socket_broadcast man page
socket_broadcast — set UDP socket to broadcast mode
int socket_broadcast(int s);
socket_broadcast sets UDP socket s to broadcast mode. socket_send4 and socket_send6 will fail to send packets to the broadcast address unless socket_broadcast is called before.
Normally socket_broadcast returns 0.
If anything goes wrong, socket_broadcast returns -1, setting errno appropriately.
strerr_die2sys(111,FATAL,"unable to create UDP socket: ");
if (socket_broadcast(s) == -1)
strerr_die2sys(111,FATAL,"unable to set broadcast mode: ");