Custom integer-like type

Hello,

I'm creating a new data in C, and everything is proceeding well, except
that the data type should be parsed on input like an integer. Maybe it's
best if I explain it with an example:

Currently, I can do this:

INSERT INTO t(my_data_type) VALUES ('1')

but I cannot do this:

INSERT INTO t(my_data_type) VALUES (1)

My type is defined as:

CREATE TYPE myint (
internallength = variable,
input = encbigint_in,
output = encbigint_out,
alignment = int4,
storage = external
);

(The "variable" length is correct in this case.)