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SCons / doc / user / troubleshoot.in

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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.

  </para>

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

    <para>

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

    </para>

    <scons_example name="explain1">
      <file name="SConstruct" printme="1">
      # Intentionally misspell the output file name in the
      # command used to create the file:
      Command('file.out', 'file.in', 'cp $SOURCE file.oout')
      </file>
      <file name="file.in">
      file.in
      </file>
    </scons_example>

    <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 on this example,
    we see that it re-runs the &cp;
    command every time:

    </para>

    <scons_output example="explain1" 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>

    <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>

    <scons_output example="explain1" os="posix">
      <scons_output_command>scons -Q --debug=explain</scons_output_command>
    </scons_output>

    <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>

    <scons_example name="explain2">
      <file name="SConstruct">
      Program('prog', ['file1.c', 'file2.c', 'file3.c'])
      </file>
      <file name="file1.c">
      file1.c
      </file>
      <file name="file2.c">
      file2.c
      </file>
      <file name="file3.c">
      file3.c
      </file>
    </scons_example>

    <scons_output example="explain2" os="posix">
      <scons_output_command>scons -Q</scons_output_command>
      <scons_output_command output="    [CHANGE THE CONTENTS OF file2.c]">edit file2.c</scons_output_command>
      <scons_output_command>scons -Q --debug=explain</scons_output_command>
    </scons_output>

    <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>

    <scons_example name="explain3">
      <file name="SConstruct">
      Program('prog', ['file1.c', 'file2.c', 'file3.c'], CPPPATH='.')
      </file>
      <file name="file1.c">
      #include &lt;hello.h&gt;
      file1.c
      </file>
      <file name="file2.c">
      file2.c
      </file>
      <file name="file3.c">
      #include &lt;hello.h&gt;
      file3.c
      </file>
      <file name="hello.h">
      #define string    "world"
      </file>
    </scons_example>

    <scons_output example="explain3" os="posix">
      <scons_output_command>scons -Q</scons_output_command>
      <scons_output_command output="    [CHANGE THE CONTENTS OF hello.h]">edit hello.h</scons_output_command>
      <scons_output_command>scons -Q --debug=explain</scons_output_command>
    </scons_output>

  </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>

    <scons_example name="Dump">
      <file name="SConstruct" print="1">
         env = Environment()
         print env.Dump()
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="Dump" os="posix" tools="gcc">
      <scons_output_command>scons</scons_output_command>
    </scons_output>

    <para>

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

    </para>

    <scons_output example="Dump" os="win32" tools="msvc">
      <scons_output_command>scons</scons_output_command>
    </scons_output>

    <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>

    <scons_example name="Dump_ENV">
      <file name="SConstruct" print="1">
         env = Environment()
         print env.Dump('ENV')
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="Dump_ENV" os="posix">
      <scons_output_command>scons</scons_output_command>
    </scons_output>

    <para>

    And the following when executed on a Windows system:

    </para>

    <scons_output example="Dump_ENV" os="win32">
      <scons_output_command>scons</scons_output_command>
    </scons_output>

  </section>