newbie libpq question...

I'm trying to use the libpq library for the first time and I'm
experiencing what seems to be a very basic problem. When I run the
following:

res = PQexec(conn, "select blah blah blah");
test_int=PQntuples(res);
fprintf(stdout,"number of rows: %f\n",test_int);
test_int=PQnfields(res);
fprintf(stdout,"number of cols: %f\n",test_int);

where "blah blah blah" is the selection criteria, I get zero rows and
zero columns. When I do the exact same "select" query through the psql
interface I get 547 rows and 3 columns. I've checked that
PQresultStatus = PGRES_TUPLES_OK, so it seems like it completed the
command correctly. I'm new to postgres and I haven't used C in a long
while so I'm a little rusty there. Am I missing something obvious?

thanks in advance for any insight,
dh