How to use the cairocffi.ffi.NULL function in cairocffi

def __init__(self, pointer, target_keep_alive=None):
        self._pointer = ffi.gc(
            pointer, _keepref(cairo, cairo.cairo_surface_destroy))
        self._check_status()
        if target_keep_alive not in (None, ffi.NULL):
            keep_alive = KeepAlive(target_keep_alive)
            _check_status(cairo.cairo_surface_set_user_data(
                self._pointer, SURFACE_TARGET_KEY, *keep_alive.closure))
            keep_alive.save()

if :obj:`with_clusters` is true, otherwise just :obj:`glyphs`.
            See :meth:`Context.show_text_glyphs` for the data structure.

        .. note::

            This method is part of
            what the cairo designers call the "toy" text API.
            It is convenient for short demos and simple programs,
            but it is not expected to be adequate
            for serious text-using applications.
            See :ref:`fonts` for details
            and :meth:`Context.show_glyphs`
            for the "real" text display API in cairo.

        """
        glyphs = ffi.new('cairo_glyph_t **', ffi.NULL)
        num_glyphs = ffi.new('int *')
        if with_clusters:
            clusters = ffi.new('cairo_text_cluster_t **', ffi.NULL)
            num_clusters = ffi.new('int *')
            cluster_flags = ffi.new('cairo_text_cluster_flags_t *')
        else:
            clusters = ffi.NULL
            num_clusters = ffi.NULL
            cluster_flags = ffi.NULL
        # TODO: Pass len_utf8 explicitly to support NULL bytes?
        status = cairo.cairo_scaled_font_text_to_glyphs(
            self._pointer, x, y, _encode_string(text), -1,
            glyphs, num_glyphs, clusters, num_clusters, cluster_flags)
        glyphs = ffi.gc(glyphs[0], _keepref(cairo, cairo.cairo_glyph_free))
        if with_clusters:
            clusters = ffi.gc(

def version_to_string(version):
        """Return the string representation of the given :ref:`PDF_VERSION`.
        See :meth:`get_versions` for a way to get
        the list of valid version ids.

        *New in cairo 1.10.*

        """
        c_string = cairo.cairo_pdf_version_to_string(version)
        if c_string == ffi.NULL:
            raise ValueError(version)
        return ffi.string(c_string).decode('ascii')

def _make_write_func(file_obj):
    """Return a CFFI callback that writes to a file-like object."""
    if file_obj is None:
        return ffi.NULL

    @ffi.callback("cairo_write_func_t", error=constants.STATUS_WRITE_ERROR)
    def write_func(_closure, data, length):
        file_obj.write(ffi.buffer(data, length))
        return constants.STATUS_SUCCESS
    return write_func

def __init__(self, target, width_in_points, height_in_points):
        if hasattr(target, 'write') or target is None:
            write_func = _make_write_func(target)
            pointer = cairo.cairo_ps_surface_create_for_stream(
                write_func, ffi.NULL, width_in_points, height_in_points)
        else:
            write_func = None
            pointer = cairo.cairo_ps_surface_create(
                _encode_filename(target), width_in_points, height_in_points)
        Surface.__init__(self, pointer, target_keep_alive=write_func)

Font variations are specified as a string with a format that is similar
        to the CSS font-variation-settings. The string contains a
        comma-separated list of axis assignments, which each assignment
        consists of a 4-character axis name and a value, separated by
        whitespace and optional equals sign.

        :param variations: the new font variations, or ``None``.

        *New in cairo 1.16.*

        *New in cairocffi 0.9.*

        """
        if variations is None:
            variations = ffi.NULL
        else:
            variations = _encode_string(variations)
        cairo.cairo_font_options_set_variations(self._pointer, variations)
        self._check_status()

def __init__(self, target, width_in_points, height_in_points):
        if hasattr(target, 'write') or target is None:
            write_func = _make_write_func(target)
            pointer = cairo.cairo_svg_surface_create_for_stream(
                write_func, ffi.NULL, width_in_points, height_in_points)
        else:
            write_func = None
            pointer = cairo.cairo_svg_surface_create(
                _encode_filename(target), width_in_points, height_in_points)
        Surface.__init__(self, pointer, target_keep_alive=write_func)

.. note::

            This method is part of
            what the cairo designers call the "toy" text API.
            It is convenient for short demos and simple programs,
            but it is not expected to be adequate
            for serious text-using applications.
            See :ref:`fonts` for details
            and :meth:`Context.show_glyphs`
            for the "real" text display API in cairo.

        """
        glyphs = ffi.new('cairo_glyph_t **', ffi.NULL)
        num_glyphs = ffi.new('int *')
        if with_clusters:
            clusters = ffi.new('cairo_text_cluster_t **', ffi.NULL)
            num_clusters = ffi.new('int *')
            cluster_flags = ffi.new('cairo_text_cluster_flags_t *')
        else:
            clusters = ffi.NULL
            num_clusters = ffi.NULL
            cluster_flags = ffi.NULL
        # TODO: Pass len_utf8 explicitly to support NULL bytes?
        status = cairo.cairo_scaled_font_text_to_glyphs(
            self._pointer, x, y, _encode_string(text), -1,
            glyphs, num_glyphs, clusters, num_clusters, cluster_flags)
        glyphs = ffi.gc(glyphs[0], _keepref(cairo, cairo.cairo_glyph_free))
        if with_clusters:
            clusters = ffi.gc(
                clusters[0], _keepref(cairo, cairo.cairo_text_cluster_free))
        _check_status(status)
        glyphs = [

def __init__(self, target, width_in_points, height_in_points):
        if hasattr(target, 'write') or target is None:
            write_func = _make_write_func(target)
            pointer = cairo.cairo_pdf_surface_create_for_stream(
                write_func, ffi.NULL, width_in_points, height_in_points)
        else:
            write_func = None
            pointer = cairo.cairo_pdf_surface_create(
                _encode_filename(target), width_in_points, height_in_points)
        Surface.__init__(self, pointer, target_keep_alive=write_func)

but it is not expected to be adequate
            for serious text-using applications.
            See :ref:`fonts` for details
            and :meth:`Context.show_glyphs`
            for the "real" text display API in cairo.

        """
        glyphs = ffi.new('cairo_glyph_t **', ffi.NULL)
        num_glyphs = ffi.new('int *')
        if with_clusters:
            clusters = ffi.new('cairo_text_cluster_t **', ffi.NULL)
            num_clusters = ffi.new('int *')
            cluster_flags = ffi.new('cairo_text_cluster_flags_t *')
        else:
            clusters = ffi.NULL
            num_clusters = ffi.NULL
            cluster_flags = ffi.NULL
        # TODO: Pass len_utf8 explicitly to support NULL bytes?
        status = cairo.cairo_scaled_font_text_to_glyphs(
            self._pointer, x, y, _encode_string(text), -1,
            glyphs, num_glyphs, clusters, num_clusters, cluster_flags)
        glyphs = ffi.gc(glyphs[0], _keepref(cairo, cairo.cairo_glyph_free))
        if with_clusters:
            clusters = ffi.gc(
                clusters[0], _keepref(cairo, cairo.cairo_text_cluster_free))
        _check_status(status)
        glyphs = [
            (glyph.index, glyph.x, glyph.y)
            for i in range(num_glyphs[0])
            for glyph in [glyphs[i]]]
        if with_clusters:
            clusters = [