pg_begintypend

Hi all,
loading a function written in C I get this error:

#create function email_in( cstring ) returns email AS 'emaildt.so', 'email_in' LANGUAGE C STRICT;
NOTICE:  type "email" is not yet defined
DETAIL:  Creating a shell type definition.
ERROR:  could not load library "emaildt.so": emaildt.so: undefined symbol: pg_begintypsend


I'm compiling my files in this way:

gcc -c  -g -fPIC -o "email.o" -I /usr/include/pgsql/server "email.c"
gcc -g -shared -fPIC -o "emaildt.so" email.o


which library I have to link in order to have the symbol pg_begintypsend ?


Regards
Gaetano Mendola