postgres functions and C++

Hello all,

Please help me with
create function days_in_month(int4, int4, int4) returns int4
as '/tmp/days.so' language 'c';

Can I write this function days_in_month in C++ ?
I've did the following:

extern "C" {
int days_in_month(int year, int mo, int day); 
} 

extern "C" { 
int days_in_month(int year, int mo, int day) { 
═══ return 13;
} 
} 

Compile with
  $ g++ -I/usr/local/qt/include -o days.so -shared days.cpp

then (as user postgres)
  my=> create function days_in_month(int4, int4, int4) returns int4
  my=> as '/tmp/days.so' language 'c';
  CREATE
  
I've tried to use new function
  my=> select days_in_month(3, 3, 3);
  ERROR:═ Can't find function days_in_month in file /tmp/days.so
  my=>
  
What is wrong ?
I'm using postgres 6.4 under RedHat.
Any help will be appreciated.

-- 
Best regards,
 Vladimir                          mailto:gsmith@eurocom.od.ua