The following code gives ENETUNREACH on RedHat 6.1 but works on other
versions of Linux:

#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define SOCKET_ERROR -1
#define closesocket close
typedef int SOCKET;
typedef struct sockaddr SOCKADDR;
typedef struct sockaddr_in SOCKADDR_IN;
#define FALSE 0
#define TRUE 1

int sendbroadcast(int sendport, const char* sendbuf, int sendbytes)
{
  SOCKET s;

  if ((s = socket(PF_INET, SOCK_DGRAM, 0)) == SOCKET_ERROR)
    return errno;

  int err = 0;

  int optval = TRUE;
  if (setsockopt(s, SOL_SOCKET, SO_BROADCAST, reinterpret_cast<const char*>
(&optval), sizeof(optval)) == SOCKET_ERROR)
    err = errno;
  else
  {
    SOCKADDR_IN sockAddr;
    memset(&sockAddr, 0, sizeof(sockAddr));

    sockAddr.sin_family = AF_INET;
    sockAddr.sin_addr.s_addr = htonl(INADDR_BROADCAST);
    sockAddr.sin_port = htons(sendport);

    int flags = 0;
    int bytes = sendto(s, sendbuf, sendbytes, flags,
reinterpret_cast<SOCKADDR*>(&sockAddr), sizeof(sockAddr));

    if (bytes == SOCKET_ERROR)
      err = errno;
  }
  closesocket(s);

  return err;
}