Problems with output function of new datatype!

hi,
I have programmed a rational datatype and provided it with an output function
rational_out.
The output function takes a rational data type as input and returns cstring as
output.
The postgres is giving an error

server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!>

The real problem starts here: when i use the function as
select rational_out(rational_type) from testtable;
it works fine and returns a cstring. But when i execute
select rational_type from testtable;
then it dies with the error i mentioned before. Why is it happeneing?
But the input function which takes a cstring as input is working fine.

I have initialised the datatype as:
CREATE OR REPLACE FUNCTION frational_in(cstring) RETURNS frational AS
'/root/pgsql/hier_farey/lib/libfrational.so.1' LANGUAGE C;
CREATE OR REPLACE FUNCTION frational_out(frational) RETURNS cstring AS
'/root/pgsql/hier_farey/lib/libfrational.so.1' LANGUAGE C;
CREATE OR REPLACE FUNCTION frational_in(cstring) RETURNS frational AS
'/root/pgsql/lib/libfrational.so.1' LANGUAGE C;
CREATE OR REPLACE FUNCTION frational_out(frational) RETURNS cstring AS
'/root/pgsql/lib/libfrational.so.1' LANGUAGE C;

Please help me out with this one,
thanks,
subhash.