Merged revisions 2454-2525 via svnmerge from

http://scons.tigris.org/svn/scons/branches/core

........
  r2455 | stevenknight | 2007-09-20 01:27:23 -0500 (Thu, 20 Sep 2007) | 2 lines

  Use ${TARGET.base} to make sure $TARGET attributes stay fixed.
........
  r2456 | stevenknight | 2007-09-25 11:52:30 -0500 (Tue, 25 Sep 2007) | 5 lines

  Issue 1734:  Avoid having content signature calculation of Alias Nodes
  consume excessive amounts of memory by having an Alias' "contents" be a
  concatenation of the children's signatures, not the children's contents.
  (Ken Deeter)
........
  r2457 | stevenknight | 2007-09-26 12:18:49 -0500 (Wed, 26 Sep 2007) | 2 lines

  Add an Options.UnknownOptions() method.
........
  r2458 | stevenknight | 2007-09-26 16:26:05 -0500 (Wed, 26 Sep 2007) | 2 lines

  Add a compatibility fnmatch.filter() function.
........
  r2459 | stevenknight | 2007-09-27 18:26:03 -0500 (Thu, 27 Sep 2007) | 3 lines

  Add a new Glob() function that matches in-memory Nodes as well as on-disk
  files (including matching repository and source directories).
........
  r2460 | stevenknight | 2007-09-28 15:01:37 -0500 (Fri, 28 Sep 2007) | 5 lines

  Issue 1020:  fix use of Clean() for files created by "side effect"
  in BuildDir() by removing the file by absolute path, not by what str()
  returns.  (It will think that the file is a source file and return a
  path to the source directory.)
........
  r2461 | stevenknight | 2007-09-29 05:39:09 -0500 (Sat, 29 Sep 2007) | 2 lines

  Update to TestCmd 0.28 modules.
........
  r2462 | stevenknight | 2007-09-29 05:49:29 -0500 (Sat, 29 Sep 2007) | 3 lines

  The RPM packaging can no longer take a "target" argument and produces
  an appropriate error message.  Update the test accordingly.
........
  r2463 | pscholl | 2007-09-30 08:57:01 -0500 (Sun, 30 Sep 2007) | 3 lines

  fix documentation issues (issue 1736 on the bugtracker)
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  r2464 | pscholl | 2007-09-30 09:39:49 -0500 (Sun, 30 Sep 2007) | 3 lines

  fix target set up if multiple package builders are specified at once.
........
  r2465 | stevenknight | 2007-10-01 13:00:44 -0500 (Mon, 01 Oct 2007) | 3 lines

  Update to TestCmd 0.29, with new methods for searching for a list
  of lines in output.
........
  r2466 | stevenknight | 2007-10-01 13:58:41 -0500 (Mon, 01 Oct 2007) | 4 lines

  Issue 1737:  Fix use of Configure() contexts with the -c (clean) and -h
  (help) options by supporting the ability to *configure* whether or no
  configure context tests are executed during those modes.
........
  r2467 | stevenknight | 2007-10-01 16:58:21 -0500 (Mon, 01 Oct 2007) | 2 lines

  Update to TestCmd 0.30, with a new TestCmd.rmdir() method.
........
  r2468 | stevenknight | 2007-10-01 17:05:44 -0500 (Mon, 01 Oct 2007) | 4 lines

  Issue 1586:  Capture a test script for "ghost" entries in .sconsign files.
  Test cases by Morten Elo Peterson and Jason Orendorff, packaged by
  Gary Oberbrunner.
........
  r2469 | stevenknight | 2007-10-04 11:21:12 -0500 (Thu, 04 Oct 2007) | 4 lines

  When cloning a construction environment, have the clone record the
  re-binding of the methods that were added to the original construction
  environment, so that further clones have their methods re-bound as well.
........
  r2470 | stevenknight | 2007-10-05 13:02:34 -0500 (Fri, 05 Oct 2007) | 3 lines

  Refactor the Glob() code for efficiency and readability.  (Greg Noel)
  Refactor Glob() unit tests for platform-independence.
........
  r2471 | stevenknight | 2007-10-09 10:49:15 -0500 (Tue, 09 Oct 2007) | 2 lines

  Back out Glob() refactoring to avoid Repository breakage.
........
  r2472 | stevenknight | 2007-10-09 12:16:33 -0500 (Tue, 09 Oct 2007) | 3 lines

  Fix ToolInitializer-related infinite recursion when the BUILDERS dict
  and the environment attributes can get out of sync.
........
  r2473 | stevenknight | 2007-10-10 14:39:19 -0500 (Wed, 10 Oct 2007) | 4 lines

  Fix a race condition in the -j sub-test by using marker directories to
  make sure (?) that the two build scripts are actually executed in parallel
  (regardless of system load).
........
  r2474 | stevenknight | 2007-10-11 12:32:07 -0500 (Thu, 11 Oct 2007) | 4 lines

  Re-fix globbing on case-insensitive systems like Windows.  Slight
  efficiency improvements as well (avoiding unnecessary calls to
  fnmatch.filter()).
........
  r2475 | stevenknight | 2007-10-11 15:04:42 -0500 (Thu, 11 Oct 2007) | 3 lines

  Refactor the Node lookup logic to fix handling Windows drive letters
  after an initial '#'.
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  r2476 | stevenknight | 2007-10-12 00:01:31 -0500 (Fri, 12 Oct 2007) | 2 lines

  Fix nested scope issues (for the benefit of older Python versions).
........
  r2477 | stevenknight | 2007-10-12 11:36:21 -0500 (Fri, 12 Oct 2007) | 2 lines

  Issue 1743:  Document '#' interpretation, with examples.
........
  r2478 | stevenknight | 2007-10-12 12:17:50 -0500 (Fri, 12 Oct 2007) | 3 lines

  Fix the ability of our default ActionFactory function to handle Nodes
  as input.
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  r2479 | stevenknight | 2007-10-12 13:53:37 -0500 (Fri, 12 Oct 2007) | 4 lines

  Enhance Options() file execution to add the file's directory to sys.path
  (and remove it afterwards) and to add a __name__ variable that can be
  used for introspecting on the file's location.
........
  r2480 | stevenknight | 2007-10-14 17:57:09 -0500 (Sun, 14 Oct 2007) | 4 lines

  Remove unnecessary os.path.normpath() calls when looking up directories
  or files by checking for whether we can just tack on a single entry name
  to the already-normalized lookup path of the directory Node.
........
  r2481 | stevenknight | 2007-10-17 01:08:47 -0500 (Wed, 17 Oct 2007) | 2 lines

  Add a GetBuildFailures() function.
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  r2482 | stevenknight | 2007-10-17 09:56:18 -0500 (Wed, 17 Oct 2007) | 2 lines

  Fix the GetBuildFailures() example in the man page.
........
  r2483 | stevenknight | 2007-10-17 10:54:21 -0500 (Wed, 17 Oct 2007) | 3 lines

  Use sys.exitfunc if there's no atexit module (Python 1.5.2).
  Sort the failure list for deterministic build output under system load.
........
  r2484 | stevenknight | 2007-10-20 12:33:07 -0500 (Sat, 20 Oct 2007) | 2 lines

  Use more efficient Decider() defaults instead of {Target,Source}Signatures().
........
  r2485 | stevenknight | 2007-10-20 17:42:27 -0500 (Sat, 20 Oct 2007) | 2 lines

  Windows portability in GetBuildFailures() test scripts.
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  r2486 | stevenknight | 2007-10-20 20:46:26 -0500 (Sat, 20 Oct 2007) | 2 lines

  Windows portability:  rename internal copy.py script, use a stub instead of tar.
........
  r2487 | stevenknight | 2007-10-24 23:36:24 -0500 (Wed, 24 Oct 2007) | 4 lines

  Whenever a script configures SConsignFile(None), make sure it uses
  stub compiler and linker scripts, not the system ones, to avoid writing
  (or trying to write) .sconsign files in system directories.
........
  r2488 | stevenknight | 2007-10-26 13:57:38 -0500 (Fri, 26 Oct 2007) | 2 lines

  Issue 1764:  Fix test-script portability issues on Solaris.
........
  r2489 | stevenknight | 2007-10-27 07:37:37 -0500 (Sat, 27 Oct 2007) | 3 lines

  Issue 1757:  add a CheckTypeSize() call to Configure contexts
  (David Cournapeau).
........
  r2490 | stevenknight | 2007-10-28 07:58:30 -0500 (Sun, 28 Oct 2007) | 2 lines

  Python 1.5.2 compatibility:  no use of +=.
........
  r2491 | stevenknight | 2007-10-29 12:13:35 -0500 (Mon, 29 Oct 2007) | 3 lines

  Issue 1758:  Fix the SCons packaging build for use with shared-lib
  versions of Python and to avoid .egg-info naming issues.
........
  r2492 | stevenknight | 2007-10-29 14:09:57 -0500 (Mon, 29 Oct 2007) | 2 lines

  Document the "expect" argument to CheckTypeSize().  (David Cournapeau)
........
  r2493 | stevenknight | 2007-11-05 20:57:27 -0600 (Mon, 05 Nov 2007) | 2 lines

  Fix use of Glob() when a pattern is below an explicitly-named subdirectory.
........
  r2495 | stevenknight | 2007-11-12 22:58:12 -0600 (Mon, 12 Nov 2007) | 5 lines

  Add a get_sources() access method to avoid an O(n^2) problem when adding
  sources to an Executor object.  The old code weeded out duplicates
  whenever a new source was added; the new code only does that when the
  source is list going to be used.
........
  r2496 | stevenknight | 2007-11-15 12:19:55 -0600 (Thu, 15 Nov 2007) | 4 lines

  Redefine the $WINDOWSPROGMANIFESTSUFFIX and
  $WINDOWSSHLIBMANIFESTSUFFIX variables so they pick up changes to
  the underlying $SHLIBSUFFIX and $PROGSUFFIX variables.
........
  r2497 | stevenknight | 2007-11-18 17:11:52 -0600 (Sun, 18 Nov 2007) | 4 lines

  Support .status and .command attributes of BuildError exceptions.
  Change Action objects to return BuildError objects (not raise them)
  when an action fails.
........
  r2498 | stevenknight | 2007-11-19 07:27:16 -0600 (Mon, 19 Nov 2007) | 3 lines

  When converting .sconsign paths to Nodes, use the more efficient
  _lookup_abs() method.
........
  r2499 | stevenknight | 2007-11-25 00:18:20 -0600 (Sun, 25 Nov 2007) | 3 lines

  Move the reflection-checking is_under() logic from the .srcdir_list()
  method to the .srcdir_duplicate() method.
........
  r2500 | stevenknight | 2007-11-25 00:31:33 -0600 (Sun, 25 Nov 2007) | 2 lines

  Have the .srcnode() method use the .srcdir_list() method.
........
  r2501 | stevenknight | 2007-11-28 22:56:39 -0600 (Wed, 28 Nov 2007) | 3 lines

  Issue 1845:  Have single-source Builders (like Object()) return
  NodeList objects even when called with multiple files.
........
  r2502 | stevenknight | 2007-11-28 23:00:46 -0600 (Wed, 28 Nov 2007) | 2 lines

  Issue 1845:  Document the NodeList behavior w.r.t Python's += operator.
........
  r2503 | stevenknight | 2007-11-29 09:35:31 -0600 (Thu, 29 Nov 2007) | 3 lines

  Issue 1840:  Fix a lot of typos in the man page and Users' Guide.
  (Malte Helmert)
........
  r2504 | stevenknight | 2007-11-29 10:41:44 -0600 (Thu, 29 Nov 2007) | 3 lines

  Issue 1841:  Fix --implicit-cache spurious rebuilds and inefficiency
  when using Builders that produce multiple targets.  (Benoit Belley)
........
  r2505 | stevenknight | 2007-11-30 17:36:03 -0600 (Fri, 30 Nov 2007) | 3 lines

  Unit test fix for Python 1.5.2, which can't .extend() lists with
  UserList objects.
........
  r2506 | stevenknight | 2007-11-30 20:37:14 -0600 (Fri, 30 Nov 2007) | 2 lines

  Python 1.5.2 portability:  use string.join(), not ' '.join().
........
  r2507 | stevenknight | 2007-11-30 21:42:19 -0600 (Fri, 30 Nov 2007) | 3 lines

  When searching directory lists like $CPPPATH, don't make Dir Nodes for
  directories that don't exist on disk.
........
  r2508 | stevenknight | 2007-12-01 00:14:35 -0600 (Sat, 01 Dec 2007) | 2 lines

  Add a Requires() function for specifying order-only prerequisites.
........
  r2509 | stevenknight | 2007-12-01 07:32:24 -0600 (Sat, 01 Dec 2007) | 3 lines

  Handle absolute paths without infinite recursion in the new code that
  searches for implicit dependencies without creating unnecessary Dir Nodes.
........
  r2510 | stevenknight | 2007-12-03 15:11:56 -0600 (Mon, 03 Dec 2007) | 2 lines

  Restore the rel_path() method, for the benefit of SConscript files using it.
........
  r2511 | stevenknight | 2007-12-04 00:34:02 -0600 (Tue, 04 Dec 2007) | 4 lines

  User's Guide updates for the Big Signature refactoring, capturing mention
  of things that still need documenting, and other changes from re-running
  the examples through the latest code.
........
  r2512 | stevenknight | 2007-12-04 08:48:51 -0600 (Tue, 04 Dec 2007) | 3 lines

  Issue 1846:  allow building only part of the dependency graph when
  BuildDir(duplicate=0) is used.  (Benoit Belley)
........
  r2513 | stevenknight | 2007-12-06 05:02:56 -0600 (Thu, 06 Dec 2007) | 3 lines

  Have the code that avoids creating unnecessary Dir Nodes when searching
  $*PATH variables handle absolute paths with Windows drive letters.
........
  r2514 | stevenknight | 2007-12-08 07:44:15 -0600 (Sat, 08 Dec 2007) | 6 lines

  Issue 1852:  Make the default behavior of
  {Source,Target}Signatures('timestamp') equivalent to
  'timestamp-match', not 'timestamp-newer'.  Fix use of CacheDir with
  Decider('timestamp-newer') by updating the modification time when copying
  files from the cache.
........
  r2515 | stevenknight | 2007-12-08 08:23:02 -0600 (Sat, 08 Dec 2007) | 4 lines

  Update the mock compiler inin/sconsoutput to use $CPPPATH.
  Capture the ripple effect in the Troubleshooting appendix.
  Also add a -t option to the mock "touch" command.
........
  r2516 | stevenknight | 2007-12-08 09:16:11 -0600 (Sat, 08 Dec 2007) | 3 lines

  Update the Dependencies chapter for use of the Decider() function,
  and to now discourage use of SourceSignatures() and TargetSignatures().
........
  r2517 | stevenknight | 2007-12-08 12:31:10 -0600 (Sat, 08 Dec 2007) | 4 lines

  Issue 1721:  On Windows, wrap __builtin__.close() and __builtin__.file()
  to disable file handle inheritance on any files opened by SCons during
  the run.
........
  r2518 | stevenknight | 2007-12-11 17:33:20 -0600 (Tue, 11 Dec 2007) | 4 lines

  Prevent the _get_str() method from causing underlying stat() values
  to be cached if we're not yet saving the string representations of
  FS.Base() Nodes.
........
  r2519 | stevenknight | 2007-12-11 23:27:05 -0600 (Tue, 11 Dec 2007) | 4 lines

  Add a warning about the unreliability of -j if the pywin32 modules aren't
  available or are old and can't suppress file handle inheritance.
  Add a release note about the change to open() and file().
........
  r2520 | stevenknight | 2007-12-11 23:28:05 -0600 (Tue, 11 Dec 2007) | 2 lines

  Add an overlooked update, fix spelling.
........
  r2521 | stevenknight | 2007-12-12 09:12:42 -0600 (Wed, 12 Dec 2007) | 3 lines

  Use &TargetSignatures; (replace missing ampersand) in the title of
  that section.  Move the &Depends; section to before the &Ignore; section.
........
  r2522 | stevenknight | 2007-12-12 09:20:46 -0600 (Wed, 12 Dec 2007) | 3 lines

  Final documentation update for checkpoint release:  propagate .in changes
  to .xml files.
........
  r2523 | stevenknight | 2007-12-12 09:29:11 -0600 (Wed, 12 Dec 2007) | 2 lines

  Update release lines for new checkpoint release.
........

1 files changed, 582 insertions, 155 deletions

diff --git a/doc/user/depends.xml b/doc/user/depends.xml
index 9e055ee..936e4b2 100644
--- a/doc/user/depends.xml
+++ b/doc/user/depends.xml
@@ -26,12 +26,12 @@
   <para>
 
   So far we've seen how &SCons; handles one-time builds.
-  But the real point of a build tool like &SCons;
+  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 be re-invoking &SCons;
+  You can see this at work simply by re-invoking &SCons;
   after building our simple &hello; example:
 
   </para>
@@ -74,33 +74,36 @@
   </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>
 
-    The other side of avoiding unnecessary rebuilds
+    Another aspect of avoiding unnecessary rebuilds
     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.
-    &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>
-    <title>MD5 Source File Signatures</title>
+    <title>Using MD5 Signatures to Decide if a File Has Changed</title>
 
       <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
@@ -150,35 +153,114 @@
       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>
 
       <programlisting>
         Program('hello.c')
-        SourceSignatures('MD5')
+        Decider('MD5')
       </programlisting>
 
+      <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>
+
+        <screen>
+           % <userinput>scons -Q hello</userinput>
+           cc -o hello.o -c hello.c
+           cc -o hello hello.o
+           % <userinput>edit hello.c</userinput>
+             [CHANGE A COMMENT IN hello.c]
+           % <userinput>scons -Q hello</userinput>
+           cc -o hello.o -c hello.c
+           scons: `hello' is up to date.
+        </screen>
+
+        <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>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
-      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
+      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>
 
       <programlisting>
         Program('hello.c')
-        SourceSignatures('timestamp')
+        Decider('timestamp-newer')
       </programlisting>
 
       <para>
@@ -199,40 +281,118 @@
          cc -o hello hello.o
       </screen>
 
-    </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>
+      <programlisting>
+        Program('hello.c')
+        Decider('make')
+      </programlisting>
 
-    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.
+
+      </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>
+
+      <programlisting>
+        Program('hello.c')
+        Decider('timestamp-match')
+      </programlisting>
+
+      <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>
+
+      <screen>
+         % <userinput>scons -Q hello</userinput>
+         cc -o hello.o -c hello.c
+         cc -o hello hello.o
+         % <userinput>touch -t 198901010000 hello.c</userinput>
+         % <userinput>scons -Q hello</userinput>
+         cc -o hello.o -c hello.c
+         scons: `hello' is up to date.
+      </screen>
+
+      <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>Build Signatures</title>
+    <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>
+
+      <programlisting>
+        Program('hello.c')
+        Decider('MD5-timestamp')
+      </programlisting>
 
       <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:
+      So configured, &SCons; will still behave like
+      it does when using <literal>Decider('MD5')</literal>:
 
       </para>
 
@@ -240,112 +400,380 @@
          % <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
+         scons: `hello' is up to date.
       </screen>
 
       <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.
+      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>
 
-      If you wish, you can
-      specify this default behavior
-      (build signatures) explicitly
-      using the &TargetSignatures; function:
+      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>
 
       <programlisting>
         Program('hello.c')
-        TargetSignatures('build')
+        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)
       </programlisting>
 
+      <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>File Contents</title>
+    <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>
 
-      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, 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>
 
       <programlisting>
-        Program('hello.c')
-        TargetSignatures('content')
+        env1 = Environment(CPPPATH = ['.'])
+        env2 = env1.Clone()
+        env2.Decider('timestamp-match')
+        env1.Program('prog-MD5', 'program1.c')
+        env2.Program('prog-timestamp', 'program2.c')
       </programlisting>
 
       <para>
 
-      So if, for example,
-      a user were to only change a comment in a C file,
-      then the rebuilt &hello_o; file
-      would be exactly the same as the one previously built
-      (assuming the compiler doesn't put any build-specific
-      information in the object file).
-      &SCons; would then realize that it would not
-      need to rebuild the &hello; program as follows:
+      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>
 
       <screen>
-         % <userinput>scons -Q hello</userinput>
-         cc -o hello.o -c hello.c
-         cc -o hello hello.o
-         % <userinput>edit hello.c</userinput>
-           [CHANGE A COMMENT IN hello.c]
-         % <userinput>scons -Q hello</userinput>
-         cc -o hello.o -c hello.c
-         scons: `hello' is up to date.
+         % <userinput>scons -Q</userinput>
+         cc -o program1.o -c -I. program1.c
+         cc -o prog-MD5 program1.o
+         cc -o program2.o -c -I. program2.c
+         cc -o prog-timestamp program2.o
+         % <userinput>touch inc.h</userinput>
+         % <userinput>scons -Q</userinput>
+         cc -o program2.o -c -I. program2.c
+         cc -o prog-timestamp program2.o
       </screen>
 
+    </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>
+
+      <programlisting>
+        Program('hello.c')
+        SourceSignatures('MD5')
+      </programlisting>
+
+      <para>
+
+      Or using time stamps:
+
+      </para>
+
+      <programlisting>
+        Program('hello.c')
+        SourceSignatures('timestamp')
+      </programlisting>
+
       <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.
+      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>
+
+      <programlisting>
+        Program('hello.c')
+        TargetSignatures('MD5')
+      </programlisting>
+
+      <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>
+
+      <programlisting>
+        Program('hello.c')
+        TargetSignatures('timestamp')
+      </programlisting>
+
+      <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>
+
+      <programlisting>
+        Program('hello.c')
+        TargetSignatures('source')
+        SourceSignatures('timestamp')
+      </programlisting>
+
+      <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>
 
@@ -359,7 +787,7 @@
     <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>
@@ -458,7 +886,7 @@
     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
@@ -703,7 +1131,44 @@
   </section>
 
   <section>
-  <title>Ignoring Dependencies:  the &Ignore; Method</title>
+  <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>
 
@@ -749,7 +1214,7 @@
 
     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;
@@ -774,45 +1239,7 @@
   </section>
 
   <section>
-  <title>Explicit Dependencies:  the &Depends; Method</title>
-
-    <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:
-
-    </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>The &AlwaysBuild; Method</title>
+  <title>The &AlwaysBuild; Function</title>
 
     <para>