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