mavlink: adapt network detection for Mac, use heap

It seems that Mac does not support the ioctl to check how big the ifconf
buffer needs to be. Therefore we just have to make a guess.
Alos, instead of allocating the variable size array on the stack, it's
probably safer to use the heap.

src/modules/mavlink/mavlink_main.cpp

@@ -998,35 +998,47 @@ Mavlink::init_udp()
 	}
 	_src_addr.sin_port = htons(_remote_port);
 
-	// Find out the required size of the buffer first.
-	// We can get the number by providing NULL to ifc_req.
 	struct ifconf ifconf;
+	int ret;
+
+#if defined(__APPLE__) && defined(__MACH__)
+	// On Mac, we can't determine the required buffer
+	// size in advance, so we just use what tends to work.
+	ifconf.ifc_len = 1024;
+#else
+	// On Linux, we can determine the required size of the
+	// buffer first by providing NULL to ifc_req.
 	ifconf.ifc_req = NULL;
 	ifconf.ifc_len = 0;
 
-	int ret = ioctl(_socket_fd, SIOCGIFCONF, &ifconf);
+	ret = ioctl(_socket_fd, SIOCGIFCONF, &ifconf);
 	if (ret != 0) {
 		PX4_WARN("getting required buffer size failed");
 		return;
 	}
+#endif
 
 	PX4_DEBUG("need to allocate %d bytes", ifconf.ifc_len);
 
-	// Allocate buffer as long as needed.
+	// Allocate buffer.
-	uint8_t ifreqs_buf[ifconf.ifc_len];
+	ifconf.ifc_req = (struct ifreq *)(new uint8_t[ifconf.ifc_len]);
-	memset(ifreqs_buf, 0, ifconf.ifc_len);
+	if (ifconf.ifc_req == nullptr) {
-	ifconf.ifc_req = (struct ifreq *)ifreqs_buf;
+		PX4_ERR("Could not allocate ifconf buffer");
+		return;
+	}
 
+	memset(ifconf.ifc_req, 0, ifconf.ifc_len);
 
 	ret = ioctl(_socket_fd, SIOCGIFCONF, &ifconf);
 	if (ret != 0) {
-		PX4_WARN("getting network config failed");
+		PX4_ERR("getting network config failed");
+		delete[] ifconf.ifc_req;
 		return;
 	}
 
 	size_t offset = 0;
 	// Later used to point to next network interface in buffer.
-	struct ifreq *cur_ifreq = (struct ifreq *)&ifreqs_buf[offset];
+	struct ifreq *cur_ifreq = (struct ifreq *)&(((uint8_t *)ifconf.ifc_req)[offset]);
 
 	// The ugly `for` construct is used because it allows to use
 	// `continue` and `break`.
@@ -1038,8 +1050,7 @@ Mavlink::init_udp()
 	     // sizeof sockaddr or ifr_addr.sa_len.
 	     offset += IF_NAMESIZE
 	               + (sizeof(struct sockaddr) > cur_ifreq->ifr_addr.sa_len ?
-		          sizeof(struct sockaddr) :
+		          sizeof(struct sockaddr) : cur_ifreq->ifr_addr.sa_len)
-		          cur_ifreq->ifr_addr.sa_len)
 #else
 	    // On Linux, it's much easier to traverse the buffer, every entry
 	    // has the constant length.
@@ -1048,14 +1059,7 @@ Mavlink::init_udp()
 	    ) {
 
 		// Point to next network interface in buffer.
-		cur_ifreq = (struct ifreq *)&ifreqs_buf[offset];
+		cur_ifreq = (struct ifreq *)&(((uint8_t *)ifconf.ifc_req)[offset]);
-
-#if defined(__APPLE__) && defined(__MACH__)
-		if (cur_ifreq->ifr_addr.sa_len == 0) {
-			// The current entry seems zero, we're probably through.
-			break;
-		}
-#endif
 
 		PX4_DEBUG("looking at %s", cur_ifreq->ifr_name);
 
@@ -1073,7 +1077,7 @@ Mavlink::init_udp()
 		strncpy(bc_ifreq.ifr_name, cur_ifreq->ifr_name, IF_NAMESIZE);
 		ret = ioctl(_socket_fd, SIOCGIFBRDADDR, &bc_ifreq);
 		if (ret != 0) {
-			PX4_INFO("getting broadcast address failed for %s", cur_ifreq->ifr_name);
+			PX4_DEBUG("getting broadcast address failed for %s", cur_ifreq->ifr_name);
 			continue;
 		}
 
@@ -1105,13 +1109,14 @@ Mavlink::init_udp()
 		}
 	}
 
-	if (!_broadcast_address_found) {
+	if (_broadcast_address_found) {
-		PX4_WARN("no broadcasting address found");
-		return;
-	}
-
 		_bcast_addr.sin_port = htons(_remote_port);
 
+	} else {
+		PX4_WARN("no broadcasting address found");
+	}
+
+	delete[] ifconf.ifc_req;
 #endif
 }