pl/pgsql, cursors and C function

Hi
I'm making my first steps in C functions. I want to avoid doing all the
SQL job in them, pl/pgsql looks a better choice. I tried to do this by
passing opened cursor from pl/pgsql function to C function.

Here is simple C function:
#include <server/postgres.h>
#include <server/executor/spi.h>
PG_FUNCTION_INFO_V1(test2);
Datum test2(PG_FUNCTION_ARGS)
{
Portal p;
int n;
p=SPI_cursor_find("xxx");
if(!p)
elog(ERROR,"Cursor error");
SPI_cursor_fetch(p,true,1);
n=SPI_processed;
PG_RETURN_INT32(n);
}

And pl/pgsql one:
CREATE OR REPLACE FUNCTION test() returns integer AS '
DECLARE
_m CURSOR FOR select id from some_table limit 1;
n integer;
BEGIN
_m=''xxx'';
open _m;
n=test2();
close _m;
return n;
END;
' language 'plpgsql';

select test();
I don't understand ERROR message at all:
ERROR: SPI_prepare() failed on "SELECT $1 "

This error is raised when trying to execute SPI_cursor_fetch. What does
it mean? What does the SPI_prepare have to already opened cursor?
Where can I find better SPI documentation than "Postgresql Server
Programming" ?

Regards,
Tomasz Myrta