Persistent variables between cross-calls in C functions

Dear all:

I could not find any way to store a value in a C function between calls
inside the same session.

For example, the following function doesn't work as expected:

/********************************/
#include "postgres.h"
#include "fmgr.h"

PG_FUNCTION_INFO_V1(next_value);

Datum next_value(PG_FUNCTION_ARGS)
{
static int n;
PG_RETURN_INT32(n);
n++;
}

/********************************/
create function next_value()
returns integer as
'next.so' language C;

/********************************/
Calling

select next_value();

will always return 0,
even if I declare 'n' as a global variable outside the C function.

Thanks a lot.

Fabio Furia Silva