Returning bigint from C extension

How can I return a bigint value from a C extension function?

I have a table with a bytea column:

CREATE TABLE mydata(data bytea);

I would like to select and index based on a subset of the data:

CREATE FUNCTION get_key(bytea) returns BIGINT AS '/lib/data.so'
LANGUAGE C IMMUTABLE;
CREATE INDEX mydata_key_idx on mydata(get_key(data));

# SELECT get_key(data) FROM mydata;

My code :

PG_FUNCTION_INFO_V1(get_key);
Datum get_key(PG_FUNCTION_ARGS)
{
  bytea *val;
  unsigned int len;
  long long res;

  val = PG_GETARG_BYTEA_P(0);
  len = VARSIZE(val) - VARHDRSZ;
  res = _extract_key(VARDATA(val), len);
  PG_FREE_IF_COPY(val, 0);

  PG_RETURN_INT32(res);
}

The PG_RETURN_INT32() macro causes the server to terminate.

It works if I convert to a cstring:

  char *result = palloc(32);
  snprintf(result, 32, "%lld", res);
  PG_RETURN_CSTRING(result);

But then I can't index it, or sort on it either.

(And, as a side note, is the above code the correct way to use bytea
data from C?)

Regards,

--
Jason Armstrong