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A Build System for Complex Projects: Part 2


It's time to look at the implementation of the core ibs. Let's examine the build_system_generator.py script that contains most of the object model, the DependencyAnalyzer.py module that contains the dependency analyzer, and the various project templates.


Let's start with the main function. Its job is to parse command-line arguments and hand them over to the generate_build_files() function. There is currently only one command-line argument -- root_dir. The root_dir is the directory that contains ibs and the source tree. It defaults to the parent directory, which works if you run it from the ibs directory (where the script is located). In addition, the save_projects argument is passed. If it's True, the generated build files are saved. If it's False, the build files are generated but not saved, which is useful during development and debugging.

def main(argv):
  """Parse command-line arguments and call generate_build_files()

  import getopt
  optionSpec = ['root_dir=']

  save_project_files = True

  root_dir = os.path.join(script_dir, '..')

    opts, args = getopt.gnu_getopt(argv, '', optionSpec)
  except Exception, e:
    print "Error parsing command line: %s" % e

  if len(args) > 0:

  for (option, val) in opts:
    if option == '--root_dir':
      root_dir = val

  # Convenience. Allows use of "~" in pathnames and other things.
  root_dir = os.path.abspath(os.path.normpath(os.path.expanduser(root_dir)))

  generate_build_files(root_dir, save_project_files)

if __name__=='__main__':

The generate_build_files() function calls its nested get_platform() function to figure out what platform it is running on (using the Python "platform" standard library module). Based on the platform, it decides what build system to target (Visual C++ 2008 or NetBeans 6). In your build system you may want to employ different logic to determine the target build system or pass it from the outside as a command-line argument.

After the target build system has been determined, the appropriate helper module is instantiated. All the helper modules reside in the 'helpers' subdirectory and are named <build system>_helper.py. For example, the Visual C++ 2008 helper is "ibs/helpers/VC_2008_helper.py". Future articles will take a closer look at these helper modules and drill into specific build systems. This article stays at the generic build-system level.

The proper helper is imported dynamically via the __import__ statement. This is a useful Pythonic technique to implement plug-ins. Note that additional target build systems can be easily added by moving the build system selection code outside and by simply adding more helper modules for the new build systems.

The next step is to select the project template's directory. The project templates for each build system reside in a sub-dir of 'project_templates' and are named after the build system. For example, the NetBeans 6 templates dir is: "ibs/project_templates/NetBeans_6".

A helper module is instantiated using an unusual syntax:

helper = getattr(helper_module, 'Helper')(templates_dir)

The getattr() call gets the Helper attribute from the current helper module. Every helper module contains a class called Helper that implements the helper interface. Then, the retrieved Helper class, which also doubles as a factory function/constructor, is invoked with the templates dir as an argument, resulting in an instantiated Helper object.

The helper is used to create a Generator object g. Several generator methods are now invoked in a sequence: generate_projects(), save_projects(), and generate_workspace_files().

def generate_build_files(root_dir, save_project_files):
  def get_platform():
    arch = platform.architecture()[0][:2]
    operating_system = platform.system().lower()
    processor = platform.processor()

    if operating_system == 'darwin' and arch == '32' and processor == 'i386':
      return 'darwin86'
    elif operating_system == 'linux':
      return 'linux' + arch
    elif operating_system == 'windows':
      return 'win' + arch
      raise Exception('Unsupported platform: ' + platform.platform())

  # Dynamically import the build system specific helper module
  p = get_platform()
  build_system = 'VC_2008' if p.startswith('win') else 'NetBeans_6'
  helper_name = build_system + '_helper'
  sys_path = sys.path[:]
  sys.path.insert(0, os.path.join(script_dir, 'helpers'))
  helper_module = __import__(helper_name)

  templates_dir = os.path.join(script_dir, 'project_templates', build_system)
  # Instantiate the Helper class
  helper = getattr(helper_module, 'Helper')(templates_dir)

  g = Generator(root_dir, p, helper)

Let's examine the Generator class. The constructor stores the root_dir, the platform, and the helper module. The helper module is used to execute all the steps that require specific build-system knowledge. This is sort of a Strategy pattern, if you will. The final line calls the internal _populate_project_list() method.

  def __init__(self, root_dir, platform, helper):
    self.root_dir = root_dir
    self.libs_dir = os.path.join(root_dir, 'src/hw')
    self.platform = platform
    self.helper = helper
    self.ignore = helper.get_ignore_string()
    self.projects = {}

The _populate_project_list() method gets the template objects for various project types from the helper (this is build-system-specific) and create a mapping between each top-level directory (hw, dlls, apps, and test) and its template object.

Then it starts drilling down the source tree starting from each top-level dir (aka parent dir) using the os.walk() function. It ignores .svn directories and sub-dirs that contain the special helper.skip_dir string. For each found sub-dir, a Project object is created and added to the project's list. This process demonstrates the flexibility of the build system generator. You can freely add new projects as long as they belong to one of the known project types and reside under the proper parent dir.

  def _populate_project_list(self):
    """Populate the list of projects to be generated

    # Map project parent directories to templates
    static_lib_template = self.helper.get_templates('static_lib')
    dynamic_lib_template = self.helper.get_templates('dynamic_lib')
    program_template = self.helper.get_templates('program')
    mapping = dict(hw=static_lib_template,
    skip_dir = self.helper.skip_dir
    for parent_dir, templates in mapping.items():
      for project_dir, s, files in os.walk(os.path.join(source_dir, parent_dir)):
        # Treat only sub-directories directly under the parent dir
        # as projects (except .svn or skip_dir)
        if '.svn' in project_dir or \
           (skip_dir and skip_dir in project_dir) or \
           os.path.basename(os.path.dirname(project_dir)) != parent_dir:

        assert not project_dir in self.projects
        #print project_dir

        project = Project(project_dir)
        project.generated = False
        project.templates = copy.deepcopy(templates)
        self.projects[project.path] = project

Now let's take a look at the Project class -- a simple container to hold the project information. The most exciting thing it does is call the DependencyAnalyzer to get its dependencies and sort them.

class Project(object):
  """The project class stores all build information of a single project

  A project has a name, a directory, a list of project templates
  and dependencies (other projects) that must be built before the project itself.
  def __init__(self, path):
    self.path = path
    self.name = os.path.basename(path)
    self.templates = {}
    dependencies = \
    self.dependencies = sorted(dependencies)

What's next? Generating the build files for all the projects. This is done by the Generator's generate_project(), which iterates over all the projects and builds them one after the other by calling _generate_project():

  def generate_projects(self):
    for p in self.projects.values():

But what about the dependency graph? The projects are built in the order they were inserted into the list. In fact, the _generate_project() method is a pretty smart cookie and it builds all its dependencies before embarking on building the project it was asked to build. This method does the heavy lifting, so I'll review it piece by piece. First, it determines if the project was already generated. project.generated is a Boolean attribute initialized to False; when _generate_project() is called for the first time on a project, it is set to True. The check ensures that a given project will only be generated once. This is necessary because any given project may be a dependency of multiple projects.

  def _generate_project(self, project):
    if project.generated:

    project.generated = True

In the next stage, all the source and header files associated with a project are collected. This process is build-system-specific, so the helper is invoked to perform it. If the project has no source files or header files, there is nothing to do and it bails out. Then the path separators (either \ or /) are replaced with the helper's separator character.

    # Collect source and header files
    project.header_files, project.source_files = self.helper.collect_files(project.path)

    # Bail out on projects with no source and/or header files
    if project.header_files == [] and project.source_files == []:

    # Make sure the paths are correct
    project.source_files = self._fix_path_separators(project.source_files)
    project.header_files = self._fix_path_separators(project.header_files)

The next stage ensures all the dependencies have been generated and iterates over them. If a dependency project hasn't been generated yet, the _generate_project() method is invoked recursively to generate it. The code doesn't detect circular dependencies at the moment, but it will not get stuck in an infinite loop because of the "generated" check. This is more an issue of code hygiene and might point to other bugs.

    # Make sure all the dependencies are in the project list
    for d in project.dependencies:
      assert d in self.projects

    dependencies = [self.projects[d] for d in project.dependencies]

    # Make sure all dependencies have been generated
    # This is recursive. It terminates as long as we have no circular
    # dependencies
    # @todo detect circular dependency
    for dep in dependencies:
      if dep.generated:

Finally, the project itself is generated. This means iterating over the project templates (there may be multiple build files per project, each with its own template), asking the helper to prepare a substitution dictionary based on various project parameters, its source and header files, and its dependencies list, and then applying the substitution dictionary to the template. The end result is that each template object now contains a textual build file.

    # Generate the project build files by creating a substitution dict
    # and applying it to each template file
    h = self.helper
    for t in project.templates:
      #print t.relative_path
      d = h.prepare_substitution_dict(project.name,

      d['Name'] = project.name
      d['name'] = project.name.lower()
      d['Platform'] = self.platform

      t.substitution_dict = d

The ultimate goal of the project generation is to apply the substitution dictionary with all its values to the template object. Let's see how it works using the Template class. The __init__() method stores all the information, reads its template file, and stores it as a string. The apply() method uses Python's string.Template object to actually perform the substitution. We will examine what a template file looks like in the next installment.

class Template(object):
  """The template class has a template file and a substitution dict

  Applying the substitution dict to template file results in a final build file
  def __init__(self, template_file, relative_path, template_type):
    @param template_file - absolute filename of the template file
    @param relative_path - relative path of this build file from the project dir
    @param template_type - static_lib, dynamic_lib or program
    self.template_file = template_file
    self.relative_path = relative_path
    self.template_type = template_type
    self.template_text = open(template_file).read()
    self.substitution_dict = {}
    self.build_file = ''

  def apply(self):
    # Generate A build file from a template file
    # (using PEP-292 string.Template substitution)
    # convert the template_text to string in case its unicode
    # (VisualStudio .vcproj files)
    t = string.Template(str(self.template_text))
    self.build_file =  t.substitute(self.substitution_dict)</code>

All the projects have been generated and the template objects contain all the generated build files. It's time to save the files via the Generator's save_projects() method. The method iterates over all the projects and within each project, it iterates over all the templates. It compares the generated build file against the exiting build file (if one exists), and if it's different and the save argument is True, it writes the new/modified build file.

  def save_projects(self, save):
    for p in self.projects.values():
      # Save build file only if modified
      for t in p.templates:
        modified = False
        filename = os.path.join(p.path, t.relative_path)
        if '%s' in filename:
          filename = filename % p.name
        if os.path.exists(filename):
          text = open(filename).read()
          text = None
          modified = True

        if text is not None:
          if t.build_file != text:
            modified = True

        # Save build file file if necessary and different
        if modified and save:
          parent_dir = os.path.dirname(filename)
          if not os.path.exists(parent_dir):
          open(filename, 'w').write(t.build_file)

If save is False and the build file was modified, the method goes into debugging mode and compares the existing build file against the generated file line by line and prints the results. This functionality is a lifesaver when developing generation code for a new build system. You will see it in action when I discuss the specific build systems in future articles.

        # Check generated file
        if modified and not save:
          dirname, basename = os.path.split(filename)
          gen_filename = os.path.join(dirname, 'gen.' + basename)
          open(gen_filename, 'w').write(t.build_file)
          real_lines = open(filename).readlines()
          gen_lines = open(gen_filename).readlines()
          if real_lines != gen_lines:
            assert len(real_lines) ==  len(gen_lines)
            for i in range(min(len(real_lines), len(gen_lines))):
              if real_lines[i] != gen_lines[i]:
                print i
                print "'%s'" % real_lines[i]
                print "'%s'" % gen_lines[i]
          if sorted(real_lines) != sorted (gen_lines):
            print filename
            assert False

Last but not least is generation of the workspace files. A workspace is just a collection of projects. It's called a Solution in Visual Studio and project group. It may or may not be trivial depending on the particular build system. The helper is doing all the work here.

  def generate_workspace_files(self):

As you can see, the generic build system has a lot of logic that can be used to generate build files for any build system. The specific work for each build system is nicely encapsulated in the helper modules.

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