AVR-Chat

Hi folks,
I've just put WinAVR on my machine (I'm not much of a C programmer) 
and tried some simple code.  Then I tried some more simple code, as 
shown here:
//***************************
#include <avr/io.h>
#include <avr/delay.h>
#include <math.h>

int main(void)
{
double trig;
DDRA=0xff;
DDRB=0xff;

	while(1)
		{
		PORTA=~PORTA;
		trig=2*M_PI;
                //trig=sin(M_PI);
		PORTB=trig;
		_delay_ms(1);
		}
return(0);
}
//***************************
That works fine.  When I simulate it in Studio, I get $06 on PortB, 
proving that M_PI is found in math.h (and if I don't include math.h, 
it gives an expected error.)
However, if I un-comment the trig=sin(M_PI) line, I get the 
error "../gcc_test.c:21: undefined reference to `sin'"

I appreciate that I'm sending a double to a port, and that might not 
be a good idea, but why is the error saying what it is?

Thanks,




Richard