Easy nios II design

Hello,

I am a newbie with Quartus II and Nios II and I have been trying to do an easy nios design in order to light on a led when pressing a button. I am working on the Altera Cyclone III EP3C25F324 board. I think the problem comes from my code in nios II that I put below. I don't really know when to write "IOWR_ALTERA_AVALON_PIO_DATA(...);" maybe the problem comes from here. Plus, when I try to write some "printf", there is an error :

collect2: ld returned 1 exit status

make: *** [button_led.elf] Error 1

Here is my program :

# include "system.h"# include "altera_avalon_pio_regs.h"# include "alt_types.h"# include "stdio.h"# include "stdlib.h"

int alt_main (void)

{

alt_u8 led = 0x2;

alt_u8 button = 0x2;

IOWR_ALTERA_AVALON_PIO_DATA(LED_PIO_BASE, led);

IOWR_ALTERA_AVALON_PIO_DATA(BUTTON_PIO_BASE, button);

while (1)

{

switch (button) {

case 0x1 :

led=0x1;

break;

case 0x2 :

led=0x2;

break;

case 0x4 :

led=0x4;

break;

case 0x8 :

led=0x8;

break;

}

}

Do someone can help me?

Thank you very much,

Myriam