Calling a SQL function inside a C function

How do I call a function defined using CREATE FUNCTION in SQL inside a C
function in an extension? I feel this should be possible as the query
parser is able to resolve the function names and arguments in a raw string
query. I want to know if there is a standard way to look up for
user-defined functions in the backend.

For example, I have a function defined in SQL:

```
CREATE FUNCTION times_two(x integer)
RETURNS integer AS $$
SELECT x*2
$$ LANGUAGE SQL;
```

Now I wish to call `times_two()` in a C extension similar to:

```
// Look up for the user-defined function times_two()
// ...

// Use the function.
Datum ret = DirectFunctionCall(times_two, Int32GetDatum(13));
```

Best,
Eric