path: root/doc/user/troubleshoot.sgml

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  <para>

  The experience of configuring any
  software build tool to build a large code base
  usually, at some point,
  involves trying to figure out why
  the tool is behaving a certain way,
  and how to get it to behave the way you want.
  &SCons; is no different.
  This appendix contains a number of
  different ways in which you can
  get some additional insight into &SCons;' behavior.

  </para>

  <para>

  Note that we're always interested in trying to
  improve how you can troubleshoot configuration problems.
  If you run into a problem that has
  you scratching your head,
  and which there just doesn't seem to be a good way to debug,
  odds are pretty good that someone else will run into
  the same problem, too.
  If so, please let the SCons development team know
  (preferably by filing a bug report
  or feature request at our project pages at tigris.org)
  so that we can use your feedback
  to try to come up with a better way to help you,
  and others, get the necessary insight into &SCons; behavior
  to help identify and fix configuration issues.

  </para>

  <section>
  <title>Why is That Target Being Rebuilt?  the &debug-explain; Option</title>

    <para>

    Let's look at a simple example of
    a misconfigured build
    that causes a target to be rebuilt
    every time &SCons; is run:

    </para>

    <programlisting>
      # Intentionally misspell the output file name in the
      # command used to create the file:
      Command('file.out', 'file.in', 'cp $SOURCE file.oout')
    </programlisting>

    <para>

    (Note to Windows users:  The POSIX &cp; command
    copies the first file named on the command line
    to the second file.
    In our example, it copies the &file_in; file
    to the &file_out; file.)

    </para>

    <para>

    Now if we run &SCons; multiple times on this example,
    we see that it re-runs the &cp;
    command every time:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      cp file.in file.oout
      % <userinput>scons -Q</userinput>
      cp file.in file.oout
      % <userinput>scons -Q</userinput>
      cp file.in file.oout
    </screen>

    <para>

    In this example,
    the underlying cause is obvious:
    we've intentionally misspelled the output file name
    in the &cp; command,
    so the command doesn't actually
    build the &file_out; file that we've told &SCons; to expect.
    But if the problem weren't obvious,
    it would be helpful
    to specify the &debug-explain; option
    on the command line
    to have &SCons; tell us very specifically
    why it's decided to rebuild the target:

    </para>

    <screen>
      % <userinput>scons -Q --debug=explain</userinput>
      scons: building `file.out' because it doesn't exist
      cp file.in file.oout
    </screen>

    <para>

    If this had been a more complicated example
    involving a lot of build output,
    having &SCons; tell us that
    it's trying to rebuild the target file
    because it doesn't exist
    would be an important clue
    that something was wrong with
    the command that we invoked to build it.

    </para>

    <para>

    The &debug-explain; option also comes in handy
    to help figure out what input file changed.
    Given a simple configuration that builds
    a program from three source files,
    changing one of the source files
    and rebuilding with the &debug-explain;
    option shows very specifically
    why &SCons; rebuilds the files that it does:

    </para>

    

    <screen>
      % <userinput>scons -Q</userinput>
      cc -o file1.o -c file1.c
      cc -o file2.o -c file2.c
      cc -o file3.o -c file3.c
      cc -o prog file1.o file2.o file3.o
      % <userinput>edit file2.c</userinput>
          [CHANGE THE CONTENTS OF file2.c]
      % <userinput>scons -Q --debug=explain</userinput>
      scons: rebuilding `file2.o' because `file2.c' changed
      cc -o file2.o -c file2.c
      scons: rebuilding `prog' because `file2.o' changed
      cc -o prog file1.o file2.o file3.o
    </screen>

    <para>

    This becomes even more helpful
    in identifying when a file is rebuilt
    due to a change in an implicit dependency,
    such as an incuded <filename>.h</filename> file.
    If the <filename>file1.c</filename>
    and <filename>file3.c</filename> files
    in our example
    both included a &hello_h; file,
    then changing that included file
    and re-running &SCons; with the &debug-explain; option
    will pinpoint that it's the change to the included file
    that starts the chain of rebuilds:

    </para>

    

    <screen>
      % <userinput>scons -Q</userinput>
      cc -o file1.o -c -I. file1.c
      cc -o file2.o -c -I. file2.c
      cc -o file3.o -c -I. file3.c
      cc -o prog file1.o file2.o file3.o
      % <userinput>edit hello.h</userinput>
          [CHANGE THE CONTENTS OF hello.h]
      % <userinput>scons -Q --debug=explain</userinput>
      scons: rebuilding `file1.o' because `hello.h' changed
      cc -o file1.o -c -I. file1.c
      scons: rebuilding `file3.o' because `hello.h' changed
      cc -o file3.o -c -I. file3.c
      scons: rebuilding `prog' because:
                 `file1.o' changed
                 `file3.o' changed
      cc -o prog file1.o file2.o file3.o
    </screen>

    <para>

    (Note that the &debug-explain; option will only tell you
    why &SCons; decided to rebuild necessary targets.
    It does not tell you what files it examined
    when deciding <emphasis>not</emphasis>
    to rebuild a target file,
    which is often a more valuable question to answer.)

    </para>

  </section>

  <section>
  <title>What's in That Construction Environment?  the &Dump; Method</title>

    <para>

    When you create a construction environment,
    &SCons; populates it
    with construction variables that are set up
    for various compilers, linkers and utilities
    that it finds on your system.
    Although this is usually helpful and what you want,
    it might be frustrating if &SCons;
    doesn't set certain variables that you
    expect to be sit.
    In situations like this,
    it's sometimes helpful to use the
    construction environment &Dump; method
    to print all or some of
    the construction variables.
    Note that the &Dump; method
    <emphasis>returns</emphasis>
    the representation of the variables
    in the environment
    for you to print (or otherwise manipulate):

    </para>

    <programlisting>
         env = Environment()
         print env.Dump()
    </programlisting>

    <para>

    On a POSIX system with gcc installed,
    this might generate:

    </para>

    <screen>
      % <userinput>scons</userinput>
      scons: Reading SConscript files ...
      { 'BUILDERS': {},
        'CONFIGUREDIR': '#/.sconf_temp',
        'CONFIGURELOG': '#/config.log',
        'CPPSUFFIXES': [ '.c',
                         '.C',
                         '.cxx',
                         '.cpp',
                         '.c++',
                         '.cc',
                         '.h',
                         '.H',
                         '.hxx',
                         '.hpp',
                         '.hh',
                         '.F',
                         '.fpp',
                         '.FPP',
                         '.m',
                         '.mm',
                         '.S',
                         '.spp',
                         '.SPP'],
        'DSUFFIXES': ['.d'],
        'Dir': &lt;SCons.Defaults.Variable_Method_Caller instance at 0xb7c3fdac&gt;,
        'Dirs': &lt;SCons.Defaults.Variable_Method_Caller instance at 0xb7c3fdcc&gt;,
        'ENV': {'PATH': '/usr/local/bin:/opt/bin:/bin:/usr/bin'},
        'ESCAPE': &lt;function escape at 0xb7ba1f0c&gt;,
        'File': &lt;SCons.Defaults.Variable_Method_Caller instance at 0xb7c3fdec&gt;,
        'IDLSUFFIXES': ['.idl', '.IDL'],
        'INSTALL': &lt;function installFunc at 0xb7c4317c&gt;,
        'INSTALLSTR': &lt;function installStr at 0xb7c431b4&gt;,
        'LATEXSUFFIXES': ['.tex', '.ltx', '.latex'],
        'LIBPREFIX': 'lib',
        'LIBPREFIXES': '$LIBPREFIX',
        'LIBSUFFIX': '.a',
        'LIBSUFFIXES': ['$LIBSUFFIX', '$SHLIBSUFFIX'],
        'MAXLINELENGTH': 128072,
        'OBJPREFIX': '',
        'OBJSUFFIX': '.o',
        'PLATFORM': 'posix',
        'PROGPREFIX': '',
        'PROGSUFFIX': '',
        'PSPAWN': &lt;function piped_env_spawn at 0xb7bb12cc&gt;,
        'RDirs': &lt;SCons.Defaults.Variable_Method_Caller instance at 0xb7c3fe0c&gt;,
        'SCANNERS': [],
        'SHELL': 'sh',
        'SHLIBPREFIX': '$LIBPREFIX',
        'SHLIBSUFFIX': '.so',
        'SHOBJPREFIX': '$OBJPREFIX',
        'SHOBJSUFFIX': '$OBJSUFFIX',
        'SPAWN': &lt;function spawnvpe_spawn at 0xb7ba1d4c&gt;,
        'TEMPFILE': &lt;class SCons.Platform.TempFileMunge at 0xb7bce89c&gt;,
        'TEMPFILEPREFIX': '@',
        'TOOLS': [],
        '_CPPDEFFLAGS': '${_defines(CPPDEFPREFIX, CPPDEFINES, CPPDEFSUFFIX, __env__)}',
        '_CPPINCFLAGS': '$( ${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)',
        '_LIBDIRFLAGS': '$( ${_concat(LIBDIRPREFIX, LIBPATH, LIBDIRSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)',
        '_LIBFLAGS': '${_concat(LIBLINKPREFIX, LIBS, LIBLINKSUFFIX, __env__)}',
        '__RPATH': '$_RPATH',
        '_concat': &lt;function _concat at 0xb7c43224&gt;,
        '_defines': &lt;function _defines at 0xb7c432cc&gt;,
        '_installStr': &lt;function installStr at 0xb7c431b4&gt;,
        '_stripixes': &lt;function _stripixes at 0xb7c43294&gt;}
      scons: done reading SConscript files.
      scons: Building targets ...
      scons: `.' is up to date.
      scons: done building targets.
    </screen>

    <para>

    On a Windows system with Visual C++
    the output might look like:

    </para>

    <screen>
      C:\><userinput>scons</userinput>
      scons: Reading SConscript files ...
      { 'BUILDERS': {'Object': &lt;SCons.Builder.CompositeBuilder instance at 0xb7b6354c&gt;, 'SharedObject': &lt;SCons.Builder.CompositeBuilder instance at 0xb7b636cc&gt;, 'StaticObject': &lt;SCons.Builder.CompositeBuilder instance at 0xb7b6354c&gt;, 'PCH': &lt;SCons.Builder.BuilderBase instance at 0xb7bd6e8c&gt;, 'RES': &lt;SCons.Builder.BuilderBase instance at 0xb7b5b9ec&gt;},
        'CC': 'cl',
        'CCCOM': &lt;SCons.Action.FunctionAction instance at 0xb7b63b6c&gt;,
        'CCCOMFLAGS': '$CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS /c $SOURCES /Fo$TARGET $CCPCHFLAGS $CCPDBFLAGS',
        'CCFLAGS': ['/nologo'],
        'CCPCHFLAGS': ['${(PCH and "/Yu%s /Fp%s"%(PCHSTOP or "",File(PCH))) or ""}'],
        'CCPDBFLAGS': ['${(PDB and "/Z7") or ""}'],
        'CFILESUFFIX': '.c',
        'CFLAGS': [],
        'CONFIGUREDIR': '#/.sconf_temp',
        'CONFIGURELOG': '#/config.log',
        'CPPDEFPREFIX': '/D',
        'CPPDEFSUFFIX': '',
        'CPPSUFFIXES': [ '.c',
                         '.C',
                         '.cxx',
                         '.cpp',
                         '.c++',
                         '.cc',
                         '.h',
                         '.H',
                         '.hxx',
                         '.hpp',
                         '.hh',
                         '.F',
                         '.fpp',
                         '.FPP',
                         '.m',
                         '.mm',
                         '.S',
                         '.spp',
                         '.SPP'],
        'CXX': '$CC',
        'CXXCOM': '$CXX $CXXFLAGS $CCCOMFLAGS',
        'CXXFILESUFFIX': '.cc',
        'CXXFLAGS': ['$CCFLAGS', '$(', '/TP', '$)'],
        'DSUFFIXES': ['.d'],
        'Dir': &lt;SCons.Defaults.Variable_Method_Caller instance at 0xb7c5adac&gt;,
        'Dirs': &lt;SCons.Defaults.Variable_Method_Caller instance at 0xb7c5adcc&gt;,
        'ENV': { 'INCLUDE': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\include',
                 'LIB': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\lib',
                 'PATH': 'C:\\Program Files\\Microsoft Visual Studio\\Common\\tools\\WIN95;C:\\Program Files\\Microsoft Visual Studio\\Common\\MSDev98\\bin;C:\\Program Files\\Microsoft Visual Studio\\Common\\tools;C:\\Program Files\\Microsoft Visual Studio/VC98\\bin',
                 'PATHEXT': '.COM;.EXE;.BAT;.CMD',
                 'SystemRoot': 'C:/WINDOWS'},
        'ESCAPE': &lt;function escape at 0xb7bcf454&gt;,
        'File': &lt;SCons.Defaults.Variable_Method_Caller instance at 0xb7c5adec&gt;,
        'IDLSUFFIXES': ['.idl', '.IDL'],
        'INCPREFIX': '/I',
        'INCSUFFIX': '',
        'INSTALL': &lt;function installFunc at 0xb7c5e17c&gt;,
        'INSTALLSTR': &lt;function installStr at 0xb7c5e1b4&gt;,
        'LATEXSUFFIXES': ['.tex', '.ltx', '.latex'],
        'LIBPREFIX': '',
        'LIBPREFIXES': ['$LIBPREFIX'],
        'LIBSUFFIX': '.lib',
        'LIBSUFFIXES': ['$LIBSUFFIX'],
        'MAXLINELENGTH': 2048,
        'MSVS': {'VERSION': '6.0', 'VERSIONS': ['6.0']},
        'MSVS_VERSION': '6.0',
        'OBJPREFIX': '',
        'OBJSUFFIX': '.obj',
        'PCHCOM': '$CXX $CXXFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS /c $SOURCES /Fo${TARGETS[1]} /Yc$PCHSTOP /Fp${TARGETS[0]} $CCPDBFLAGS $PCHPDBFLAGS',
        'PCHPDBFLAGS': ['${(PDB and "/Yd") or ""}'],
        'PLATFORM': 'win32',
        'PROGPREFIX': '',
        'PROGSUFFIX': '.exe',
        'PSPAWN': &lt;function piped_spawn at 0xb7bcf3ac&gt;,
        'RC': 'rc',
        'RCCOM': '$RC $_CPPDEFFLAGS $_CPPINCFLAGS $RCFLAGS /fo$TARGET $SOURCES',
        'RCFLAGS': [],
        'RDirs': &lt;SCons.Defaults.Variable_Method_Caller instance at 0xb7c5ae0c&gt;,
        'SCANNERS': [],
        'SHCC': '$CC',
        'SHCCCOM': &lt;SCons.Action.FunctionAction instance at 0xb7b63bcc&gt;,
        'SHCCFLAGS': ['$CCFLAGS'],
        'SHCFLAGS': ['$CFLAGS'],
        'SHCXX': '$CXX',
        'SHCXXCOM': '$SHCXX $SHCXXFLAGS $CCCOMFLAGS',
        'SHCXXFLAGS': ['$CXXFLAGS'],
        'SHELL': None,
        'SHLIBPREFIX': '',
        'SHLIBSUFFIX': '.dll',
        'SHOBJPREFIX': '$OBJPREFIX',
        'SHOBJSUFFIX': '$OBJSUFFIX',
        'SPAWN': &lt;function spawn at 0xb7bcf41c&gt;,
        'STATIC_AND_SHARED_OBJECTS_ARE_THE_SAME': 1,
        'TEMPFILE': &lt;class SCons.Platform.TempFileMunge at 0xb7be989c&gt;,
        'TEMPFILEPREFIX': '@',
        'TOOLS': ['msvc'],
        '_CPPDEFFLAGS': '${_defines(CPPDEFPREFIX, CPPDEFINES, CPPDEFSUFFIX, __env__)}',
        '_CPPINCFLAGS': '$( ${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)',
        '_LIBDIRFLAGS': '$( ${_concat(LIBDIRPREFIX, LIBPATH, LIBDIRSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)',
        '_LIBFLAGS': '${_concat(LIBLINKPREFIX, LIBS, LIBLINKSUFFIX, __env__)}',
        '_concat': &lt;function _concat at 0xb7c5e224&gt;,
        '_defines': &lt;function _defines at 0xb7c5e2cc&gt;,
        '_installStr': &lt;function installStr at 0xb7c5e1b4&gt;,
        '_stripixes': &lt;function _stripixes at 0xb7c5e294&gt;}
      scons: done reading SConscript files.
      scons: Building targets ...
      scons: `.' is up to date.
      scons: done building targets.
    </screen>

    <para>

    The construction environments in these examples have
    actually been restricted to just gcc and Visual C++,
    respectively.
    In a real-life situation,
    the construction environments will
    likely contain a great many more variables.

    </para>

    <para>

    To make it easier to see just what you're
    interested in,
    the &Dump; method allows you to
    specify a specific constrcution variable
    that you want to disply.
    For example,
    it's not unusual to want to verify
    the external environment used to execute build commands,
    to make sure that the PATH and other
    environment variables are set up the way they should be.
    You can do this as follows:

    </para>

    <programlisting>
         env = Environment()
         print env.Dump('ENV')
    </programlisting>

    <para>

    Which might display the following when executed on a POSIX system:

    </para>

    <screen>
      % <userinput>scons</userinput>
      scons: Reading SConscript files ...
      {'PATH': '/usr/local/bin:/opt/bin:/bin:/usr/bin'}
      scons: done reading SConscript files.
      scons: Building targets ...
      scons: `.' is up to date.
      scons: done building targets.
    </screen>

    <para>

    And the following when executed on a Windows system:

    </para>

    <screen>
      C:\><userinput>scons</userinput>
      scons: Reading SConscript files ...
      { 'INCLUDE': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\include',
        'LIB': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\lib',
        'PATH': 'C:\\Program Files\\Microsoft Visual Studio\\Common\\tools\\WIN95;C:\\Program Files\\Microsoft Visual Studio\\Common\\MSDev98\\bin;C:\\Program Files\\Microsoft Visual Studio\\Common\\tools;C:\\Program Files\\Microsoft Visual Studio/VC98\\bin',
        'PATHEXT': '.COM;.EXE;.BAT;.CMD',
        'SystemRoot': 'C:/WINDOWS'}
      scons: done reading SConscript files.
      scons: Building targets ...
      scons: `.' is up to date.
      scons: done building targets.
    </screen>

  </section>

  <section>

  <title>What Dependencies Does &SCons; Know About?  the &tree; Option</title>

    <para>

    Sometimes the best way to try to figure out what
    &SCons; is doing is simply to take a look at the
    dependency graph that it constructs
    based on your &SConscript; files.
    The <literal>--tree</literal> option
    will display all or part of the
    &SCons; dependency graph in an
    "ASCII art" graphical format
    that shows the dependency hierarchy.

    </para>

    <para>

    For example, given the following input &SConstruct; file:

    </para>

    <programlisting>
         env = Environment(CPPPATH = ['.'])
         env.Program('prog', ['f1.c', 'f2.c', 'f3.c'])
    </programlisting>

    <para>

    Running &SCons; with the <literal>--tree=all</literal>
    option yields:

    </para>

    <screen>
      % <userinput>scons -Q --tree=all</userinput>
      cc -o f1.o -c -I. f1.c
      cc -o f2.o -c -I. f2.c
      cc -o f3.o -c -I. f3.c
      cc -o prog f1.o f2.o f3.o
      +-.
        +--
        +-SConstruct
        +-f1.c
        +-f1.o
        | +-f1.c
        | +-inc.h
        +-f2.c
        +-f2.o
        | +-f2.c
        | +-inc.h
        +-f3.c
        +-f3.o
        | +-f3.c
        | +-inc.h
        +-inc.h
        +-prog
          +-f1.o
          | +-f1.c
          | +-inc.h
          +-f2.o
          | +-f2.c
          | +-inc.h
          +-f3.o
            +-f3.c
            +-inc.h
    </screen>

    <para>

    The tree will also be printed when the
    <literal>-n</literal> (no execute) option is used,
    which allows you to examine the dependency graph
    for a configuration without actually
    rebuilding anything in the tree.

    </para>

    <para>

    The <literal>--tree</literal> option only prints
    the dependency graph for the specified targets
    (or the default target(s) if none are specified on the command line).
    So if you specify a target like <filename>f2.o</filename>
    on the command line,
    the <literal>--tree</literal> option will only
    print the dependency graph for that file:

    </para>

    <screen>
      % <userinput>scons -Q --tree=all f2.o</userinput>
      cc -o f2.o -c -I. f2.c
      +-f2.o
        +-f2.c
        +-inc.h
    </screen>

    <para>

    This is, of course, useful for
    restricting the output from a very large
    build configuration to just a
    portion in which you're interested.
    Multiple targets are fine,
    in which case a tree will be printed
    for each specified target:

    </para>

    <screen>
      % <userinput>scons -Q --tree=all f1.o f3.o</userinput>
      cc -o f1.o -c -I. f1.c
      +-f1.o
        +-f1.c
        +-inc.h
      cc -o f3.o -c -I. f3.c
      +-f3.o
        +-f3.c
        +-inc.h
    </screen>

    <para>

    The <literal>status</literal> argument may be used
    to tell &SCons; to print status information about
    each file in the dependency graph:

    </para>

    <screen>
      % <userinput>scons -Q --tree=status</userinput>
      cc -o f1.o -c -I. f1.c
      cc -o f2.o -c -I. f2.c
      cc -o f3.o -c -I. f3.c
      cc -o prog f1.o f2.o f3.o
       E         = exists
        R        = exists in repository only
         b       = implicit builder
         B       = explicit builder
          S      = side effect
           P     = precious
            A    = always build
             C   = current
              N  = no clean
               H = no cache
      
      [E b      ]+-.
      [         ]  +--
      [E        ]  +-SConstruct
      [E        ]  +-f1.c
      [E B   C  ]  +-f1.o
      [E        ]  | +-f1.c
      [E        ]  | +-inc.h
      [E        ]  +-f2.c
      [E B   C  ]  +-f2.o
      [E        ]  | +-f2.c
      [E        ]  | +-inc.h
      [E        ]  +-f3.c
      [E B   C  ]  +-f3.o
      [E        ]  | +-f3.c
      [E        ]  | +-inc.h
      [E        ]  +-inc.h
      [E B   C  ]  +-prog
      [E B   C  ]    +-f1.o
      [E        ]    | +-f1.c
      [E        ]    | +-inc.h
      [E B   C  ]    +-f2.o
      [E        ]    | +-f2.c
      [E        ]    | +-inc.h
      [E B   C  ]    +-f3.o
      [E        ]      +-f3.c
      [E        ]      +-inc.h
    </screen>

    <para>

    Note that <literal>--tree=all,status</literal> is equivalent;
    the <literal>all</literal>
    is assumed if only <literal>status</literal> is present.
    As an alternative to <literal>all</literal>,
    you can specify <literal>--tree=derived</literal>
    to have &SCons; only print derived targets
    in the tree output,
    skipping source files
    (like <filename>.c</filename> and <filename>.h</filename> files):

    </para>

    <screen>
      % <userinput>scons -Q --tree=derived</userinput>
      cc -o f1.o -c -I. f1.c
      cc -o f2.o -c -I. f2.c
      cc -o f3.o -c -I. f3.c
      cc -o prog f1.o f2.o f3.o
      +-.
    </screen>

    <para>

    You can use the <literal>status</literal>
    modifier with <literal>derived</literal> as well:

    </para>

    <screen>
      % <userinput>scons -Q --tree=derived,status</userinput>
      cc -o f1.o -c -I. f1.c
      cc -o f2.o -c -I. f2.c
      cc -o f3.o -c -I. f3.c
      cc -o prog f1.o f2.o f3.o
       E         = exists
        R        = exists in repository only
         b       = implicit builder
         B       = explicit builder
          S      = side effect
           P     = precious
            A    = always build
             C   = current
              N  = no clean
               H = no cache
      
      [E b      ]+-.
      [E B   C  ]  +-f1.o
      [E B   C  ]  +-f2.o
      [E B   C  ]  +-f3.o
      [E B   C  ]  +-prog
      [E B   C  ]    +-f1.o
      [E B   C  ]    +-f2.o
      [E B   C  ]    +-f3.o
    </screen>

    <para>

    Note that the order of the <literal>--tree=</literal>
    arguments doesn't matter;
    <literal>--tree=status,derived</literal> is
    completely equivalent.

    </para>

    <para>

    The default behavior of the <literal>--tree</literal> option
    is to repeat all of the dependencies each time the library dependency
    (or any other dependency file) is encountered in the tree.
    If certain target files share other target files,
    such as two programs that use the same library:

    </para>

    <programlisting>
         env = Environment(CPPPATH = ['.'],
                           LIBS = ['foo'],
                           LIBPATH = ['.'])
         env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
         env.Program('prog1.c')
         env.Program('prog2.c')
    </programlisting>

    <para>

    Then there can be a <emphasis>lot</emphasis> of repetition in the
    <literal>--tree=</literal> output:

    </para>

    <screen>
      % <userinput>scons -Q --tree=all</userinput>
      cc -o f1.o -c -I. f1.c
      cc -o f2.o -c -I. f2.c
      cc -o f3.o -c -I. f3.c
      ar rc libfoo.a f1.o f2.o f3.o
      ranlib libfoo.a
      cc -o prog1.o -c -I. prog1.c
      cc -o prog1 prog1.o -L. -lfoo
      cc -o prog2.o -c -I. prog2.c
      cc -o prog2 prog2.o -L. -lfoo
      +-.
        +--
        +-SConstruct
        +-f1.c
        +-f1.o
        | +-f1.c
        | +-inc.h
        +-f2.c
        +-f2.o
        | +-f2.c
        | +-inc.h
        +-f3.c
        +-f3.o
        | +-f3.c
        | +-inc.h
        +-inc.h
        +-libfoo.a
        | +-f1.o
        | | +-f1.c
        | | +-inc.h
        | +-f2.o
        | | +-f2.c
        | | +-inc.h
        | +-f3.o
        |   +-f3.c
        |   +-inc.h
        +-prog1
        | +-prog1.o
        | | +-prog1.c
        | | +-inc.h
        | +-libfoo.a
        |   +-f1.o
        |   | +-f1.c
        |   | +-inc.h
        |   +-f2.o
        |   | +-f2.c
        |   | +-inc.h
        |   +-f3.o
        |     +-f3.c
        |     +-inc.h
        +-prog1.c
        +-prog1.o
        | +-prog1.c
        | +-inc.h
        +-prog2
        | +-prog2.o
        | | +-prog2.c
        | | +-inc.h
        | +-libfoo.a
        |   +-f1.o
        |   | +-f1.c
        |   | +-inc.h
        |   +-f2.o
        |   | +-f2.c
        |   | +-inc.h
        |   +-f3.o
        |     +-f3.c
        |     +-inc.h
        +-prog2.c
        +-prog2.o
          +-prog2.c
          +-inc.h
    </screen>

    <para>

    In a large configuration with many internal libraries
    and include files,
    this can very quickly lead to huge output trees.
    To help make this more manageable,
    a <literal>prune</literal> modifier may
    be added to the option list,
    in which case &SCons;
    will print the name of a target that has
    already been visited during the tree-printing
    in <literal>[square brackets]</literal>
    as an indication that the dependencies
    of the target file may be found
    by looking farther up the tree:

    </para>

    <screen>
      % <userinput>scons -Q --tree=prune</userinput>
      cc -o f1.o -c -I. f1.c
      cc -o f2.o -c -I. f2.c
      cc -o f3.o -c -I. f3.c
      ar rc libfoo.a f1.o f2.o f3.o
      ranlib libfoo.a
      cc -o prog1.o -c -I. prog1.c
      cc -o prog1 prog1.o -L. -lfoo
      cc -o prog2.o -c -I. prog2.c
      cc -o prog2 prog2.o -L. -lfoo
      +-.
        +--
        +-SConstruct
        +-f1.c
        +-f1.o
        | +-[f1.c]
        | +-inc.h
        +-f2.c
        +-f2.o
        | +-[f2.c]
        | +-[inc.h]
        +-f3.c
        +-f3.o
        | +-[f3.c]
        | +-[inc.h]
        +-[inc.h]
        +-libfoo.a
        | +-[f1.o]
        | +-[f2.o]
        | +-[f3.o]
        +-prog1
        | +-prog1.o
        | | +-prog1.c
        | | +-[inc.h]
        | +-[libfoo.a]
        +-[prog1.c]
        +-[prog1.o]
        +-prog2
        | +-prog2.o
        | | +-prog2.c
        | | +-[inc.h]
        | +-[libfoo.a]
        +-[prog2.c]
        +-[prog2.o]
    </screen>

    <para>

    Like the <literal>status</literal> keyword,
    the <literal>prune</literal> argument by itself
    is equivalent to <literal>--tree=all,prune</literal>.

    </para>

  </section>

  <section>

  <title>How is &SCons; Constructing the Command Lines It Executes?  the &debug-presub; Option</title>

    <para>

    Sometimes it's useful to look at the
    pre-substitution string
    that &SCons; uses to generate
    the command lines it executes.
    This can be done with the &debug-presub; option:

    </para>

    

    <!--

    Have to capture output here, otherwise the - -debug=presub output
    shows the Python functions from the sconsdoc.py execution wrapper
    used to generate this manual, not the underlying command-line strings.

    <scons_output example="presub">
      <scons_output_command>scons -Q - -debug=presub</scons_output_command>
    </scons_output>

    -->

    <screen>
      % <userinput>scons -Q --debug=presub</userinput>
      Building prog.o with action:
        $CC -o $TARGET -c $CFLAGS $CCFLAGS $_CCOMCOM $SOURCES
      cc -o prog.o -c -I. prog.c
      Building prog with action:
        $SMART_LINKCOM
      cc -o prog prog.o
    </screen>

  </section>

  <section>

  <title>Where is &SCons; Searching for Libraries?  the &debug-findlibs; Option</title>

    <para>

    To get some insight into what library names
    &SCons; is searching for,
    and in which directories it is searching,
    Use the <literal>--debug=findlibs</literal> option.
    Given the following input &SConstruct; file:

    </para>

    <programlisting>
        env = Environment(LIBPATH = ['libs1', 'libs2'])
        env.Program('prog.c', LIBS=['foo', 'bar'])
    </programlisting>

    <para>

    And the libraries <filename>libfoo.a</filename>
    and <filename>libbar.a</filename>
    in <filename>libs1</filename> and <filename>libs2</filename>,
    respectively,
    use of the <literal>--debug=findlibs</literal> option yields:

    </para>

    <screen>
      % <userinput>scons -Q --debug=findlibs</userinput>
        findlibs: looking for 'libfoo.a' in 'libs1' ...
        findlibs: ... FOUND 'libfoo.a' in 'libs1'
        findlibs: looking for 'libfoo.so' in 'libs1' ...
        findlibs: looking for 'libfoo.so' in 'libs2' ...
        findlibs: looking for 'libbar.a' in 'libs1' ...
        findlibs: looking for 'libbar.a' in 'libs2' ...
        findlibs: ... FOUND 'libbar.a' in 'libs2'
        findlibs: looking for 'libbar.so' in 'libs1' ...
        findlibs: looking for 'libbar.so' in 'libs2' ...
      cc -o prog.o -c prog.c
      cc -o prog prog.o -Llibs1 -Llibs2 -lfoo -lbar
    </screen>

  </section>

  <!--

  <section>

  <title>What Implicit Dependencies Did the &SCons; Scanner find?  the &debug-includes; Option</title>

    <para>

    XXX explain the - - debug=includes option

    </para>

    <scons_example name="includes">
      <file name="SConstruct" printme="1">
        env = Environment(CPPPATH = ['inc1', 'inc2'])
        env.Program('prog.c')
      </file>
      <file name="prog.c">
      #include "file1.h"
      #include "file2.h"
      prog.c
      </file>
      <file name="inc1/file1.h">
      inc1/file1.h
      </file>
      <file name="inc2/file2.h">
      inc2/file2.h
      </file>
    </scons_example>

    <scons_output example="includes">
      <scons_output_command>scons -Q - - debug=includes prog</scons_output_command>
    </scons_output>

  </section>

  -->

  <section>

  <title>Where is &SCons; Blowing Up?  the &debug-stacktrace; Option</title>

    <para>

    In general, &SCons; tries to keep its error
    messages short and informative.
    That means we usually try to avoid showing
    the stack traces that are familiar
    to experienced Python programmers,
    since they usually contain much more
    information than is useful to most people.

    </para>

    <para>

    For example, the following &SConstruct; file:

    </para>

    <programlisting>
         Program('prog.c')
    </programlisting>

    <para>

    Generates the following error if the
    <filename>prog.c</filename> file
    does not exist:

    </para>

    <screen>
      % <userinput>scons -Q</userinput>
      scons: *** Source `prog.c' not found, needed by target `prog.o'.  Stop.
    </screen>

    <para>

    In this case,
    the error is pretty obvious.
    But if it weren't,
    and you wanted to try to get more information
    about the error,
    the &debug-stacktrace; option
    would show you exactly where in the &SCons; source code
    the problem occurs:

    </para>

    <screen>
      % <userinput>scons -Q --debug=stacktrace</userinput>
      scons: *** Source `prog.c' not found, needed by target `prog.o'.  Stop.
      scons: internal stack trace:
        File "/home/knight/SCons/scons.0.96.C763/bootstrap/src/engine/SCons/Job.py", line 111, in start
          task.prepare()
        File "/home/knight/SCons/scons.0.96.C763/bootstrap/src/engine/SCons/Taskmaster.py", line 166, in prepare
          t.prepare()
        File "/home/knight/SCons/scons.0.96.C763/bootstrap/src/engine/SCons/Node/FS.py", line 2137, in prepare
          SCons.Node.Node.prepare(self)
        File "/home/knight/SCons/scons.0.96.C763/bootstrap/src/engine/SCons/Node/__init__.py", line 806, in prepare
          raise SCons.Errors.StopError, desc
    </screen>

    <para>

    Of course, if you do need to dive into the &SCons; source code,
    we'd like to know if, or how,
    the error messages or troubleshooting options
    could have been improved to avoid that.
    Not everyone has the necessary time or
    Python skill to dive into the source code,
    and we'd like to improve &SCons;
    for those people as well...

    </para>

  </section>

  <section>

  <title>How is &SCons; Making Its Decisions?  the &taskmastertrace; Option</title>

    <para>

    The internal &SCons; subsystem that handles walking
    the dependency graph
    and controls the decision-making about what to rebuild
    is the <literal>Taskmaster</literal>.
    &SCons; supports a <literal>--taskmastertrace</literal>
    option that tells the Taskmaster to print
    information about the children (dependencies)
    of the various Nodes on its walk down the graph,
    which specific dependent Nodes are being evaluated,
    and in what order.

    </para>

    <para>

    The <literal>--taskmastertrace</literal> option
    takes as an argument the name of a file in
    which to put the trace output,
    with <filename>-</filename> (a single hyphen)
    indicating that the trace messages
    should be printed to the standard output:

    </para>

    <programlisting>
      env = Environment(CPPPATH = ['.'])
      env.Program('prog.c')
    </programlisting>

    <screen>
      % <userinput>scons -Q --taskmastertrace=- prog</userinput>
      Taskmaster: 'prog': children:
          ['prog.o']
          waiting on unstarted children:
          ['prog.o']
      Taskmaster: 'prog.o': children:
          ['inc.h', 'prog.c']
          evaluating prog.o
      cc -o prog.o -c -I. prog.c
      Taskmaster: 'prog': children:
          ['prog.o']
          evaluating prog
      cc -o prog prog.o
      Taskmaster: 'prog': already handled (executed)
    </screen>

    <para>

    The <literal>--taskmastertrace</literal> option
    doesn't provide information about the actual
    calculations involved in deciding if a file is up-to-date,
    but it does show all of the dependencies
    it knows about for each Node,
    and the order in which those dependencies are evaluated.
    This can be useful as an alternate way to determine
    whether or not your &SCons; configuration,
    or the implicit dependency scan,
    has actually identified all the correct dependencies
    you want it to.

    </para>

  </section>

  <!--

  <section>

  <title>Where Are My Build Bottlenecks?  the &profile; Option</title>

    <para>

    XXX explain the - - profile= option

    </para>

  </section>

  -->

  <!--

  <section>
  <title>Troubleshooting Shared Caching:  the &cache-debug; Option</title>

    <para>

    XXX describe the - - cache-debug option
    XXX maybe point to the caching.in chapter?

    </para>

  </section>

  -->