path: root/doc/user/output.in

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  <para>

  A key aspect of creating a usable build configuration
  is providing good output from the build
  so its users can readily understand
  what the build is doing
  and get information about how to control the build.
  &SCons; provides several ways of
  controlling output from the build configuration
  to help make the build
  more useful and understandable.

  </para>

  <section>
  <title>Providing Build Help:  the &Help; Function</title>

    <para>

    It's often very useful to be able to give
    users some help that describes the
    specific targets, build options, etc.,
    that can be used for your build.
    &SCons; provides the &Help; function
    to allow you to specify this help text:

    </para>

    <scons_example name="ex1">
       <file name="SConstruct" printme="1">
       Help("""
       Type: 'scons program' to build the production program,
             'scons debug' to build the debug version.
       """)
       </file>
    </scons_example>

    <para>

    (Note the above use of the Python triple-quote syntax,
    which comes in very handy for
    specifying multi-line strings like help text.)

    </para>

    <para>

    When the &SConstruct; or &SConscript; files
    contain such a call to the &Help; function,
    the specified help text will be displayed in response to
    the &SCons; <literal>-h</literal> option:

    </para>

    <scons_output example="ex1">
       <scons_output_command>scons -h</scons_output_command>
    </scons_output>

    <para>

    The &SConscript; files may contain
    multiple calls to the &Help; function,
    in which case the specified text(s)
    will be concatenated when displayed.
    This allows you to split up the
    help text across multiple &SConscript; files.
    In this situation, the order in
    which the &SConscript; files are called
    will determine the order in which the &Help; functions are called,
    which will determine the order in which
    the various bits of text will get concatenated.

    </para>

    <para>

    Another use would be to make the help text conditional
    on some variable.
    For example, suppose you only want to display
    a line about building a Windows-only
    version of a program when actually
    run on Windows.
    The following &SConstruct; file:

    </para>

    <scons_example name="ex2">
       <file name="SConstruct" printme="1">
       env = Environment()

       Help("\nType: 'scons program' to build the production program.\n")

       if env['PLATFORM'] == 'win32':
           Help("\nType: 'scons windebug' to build the Windows debug version.\n")
       </file>
    </scons_example>

    <para>

    Will display the complete help text on Windows:

    </para>

    <scons_output example="ex2" os="win32">
       <scons_output_command>scons -h</scons_output_command>
    </scons_output>

    <para>

    But only show the relevant option on a Linux or UNIX system:

    </para>

    <scons_output example="ex2" os="posix">
       <scons_output_command>scons -h</scons_output_command>
    </scons_output>

    <para>

    If there is no &Help; text in the &SConstruct; or
    &SConscript; files,
    &SCons; will revert to displaying its
    standard list that describes the &SCons; command-line
    options.
    This list is also always displayed whenever
    the <literal>-H</literal> option is used.

    </para>

  </section>

  <section>
  <title>Controlling How &SCons; Prints Build Commands:  the <envar>$*COMSTR</envar> Variables</title>

    <para>

    Sometimes the commands executed
    to compile object files or link programs
    (or build other targets)
    can get very long,
    long enough to make it difficult for users
    to distinguish error messages or
    other important build output
    from the commands themselves.
    All of the default <envar>$*COM</envar> variables
    that specify the command lines
    used to build various types of target files
    have a corresponding <envar>$*COMSTR</envar> variable
    that can be set to an alternative
    string that will be displayed
    when the target is built.

    </para>

    <para>

    For example, suppose you want to
    have &SCons; display a
    <literal>"Compiling"</literal>
    message whenever it's compiling an object file,
    and a
    <literal>"Linking"</literal>
    when it's linking an executable.
    You could write a &SConstruct; file
    that looks like:

    </para>

    <scons_example name="COMSTR">
       <file name="SConstruct" printme="1">
       env = Environment(CCCOMSTR = "Compiling $TARGET",
                         LINKCOMSTR = "Linking $TARGET")
       env.Program('foo.c')
       </file>
       <file name="foo.c">
       foo.c
       </file>
    </scons_example>

    <para>

    Which would then yield the output:

    </para>

    <!--

    <scons_output example="COMSTR" os="posix">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    -->

    <screen>
       % <userinput>scons -Q</userinput>
       Compiling foo.o
       Linking foo
    </screen>

    <para>

    &SCons; performs complete variable substitution
    on <envar>$*COMSTR</envar> variables,
    so they have access to all of the
    standard variables like &cv-TARGET; &cv-SOURCES;, etc.,
    as well as any construction variables
    that happen to be configured in
    the construction environment
    used to build a specific target.

    </para>

    <para>

    Of course, sometimes it's still important to
    be able to see the exact command
    that &SCons; will execute to build a target.
    For example, you may simply need to verify
    that &SCons; is configured to supply
    the right options to the compiler,
    or a developer may want to
    cut-and-paste a compile command
    to add a few options
    for a custom test.

    </para>

    <para>

    One common way to give users
    control over whether or not
    &SCons; should print the actual command line
    or a short, configured summary
    is to add support for a
    <varname>VERBOSE</varname>
    command-line variable to your &SConstruct; file.
    A simple configuration for this might look like:

    </para>

    <scons_example name="COMSTR-VERBOSE">
       <file name="SConstruct" printme="1">
       env = Environment()
       if ARGUMENTS.get('VERBOSE') != "1':
           env['CCCOMSTR'] = "Compiling $TARGET"
           env['LINKCOMSTR'] = "Linking $TARGET"
       env.Program('foo.c')
       </file>
       <file name="foo.c">
       foo.c
       </file>
    </scons_example>

    <para>


    By only setting the appropriate
    <envar>$*COMSTR</envar> variables
    if the user specifies
    <literal>VERBOSE=1</literal>
    on the command line,
    the user has control
    over how &SCons;
    displays these particular command lines:

    </para>

    <!--

    <scons_output example="COMSTR-VERBOSE" os="posix">
       <scons_output_command>scons -Q</scons_output_command>
       <scons_output_command>scons -Q -c</scons_output_command>
       <scons_output_command>scons -Q VERBOSE=1</scons_output_command>
    </scons_output>

    -->

    <screen>
       % <userinput>scons -Q</userinput>
       Compiling foo.o
       Linking foo
       % <userinput>scons -Q -c</userinput>
       Removed foo.o
       Removed foo
       % <userinput>scons -Q VERBOSE=1</userinput>
       cc -o foo.o -c foo.c
       cc -o foo foo.o
    </screen>

  </section>

  <section>
  <title>Providing Build Progress Output:  the &Progress; Function</title>

    <para>

    Another aspect of providing good build output
    is to give the user feedback
    about what &SCons; is doing
    even when nothing is being built at the moment.
    This can be especially true for large builds
    when most of the targets are already up-to-date.
    Because &SCons; can take a long time
    making absolutely sure that every
    target is, in fact, up-to-date
    with respect to a lot of dependency files,
    it can be easy for users to mistakenly
    conclude that &SCons; is hung
    or that there is some other problem with the build.

    </para>

    <para>

    One way to deal with this perception
    is to configure &SCons; to print something to
    let the user know what it's "thinking about."
    The &Progress; function
    allows you to specify a string
    that will be printed for every file
    that &SCons; is "considering"
    while it is traversing the dependency graph
    to decide what targets are or are not up-to-date.

    </para>

    <scons_example name="Progress-TARGET">
      <file name="SConstruct" printme="1">
        Progress('Evaluating $TARGET\n')
        Program('f1.c')
        Program('f2.c')
      </file>
      <file name="f1.c">
        f1.c
      </file>
      <file name="f2.c">
        f2.c
      </file>
    </scons_example>

    <para>

    Note that the &Progress; function does not
    arrange for a newline to be printed automatically
    at the end of the string (as does the Python
    <literal>print</literal> statement),
    and we must specify the
    <literal>\n</literal>
    that we want printed at the end of the configured string.
    This configuration, then,
    will have &SCons;
    print that it is <literal>Evaluating</literal>
    each file that it encounters
    in turn as it traverses the dependency graph:

    </para>

    <scons_output example="Progress-TARGET" os="posix">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    Of course, normally you don't want to add
    all of these additional lines to your build output,
    as that can make it difficult for the user
    to find errors or other important messages.
    A more useful way to display
    this progress might be
    to have the file names printed
    directly to the user's screen,
    not to the same standard output
    stream where build output is printed,
    and to use a carriage return character
    (<literal>\r</literal>)
    so that each file name gets re-printed on the same line.
    Such a configuration would look like:

    </para>

    <sconstruct>
        Progress('$TARGET\r',
                 file=open('/dev/tty', 'w'),
                 overwrite=True)
        Program('f1.c')
        Program('f2.c')
    </sconstruct>

    <para>

    Note that we also specified the
    <literal>overwrite=True</literal> argument
    to the &Progress; function,
    which causes &SCons; to
    "wipe out" the previous string with space characters
    before printing the next &Progress; string.
    Without the
    <literal>overwrite=True</literal> argument,
    a shorter file name would not overwrite
    all of the charactes in a longer file name that 
    precedes it,
    making it difficult to tell what the
    actual file name is on the output.
    Also note that we opened up the
    <filename>/dev/tty</filename> file
    for direct access (on POSIX) to
    the user's screen.
    On Windows, the equivalent would be to open
    the <filename>con:</filename> file name.

    </para>

    <para>

    Also, it's important to know that although you can use
    <literal>$TARGET</literal> to substitute the name of
    the node in the string,
    the &Progress; function does <emphasis>not</emphasis>
    perform general variable substitution
    (because there's not necessarily a construction
    environment involved in evaluating a node
    like a source file, for example).

    </para>

    <para>

    You can also specify a list of strings
    to the &Progress; function,
    in which case &SCons; will
    display each string in turn.
    This can be used to implement a "spinner"
    by having &SCons; cycle through a
    sequence of strings:

    </para>

    <sconstruct>
        Progress(['-\r', '\\\r', '|\r', '/\r'], interval=5)
        Program('f1.c')
        Program('f2.c')
    </sconstruct>

    <para>

    Note that here we have also used the
    <literal>interval=</literal>
    keyword argument to have &SCons;
    only print a new "spinner" string
    once every five evaluated nodes.
    Using an <literal>interval=</literal> count,
    even with strings that use <literal>$TARGET</literal> like
    our examples above,
    can be a good way to lessen the
    work that &SCons; expends printing &Progress; strings,
    while still giving the user feedback
    that indicates &SCons; is still
    working on evaluating the build.

    </para>

    <para>

    Lastly, you can have direct control
    over how to print each evaluated node
    by passing a Python function
    (or other Python callable)
    to the &Progress; function.
    Your function will be called
    for each evaluated node,
    allowing you to
    implement more sophisticated logic
    like adding a counter:

    </para>

    <scons_example name="Progress-callable">
      <file name="SConstruct" printme="1">
        screen = open('/dev/tty', 'w')
        count = 0
        def progress_function(node)
            count += 1
            screen.write('Node %4d: %s\r' % (count, node))

        Progress(progress_function)
      </file>
    </scons_example>

    <para>

    Of course, if you choose,
    you could completely ignore the
    <varname>node</varname> argument to the function,
    and just print a count,
    or anything else you wish.

    </para>

    <para>

    (Note that there's an obvious follow-on question here:
    how would you find the total number of nodes
    that <emphasis>will be</emphasis>
    evaluated so you can tell the user how
    close the build is to finishing?
    Unfortunately, in the general case,
    there isn't a good way to do that,
    short of having &SCons; evaluate its
    dependency graph twice,
    first to count the total and
    the second time to actually build the targets.
    This would be necessary because
    you can't know in advance which
    target(s) the user actually requested
    to be built.
    The entire build may consist of thousands of Nodes,
    for example,
    but maybe the user specifically requested
    that only a single object file be built.)

    </para>

  </section>

  <section>
  <title>Printing Detailed Build Status:  the &GetBuildFailures; Function</title>

    <para>

    SCons, like most build tools, returns zero status to
    the shell on success and nonzero status on failure.
    Sometimes it's useful to give more information about
    the build status at the end of the run, for instance
    to print an informative message, send an email, or
    page the poor slob who broke the build.

    </para>

    <para>

    SCons provides a &GetBuildFailures; method that
    you can use in a python <function>atexit</function> function
    to get a list of objects describing the actions that failed
    while attempting to build targets.  There can be more
    than one if you're using <literal>-j</literal>.  Here's a 
    simple example:

    </para>

    <scons_example name="gbf1">
      <file name="SConstruct" printme="1">
        import atexit

        def print_build_failures():
            from SCons.Script import GetBuildFailures
            for bf in GetBuildFailures():
                print "%s failed: %s" % (bf.node, bf.errstr)
        atexit.register(print_build_failures)
      </file>
    </scons_example>

    <para>

    The <function>atexit.register</function> call
    registers <function>print_build_failures</function>
    as an <function>atexit</function> callback, to be called
    before &SCons; exits.  When that function is called,
    it calls &GetBuildFailures; to fetch the list of failed objects.
    See the man page
    for the detailed contents of the returned objects;
    some of the more useful attributes are 
    <literal>.node</literal>,
    <literal>.errstr</literal>,
    <literal>.filename</literal>, and
    <literal>.command</literal>.
    The <literal>filename</literal> is not necessarily
    the same file as the <literal>node</literal>; the
    <literal>node</literal> is the target that was
    being built when the error occurred, while the 
    <literal>filename</literal>is the file or dir that
    actually caused the error.
    Note:  only call &GetBuildFailures; at the end of the
    build; calling it at any other time is undefined.

    </para>

    <para>   

    Here is a more complete example showing how to
    turn each element of &GetBuildFailures; into a string:

    </para>

    <scons_example name="gbf2">
      <file name="SConstruct" printme="1">
        # Make the build fail if we pass fail=1 on the command line
        if ARGUMENTS.get('fail', 0):
            Command('target', 'source', ['/bin/false'])

        def bf_to_str(bf):
            """Convert an element of GetBuildFailures() to a string
            in a useful way."""
            import SCons.Errors
            if bf is None: # unknown targets product None in list
                return '(unknown tgt)'
            elif isinstance(bf, SCons.Errors.StopError):
                return str(bf)
            elif bf.node:
                return str(bf.node) + ': ' + bf.errstr
            elif bf.filename:
                return bf.filename + ': ' + bf.errstr
            return 'unknown failure: ' + bf.errstr
        import atexit

        def build_status():
            """Convert the build status to a 2-tuple, (status, msg)."""
            from SCons.Script import GetBuildFailures
            bf = GetBuildFailures()
            if bf:
                # bf is normally a list of build failures; if an element is None,
                # it's because of a target that scons doesn't know anything about.
                status = 'failed'
                failures_message = "\n".join(["Failed building %s" % bf_to_str(x)
                                   for x in bf if x is not None])
            else:
                # if bf is None, the build completed successfully.
                status = 'ok'
                failures_message = ''
            return (status, failures_message)

        def display_build_status():
            """Display the build status.  Called by atexit.
            Here you could do all kinds of complicated things."""
            status, failures_message = build_status()
            if status == 'failed':
               print "FAILED!!!!"  # could display alert, ring bell, etc.
            elif status == 'ok':
               print "Build succeeded."
            print failures_message

        atexit.register(display_build_status)
      </file>
    </scons_example>

    <para>
    
    When this runs, you'll see the appropriate output:

    </para>

    <scons_output example="gbf2">
          <scons_output_command>scons -Q</scons_output_command>
          <scons_output_command>scons -Q fail=1</scons_output_command>
    </scons_output>

  </section>