Finished adding a '-w' option.

src/modules/mavlink/mavlink_main.cpp

@@ -167,12 +167,12 @@ mavlink_send_uart_bytes(mavlink_channel_t channel, const uint8_t *ch, int length
 	int buf_free = 0;
 
 	if (instance->get_flow_control_enabled()
-	    && ioctl(uart, FIONWRITE, (unsigned long)&buf_free) == 0) {
+		&& ioctl(uart, FIONWRITE, (unsigned long)&buf_free) == 0) {
 
 		if (buf_free == 0) {
 
 			if (last_write_times[(unsigned)channel] != 0 &&
-			    hrt_elapsed_time(&last_write_times[(unsigned)channel]) > 500 * 1000UL) {
+				hrt_elapsed_time(&last_write_times[(unsigned)channel]) > 500 * 1000UL) {
 
 				warnx("DISABLING HARDWARE FLOW CONTROL");
 				instance->enable_flow_control(false);
@@ -186,12 +186,17 @@ mavlink_send_uart_bytes(mavlink_channel_t channel, const uint8_t *ch, int length
 		}
 	}
 
-	ssize_t ret = write(uart, ch, desired);
-
-	if (ret != desired) {
-		// XXX do something here, but change to using FIONWRITE and OS buf size for detection
+	/* If the wait until transmit flag is on, only transmit after we've received messages.
+	   Otherwise, transmit all the time. */
+	if (instance->should_transmit()) {
+	   ssize_t ret = write(uart, ch, desired);
+		if (ret != desired) {
+			// XXX do something here, but change to using FIONWRITE and OS buf size for detection
+		}
 	}
 
+
+
 }
 
 static void usage(void);
@@ -204,6 +209,8 @@ Mavlink::Mavlink() :
 	_task_running(false),
 	_hil_enabled(false),
 	_is_usb_uart(false),
+	_wait_to_transmit(false),
+	_received_messages(false),
 	_main_loop_delay(1000),
 	_subscriptions(nullptr),
 	_streams(nullptr),
@@ -1768,7 +1775,7 @@ Mavlink::task_main(int argc, char *argv[])
 	 * set error flag instead */
 	bool err_flag = false;
 
-	while ((ch = getopt(argc, argv, "b:r:d:m:fpv")) != EOF) {
+	while ((ch = getopt(argc, argv, "b:r:d:m:fpvw")) != EOF) {
 		switch (ch) {
 		case 'b':
 			_baudrate = strtoul(optarg, NULL, 10);
@@ -1820,6 +1827,10 @@ Mavlink::task_main(int argc, char *argv[])
 			_verbose = true;
 			break;
 
+		case 'w':
+			_wait_to_transmit = true;
+			break;
+
 		default:
 			err_flag = true;
 			break;
@@ -2164,11 +2175,11 @@ Mavlink::start(int argc, char *argv[])
 	// task - start_helper() only returns
 	// when the started task exits.
 	task_spawn_cmd(buf,
-		       SCHED_DEFAULT,
-		       SCHED_PRIORITY_DEFAULT,
-		       2048,
-		       (main_t)&Mavlink::start_helper,
-		       (const char **)argv);
+			   SCHED_DEFAULT,
+			   SCHED_PRIORITY_DEFAULT,
+			   2048,
+			   (main_t)&Mavlink::start_helper,
+			   (const char **)argv);
 
 	// Ensure that this shell command
 	// does not return before the instance
@@ -2264,7 +2275,7 @@ Mavlink::stream(int argc, char *argv[])
 
 static void usage()
 {
-	warnx("usage: mavlink {start|stop-all|stream} [-d device] [-b baudrate] [-r rate] [-m mode] [-s stream] [-f] [-p] [-v]");
+	warnx("usage: mavlink {start|stop-all|stream} [-d device] [-b baudrate] [-r rate] [-m mode] [-s stream] [-f] [-p] [-v] [-w]");
 }
 
 int mavlink_main(int argc, char *argv[])

src/modules/mavlink/mavlink_main.h

@@ -202,6 +202,14 @@ public:
 
 	int get_mavlink_fd() { return _mavlink_fd; }
 
+
+    /* Functions for waiting to start transmission until message received. */
+    void set_has_received_messages(bool received_messages) { _received_messages = received_messages; }
+    bool get_has_received_messages() { return _received_messages; }
+    void set_wait_to_transmit(bool wait) { _wait_to_transmit = wait; }
+    bool get_wait_to_transmit() { return _wait_to_transmit; }
+    bool should_transmit() { return (!_wait_to_transmit || (_wait_to_transmit && _received_messages)); }
+
 protected:
 	Mavlink	*next;
 
@@ -216,6 +224,8 @@ private:
 	/* states */
 	bool		_hil_enabled;		/**< Hardware In the Loop mode */
 	bool		_is_usb_uart;		/**< Port is USB */
+    bool        _wait_to_transmit;  /**< Wait to transmit until received messages. */
+    bool        _received_messages; /**< Whether we've received valid mavlink messages. */
 
 	unsigned	_main_loop_delay;		/**< mainloop delay, depends on data rate */
 
@@ -270,6 +280,8 @@ private:
 
 	pthread_mutex_t _message_buffer_mutex;
 
+
+
 	/**
 	 * Send one parameter.
 	 *

src/modules/mavlink/mavlink_receiver.cpp

@@ -181,6 +181,10 @@ MavlinkReceiver::handle_message(mavlink_message_t *msg)
 			break;
 		}
 	}
+
+    /* If we've received a valid message, mark the flag indicating so.
+       This is used in the '-w' command-line flag. */
+    _mavlink->set_has_received_messages(true);
 }
 
 void