path: root/doc/user/sconf.in

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  <para>

  &SCons; has integrated support for multi-platform build configuration
  similar to that offered by GNU &Autoconf;,
  such as
  figuring out what libraries or header files
  are available on the local system.
  This section describes how to use
  this &SCons feature.

  </para>

  <note>
  <para>
  This chapter is still under development,
  so not everything is explained as well as it should be.
  See the &SCons; man page for additional information.
  </para>
  </note>

  <section>
  <title>&Configure_Contexts;</title>

    <para>

    The basic framework for multi-platform build configuration
    in &SCons; is to attach a &configure_context; to a
    construction environment by calling the &Configure; function,
    perform a number of checks for
    libraries, functions, header files, etc.,
    and to then call the configure context's &Finish; method
    to finish off the configuration:

    </para>

    <sconstruct>
    env = Environment()
    conf = Configure(env)
    # Checks for libraries, header files, etc. go here!
    env = conf.Finish()
    </sconstruct>

    <para>

    &SCons; provides a number of basic checks,
    as well as a mechanism for adding your own custom checks.

    </para>

    <para>

    Note that &SCons; uses its own dependency
    mechanism to determine when a check
    needs to be run--that is,
    &SCons; does not run the checks
    every time it is invoked,
    but caches the values returned by previous checks
    and uses the cached values unless something has changed.
    This saves a tremendous amount
    of developer time while working on
    cross-platform build issues.

    </para>

    <para>

    The next sections describe
    the basic checks that &SCons; supports,
    as well as how to add your own custom checks.

    </para>

  </section>

  <section>
  <title>Checking for the Existence of Header Files</title>

    <para>

    Testing the existence of a header file
    requires knowing what language the header file is.
    A configure context has a &CheckCHeader; method
    that checks for the existence of a C header file:

    </para>

    <sconstruct>
    env = Environment()
    conf = Configure(env)
    if not conf.CheckCHeader('math.h'):
        print 'Math.h must be installed!'
        Exit(1)
    if conf.CheckCHeader('foo.h'):
        conf.env.Append('-DHAS_FOO_H')
    env = conf.Finish()
    </sconstruct>

    <para>

    Note that you can choose to terminate
    the build if a given header file doesn't exist,
    or you can modify the contstruction environment
    based on the existence of a header file.

    </para>

    <para>

    If you need to check for the existence
    a C++ header file,
    use the &CheckCXXHeader; method:

    </para>

    <sconstruct>
    env = Environment()
    conf = Configure(env)
    if not conf.CheckCXXHeader('vector.h'):
        print 'vector.h must be installed!'
        Exit(1)
    env = conf.Finish()
    </sconstruct>

  </section>

  <section>
  <title>Checking for the Availability of a Function</title>

    <para>

    Check for the availability of a specific function
    using the &CheckFunc; method:

    </para>

    <sconstruct>
    env = Environment()
    conf = Configure(env)
    if not conf.CheckFunc('strcpy'):
        print 'Did not find strcpy(), using local version'
        conf.env.Append('-Dstrcpy=my_local_strcpy')
    env = conf.Finish()
    </sconstruct>

  </section>

  <section>
  <title>Checking for the Availability of a Library</title>

    <para>

    Check for the availability of a library
    using the &CheckLib; method.
    You only specify the basename of the library,
    you don't need to add a <literal>lib</literal>
    prefix or a <literal>.a</literal> or <literal>.lib</literal> suffix:

    </para>

    <sconstruct>
    env = Environment()
    conf = Configure(env)
    if not conf.CheckLib('m'):
        print 'Did not find libm.a or m.lib, exiting!'
        Exit(1)
    env = conf.Finish()
    </sconstruct>

    <para>

    Because the ability to use a library successfully
    often depends on having access to a header file
    that describes the library's interface,
    you can check for a library
    <emphasis>and</emphasis> a header file
    at the same time by using the
    &CheckLibWithHeader; method:

    </para>

    <sconstruct>
    env = Environment()
    conf = Configure(env)
    if not conf.CheckLibWithHeader('m', 'math.h'):
        print 'Did not find libm.a or m.lib, exiting!'
        Exit(1)
    env = conf.Finish()
    </sconstruct>

    <para>

    This is essentially shorthand for
    separate calls to the &CheckHeader; and &CheckLib;
    functions.

    </para>

  </section>

  <section>
  <title>Checking for the Availability of a &typedef;</title>

    <para>

    Check for the availability of a &typedef;
    by using the &CheckType; method:

    </para>

    <sconstruct>
    env = Environment()
    conf = Configure(env)
    if not conf.CheckType('off_t'):
        print 'Did not find off_t typedef, assuming int'
        conf.env.Append(CCFLAGS = '-Doff_t=int')
    env = conf.Finish()
    </sconstruct>

    <para>

    You can also add a string that will be
    placed at the beginning of the test file
    that will be used to check for the &typedef;.
    This provide a way to specify
    files that must be included to find the &typedef;:

    </para>

    <sconstruct>
    env = Environment()
    conf = Configure(env)
    if not conf.CheckType('off_t', '#include &amp;lt;sys/types.h&amp;gt;\n'):
        print 'Did not find off_t typedef, assuming int'
        conf.env.Append(CCFLAGS = '-Doff_t=int')
    env = conf.Finish()
    </sconstruct>

  </section>

  <section>
  <title>Adding Your Own Custom Checks</title>

    <para>

    A custom check is a Python function
    that checks for a certain condition to exist
    on the running system,
    usually using methods that &SCons;
    supplies to take care of the details
    of checking whether a compilation succeeds,
    a link succeeds,
    a program is runnable,
    etc.
    A simple custom check for the existence of
    a specific library might look as follows:

    </para>

    <sconstruct>
    mylib_test_source_file = """
    #include &amp;lt;mylib.h&amp;gt;
    int main(int argc, char **argv)
    {
        MyLibrary mylib(argc, argv);
        return 0;
    }
    """

    def CheckMyLibrary(context):
        context.Message('Checking for MyLibrary...')
        result = context.TryLink(mylib_test_source_file, '.c')
        context.Result(result)
        return result
    </sconstruct>

    <para>

    The &Message; and &Result; methods
    should typically begin and end a custom check to
    let the user know what's going on:
    the &Message; call prints the
    specified message (with no trailing newline)
    and the &Result; call prints
    <literal>ok</literal> if the check succeeds and
    <literal>failed</literal> if it doesn't.
    The &TryLink; method
    actually tests for whether the
    specified program text
    will successfully link.

    </para>

    <para>

    (Note that a custom check can modify
    its check based on any arguments you
    choose to pass it,
    or by using or modifying the configure context environment
    in the <literal>context.env</literal> attribute.)

    </para>

    <para>

    This custom check function is
    then attached to the &configure_context;
    by passing a dictionary
    to the &Configure; call
    that maps a name of the check
    to the underlying function:

    </para>

    <sconstruct>
    env = Environment()
    conf = Configure(env, custom_tests = {'CheckMyLibrary' : CheckMyLibrary})
    </sconstruct>

    <para>

    You'll typically want to make
    the check and the function name the same,
    as we've done here,
    to avoid potential confusion.

    </para>

    <para>

    We can then put these pieces together
    and actually call the <literal>CheckMyLibrary</literal> check
    as follows:

    </para>

    <sconstruct>
    mylib_test_source_file = """
    #include &amp;lt;mylib.h&amp;gt;
    int main(int argc, char **argv)
    {
        MyLibrary mylib(argc, argv);
        return 0;
    }
    """

    def CheckMyLibrary(context):
        context.Message('Checking for MyLibrary... ')
        result = context.TryLink(mylib_test_source_file, '.c')
        context.Result(result)
        return result

    env = Environment()
    conf = Configure(env, custom_tests = {'CheckMyLibrary' : CheckMyLibrary})
    if not conf.CheckMyLibrary():
        print 'MyLibrary is not installed!'
        Exit(1)
    env = conf.Finish()

    # We would then add actual calls like Program() to build
    # something using the "env" construction environment.
    </sconstruct>

    <para>

    If MyLibrary is not installed on the system,
    the output will look like:

    </para>

    <screen>
    % <userinput>scons</userinput>
    scons: Reading SConscript file ...
    Checking for MyLibrary... failed
    MyLibrary is not installed!
    </screen>

    <para>

    If MyLibrary is installed,
    the output will look like:

    </para>

    <screen>
    % <userinput>scons</userinput>
    scons: Reading SConscript file ...
    Checking for MyLibrary... failed
    scons: done reading SConscript
    scons: Building targets ...
        .
        .
        .
    </screen>

  </section>

  <section>
  <title>Not Configuring When Cleaning Targets</title>

    <para>

    Using multi-platform configuration
    as described in the previous sections
    will run the configuration commands
    even when invoking
    <userinput>scons -c</userinput>
    to clean targets:

    </para>

    <screen>
    % <userinput>scons -Q -c</userinput>
    Checking for MyLibrary... ok
    Removed foo.o
    Removed foo
    </screen>

    <para>

    Although running the platform checks
    when removing targets doesn't hurt anything,
    it's usually unnecessary.
    You can avoid this by using the
    &GetOption(); method to
    check whether the <option>-c</option> (clean)
    option has been invoked on the command line:

    </para>

    <sconstruct>
    env = Environment()
    if not env.GetOption('clean'):
        conf = Configure(env, custom_tests = {'CheckMyLibrary' : CheckMyLibrary})
        if not conf.CheckMyLibrary():
            print 'MyLibrary is not installed!'
            Exit(1)
        env = conf.Finish()
    </sconstruct>

    <screen>
    % <userinput>scons -Q -c</userinput>
    Removed foo.o
    Removed foo
    </screen>

  </section>

  <!--

  <section>
  <title>Controlling Configuration:  the &config; Option</title>

    <para>

    XXX -D, -u and -U

    </para>

  </section>

  -->