Support broadcast destination for udp protocol

Use the required socket option SO_BROADCAST to be able to stream to a broadcast
address.

Prior to the patch, trying to stream to a broadcast address was resulting to the
following error:

av_interleaved_write_frame(): Permission denied

The patch has been tested with:

ffmpeg -f v4l2 -framerate 30 -input_format yuyv422 -video_size 640x480 -i /dev/video0 \
 -c:v libx264 -profile:v high -preset ultrafast -tune zerolatency -b:v 500k -pix_fmt yuv420p \
 -f mpegts udp://192.168.1.255:5004?broadcast=1

I have added an option to let the user explicitly request broadcast in order to avoid
ffmpeg to broadcast unintentionally.
Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/udp.c

@@ -62,6 +62,7 @@ typedef struct {
     int ttl;
     int buffer_size;
     int is_multicast;
+    int is_broadcast;
     int local_port;
     int reuse_socket;
     int overrun_nonfatal;
@@ -96,6 +97,7 @@ static const AVOption options[] = {
 {"localaddr", "choose local IP address", OFFSET(local_addr), AV_OPT_TYPE_STRING, {.str = ""}, 0, 0, D|E },
 {"pkt_size", "set size of UDP packets", OFFSET(packet_size), AV_OPT_TYPE_INT, {.i64 = 1472}, 0, INT_MAX, D|E },
 {"reuse", "explicitly allow or disallow reusing UDP sockets", OFFSET(reuse_socket), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, D|E },
+{"broadcast", "explicitly allow or disallow broadcast destination", OFFSET(is_broadcast), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, E },
 {"ttl", "set the time to live value (for multicast only)", OFFSET(ttl), AV_OPT_TYPE_INT, {.i64 = 16}, 0, INT_MAX, E },
 {"connect", "set if connect() should be called on socket", OFFSET(is_connected), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, D|E },
 /* TODO 'sources', 'block' option */
@@ -602,6 +604,8 @@ static int udp_open(URLContext *h, const char *uri, int flags)
         }
         if (!is_output && av_find_info_tag(buf, sizeof(buf), "timeout", p))
             s->timeout = strtol(buf, NULL, 10);
+        if (is_output && av_find_info_tag(buf, sizeof(buf), "broadcast", p))
+            s->is_broadcast = strtol(buf, NULL, 10);
     }
     /* handling needed to support options picking from both AVOption and URL */
     s->circular_buffer_size *= 188;
@@ -642,6 +646,11 @@ static int udp_open(URLContext *h, const char *uri, int flags)
             goto fail;
     }
 
+    if (s->is_broadcast) {
+        if (setsockopt (udp_fd, SOL_SOCKET, SO_BROADCAST, &(s->is_broadcast), sizeof(s->is_broadcast)) != 0)
+           goto fail;
+    }
+
     /* If multicast, try binding the multicast address first, to avoid
      * receiving UDP packets from other sources aimed at the same UDP
      * port. This fails on windows. This makes sending to the same address