path: root/doc/user/misc.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-misc"
         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>Miscellaneous Functionality</title>

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-->

  <para>

  &SCons; supports a lot of additional functionality
  that doesn't readily fit into the other chapters.

  </para>

  <section>
  <title>Verifying the Python Version:  the &EnsurePythonVersion; Function</title>

    <para>

    Although the &SCons; code itself will run
    on any 2.x Python version 2.7 or later,
    you are perfectly free to make use of
    Python syntax and modules from later versions
    when writing your &SConscript; files
    or your own local modules.
    If you do this, it's usually helpful to
    configure &SCons; to exit gracefully with an error message
    if it's being run with a version of Python
    that simply won't work with your code.
    This is especially true if you're going to use &SCons;
    to build source code that you plan to distribute publicly,
    where you can't be sure of the Python version
    that an anonymous remote user might use
    to try to build your software.

    </para>

    <para>

    &SCons; provides an &EnsurePythonVersion; function for this.
    You simply pass it the major and minor versions
    numbers of the version of Python you require:

    </para>

    <!--

    TODO:  Figure out how to generate the error message
    regardless of executing Python version by faking out
    the infrastructure in some way.

    <scons_example name="misc_EnsurePythonVersion">
      <file name="SConstruct" printme="1">
EnsurePythonVersion(2, 5)
      </file>
    </scons_example>

    -->

    <sconstruct>
EnsurePythonVersion(2, 5)
    </sconstruct>

    <para>

    And then &SCons; will exit with the following error
    message when a user runs it with an unsupported
    earlier version of Python:

    </para>

    <!--

    TODO:  Figure out how to generate the error message
    regardless of executing Python version by faking out
    the infrastructure in some way.

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

    -->

    <screen>
% <userinput>scons -Q</userinput>
Python 2.5 or greater required, but you have Python 2.3.6
    </screen>

  </section>

  <section>
  <title>Verifying the SCons Version:  the &EnsureSConsVersion; Function</title>

    <para>

    You may, of course, write your &SConscript; files
    to use features that were only added in
    recent versions of &SCons;.
    When you publicly distribute software that is built using &SCons;,
    it's helpful to have &SCons;
    verify the version being used and
    exit gracefully with an error message
    if the user's version of &SCons; won't work
    with your &SConscript; files.
    &SCons; provides an &EnsureSConsVersion; function
    that verifies the version of &SCons;
    in the same
    the &EnsurePythonVersion; function
    verifies the version of Python,
    by passing in the major and minor versions
    numbers of the version of SCons you require:

    </para>

    <!--

    TODO:  Figure out how to generate the error message
    regardless of executing SCons version by faking out
    the infrastructure in some way.

    <scons_example name="misc_EnsureSConsVersion">
      <file name="SConstruct" printme="1">
EnsureSConsVersion(1, 0)
      </file>
    </scons_example>

    -->

    <sconstruct>
EnsureSConsVersion(1, 0)
    </sconstruct>

    <para>

    And then &SCons; will exit with the following error
    message when a user runs it with an unsupported
    earlier version of &SCons;:

    </para>

    <!--

    TODO:  Figure out how to generate the error message
    regardless of executing SCons version by faking out
    the infrastructure in some way.

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

    -->

    <screen>
% <userinput>scons -Q</userinput>
SCons 1.0 or greater required, but you have SCons 0.98.5
    </screen>

  </section>

  <section>
  <title>Explicitly Terminating &SCons; While Reading &SConscript; Files:  the &Exit; Function</title>

    <para>

    &SCons; supports an &Exit; function
    which can be used to terminate &SCons;
    while reading the &SConscript; files,
    usually because you've detected a condition
    under which it doesn't make sense to proceed:

    </para>

    <scons_example name="misc_Exit">
      <file name="SConstruct" printme="1">
if ARGUMENTS.get('FUTURE'):
    print("The FUTURE option is not supported yet!")
    Exit(2)
env = Environment()
env.Program('hello.c')
      </file>
      <file name="hello.c">
hello.c
      </file>
    </scons_example>

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

    <para>

    The &Exit; function takes as an argument
    the (numeric) exit status that you want &SCons; to exit with.
    If you don't specify a value,
    the default is to exit with <literal>0</literal>,
    which indicates successful execution.

    </para>

    <para>

    Note that the &Exit; function
    is equivalent to calling the Python
    <function>sys.exit</function> function
    (which the it actually calls),
    but because &Exit; is a &SCons; function,
    you don't have to import the Python
    <literal>sys</literal> module to use it.

    </para>

  </section>

  <section>
  <title>Searching for Files:  the &FindFile; Function</title>

    <para>

     The &FindFile; function searches for a file in a list of directories.
     If there is only one directory, it can be given as a simple string.
     The function returns a File node if a matching file exists,
     or None if no file is found.
     (See the documentation for the &Glob; function for an alternative way
     of searching for entries in a directory.)

    </para>

    <scons_example name="misc_FindFile1a">
      <file name="SConstruct" printme="1">
# one directory
print("%s"%FindFile('missing', '.'))
t = FindFile('exists', '.')
print("%s %s"%(t.__class__, t))
      </file>
      <file name="exists">
 exists
      </file>
    </scons_example>

    <scons_output example="misc_FindFile1a" os="posix" suffix="1">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <scons_example name="misc_FindFile1b">
      <file name="SConstruct" printme="1">
# several directories
includes = [ '.', 'include', 'src/include']
headers = [ 'nonesuch.h', 'config.h', 'private.h', 'dist.h']
for hdr in headers:
    print('%-12s: %s'%(hdr, FindFile(hdr, includes)))
      </file>
      <file name="config.h">
exists
      </file>
      <directory name="src"></directory>
      <directory name="src/include"></directory>
      <file name="src/include/private.h">
exists
      </file>
      <directory name="include"></directory>
      <file name="include/dist.h">
exists
      </file>
    </scons_example>

    <scons_output example="misc_FindFile1b" os="posix" suffix="1">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

   <!-- The man page says this should work, but it fails.
    <para>

     If the 'file' parameter is a list of files,
     a list of File nodes is returned.

    </para>

    <scons_example name="misc_FindFile1c">
      <file name="SConstruct" printme="1">
# several directories
includes = [ '.', 'include', 'src/include']
headers = [ 'nonesuch.h', 'config.h', 'private.h', 'dist.h']
print(FindFile(headers, includes))
      </file>
      <file name="config.h">
exists
      </file>
      <directory name="src"></directory>
      <directory name="src/include"></directory>
      </file>
      <file name="src/include/private.h">
exists
      <directory name="include"></directory>
      </file>
      <file name="include/dist.h">
exists
    </scons_example>

    <scons_output example="misc_FindFile1c" os="posix" suffix="1">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>
   -->

    <para>

     If the file exists in more than one directory,
     only the first occurrence is returned.

    </para>

    <scons_example name="misc_FindFile1d">
      <file name="SConstruct" printme="1">
print(FindFile('multiple', ['sub1', 'sub2', 'sub3']))
print(FindFile('multiple', ['sub2', 'sub3', 'sub1']))
print(FindFile('multiple', ['sub3', 'sub1', 'sub2']))
      </file>
      <directory name="sub1"></directory>
      <file name="sub1/multiple">
exists
      </file>
      <directory name="sub2"></directory>
      <file name="sub2/multiple">
exists
      </file>
      <directory name="sub3"></directory>
      <file name="sub3/multiple">
exists
      </file>
    </scons_example>

    <scons_output example="misc_FindFile1d" os="posix" suffix="1">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

   <!-- file may be a list of file names or a single file name. -->

    <para>

    In addition to existing files, &FindFile; will also find derived files
    (that is, non-leaf files) that haven't been built yet.
    (Leaf files should already exist, or the build will fail!)

    </para>

    <scons_example name="misc_FindFile2">
      <file name="SConstruct" printme="1">
# Neither file exists, so build will fail
Command('derived', 'leaf', 'cat >$TARGET $SOURCE')
print(FindFile('leaf', '.'))
print(FindFile('derived', '.'))
      </file>
    </scons_example>

    <scons_output example="misc_FindFile2" os="posix" suffix="1">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <scons_example name="misc_FindFile2">
      <file name="SConstruct" printme="1">
# Only 'leaf' exists
Command('derived', 'leaf', 'cat >$TARGET $SOURCE')
print(FindFile('leaf', '.'))
print(FindFile('derived', '.'))
      </file>
      <file name="leaf">
leaf
      </file>
    </scons_example>

    <scons_output example="misc_FindFile2" os="posix" suffix="2">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    If a source file exists, &FindFile; will correctly return the name
    in the build directory.

    </para>

    <scons_example name="misc_FindFile3">
      <file name="SConstruct" printme="1">
# Only 'src/leaf' exists
VariantDir('build', 'src')
print(FindFile('leaf', 'build'))
      </file>
      <directory name="src"></directory>
      <file name="src/leaf">
leaf
      </file>
    </scons_example>

    <scons_output example="misc_FindFile3" os="posix" suffix="1">
       <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

  </section>

  <section>
  <title>Handling Nested Lists:  the &Flatten; Function</title>

    <para>

    &SCons; supports a &Flatten; function
    which takes an input Python sequence
    (list or tuple)
    and returns a flattened list
    containing just the individual elements of
    the sequence.
    This can be handy when trying to examine
    a list composed of the lists
    returned by calls to various Builders.
    For example, you might collect
    object files built in different ways
    into one call to the &Program; Builder
    by just enclosing them in a list, as follows:

    </para>

    <scons_example name="misc_Flatten1">
      <file name="SConstruct" printme="1">
objects = [
    Object('prog1.c'),
    Object('prog2.c', CCFLAGS='-DFOO'),
]
Program(objects)
      </file>
      <file name="prog1.c">
prog1.c
      </file>
      <file name="prog2.c">
prog2.c
      </file>
    </scons_example>

    <para>

    Because the Builder calls in &SCons;
    flatten their input lists,
    this works just fine to build the program:

    </para>

    <scons_output example="misc_Flatten1" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    But if you were debugging your build
    and wanted to print the absolute path
    of each object file in the
    <varname>objects</varname> list,
    you might try the following simple approach,
    trying to print each Node's
    <literal>abspath</literal>
    attribute:

    </para>

    <scons_example name="misc_Flatten2">
      <file name="SConstruct" printme="1">
objects = [
    Object('prog1.c'),
    Object('prog2.c', CCFLAGS='-DFOO'),
]
Program(objects)

for object_file in objects:
    print(object_file.abspath)
      </file>
      <file name="prog1.c">
prog1.c
      </file>
      <file name="prog2.c">
prog2.c
      </file>
    </scons_example>

    <para>

    This does not work as expected
    because each call to <function>str</function>
    is operating an embedded list returned by
    each &Object; call,
    not on the underlying Nodes within those lists:

    </para>

    <scons_output example="misc_Flatten2" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    <para>

    The solution is to use the &Flatten; function
    so that you can pass each Node to
    the <function>str</function> separately:

    </para>

    <scons_example name="misc_Flatten3">
      <file name="SConstruct" printme="1">
objects = [
    Object('prog1.c'),
    Object('prog2.c', CCFLAGS='-DFOO'),
]
Program(objects)

for object_file in Flatten(objects):
    print(object_file.abspath)
      </file>
      <file name="prog1.c">
prog1.c
      </file>
      <file name="prog2.c">
prog2.c
      </file>
    </scons_example>

    <!--

    TODO:  can't use this now because it displays the temporary path name

    <scons_output example="misc_Flatten3" suffix="1">
      <scons_output_command>scons -Q</scons_output_command>
    </scons_output>

    -->

    <screen>
% <userinput>scons -Q</userinput>
/home/me/project/prog1.o
/home/me/project/prog2.o
cc -o prog1.o -c prog1.c
cc -o prog2.o -c -DFOO prog2.c
cc -o prog1 prog1.o prog2.o
    </screen>

  </section>

  <section>
  <title>Finding the Invocation Directory:  the &GetLaunchDir; Function</title>

    <para>

    If you need to find the directory from
    which the user invoked the &scons; command,
    you can use the &GetLaunchDir; function:

    </para>

    <sconstruct>
env = Environment(
    LAUNCHDIR = GetLaunchDir(),
)
env.Command('directory_build_info',
            '$LAUNCHDIR/build_info'
            Copy('$TARGET', '$SOURCE'))
    </sconstruct>

    <para>

    Because &SCons; is usually invoked from the top-level
    directory in which the &SConstruct; file lives,
    the Python <function>os.getcwd()</function>
    is often equivalent.
    However, the &SCons;
    <literal>-u</literal>,
    <literal>-U</literal>
    and
    <literal>-D</literal>
    command-line options,
    when invoked from a subdirectory,
    will cause &SCons; to change to the directory
    in which the &SConstruct; file is found.
    When those options are used,
    &GetLaunchDir; will still return the path to the
    user's invoking subdirectory,
    allowing the &SConscript; configuration
    to still get at configuration (or other) files
    from the originating directory.

    </para>

  </section>

  <section>
  <title>Virtual environments (virtualenvs)</title>

    <para>

      Virtualenv is a tool to create isolated Python environments.
      A python application (such as SCons) may be executed within
      an activated virtualenv. The activation of virtualenv modifies
      current environment by defining some virtualenv-specific variables
      and modifying search PATH, such that executables installed within
      virtualenv's home directory are preferred over the ones installed
      outside of it.

    </para>

    <para>

      Normally, SCons uses hard-coded PATH when searching for external
      executables, so it always picks-up executables from these pre-defined
      locations. This applies also to python interpreter, which is invoked
      by some custom SCons tools or test suites. This means, when running
      SCons in a virtualenv, an eventual invocation of python interpreter from
      SCons script will most probably jump out of virtualenv and execute
      python executable found in hard-coded SCons PATH, not the one which is
      executing SCons. Some users may consider this as an inconsistency.

    </para>

    <para>
      This issue may be overcome by using <literal>--enable-virtualenv</literal>
      option. The option automatically imports virtualenv-related environment
      variables to all created construction environment <literal>env['ENV']</literal>,
      and modifies SCons PATH appropriately to prefer virtualenv's executables.
      Setting environment variable <literal>SCONS_ENABLE_VIRTUALENV=1</literal>
      will have same effect. If virtualenv support is enabled system-vide
      by the environment variable, it may be suppressed with
      <literal>--ignore-virtualenv</literal> option.
    </para>

    <para>
      Inside of SConscript, a global function <literal>Virtualenv</literal> is
      available. It returns a path to virtualenv's home directory, or
      <literal>None</literal> if SCons is not running from virtualenv. Note,
      that this function returns a path even if SCons is run from an
      unactivated virtualenv.
    </para>

  </section>

</chapter>