LinuxQuestions.org - sendto failed -1 : Invalid argument

I'm not sure why the sndto() is failing. I am a newbie so any pointers would be great!

STATUS send_request(const packet_t * pkt)
{

int fd = -1; /* socket filedescriptor */
int result = 0; /* sendto status */
struct sockaddr_in sa; /* structure for handling internet addressing */

/* create UDP socket PF_PACKET for post 2.0 kernel */
fd = socket(PF_PACKET, SOCK_DGRAM, IPPROTO_UDP);
if (-1 == fd)
{
/* error case */
printf ("%s:%d socket failed %s", __FUNCTION__, __LINE__, strerror(errno) );
return FAILED;
}

/* setup sockaddr_in struct */
memset((char *) &sa, 0, sizeof(sa));
sa.sin_family = AF_INET;
sa.sin_port = htons(SEND_PORT);
/* if (0 == inet_aton(SVR_IP, &sa.sin_addr))
{
// error case
printf ("%s:%d inet_aton failed", __FUNCTION__, __LINE__ );
if (-1 == (close(fd)))
{
printf ("%s:%d close failed %s", __FUNCTION__, __LINE__, strerror(errno) );
}
return FAILED;
}
*/

/* send packet */
printf ("%s:%d sendto fd:%d sizeof(packet_t):%d sizeof(pkt):%d", __FUNCTION__, __LINE__,fd, sizeof(packet_t), sizeof(pkt));

result = sendto(fd, pkt, sizeof(packet_t), 0, (struct sockaddr *)&sa, sizeof(sa));
if(-1 == result)
{
/* error case */
printf ("%s:%d sendto failed %d : %s", __FUNCTION__, __LINE__,result, strerror(errno) );
if (-1 == (close(fd)))
{
printf ("%s:%d close failed %s", __FUNCTION__, __LINE__, strerror(errno) );
}
return FAILED;
}
else if (sizeof(packet_t) == result)
{
/* success case */
printf ("%s:%d sendto successful", __FUNCTION__, __LINE__ );
if (-1 == (close(fd)))
{
printf ("%s:%d close failed %s", __FUNCTION__, __LINE__, strerror(errno) );
return FAILED;
}
}
else
{
/* catch all */
printf ("%s:%d sendto %d : %s", __FUNCTION__, __LINE__,result, strerror(errno) );
if (-1 == (close(fd)))
{
printf ("%s:%d close failed %s", __FUNCTION__, __LINE__, strerror(errno) );
}
return FAILED;
}

return SUCCESS;
}