Error using home-made functions

Hello, 

I still have the same mistake after upgrading to v6.4.2 : 
add_one is a simple function that add 1 to a given int. It has been
compiled as a .so using gcc. The first time I use it, it gives an error
message, and the second and following times, it seems to work well : 

template1=> select add_one(1) ;
ERROR:  Load of file /usr/local/pgsql/lib/add_one.so failed:
/usr/local/pgsql/lib/add_one.so: undefined symbol: mem_set
template1=> select add_one(1) ;
add_one
-------     2
(1 row)
template1=>

Any help would be much appreciated, 

Jose Paumard