How to use the pygit2.ffi.C function in pygit2

def favor_to_enum(favor):
            if favor == 'normal':
                return C.GIT_MERGE_FILE_FAVOR_NORMAL
            elif favor == 'ours':
                return C.GIT_MERGE_FILE_FAVOR_OURS
            elif favor == 'theirs':
                return C.GIT_MERGE_FILE_FAVOR_THEIRS
            elif favor == 'union':
                return C.GIT_MERGE_FILE_FAVOR_UNION
            else:
                return None

def __del__(self):
        if not self.from_iterator:
            C.git_config_entry_free(self._entry)

def __del__(self):
        C.git_config_iterator_free(self._iter)

def set_head(self, target):
        """
        Set HEAD to point to the given target.

        Parameters:

        target
            The new target for HEAD. Can be a string or Oid (to detach).
        """

        if isinstance(target, Oid):
            oid = ffi.new('git_oid *')
            ffi.buffer(oid)[:] = target.raw[:]
            err = C.git_repository_set_head_detached(self._repo, oid)
            check_error(err)
            return

        # if it's a string, then it's a reference name
        err = C.git_repository_set_head(self._repo, to_bytes(target))
        check_error(err)

def get_xdg_config():
        """Return a  object representing the global configuration file.
        """
        return Config._from_found_config(C.git_config_find_xdg)

# This file is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; see the file COPYING.  If not, write to
# the Free Software Foundation, 51 Franklin Street, Fifth Floor,
# Boston, MA 02110-1301, USA.

# Import from pygit2
from .ffi import ffi, C
from _pygit2 import GitError


value_errors = set([C.GIT_EEXISTS, C.GIT_EINVALIDSPEC, C.GIT_EAMBIGUOUS])

def check_error(err, io=False):
    if err >= 0:
        return

    # Error message
    giterr = C.git_error_last()
    if giterr != ffi.NULL:
        message = ffi.string(giterr.message).decode('utf8')
    else:
        message = "err %d (no message provided)" % err

    # Translate to Python errors
    if err in value_errors:
        raise ValueError(message)

    def fetch(self, refspecs=None, message=None, callbacks=None, prune=C.GIT_FETCH_PRUNE_UNSPECIFIED):
        """Perform a fetch against this remote. Returns a 
        object.

        Parameters:

        prune : enum
            Either , , or
            . The first uses the configuration from the
            repo, the second will remove any remote branch in the local
            repository that does not exist in the remote and the last will
            always keep the remote branches
        """

        fetch_opts = ffi.new('git_fetch_options *')
        err = C.git_fetch_init_options(fetch_opts, C.GIT_FETCH_OPTIONS_VERSION)

def open(self):
        """Open the repository for a submodule."""
        crepo = ffi.new('git_repository **')
        err = C.git_submodule_open(crepo, self._subm)
        check_error(err)

        return self._repo._from_c(crepo[0], True)

Push the given refspec to the remote. Raises ``GitError`` on protocol
        error or unpack failure.

        When the remote has a githook installed, that denies the reference this
        function will return successfully. Thus it is strongly recommended to
        install a callback, that implements
        :py:meth:`RemoteCallbacks.push_update_reference` and check the passed
        parameters for successfull operations.

        Parameters:

        specs : [str]
            Push refspecs to use.
        """
        push_opts = ffi.new('git_push_options *')
        err = C.git_push_init_options(push_opts, C.GIT_PUSH_OPTIONS_VERSION)

        if callbacks is None:
            callbacks = RemoteCallbacks()

        callbacks._fill_push_options(push_opts)
        # Build custom callback structure

        try:
            with StrArray(specs) as refspecs:
                err = C.git_remote_push(self._remote, refspecs, push_opts)
                check_error(err)
        finally:
            callbacks._self_handle = None