

  <!--

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

    <para>

    XXX Salt() (are we going to implement this ?)

        original Cons classic POD documentation:

=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>

  </section>

  -->