SETOF function call

I hope I am on good list, else point me where I should ask.

I create a C function like this:

CREATE
-- IFUPDATE or REPLACE
FUNCTION rpmquery(rpmheader, INT) RETURNS SETOF TEXT AS
'MODULE_PATHNAME', 'header_query' LANGUAGE C IMMUTABLE STRICT;

(do not take care, code come from the .sql.in)

And the function works well, rpmheader is a new type with its own in/out
function.

But I have a problem, the second argument is an integer, ie an internal value.
So I want to create a function wrapper as rpmquery(rpmheader, TEXT), the goal
of this function is to first convert the "human text value" to "internal
integer value" and then call the first function.

But I am unable to to create a single function like I allready did with
function returning single Datum:

PG_FUNCTION_INFO_V1(header_querytxt);

Datum
header_querytxt(PG_FUNCTION_ARGS)
{
elog(NOTICE, "querytxt");
Datum tag;
/* converting TEXT value to integer one */
tag = DirectFunctionCall1(rpmtagvalue, PG_GETARG_TEXT_P(1));
elog(NOTICE, "querytxt %d", tag);
/* header_query = function accepting integer instead text)
PG_RETURN_POINTER(DirectFunctionCall2(header_query,
PG_GETARG_BYTEA_P_COPY(0), tag));
}

I get a segfault just after second elog. This is maybe simple for anyone
reading this list, but I am unable to find an example, or a documention to do
this.
A simple example, or a URL is welcome.