path: root/doc/user/troubleshoot.xml

<?xml version='1.0'?>
<!DOCTYPE sconsdoc [
    <!ENTITY % scons SYSTEM "../scons.mod">
    %scons;

    <!ENTITY % builders-mod SYSTEM "../generated/builders.mod">
    %builders-mod;
    <!ENTITY % functions-mod SYSTEM "../generated/functions.mod">
    %functions-mod;
    <!ENTITY % tools-mod SYSTEM "../generated/tools.mod">
    %tools-mod;
    <!ENTITY % variables-mod SYSTEM "../generated/variables.mod">
    %variables-mod;
]>

<chapter id="chap-troubleshooting"
         xmlns="http://www.scons.org/dbxsd/v1.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 http://www.scons.org/dbxsd/v1.0/scons.xsd">
<title>Troubleshooting</title>

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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>

    <scons_example name="troubleshoot_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 times on this example,
    we see that it re-runs the &cp;
    command every time:

    </para>

    <scons_output example="troubleshoot_explain1" os="posix" suffix="1">
      <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="troubleshoot_explain1" os="posix" suffix="2">
      <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>
    Note that you can also use
    <option>--warn=target-not-built</option> which checks
    whether or not expected targets exist after a build rule is
    executed.
    </para>

    <scons_output example="troubleshoot_explain1" os="posix" suffix="3">
      <scons_output_command>scons -Q --warn=target-not-built</scons_output_command>
    </scons_output>

    <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="troubleshoot_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="troubleshoot_explain2" os="posix" suffix="1">
      <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="troubleshoot_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="troubleshoot_explain3" os="posix" suffix="1">
      <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>

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

    <para>

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

    </para>

    <scons_output example="troubleshoot_Dump" os="posix" tools="gcc" suffix="1">
      <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="troubleshoot_Dump" os="win32" tools="msvc" suffix="2">
      <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.
    Also note that we've massaged the example output above
    to make the memory address of all objects a constant 0x700000.
    In reality, you would see a different hexadecimal
    number for each object.

    </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="troubleshoot_Dump_ENV">
      <file name="SConstruct" printme="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="troubleshoot_Dump_ENV" os="posix" suffix="1">
      <scons_output_command>scons</scons_output_command>
    </scons_output>

    <para>

    And the following when executed on a Windows system:

    </para>

    <scons_output example="troubleshoot_Dump_ENV" os="win32" suffix="2">
      <scons_output_command>scons</scons_output_command>
    </scons_output>

  </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 <option>--tree</option> 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>

    <scons_example name="troubleshoot_tree1">
      <file name="SConstruct" printme="1">
env = Environment(CPPPATH = ['.'])
env.Program('prog', ['f1.c', 'f2.c', 'f3.c'])
      </file>
      <file name="f1.c">
#include "inc.h"
      </file>
      <file name="f2.c">
#include "inc.h"
      </file>
      <file name="f3.c">
#include "inc.h"
      </file>
      <file name="inc.h">
inc.h
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="troubleshoot_tree1" suffix="1">
      <scons_output_command>scons -Q --tree=all</scons_output_command>
    </scons_output>

    <para>

    The tree will also be printed when the
    <option>-n</option> (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>

    By default &SCons; uses "ASCII art" to draw the tree. It is
    possible to use line-drawing characters (Unicode calls these
    Box Drawing) to make a nicer display. To do this, add the
    <option>linedraw</option> qualifier:

    </para>

    <scons_output example="troubleshoot_tree1" suffix="2">
      <scons_output_command>scons -Q --tree=all,linedraw</scons_output_command>
    </scons_output>

    <para>

    The <option>--tree</option> 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 <option>--tree</option> option will only
    print the dependency graph for that file:

    </para>

    <scons_output example="troubleshoot_tree1" suffix="3">
      <scons_output_command>scons -Q --tree=all f2.o</scons_output_command>
    </scons_output>

    <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>

    <scons_output example="troubleshoot_tree1" suffix="4">
      <scons_output_command>scons -Q --tree=all f1.o f3.o</scons_output_command>
    </scons_output>

    <para>

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

    </para>

    <scons_output example="troubleshoot_tree1" suffix="5">
      <scons_output_command>scons -Q --tree=status</scons_output_command>
    </scons_output>

    <para>

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

    </para>

    <scons_output example="troubleshoot_tree1" suffix="6">
      <scons_output_command>scons -Q --tree=derived</scons_output_command>
    </scons_output>

    <para>

    You can use the <parameter class="option">status</parameter>
    modifier with <parameter class="option">derived</parameter> as well:

    </para>

    <scons_output example="troubleshoot_tree1" suffix="7">
      <scons_output_command>scons -Q --tree=derived,status</scons_output_command>
    </scons_output>

    <para>

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

    </para>

    <para>

    The default behavior of the <option>--tree</option> 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>

    <scons_example name="troubleshoot_tree2">
      <file name="SConstruct" printme="1">
env = Environment(CPPPATH = ['.'],
                  LIBS = ['foo'],
                  LIBPATH = ['.'])
env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
env.Program('prog1.c')
env.Program('prog2.c')
      </file>
      <file name="prog1.c">
#include "inc.h"
      </file>
      <file name="prog2.c">
#include "inc.h"
      </file>
      <file name="f1.c">
#include "inc.h"
      </file>
      <file name="f2.c">
#include "inc.h"
      </file>
      <file name="f3.c">
#include "inc.h"
      </file>
      <file name="inc.h">
inc.h
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="troubleshoot_tree2" suffix="1">
      <scons_output_command>scons -Q --tree=all</scons_output_command>
    </scons_output>

    <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 <parameter class="option">prune</parameter> 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 square brackets (<literal>[]</literal>)
    as an indication that the dependencies
    of the target file may be found
    by looking farther up the tree:

    </para>

    <scons_output example="troubleshoot_tree2" suffix="2">
      <scons_output_command>scons -Q --tree=prune</scons_output_command>
    </scons_output>

    <para>

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

    </para>

  </section>

  <section>

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

    <para>

    Sometimes the command lines that &SCons; executes don't
    come out looking as you expect. In this case it may be
    useful to look at the strings before &SCons; performs
    substitution on them.
    This can be done with the &debug-presub; option:

    </para>

    <scons_example name="troubleshoot_presub">
      <file name="SConstruct">
env = Environment(CPPPATH = ['.'])
env.Program('prog', 'prog.c')
      </file>
      <file name="prog.c">
prog.c
      </file>
    </scons_example>

    <!--

    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="troubleshoot_presub" suffix="1">
      <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 &debug-findlibs; option.
    Given the following input &SConstruct; file:

    </para>

    <scons_example name="troubleshoot_findlibs">
      <file name="SConstruct" printme="1">
env = Environment(LIBPATH = ['libs1', 'libs2'])
env.Program('prog.c', LIBS=['foo', 'bar'])
      </file>
      <file name="prog.c">
prog.c
      </file>
      <file name="libs1/libfoo.a">
libs1/libfoo.a
      </file>
      <file name="libs2/libbar.a">
libs2/libbar.a
      </file>
    </scons_example>

    <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 &debug-findlibs; option yields:

    </para>

    <scons_output example="troubleshoot_findlibs" suffix="1">
      <scons_output_command>scons -Q --debug=findlibs</scons_output_command>
    </scons_output>

  </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="troubleshoot_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="troubleshoot_includes" suffix="1">
      <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>

    <scons_example name="troubleshoot_stacktrace">
      <file name="SConstruct" printme="1">
Program('prog.c')
      </file>
    </scons_example>

    <para>

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

    </para>

    <scons_output example="troubleshoot_stacktrace" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <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>

    <scons_output example="troubleshoot_stacktrace" suffix="2">
      <scons_output_command>scons -Q --debug=stacktrace</scons_output_command>
    </scons_output>

    <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 <firstterm>Taskmaster</firstterm>.
    &SCons; supports a &taskmastertrace;
    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 &taskmastertrace; 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>

    <scons_example name="troubleshoot_taskmastertrace">
      <file name="SConstruct" printme="1">
env = Environment(CPPPATH = ['.'])
env.Program('prog.c')
      </file>
      <file name="prog.c">
#include "inc.h"
prog.c
      </file>
      <file name="inc.h">
#define   STRING  "one"
      </file>
    </scons_example>

    <scons_output example="troubleshoot_taskmastertrace" os="posix" suffix="1">
      <scons_output_command>scons -Q --taskmastertrace=- prog</scons_output_command>
    </scons_output>

    <para>

    The &taskmastertrace; 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>Watch &SCons; prepare targets for building: the &debug-prepare; Option</title>

    <para>

    Sometimes SCons doesn't build the target you want
    and it's difficult to figure out why.  You can use
    the &debug-prepare; option
    to see all the targets &SCons; is considering, and whether
    they are already up-to-date or not. The message is
    printed before &SCons; decides whether to build the target.
    </para>

  </section>

  <section>

  <title>Why is a file disappearing?  the &debug-duplicate; Option</title>

    <para>

    When using the &Duplicate; option to create variant dirs,
    sometimes you may find files not getting linked or copied to where you
    expect (or not at all), or files mysteriously disappearing.
    These are usually because of a misconfiguration of some kind in the
    SConscript files, but they can be tricky to debug.  The
    &debug-duplicate; option shows each time a variant file is
    unlinked and relinked from its source (or copied, depending on
    settings), and also shows a message for removing "stale"
    variant-dir files that no longer have a corresponding source file.
    It also prints a line for each target that's removed just before
    building, since that can also be mistaken for the same thing.

    </para>

  </section>

  <section>

  <title>Keep it simple</title>

  <para>
  Over the years, many developers have chosen to dive in and make vastly
  complicated build systems out of &SCons;, which sometimes don't work
  quite as expected.  As a general rule, make sure you
  <emphasis>need</emphasis> to reach for a complex solution before you do so.
  &SCons; is mature software and has evolved over time to meet a lot
  of feature requests, so there is often an easier way to do something
  if you can just find it.  The &SCons; community can be helpful here -
  the discussion lists and chat channels can be a way to find out if
  something can be done an easier way before embarking on an implementation.
  </para>

  <para>
  When something does misbehave, trying to isolate the problem to
  a simple test case can really help.  The work to create a reproducer
  often helps you spot the issue yourself, and a simple example is much
  easier for others to look over and possibly spot logical flaws,
  misuse of the API, or other ways something could have been done.
  In addition, if it turns out there's actually a real &SCons; bug
  (we believe it's a high quality piece of software, but all software
  has some bugs), it's very likely the bug filing will result in a
  request for a simple reproducer anyway.
  </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>

  -->

</chapter>