UDP-Broadcast on all interfaces

From Jeremy's solution on UNIX Socket FAQ:

#include <stdio.h>

#ifdef WIN32
# include <windows.h>
# include <winsock.h>
# include <iphlpapi.h>
#else
# include <unistd.h>
# include <stdlib.h>
# include <sys/socket.h>
# include <netdb.h>
# include <netinet/in.h>
# include <net/if.h>
# include <sys/ioctl.h>
#endif

#include <string.h>
#include <sys/stat.h>

typedef unsigned long uint32;

#if defined(__FreeBSD__) || defined(BSD) || defined(__APPLE__) || defined(__linux__)
# define USE_GETIFADDRS 1
# include <ifaddrs.h>
static uint32 SockAddrToUint32(struct sockaddr * a)
{
   return ((a)&&(a->sa_family == AF_INET)) ? ntohl(((struct sockaddr_in *)a)->sin_addr.s_addr) : 0;
}
#endif

// convert a numeric IP address into its string representation
static void Inet_NtoA(uint32 addr, char * ipbuf)
{
   sprintf(ipbuf, "%li.%li.%li.%li", (addr>>24)&0xFF, (addr>>16)&0xFF, (addr>>8)&0xFF, (addr>>0)&0xFF);
}

// convert a string represenation of an IP address into its numeric equivalent
static uint32 Inet_AtoN(const char * buf)
{
   // net_server inexplicably doesn't have this function; so I'll just fake it
   uint32 ret = 0;
   int shift = 24;  // fill out the MSB first
   bool startQuad = true;
   while((shift >= 0)&&(*buf))
   {
      if (startQuad)
      {
         unsigned char quad = (unsigned char) atoi(buf);
         ret |= (((uint32)quad) << shift);
         shift -= 8;
      }
      startQuad = (*buf == '.');
      buf++;
   }
   return ret;
}

static void PrintNetworkInterfaceInfos()
{
#if defined(USE_GETIFADDRS)
   // BSD-style implementation
   struct ifaddrs * ifap;
   if (getifaddrs(&ifap) == 0)
   {
      struct ifaddrs * p = ifap;
      while(p)
      {
         uint32 ifaAddr  = SockAddrToUint32(p->ifa_addr);
         uint32 maskAddr = SockAddrToUint32(p->ifa_netmask);
         uint32 dstAddr  = SockAddrToUint32(p->ifa_dstaddr);
         if (ifaAddr > 0)
         {
            char ifaAddrStr[32];  Inet_NtoA(ifaAddr,  ifaAddrStr);
            char maskAddrStr[32]; Inet_NtoA(maskAddr, maskAddrStr);
            char dstAddrStr[32];  Inet_NtoA(dstAddr,  dstAddrStr);
            printf("  Found interface:  name=[%s] desc=[%s] address=[%s] netmask=[%s] broadcastAddr=[%s]\n", p->ifa_name, "unavailable", ifaAddrStr, maskAddrStr, dstAddrStr);
         }
         p = p->ifa_next;
      }
      freeifaddrs(ifap);
   }
#elif defined(WIN32)
   // Windows XP style implementation

   // Adapted from example code at http://msdn2.microsoft.com/en-us/library/aa365917.aspx
   // Now get Windows' IPv4 addresses table.  Once again, we gotta call GetIpAddrTable()
   // multiple times in order to deal with potential race conditions properly.
   MIB_IPADDRTABLE * ipTable = NULL;
   {
      ULONG bufLen = 0;
      for (int i=0; i<5; i++)
      {
         DWORD ipRet = GetIpAddrTable(ipTable, &bufLen, false);
         if (ipRet == ERROR_INSUFFICIENT_BUFFER)
         {
            free(ipTable);  // in case we had previously allocated it
            ipTable = (MIB_IPADDRTABLE *) malloc(bufLen);
         }
         else if (ipRet == NO_ERROR) break;
         else
         {
            free(ipTable);
            ipTable = NULL;
            break;
         }
     }
   }

   if (ipTable)
   {
      // Try to get the Adapters-info table, so we can given useful names to the IP
      // addresses we are returning.  Gotta call GetAdaptersInfo() up to 5 times to handle
      // the potential race condition between the size-query call and the get-data call.
      // I love a well-designed API :^P
      IP_ADAPTER_INFO * pAdapterInfo = NULL;
      {
         ULONG bufLen = 0;
         for (int i=0; i<5; i++)
         {
            DWORD apRet = GetAdaptersInfo(pAdapterInfo, &bufLen);
            if (apRet == ERROR_BUFFER_OVERFLOW)
            {
               free(pAdapterInfo);  // in case we had previously allocated it
               pAdapterInfo = (IP_ADAPTER_INFO *) malloc(bufLen);
            }
            else if (apRet == ERROR_SUCCESS) break;
            else
            {
               free(pAdapterInfo);
               pAdapterInfo = NULL;
               break;
            }
         }
      }

      for (DWORD i=0; i<ipTable->dwNumEntries; i++)
      {
         const MIB_IPADDRROW & row = ipTable->table[i];

         // Now lookup the appropriate adaptor-name in the pAdaptorInfos, if we can find it
         const char * name = NULL;
         const char * desc = NULL;
         if (pAdapterInfo)
         {
            IP_ADAPTER_INFO * next = pAdapterInfo;
            while((next)&&(name==NULL))
            {
               IP_ADDR_STRING * ipAddr = &next->IpAddressList;
               while(ipAddr)
               {
                  if (Inet_AtoN(ipAddr->IpAddress.String) == ntohl(row.dwAddr))
                  {
                     name = next->AdapterName;
                     desc = next->Description;
                     break;
                  }
                  ipAddr = ipAddr->Next;
               }
               next = next->Next;
            }
         }
         char buf[128];
         if (name == NULL)
         {
            sprintf(buf, "unnamed-%i", i);
            name = buf;
         }

         uint32 ipAddr  = ntohl(row.dwAddr);
         uint32 netmask = ntohl(row.dwMask);
         uint32 baddr   = ipAddr & netmask;
         if (row.dwBCastAddr) baddr |= ~netmask;

         char ifaAddrStr[32];  Inet_NtoA(ipAddr,  ifaAddrStr);
         char maskAddrStr[32]; Inet_NtoA(netmask, maskAddrStr);
         char dstAddrStr[32];  Inet_NtoA(baddr,   dstAddrStr);
         printf("  Found interface:  name=[%s] desc=[%s] address=[%s] netmask=[%s] broadcastAddr=[%s]\n", name, desc?desc:"unavailable", ifaAddrStr, maskAddrStr, dstAddrStr);
      }

      free(pAdapterInfo);
      free(ipTable);
   }
#else
   // Dunno what we're running on here!
#  error "Don't know how to implement PrintNetworkInterfaceInfos() on this OS!"
#endif
}

int main(int, char **)
{
   PrintNetworkInterfaceInfos();
   return 0;
}

First of all, you should consider broadcast obsolete, specially INADDR_BROADCAST (255.255.255.255). Your question highlights exactly one of the reasons that broadcast is unsuitable. It should die along with IPv4 (hopefully). Note that IPv6 doesn't even have a concept of broadcast (multicast is used, instead).

INADDR_BROADCAST is limited to the local link. Nowadays, it's only visible use is for DHCP auto-configuration, since at such time, the client will not know yet in what network it is connected to.

With a single sendto(), only a single packet is generated, and the outgoing interface is determined by the operating system's routing table (ip route on linux). You can't have a single sendto() generate more than one packet, you would have to iterate over all interfaces, and either use raw sockets or bind the socket to a device using setsockopt(..., SOL_SOCKET, SO_BINDTODEVICE, "ethX") to send each packet bypassing the OS routing table (this requires root privileges). Not a good solution.

Instead, since INADDR_BROADCAST is not routed anyway, you can achieve almost the same thing by iterating over each interface, and sending the packet to its broadcast address. For example, assuming that your networks have 255.255.255.0 (/24) masks, the broadcast addresses are 192.168.1.255 and 192.168.2.255. Call sendto() once for each of these addresses and you will have accomplished your goal.

---

Edit: fixed information regarding to INADDR_BROADCAST, and complementing the answer with information about SO_BINDTODEVICE.

You can't have a single sendto() generate a packet on every interface - in general (fragmentation notwithstanding) it's one packet transmitted for each sendto().

You'll need to transmit the packet once for each interface and either:

1. use low-level (setsockopt()?) calls to select the outbound interface

2. send to the specific broadcast address for each known interface

the latter is however not suitable if you're trying to do some sort of discovery mechanism, such that the devices you're expecting to respond aren't actually correctly configured with an IP address in the same subnet as the interface they're connected to.