I'm trying to send out a broadcast packet. I'm getting this error and i've been bangking my head on this for a while now. The following code below compiles but when I run it I get this error. 192.168.0.255 sendto:: Invalid argument As you can see the dst contains the correct information but sendto function keeps giving me the Invalid argument. All the arguments are valid and correct it seems (ie, no null). Thanks for your help. Hopefully this makes sense.

#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <string.h>
#include <stdio.h>

int main()
{
  int s, on = 1;
  struct sockaddr_in sin;
  struct sockaddr_in dst;
  struct ifconf ifc;
  struct ifreq *ifr;
  char buf[2048];
  char buf2[] = "HELLO MOTHER";
  int n;

  s = socket(AF_INET, SOCK_DGRAM, 0);
  if(s < 0)
    perror("socket:");

  if(setsockopt(s, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on)) < 0)
    perror("setsockopt:");

  sin.sin_family = AF_INET;
  sin.sin_addr.s_addr = htonl(INADDR_ANY);
  sin.sin_port = htons(2190);

  if(bind(s, (struct sockaddr *)&sin, sizeof(sin)) < 0)
    perror("bind:");

  ifc.ifc_len = sizeof(buf);
  ifc.ifc_buf = buf;
  if(ioctl(s, SIOCGIFCONF, (char *)&ifc) < 0)
    perror("ioctl-conf:");

  ifr = ifc.ifc_req;
  for (n=ifc.ifc_len/sizeof(struct ifreq); --n >= 0; ifr++)
  {
    if(!strcmp(ifr->ifr_name, "eth0"))
    {
      if(ioctl(s, SIOCGIFFLAGS, (char *)ifr) < 0)
        perror("ioctl-flag:");
      break;
    }
  }

  if (ifr->ifr_flags & IFF_BROADCAST)
  {
    if(ioctl(s, SIOCGIFBRDADDR, (char *) ifr) < 0)
      perror("ioctl-addr:");
    memcpy((char *) &dst, (char *) &ifr->ifr_broadaddr, sizeof(ifr->ifr_broadaddr));
  } else
    printf("Not able to broadcast");

  printf("%s\n", inet_ntoa(dst.sin_addr));

  if(sendto(s, buf2, strlen(buf2), 0, (struct sockaddr *)&dst, sizeof(dst)) < 0)
    perror("sendto:");

  return 0;
} 

You forgot to set dst.sin_port. In a debug build it would be 0x00, so it would be an invalid argument.

Have u specified "dst"'s port?
In terms of UDP broadcast, u need specify a port number in order to send datagram.
And, UDP broadcast has a special broadcasting address: 255.255.255.255. That define is INADDR_BROADCAST, u haven't refered in ur program.

Here is a simple UDP broadcast code as follows.

SOCKET s;
BOOL bBroadcast;
char* sMsg = "I love u";
SOCKADDR_IN dst;

s = socket(AF_INET, SOCK_DGRAM, 0);
bBroadcast = TRUE;
setsockopt(s, SOL_SOCKET, SO_BROADCAST, (char*)&bBroadcast, sizeof(BOOL));
dst.sin_family = AF_INET;
dst.sin_adddr.s_addr = inet_addr(INADDR_BROADCAST);
dst.sin_port = htons(8000);
sendto(s, sMsg, strlen(sMsg), 0, (SOCKADDR*)&dst, sizeof(dst));

In fact, I myself don't like Broadcast, if concurrently a lot of processes send broadcast message, the network immediately slows to a standstill and everyone gets frustrated.