AVR Freaks

I am getting the following error while building the code "undefined reference to main"
pls help me

sr i am trying to compile a simple program of lcd interfacing using Avr studio 4 and winavr.......bt sr at the time of building it it shows an error that "AVR Simulator: Invalid opcode 0xffff at address 0x001c00
"sr what should i do now........plz help me

sr my code is........

#include
#include

#define LCD_DATA PORTA // LCD data port
#define ctrl PORTB
#define en PB2 // enable signal
#define rw PB1 // read/write signal
#define rs PB0 // register select signal

void LCD_cmd(unsigned char cmd);
void init_LCD(void);
void LCD_write(unsigned char data);

int main()
{
DDRA=0xff; // making LCD_DATA port as output port
DDRB=0x07; // making signal as out put
init_LCD(); // initialization of LCD
_delay_ms(50); // delay of 50 milli seconds
LCD_write('A'); // call a function to write A on LCD
return 0;
}

void init_LCD(void)
{
LCD_cmd(0x38); // initialization of 16X2 LCD in 8bit mode
_delay_ms(1);

LCD_cmd(0x01); // clear LCD
_delay_ms(1);

LCD_cmd(0x0E); // cursor ON
_delay_ms(1);

LCD_cmd(0x80); // ---8 go to first line and --0 is for 0th position
delay_ms(1);
return;
}

void LCD_cmd(unsigned char cmd)
{
LCD_DATA=cmd;
ctrl =(0<<rs)|(0<<rw)|(1<<en); // RS and RW as LOW and EN as HIGH
_delay_ms(1);
ctrl =(0<<rs)|(0<<rw)|(0<<en); // RS, RW , LOW and EN as LOW
_delay_ms(50);
return;
}

void LCD_write(unsigned char data)
{
LCD_DATA= data;
ctrl = (1<<rs)|(0<<rw)|(1<<en); // RW as LOW and RS, EN as HIGH
_delay_ms(1);
ctrl = (1<<rs)|(0<<rw)|(0<<en); // EN and RW as LOW and RS HIGH
_delay_ms(50); // delay to get things executed
return ;
}

and my errors are.........

rm -rf bbb.o bbb.elf dep/* bbb.hex bbb.eep bbb.lss bbb.map
Build succeeded with 0 Warnings...
avr-gcc -mmcu=atmega16 -Wall -gdwarf-2 -Os -std=gnu99 -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -MD -MP -MT bbb.o -MF dep/bbb.o.d -c ../bbb.c
In file included from ../bbb.c:2:
c:/winavr-20100110/lib/gcc/../../avr/include/util/delay.h:85:3: warning: #warning "F_CPU not defined for "
avr-gcc -mmcu=atmega16 -Wl,-Map=bbb.map bbb.o -o bbb.elf
avr-objcopy -O ihex -R .eeprom -R .fuse -R .lock -R .signature bbb.elf bbb.hex
avr-objcopy -j .eeprom --set-section-flags=.eeprom="alloc,load" --change-section-lma .eeprom=0 --no-change-warnings -O ihex bbb.elf bbb.eep || exit 0
avr-objdump -h -S bbb.elf > bbb.lss
Build succeeded with 1 Warnings...

Sr what should i do to remove this error........

c:/winavr-20100110/lib/gcc/../../avr/include/util/delay.h:85:3: warning: #warning "F_CPU not defined for "
.......

sr still is a problem........bt its only a warning...and when i double click at it....a yellow colour's arrow points to my programs particular line that is "DDRA=0xff".....and the warning is

AVR Simulator: Invalid opcode 0xffff at address 0x001c00