Registring a C function in PostgreSQL

I have followed the instructions in the Bruje Momjian´s book to register a
function, and i got this message:

 ERROR:/load of file /home/postgres/ctof.so failed: /home/postgres/ctof.so
ELF file´s phentsize not the expected size.

What is going on? I have tried to get more documentation about how to use C
functions in PostgreSQL, but I cant.

 Many thanks in advance

 Miguel