Fix the Requires() examples by adding a sleep command to the

[scons.git] / doc / user / depends.in

diff --git a/doc/user/depends.in b/doc/user/depends.in
index 0262924593b30fc60df6bdb56693244c2ac6e7c2..8ba05048affde3b379e687eb68d4d7bda9606bb1 100644 (file)
--- a/doc/user/depends.in
+++ b/doc/user/depends.in
@@ -21,38 +21,17 @@
   OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
   WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
   OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
   WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 

--->
-
-<!--
-
-=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.
 -->
 
   <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 only the necessary things
+  But one of the main functions of a build tool like &SCons;
+  is to rebuild only what is necessary

   when source files change--or, put another way,
   &SCons; should <emphasis>not</emphasis>
   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 be re-invoking &SCons;
+  waste time rebuilding things that don't need to be rebuilt.
+  You can see this at work simply by re-invoking &SCons;

   after building our simple &hello; example:
 
   </para>
   after building our simple &hello; example:
 
   </para>


@@ -96,33 +75,36 @@ operating system on which the build is performed (as reported by C<uname
   </para>
 
   <section>
   </para>
 
   <section>

-  <title>Deciding When a Source File Has Changed:  the &SourceSignatures; Function</title>
+  <title>Deciding When an Input File Has Changed:  the &Decider; Function</title>

 
     <para>
 
 
     <para>
 

-    The other side of avoiding unnecessary rebuilds
+    Another aspect of avoiding unnecessary rebuilds

     is the fundamental build tool behavior
     of <emphasis>rebuilding</emphasis>
     is the fundamental build tool behavior
     of <emphasis>rebuilding</emphasis>

-    things when a source file changes,
+    things when an input file changes,

     so that the built software is up to date.
     so that the built software is up to date.

-    &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.
+    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>
 
     </para>
 
     <section>

-    <title>MD5 Source File Signatures</title>
+    <title>Using MD5 Signatures to Decide if a File Has Changed</title>

 
       <para>
 
       By default,
 
       <para>
 
       By default,

-      &SCons; keeps track of whether a source file has changed
-      based on the file's contents,
-      not the modification time.
+      &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
       This means that you may be surprised by the
       default &SCons; behavior if you are used to the
       &Make; convention of forcing


@@ -164,36 +146,110 @@ operating system on which the build is performed (as reported by C<uname
       Note that you can, if you wish,
       specify this default behavior
       (MD5 signatures) explicitly
       Note that you can, if you wish,
       specify this default behavior
       (MD5 signatures) explicitly

-      using the &SourceSignatures; function as follows:
+      using the &Decider; function as follows:

 
       </para>
 
       <sconstruct>
         Program('hello.c')
 
       </para>
 
       <sconstruct>
         Program('hello.c')

-        SourceSignatures('MD5')
+        Decider('MD5')

       </sconstruct>
 
       </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>
     </section>
 
     <section>

-    <title>Source File Time Stamps</title>
+    <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
 
       <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;
+      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
+      that a target must be rebuilt
+      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>
 
       function as follows:
 
       </para>
 

-      <scons_example name="ex2">
+      <scons_example name="newer">

         <file name="SConstruct" printme="1">
         Program('hello.c')
         <file name="SConstruct" printme="1">
         Program('hello.c')

-        SourceSignatures('timestamp')
+        Decider('timestamp-newer')

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


@@ -208,117 +264,245 @@ operating system on which the build is performed (as reported by C<uname
 
       </para>
 
 
       </para>
 

-      <scons_output example="ex2" os="posix">
+      <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>
 
          <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>
 

-    </section>
+      <para>

 
 

-  </section>
+      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:

 
 

-  <section>
-  <title>Deciding When a Target File Has Changed:  the &TargetSignatures; Function</title>
+      </para>

 
 

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

 
 

-    As you've just seen,
-    &SCons; uses signatures to decide whether a 
-    target file is up to date or must be rebuilt.
-    When a target file depends on another target file,
-    &SCons; allows you to configure separately
-    how the signatures of "intermediate" target files
-    are used when deciding if a dependent target file
-    must be rebuilt.
+      <para>

 
 

-    </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.

 
 

-    <section>
-    <title>Build Signatures</title>
+      </para>

 
       <para>
 
 
       <para>
 

-      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 the &hello_c; file causes
-      the &hello_o; file to be rebuilt,
-      which in turn causes the
-      &hello; program to be rebuilt:
+      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>
 
 
       </para>
 

-      <scons_output example="ex1" os="posix">
+      <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>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>touch -t 198901010000 hello.c</scons_output_command>

          <scons_output_command>scons -Q hello</scons_output_command>
       </scons_output>
 
       <para>
 
          <scons_output_command>scons -Q hello</scons_output_command>
       </scons_output>
 
       <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.
+      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>
 
 
       </para>
 

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

       <para>
 
       <para>
 

-      If you wish, you can
-      specify this default behavior
-      (build signatures) explicitly
-      using the &TargetSignatures; function:
+      As a performance enhancement,
+      &SCons; provides a way to use
+      MD5 checksums of file contents
+      but to read those contents
+      only when the file's timestamp has changed.
+      To do this, call the &Decider;
+      function with <literal>'MD5-timestamp'</literal>
+      argument as follows:

 
       </para>
 
 
       </para>
 

-      <sconstruct>
+      <scons_example name="MD5-timestamp">
+        <file name="SConstruct" printme="1">

         Program('hello.c')
         Program('hello.c')

-        TargetSignatures('build')
-      </sconstruct>
+        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>
+
+      <!--
+
+      We want to generate the output as follows,
+      but our "surrogate" system for generating the
+      output seems to get this wrong.
+      Just in-line the output for now.
+
+      <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>
+
+      -->
+
+      <screen>
+         % <userinput>scons -Q hello</userinput>
+         cc -o hello.o -c hello.c
+         cc -o hello hello.o
+         % <userinput>touch hello.c</userinput>
+         % <userinput>scons -Q hello</userinput>
+         scons: `hello' is up to date.
+         % <userinput>edit hello.c</userinput>
+             [CHANGE THE CONTENTS OF hello.c]
+         % <userinput>scons -Q hello</userinput>
+         cc -o hello.o -c hello.c
+         cc -o hello hello.o
+      </screen>
+
+      <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 enough 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 apply 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>
 
     </section>
 
     <section>

-    <title>File Contents</title>
+    <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>
 
 
       <para>
 

-      Sometimes a source file can be 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.
-      If so, then any other target files
-      that depend on such a built-but-not-changed target
-      file actually need not be rebuilt.
-      You can make &SCons;
-      realize that it does not need to rebuild
-      a dependent target file in this situation
-      using the &TargetSignatures; function as follows:
+      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 want to open the input file only if its timestamp has changed.
+      This could done with a custom
+      &Decider; function that might look something like this:

 
       </para>
 
 
       </para>
 

-      <scons_example name="ex3">
+      <scons_example name="function">

         <file name="SConstruct" printme="1">
         Program('hello.c')
         <file name="SConstruct" printme="1">
         Program('hello.c')

-        TargetSignatures('content')
+        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>
         <file name="hello.c">
         int main() { printf("Hello, world!\n"); }


@@ -327,35 +511,348 @@ operating system on which the build is performed (as reported by C<uname
 
       <para>
 
 
       <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:
+      Note that in the function definition,
+      the <varname>dependency</varname>
+      (input file) is the first argument,
+      and then the &target;.
+      Both of these are passed to the functions as
+      SCons &Node; objects,
+      which we convert to strings using the Python
+      <function>str()</function>.

 
       </para>
 
 
       </para>
 

-      <scons_output example="ex3" 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>
+      <para>
+
+      The third argument, <varname>prev_ni</varname>,
+      is an object that holds the
+      signature or timestamp information
+      that was recorded about the dependency
+      the last time the target was built.
+      A <varname>prev_ni</varname> object can hold
+      different information,
+      depending on the type of thing that the
+      <varname>dependency</varname> argument represents.
+      For normal files,
+      the <varname>prev_ni</varname> object
+      has the following attributes:
+
+      </para>
+
+      <variablelist>
+
+        <varlistentry>
+        <term>.csig</term>
+
+        <listitem>
+        <para>
+        The <emphasis>content signature</emphasis>,
+        or MD5 checksum, of the contents of the
+        <varname>dependency</varname>
+        file the list time the &target; was built.
+        </para>
+        </listitem>
+
+        </varlistentry>
+
+        <varlistentry>
+        <term>.size</term>
+
+        <listitem>
+        <para>
+        The size in bytes of the <varname>dependency</varname>
+        file the list time the target was built.
+        </para>
+        </listitem>
+
+        </varlistentry>
+
+        <varlistentry>
+        <term>.timestamp</term>
+
+        <listitem>
+        <para>
+        The modification time of the <varname>dependency</varname>
+        file the list time the &target; was built.
+        </para>
+        </listitem>
+
+        </varlistentry>
+
+      </variablelist>
+
+      <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 using 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>
 
       </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>
 
       <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.
+      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>
 
 
       </para>
 


@@ -369,12 +866,12 @@ operating system on which the build is performed (as reported by C<uname
     <para>
 
     Now suppose that our "Hello, World!" program
     <para>
 
     Now suppose that our "Hello, World!" program

-    actually has a <literal>#include</literal> line
+    actually has an <literal>#include</literal> line

     to include the &hello_h; file in the compilation:
 
     </para>
 
     to include the &hello_h; file in the compilation:
 
     </para>
 

-    <scons_example name="ex4">
+    <scons_example name="include">

       <file name="SConstruct">
        Program('hello.c', CPPPATH = '.')
       </file>
       <file name="SConstruct">
        Program('hello.c', CPPPATH = '.')
       </file>


@@ -397,7 +894,7 @@ operating system on which the build is performed (as reported by C<uname
 
     </para>
 
 
     </para>
 

-    <scons_example_file example="ex4"  name="hello.h">
+    <scons_example_file example="include"  name="hello.h">

     </scons_example_file>
 
     <para>
     </scons_example_file>
 
     <para>


@@ -410,12 +907,12 @@ operating system on which the build is performed (as reported by C<uname
 
     </para>
 
 
     </para>
 

-    <scons_example_file example="ex4"  name="SConstruct">
+    <scons_example_file example="include"  name="SConstruct">

     </scons_example_file>
 
     <para>
 
     </scons_example_file>
 
     <para>
 

-    The &cv-CPPPATH; value
+    The &cv-link-CPPPATH; value

     tells &SCons; to look in the current directory
     (<literal>'.'</literal>)
     for any files included by C source files
     tells &SCons; to look in the current directory
     (<literal>'.'</literal>)
     for any files included by C source files


@@ -424,7 +921,7 @@ operating system on which the build is performed (as reported by C<uname
 
     </para>
 
 
     </para>
 

-    <scons_output example="ex4" os="posix">
+    <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_command>scons -Q hello</scons_output_command>
        <scons_output_command output="    [CHANGE THE CONTENTS OF hello.h]">edit hello.h</scons_output_command>


@@ -462,11 +959,11 @@ operating system on which the build is performed (as reported by C<uname
 
     <para>
 
 
     <para>
 

-    Like the &cv-LIBPATH; variable,
+    Like the &cv-link-LIBPATH; variable,

     the &cv-CPPPATH; variable
     may be a list of directories,
     or a string separated by
     the &cv-CPPPATH; variable
     may be a list of directories,
     or a string separated by

-    the system-specific path separate character
+    the system-specific path separation character

     (':' on POSIX/Linux, ';' on Windows).
     Either way, &SCons; creates the
     right command-line options
     (':' on POSIX/Linux, ';' on Windows).
     Either way, &SCons; creates the
     right command-line options


@@ -572,22 +1069,46 @@ operating system on which the build is performed (as reported by C<uname
        SetOption('implicit_cache', 1)
     </sconstruct>
 
        SetOption('implicit_cache', 1)
     </sconstruct>
 

-    <!--
-

     <para>
     <para>

-    
-    XXX

 
 

-    </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>

 
 

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

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


@@ -633,7 +1154,7 @@ operating system on which the build is performed (as reported by C<uname
       and re-scans the file for any updated
       implicit dependency information.
       Sometimes, however, you may want
       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, 
+      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
       even if the source files changed.
       This can speed up a build for example,
       when you have changed your source files


@@ -665,14 +1186,268 @@ operating system on which the build is performed (as reported by C<uname
 
     </section>
 
 
     </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>
+
+    <para>
+
+    Note that the dependency
+    (the second argument to &Depends;)
+    may also be a list of Node objects
+    (for example, as returned by a call to a Builder):
+
+    </para>
+
+    <programlisting>
+       hello = Program('hello.c')
+       goodbye = Program('goodbye.c')
+       Depends(hello, goodbye)
+    </programlisting>
+
+    <para>
+
+    in which case the dependency or dependencies
+    will be built before the target(s):
+
+    </para>
+
+    <screen>
+       % <userinput>scons -Q hello</userinput>
+       cc -c goodbye.c -o goodbye.o
+       cc -o goodbye goodbye.o
+       cc -c hello.c -o hello.o
+       cc -o hello hello.o
+    </screen>
+

   </section>
 
   <section>
   </section>
 
   <section>

-  <title>Ignoring Dependencies:  the &Ignore; Method</title>
+  <title>Dependencies From External Files:  the &ParseDepends;
+  Function</title>

 
     <para>
 
 
     <para>
 

-    Sometimes it makes sense 
+    &SCons; has built-in scanners for a number of languages. Sometimes
+    these scanners fail to extract certain implicit dependencies due
+    to limitations of the scanner implementation.
+
+    </para>
+
+    <para>
+
+    The following example illustrates a case where the built-in C
+    scanner is unable to extract the implicit dependency on a header
+    file.
+
+    </para>
+
+    <scons_example name="macroinc">
+      <file name="hello.c" printme="1">
+      #define FOO_HEADER &lt;foo.h&gt;
+      #include FOO_HEADER
+
+      int main() {
+          return FOO;
+      }
+      </file>
+      <file name="SConstruct">
+      Program('hello', 'hello.c', CPPPATH='.')
+      </file>
+      <file name="foo.h">
+      #define FOO 42
+      </file>
+    </scons_example>
+
+    <scons_output example="macroinc" os="posix">
+      <scons_output_command>scons -Q</scons_output_command>
+      <scons_output_command output="   [CHANGE CONTENTS OF foo.h]"
+      >edit foo.h</scons_output_command>
+      <scons_output_command>scons -Q</scons_output_command>
+    </scons_output>
+
+    <para>
+
+    Apparently, the scanner does not know about the header dependency.
+    Being not a full-fledged C preprocessor, the scanner does not
+    expand the macro.
+
+    </para>
+
+    <para>
+
+    In these cases, you may also use the compiler to extract the
+    implicit dependencies. &ParseDepends; can parse the contents of
+    the compiler output in the style of &Make;, and explicitly
+    establish all of the listed dependencies.
+
+    </para>
+
+    <para>
+
+    The following example uses &ParseDepends; to process a compiler
+    generated dependency file which is generated as a side effect
+    during compilation of the object file:
+
+    </para>
+
+    <!-- XXX The ParseDepends example below fakes proper working by a
+    priori specification of the dependency file. The produced hello.d
+    file is not found (or used) for unknown reasons. -->
+
+    <scons_example name="parsedep">
+      <file name="hello.c">
+      #define FOO_HEADER &lt;foo.h&gt;
+      #include FOO_HEADER
+
+      int main() {
+          return FOO;
+      }
+      </file>
+      <file name="SConstruct" printme="1">
+      obj = Object('hello.c', CCFLAGS='-MD -MF hello.d', CPPPATH='.')
+      SideEffect('hello.d', obj)
+      ParseDepends('hello.d')
+      Program('hello', obj)
+      </file>
+      <file name="foo.h">
+      #define FOO 42
+      </file>
+      <file name="hello.d">
+      hello.o: hello.c foo.h
+      </file>
+    </scons_example>
+
+    <scons_output example="parsedep" os="posix">
+      <scons_output_command>scons -Q</scons_output_command>
+      <scons_output_command output="   [CHANGE CONTENTS OF foo.h]"
+      >edit foo.h</scons_output_command>
+      <scons_output_command>scons -Q</scons_output_command>
+    </scons_output>
+
+    <para>
+
+    Parsing dependencies from a compiler-generated
+    <filename>.d</filename> file has a chicken-and-egg problem, that
+    causes unnecessary rebuilds:
+
+    </para>
+
+    <scons_example name="parsedeprebuild">
+      <file name="hello.c">
+      #define FOO_HEADER &lt;foo.h&gt;
+      #include FOO_HEADER
+
+      int main() {
+          return FOO;
+      }
+      </file>
+      <file name="SConstruct">
+      obj = Object('hello.c', CCFLAGS='-MD -MF hello.d', CPPPATH='.')
+      SideEffect('hello.d', obj)
+      ParseDepends('hello.d')
+      Program('hello', obj)
+      </file>
+      <file name="foo.h">
+      #define FOO 42
+      </file>
+    </scons_example>
+
+    <!--
+    <scons_output example="parsedeprebuild" os="posix">
+      <scons_output_command>scons -Q</scons_output_command>
+      <scons_output_command>scons -Q</scons_output_command>
+      <scons_output_command>scons -Q</scons_output_command>
+    </scons_output>
+    -->
+
+    <screen>
+      % <userinput>scons -Q</userinput>
+      cc -o hello.o -c -MD -MF hello.d -I. hello.c
+      cc -o hello hello.o
+      % <userinput>scons -Q --debug=explain</userinput>
+      scons: rebuilding `hello.o' because `foo.h' is a new dependency
+      cc -o hello.o -c -MD -MF hello.d -I. hello.c
+      % <userinput>scons -Q</userinput>
+      scons: `.' is up to date.
+    </screen>
+
+    <para>
+
+    In the first pass, the dependency file is generated while the
+    object file is compiled. At that time, &SCons; does not know about
+    the dependency on <filename>foo.h</filename>. In the second pass,
+    the object file is regenerated because <filename>foo.h</filename>
+    is detected as a new dependency.
+
+    </para>
+
+    <para>
+
+    &ParseDepends; immediately reads the specified file at invocation
+    time and just returns if the file does not exist. A dependency
+    file generated during the build process is not automatically
+    parsed again. Hence, the compiler-extracted dependencies are not
+    stored in the signature database during the same build pass. This
+    limitation of &ParseDepends; leads to unnecessary recompilations.
+    Therefore, &ParseDepends; should only be used if scanners are not
+    available for the employed language or not powerful enough for the
+    specific task.
+
+    </para>
+
+  </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,
     to not rebuild a program,
     even if a dependency file changes.
     In this case,


@@ -683,11 +1458,12 @@ operating system on which the build is performed (as reported by C<uname
 
     <scons_example name="ignore">
       <file name="SConstruct" printme="1">
 
     <scons_example name="ignore">
       <file name="SConstruct" printme="1">

-      hello = Program('hello.c')
-      Ignore(hello, 'hello.h')
+      hello_obj=Object('hello.c')
+      hello = Program(hello_obj)
+      Ignore(hello_obj, 'hello.h')

       </file>
       <file name="hello.c">
       </file>
       <file name="hello.c">

-      #include "hello.h" 
+      #include "hello.h"

       int main() { printf("Hello, %s!\n", string); }
       </file>
       <file name="hello.h">
       int main() { printf("Hello, %s!\n", string); }
       </file>
       <file name="hello.h">


@@ -695,7 +1471,7 @@ operating system on which the build is performed (as reported by C<uname
       </file>
     </scons_example>
 
       </file>
     </scons_example>
 

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

 
     <!--
     <scons_output example="ignore">
 
     <!--
     <scons_output example="ignore">


@@ -723,7 +1499,7 @@ operating system on which the build is performed (as reported by C<uname
 
     Now, the above example is a little contrived,
     because it's hard to imagine a real-world situation
 
     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;
+    where you wouldn't want to rebuild &hello;

     if the &hello_h; file changed.
     A more realistic example
     might be if the &hello;
     if the &hello_h; file changed.
     A more realistic example
     might be if the &hello;


@@ -741,58 +1517,299 @@ operating system on which the build is performed (as reported by C<uname
     </para>
 
     <programlisting>
     </para>
 
     <programlisting>

-       hello = Program('hello.c')
+       hello = Program('hello.c', CPPPATH=['/usr/include'])

        Ignore(hello, '/usr/include/stdio.h')
     </programlisting>
 
        Ignore(hello, '/usr/include/stdio.h')
     </programlisting>
 

+    <para>
+    &Ignore; can also be used to prevent a generated file from being built 
+    by default. This is due to the fact that directories depend on 
+    their contents.  So to ignore a generated file from the default build, 
+    you specify that the directory should ignore the generated file.
+    Note that the file will still be built if the user specifically 
+    requests the target on scons command line, or if the file is
+    a dependency of another file which is requested and/or is built
+    by default.
+    </para>
+
+    <scons_example name="ignore_explicit">
+      <file name="SConstruct" printme="1">
+      hello_obj=Object('hello.c')
+      hello = Program(hello_obj)
+      Ignore('.',[hello,hello_obj])
+      </file>
+      <file name="hello.c">
+      #include "stdio.h"
+      int main() { printf("Hello!\n"); }
+      </file>
+    </scons_example>
+
+    <scons_output example="ignore_explicit" os="posix">
+      <scons_output_command>scons -Q</scons_output_command>
+      <scons_output_command>scons -Q hello</scons_output_command>
+      <scons_output_command>scons -Q hello</scons_output_command>
+    </scons_output>

   </section>
 
   <section>
   </section>
 
   <section>

-  <title>Explicit Dependencies:  the &Depends; Method</title>
+  <title>Order-Only Dependencies:  the &Requires; Function</title>

 
     <para>
 
 
     <para>
 

-    On the other hand,
-    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:
+    Occasionally,
+    it may be useful to specify that a certain
+    file or directory must, if necessary,
+    be built or created before some other target is built,
+    but that changes to that file or directory
+    do <emphasis>not</emphasis>
+    require that the target itself be rebuilt.
+    Such a relationship is called an
+    <emphasis>order-only dependency</emphasis>
+    because it only affects the order in which
+    things must be built--the dependency before the target--but
+    it is not a strict dependency relationship
+    because the target should not
+    change in response to changes in the dependent file.

 
     </para>
 
 
     </para>
 

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

 
 

-    <!-- XXX mention that you can use arrays for target and source? -->
+    For example, suppose that you want to create a file
+    every time you run a build
+    that identifies the time the build was performed,
+    the version number, etc.,
+    and which is included in every program that you build.
+    The version file's contents will change every build.
+    If you specify a normal dependency relationship,
+    then every program that depends on
+    that file would be rebuilt every time you ran &SCons;.
+    For example, we could use some Python code in
+    a &SConstruct; file to create a new <filename>version.c</filename> file
+    with a string containing the current date every time
+    we run &SCons;,
+    and then link a program with the resulting object file
+    by listing <filename>version.c</filename> in the sources:

 
 

-    <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>
+    </para>
+
+    <scons_example name="no-Requires">
+      <file name="SConstruct" printme="1">
+      import time
+
+      version_c_text = """
+      char *date = "%s";
+      """ % time.ctime(time.time())
+      open('version.c', 'w').write(version_c_text)
+
+      hello = Program(['hello.c', 'version.c'])
+      </file>
+      <file name="hello.c">
+      extern char *date;
+      int main() { printf("Hello, %s!  I was built: %s\n", date); }
+      </file>
+    </scons_example>
+
+    <para>
+
+    If we list <filename>version.c</filename> as an actual source file,
+    though, then the <filename>version.o</filename> file
+    will get rebuilt every time we run &SCons;
+    (because the &SConstruct; file itself changes
+    the contents of <filename>version.c</filename>)
+    and the <filename>hello</filename> executable
+    will get re-linked every time
+    (because the <filename>version.o</filename> file changes):
+
+    </para>
+
+    <scons_output example="no-Requires">
+      <scons_output_command>scons -Q hello</scons_output_command>
+      <scons_output_command>sleep 1</scons_output_command>
+      <scons_output_command>scons -Q hello</scons_output_command>
+      <scons_output_command>sleep 1</scons_output_command>
+      <scons_output_command>scons -Q hello</scons_output_command>
+    </scons_output>
+
+    <para>
+
+    (Note that for the above example to work,
+    we &sleep; for one second in between each run,
+    so that the &SConstruct; file will create a
+    <filename>version.c</filename> file with a time string
+    that's one second later than the previous run.)
+
+    </para>
+
+    <para>
+
+    One solution is to use the &Requires; function
+    to specify that the <filename>version.o</filename>
+    must be rebuilt before it is used by the link step,
+    but that changes to <filename>version.o</filename>
+    should not actually cause the <filename>hello</filename>
+    executable to be re-linked:
+
+    </para>
+
+    <scons_example name="Requires">
+      <file name="SConstruct" printme="1">
+      import time
+
+      version_c_text = """
+      char *date = "%s";
+      """ % time.ctime(time.time())
+      open('version.c', 'w').write(version_c_text)
+
+      version_obj = Object('version.c')
+
+      hello = Program('hello.c',
+                      LINKFLAGS = str(version_obj[0]))
+
+      Requires(hello, version_obj)
+      </file>
+      <file name="hello.c">
+      extern char *date;
+      int main() { printf("Hello, %s!  I was built: %s\n", date); }
+      </file>
+    </scons_example>
+
+    <para>
+
+    Notice that because we can no longer list <filename>version.c</filename>
+    as one of the sources for the <filename>hello</filename> program,
+    we have to find some other way to get it into the link command line.
+    For this example, we're cheating a bit and stuffing the
+    object file name (extracted from <literal>version_obj</literal>
+    list returned by the &b-Object; call)
+    into the &cv-link-LINKFLAGS; variable,
+    because &cv-LINKFLAGS; is already included
+    in the &cv-link-LINKCOM; command line.
+
+    </para>
+
+    <para>
+
+    With these changes,
+    we get the desired behavior of only
+    re-linking the <filename>hello</filename> executable
+    when the <filename>hello.c</filename> has changed,
+    even though the <filename>version.o</filename> is rebuilt
+    (because the &SConstruct; file still changes the
+    <filename>version.c</filename> contents directly each run):
+
+    </para>
+
+    <scons_output example="Requires">
+      <scons_output_command>scons -Q hello</scons_output_command>
+      <scons_output_command>sleep 1</scons_output_command>
+      <scons_output_command>scons -Q hello</scons_output_command>
+      <scons_output_command>sleep 1</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_command>sleep 1</scons_output_command>
+      <scons_output_command>scons -Q hello</scons_output_command>
+    </scons_output>

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

-    XXX
+    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>
 
 
     </para>