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