code: def application(environ, start_response):
      
      # the environment variable CONTENT_LENGTH may be empty or missing
      try:
          request_body_size = int(environ.get('CONTENT_LENGTH', 0))
      except ValueError:
          request_body_size = 0
  
      # When the method is POST the query string will be sent
      # in the HTTP request body which is passed by the WSGI server
      # in the file like wsgi.input environment variable.
      if request_body_size > 0:
          request_body = environ['wsgi.input'].read(request_body_size)
      else:
          request_body = environ['wsgi.input'].read()
      form = cgi.parse_qs(request_body)
  
      status = '200 OK'
      start_response(status, get_response_headers())
      for output in checklink(form=form, env=environ):
          yield output
docs: WSGI interface: start an URL check.