Linking in sin() as a C function

I tried the following, one at a time, to create sin() for PostgreSQL:

CREATE FUNCTION sin(float8) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
CREATE FUNCTION sin(float4) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';
CREATE FUNCTION sin(float4) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
CREATE FUNCTION sin(float8) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';

Each of these gave different and odd results (and a 'segmentation fault'
at one point), but none of them gave the right answer. What am I doing
wrong?