Calling methods and receiving signals using low-level APIs - api

Question

I am trying to call the method ReadLocalBdAddrReq and receive its signal ReadLocalBdAddrCfm on dbus using the dbus low level APIs.
I have written the following code with the help of some forum posts and a dbus tutorial.
The thing is, I am not able to receive the signals back. The code is incomplete at some places as I didn't know what should be done.
So please help me so that I can receive the signals for the methods called.
Here the code I have written. Please correct any mistakes I've made.
#include <stdlib.h>
#include <stdio.h>
#include <dbus/dbus.h>
#define OBJ_PATH "/bt/cm"
static dbus_bool_t add_watch(DBusWatch *watch, void *data)
{
if (!dbus_watch_get_enabled(watch))
return TRUE;
int fd = dbus_watch_get_unix_fd(watch);
unsigned int flags = dbus_watch_get_flags(watch);
int f = 0;;
if (flags & DBUS_WATCH_READABLE) {
f |= DBUS_WATCH_READABLE;
printf("Readable\n");
}
if (flags & DBUS_WATCH_WRITABLE) {
printf("Writeable\n");
f |= DBUS_WATCH_WRITABLE;
}
/* this should not be here */
if (dbus_watch_handle(watch, f) == FALSE)
printf("dbus_watch_handle() failed\n");
return TRUE;
}
static void remove_watch(DBusWatch *watch, void *data)
{
printf("In remove watch with fd = [%d]\n",dbus_watch_get_unix_fd(watch));
}
static void toggel_watch(DBusWatch *watch, void *data)
{
printf("In toggel watch\n");
/*
if (dbus_watch_get_enabled(watch))
add_watch(watch, data);
else
remove_watch(watch, data);
*/
}
/* timeout functions */
static dbus_bool_t add_time(DBusTimeout *timeout, void *data)
{
/* Incomplete */
printf("In add_time\n");
if (!dbus_timeout_get_enabled(timeout))
return TRUE;
//dbus_timeout_handle(timeout);
return 0;
}
static void remove_time(DBusTimeout *timeout, void *data)
{
/* Incomplete */
printf("In remove_time\n");
}
static void toggel_time(DBusTimeout *timeout, void *data)
{
/* Incomplete */
printf("In toggel_time\n");
/*
if (dbus_timeout_get_enabled(timeout))
add_timeout(timeout, data);
else
remove_timeout(timeout, data);
*/
}
/* message filter -- handlers to run on all incoming messages*/
static DBusHandlerResult filter (DBusConnection *connection, DBusMessage *message, void *user_data)
{
printf("In filter\n");
char *deviceaddr;
if (dbus_message_is_signal(message, "com.bluegiga.v2.bt.cm", "ReadLocalBdAddrCfm")) {
printf("Signal received is ReadLocalBdAddrCfm\n");
if ((dbus_message_get_args(message,NULL,DBUS_TYPE_STRING, &deviceaddr,DBUS_TYPE_INVALID) == FALSE))
{
printf("Could not get the arguments from the message received\n");
return -2;
}
printf("Got Signal and device address is [%s]\n", deviceaddr);
}
return 0;
}
/* dispatch function-- simply save an indication that messages should be dispatched later, when the main loop is re-entered*/
static void dispatch_status(DBusConnection *connection, DBusDispatchStatus new_status, void *data)
{
printf("In dispatch_status\n");
if (new_status == DBUS_DISPATCH_DATA_REMAINS)
{
printf("new dbus dispatch status: DBUS_DISPATCH_DATA_REMAINS [%d]",new_status);
}
}
/* unregister function */
void unregister_func(DBusConnection *connection, void *user_data)
{
}
/* message function - Called when a message is sent to a registered object path. */
static DBusHandlerResult message_func(DBusConnection *connection, DBusMessage *message, void *data)
{
printf("Message [%s] is sent to [%s] from interface [%s] on path [%s] \n",dbus_message_get_member(message),dbus_message_get_destination(message),
dbus_message_get_interface(message),dbus_message_get_path(message)); return 0;
}
DBusObjectPathVTable table = {
.unregister_function = unregister_func,
.message_function = message_func,
};
int main(void) {
DBusMessage* msg;
DBusMessageIter args;
DBusConnection* conn;
DBusError err;
DBusPendingCall* pending;
int ret;
//unsigned int level;
char* appHandle = NULL;
//int *context;
int msg_serial;
int open;
char *deviceaddr;
dbus_error_init(&err);
// connect to the system bus and check for errors
conn = dbus_bus_get(DBUS_BUS_SYSTEM, &err);
if (dbus_error_is_set(&err)) {
fprintf(stderr, "Connection Error (%s)\n", err.message);
dbus_error_free(&err);
}
if (NULL == conn) {
exit(1);
}
if (!dbus_connection_set_watch_functions(conn, add_watch, remove_watch, toggel_watch, NULL, NULL))
{
printf("Error in dbus_set_watch_functions\n");
dbus_connection_unref(conn);
return -1;
}
/* These functions are responsible for making the application's main loop aware of timeouts */
if (!dbus_connection_set_timeout_functions(conn, add_time, remove_time, toggel_time, NULL, NULL))
{
printf("Error in dbus_set_timeout_functions\n");
dbus_connection_unref(conn);
return -1;
}
/* Used to register the handler functions run on incoming messages*/
if (!dbus_connection_add_filter(conn, filter, NULL, NULL))
{
printf("Error in adding filter\n");
dbus_connection_unref(conn);
return -1;
}
/* Filter added for incoming messages */
/* Set a function to be invoked when the dispatch status changes */
dbus_connection_set_dispatch_status_function(conn, dispatch_status, NULL ,NULL);
/* Register a handler for messages sent to a given path */
if(!dbus_connection_register_object_path(conn, OBJ_PATH, &table, NULL))
{
printf("Error in registering object\n");
return -1;
}
/* sending messages to the outgoing queue */
msg = dbus_message_new_method_call("com.bluegiga.v2.bt.cm", // target for the method call
OBJ_PATH, // object to call on
"com.bluegiga.v2.bt.cm", // interface to call on
"ReadLocalBdAddrReq"); // method name
if (NULL == msg) {
fprintf(stderr, "Message Null\n");
exit(1);
}
dbus_message_iter_init_append(msg, &args);
if (!dbus_message_iter_append_basic(&args, DBUS_TYPE_UINT16,&appHandle)) {
fprintf(stderr, "Out Of Memory!\n");
exit(1);
}
fprintf(stderr, "Sending the connections\n");
// send message and get a handle for a reply
if (!dbus_connection_send (conn, msg, &msg_serial)) {
fprintf(stderr, "Out Of Memory!\n");
exit(1);
}
fprintf(stderr, "Connection sent and the msg serial is %d\n",msg_serial);
/* Message sent over */
/* not sure whether this should be here or above watch */
while (dbus_connection_get_dispatch_status(conn) == DBUS_DISPATCH_DATA_REMAINS)
{
//printf("Entered in dispatch\n");
/* Processes any incoming data. will call the filters registered by add_filer*/
dbus_connection_dispatch(conn);
}
return 0;
}
After I run this program it has the following output:
Readable
Sending the connections
Connection sent and the msg serial is
2(DBUS_MESSAGE_TYPE_METHOD_RETURN)
If the connection was sent to the object path then message_func should have been called correctly, but it never is called. Have I made any mistake in sending the method call?

Answer 1

You are missing the event loop which is otherwise available by default if you choose to go with one of the bindings. When you get a call to add_watch, libdbus expects that the application will attach an IO handler to it. The IOHandler added by application will watch for an activity on the fd (filedescriptor) queried for the watch. Whenever there is an activity on that file descriptor, the IOHandler will trigger a callback with appropriate flags that you need to convert to DBUS flags before calling dbus_watch_handle.
Suggest that you use glib if you don't know how to use event loops. I am able to get it if I use libUV or libEV as low footprint event loop.