SCons / doc / user / sconf.xml

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<!--

  __COPYRIGHT__

  Permission is hereby granted, free of charge, to any person obtaining
  a copy of this software and associated documentation files (the
  "Software"), to deal in the Software without restriction, including
  without limitation the rights to use, copy, modify, merge, publish,
  distribute, sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:

  The above copyright notice and this permission notice shall be included
  in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
  KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
  WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

-->

  <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>

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

    <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>

    <programlisting>
    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()
    </programlisting>

    <para>

    Note that you can choose to terminate
    the build if a given header file doesn't exist,
    or you can modify the construction 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>

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

  </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>

    <programlisting>
    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()
    </programlisting>

  </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>

    <programlisting>
    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()
    </programlisting>

    <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>

    <programlisting>
    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()
    </programlisting>

    <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>

    <programlisting>
    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()
    </programlisting>

    <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>

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

  </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>

    <programlisting>
    mylib_test_source_file = """
    #include &lt;mylib.h&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
    </programlisting>

    <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>

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

    <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>

    <programlisting>
    mylib_test_source_file = """
    #include &lt;mylib.h&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.
    </programlisting>

    <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>

    <programlisting>
    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()
    </programlisting>

    <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>

  -->
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