y* and z* result is a Py_buffer, but in C you have to pass a reference to the result variable using &result. Full example:

static PyObject *
getargs_y_star(PyObject *self, PyObject *args)
{
    Py_buffer buffer;
    PyObject *bytes;
    if (!PyArg_ParseTuple(args, "y*", &buffer))
        return NULL;
    bytes = PyBytes_FromStringAndSize(buffer.buf, buffer.len);
    PyBuffer_Release(&buffer);
    return bytes;
}

Another example: "s" format result is char* (and not char**). You also have to pass a reference using &:

static PyObject *
getargs_s(PyObject *self, PyObject *args)
{
    char *str;
    if (!PyArg_ParseTuple(args, "s", &str))
        return NULL;
    return PyBytes_FromString(str);
}