DBUS

D-Bus is an inter-process communication (IPC) open-source system for software applications to communicate with one another. Heavily influenced by KDE 2–3's DCOP system, D-Bus has replaced DCOP in the KDE 4 release. An implementation of D-Bus supports most POSIX operating systems, and a port for Windows exists. It is used by Qt 4 and GNOME. In GNOME it has gradually replaced most parts of the earlier Bonobo mechanism.
Red Hat operates as the primary developer of D-Bus as part of the freedesktop.org project.

 Example

#define DBUS_API_SUBJECT_TO_CHANGE
#include <dbus/dbus.h>
#include <stdbool.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

/**
 * Connect to the DBUS bus and send a broadcast signal
 */
void sendsignal(char* sigvalue)
{
   DBusMessage* msg;
   DBusMessageIter args;
   DBusConnection* conn;
   DBusError err;
   int ret;
   dbus_uint32_t serial = 0;

   printf("Sending signal with value %s\n", sigvalue);

   // initialise the error value
   dbus_error_init(&err);

   // connect to the DBUS 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);
   }

   // register our name on the bus, and check for errors
   ret = dbus_bus_request_name(conn, "test.signal.source", DBUS_NAME_FLAG_REPLACE_EXISTING , &err);
   if (dbus_error_is_set(&err)) {
      fprintf(stderr, "Name Error (%s)\n", err.message);
      dbus_error_free(&err);
   }
   if (DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER != ret) {
      exit(1);
   }

   // create a signal & check for errors
   msg = dbus_message_new_signal("/test/signal/Object", // object name of the signal
                                 "test.signal.Type", // interface name of the signal
                                 "Test"); // name of the signal
   if (NULL == msg)
   {
      fprintf(stderr, "Message Null\n");
      exit(1);
   }

   // append arguments onto signal
   dbus_message_iter_init_append(msg, &args);
   if (!dbus_message_iter_append_basic(&args, DBUS_TYPE_STRING, &sigvalue)) {
      fprintf(stderr, "Out Of Memory!\n");
      exit(1);
   }

   // send the message and flush the connection
   if (!dbus_connection_send(conn, msg, &serial)) {
      fprintf(stderr, "Out Of Memory!\n");
      exit(1);
   }
   dbus_connection_flush(conn);
  
   printf("Signal Sent\n");
  
   // free the message and close the connection
   dbus_message_unref(msg);
   dbus_connection_close(conn);
}

/**
 * Call a method on a remote object
 */
void query(char* param)
{
   DBusMessage* msg;
   DBusMessageIter args;
   DBusConnection* conn;
   DBusError err;
   DBusPendingCall* pending;
   int ret;
   bool stat;
   dbus_uint32_t level;

   printf("Calling remote method with %s\n", param);

   // initialiset the errors
   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);
   }

   // request our name on the bus
   ret = dbus_bus_request_name(conn, "test.method.caller", DBUS_NAME_FLAG_REPLACE_EXISTING , &err);
   if (dbus_error_is_set(&err)) {
      fprintf(stderr, "Name Error (%s)\n", err.message);
      dbus_error_free(&err);
   }
   if (DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER != ret) {
      exit(1);
   }

   // create a new method call and check for errors
   msg = dbus_message_new_method_call("test.method.server", // target for the method call
                                      "/test/method/Object", // object to call on
                                      "test.method.Type", // interface to call on
                                      "Method"); // method name
   if (NULL == msg) {
      fprintf(stderr, "Message Null\n");
      exit(1);
   }

   // append arguments
   dbus_message_iter_init_append(msg, &args);
   if (!dbus_message_iter_append_basic(&args, DBUS_TYPE_STRING, &param)) {
      fprintf(stderr, "Out Of Memory!\n");
      exit(1);
   }
  
   // send message and get a handle for a reply
   if (!dbus_connection_send_with_reply (conn, msg, &pending, -1)) { // -1 is default timeout
      fprintf(stderr, "Out Of Memory!\n");
      exit(1);
   }
   if (NULL == pending) {
      fprintf(stderr, "Pending Call Null\n");
      exit(1);
   }
   dbus_connection_flush(conn);
  
   printf("Request Sent\n");
  
   // free message
   dbus_message_unref(msg);
  
   // block until we recieve a reply
   dbus_pending_call_block(pending);

   // get the reply message
   msg = dbus_pending_call_steal_reply(pending);
   if (NULL == msg) {
      fprintf(stderr, "Reply Null\n");
      exit(1);
   }
   // free the pending message handle
   dbus_pending_call_unref(pending);

   // read the parameters
   if (!dbus_message_iter_init(msg, &args))
      fprintf(stderr, "Message has no arguments!\n");
   else if (DBUS_TYPE_BOOLEAN != dbus_message_iter_get_arg_type(&args))
      fprintf(stderr, "Argument is not boolean!\n");
   else
      dbus_message_iter_get_basic(&args, &stat);

   if (!dbus_message_iter_next(&args))
      fprintf(stderr, "Message has too few arguments!\n");
   else if (DBUS_TYPE_UINT32 != dbus_message_iter_get_arg_type(&args))
      fprintf(stderr, "Argument is not int!\n");
   else
      dbus_message_iter_get_basic(&args, &level);

   printf("Got Reply: %d, %d\n", stat, level);
  
   // free reply and close connection
   dbus_message_unref(msg);  
   dbus_connection_close(conn);
}

void reply_to_method_call(DBusMessage* msg, DBusConnection* conn)
{
   DBusMessage* reply;
   DBusMessageIter args;
   bool stat = true;
   dbus_uint32_t level = 21614;
   dbus_uint32_t serial = 0;
   char* param = "";

   // read the arguments
   if (!dbus_message_iter_init(msg, &args))
      fprintf(stderr, "Message has no arguments!\n");
   else if (DBUS_TYPE_STRING != dbus_message_iter_get_arg_type(&args))
      fprintf(stderr, "Argument is not string!\n");
   else
      dbus_message_iter_get_basic(&args, &param);

   printf("Method called with %s\n", param);

   // create a reply from the message
   reply = dbus_message_new_method_return(msg);

   // add the arguments to the reply
   dbus_message_iter_init_append(reply, &args);
   if (!dbus_message_iter_append_basic(&args, DBUS_TYPE_BOOLEAN, &stat)) {
      fprintf(stderr, "Out Of Memory!\n");
      exit(1);
   }
   if (!dbus_message_iter_append_basic(&args, DBUS_TYPE_UINT32, &level)) {
      fprintf(stderr, "Out Of Memory!\n");
      exit(1);
   }

   // send the reply && flush the connection
   if (!dbus_connection_send(conn, reply, &serial)) {
      fprintf(stderr, "Out Of Memory!\n");
      exit(1);
   }
   dbus_connection_flush(conn);

   // free the reply
   dbus_message_unref(reply);
}

/**
 * Server that exposes a method call and waits for it to be called
 */
void listen()
{
   DBusMessage* msg;
   DBusMessage* reply;
   DBusMessageIter args;
   DBusConnection* conn;
   DBusError err;
   int ret;
   char* param;

   printf("Listening for method calls\n");

   // initialise the error
   dbus_error_init(&err);
  
   // connect to the 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) {
      fprintf(stderr, "Connection Null\n");
      exit(1);
   }
  
   // request our name on the bus and check for errors
   ret = dbus_bus_request_name(conn, "test.method.server", DBUS_NAME_FLAG_REPLACE_EXISTING , &err);
   if (dbus_error_is_set(&err)) {
      fprintf(stderr, "Name Error (%s)\n", err.message);
      dbus_error_free(&err);
   }
   if (DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER != ret) {
      fprintf(stderr, "Not Primary Owner (%d)\n", ret);
      exit(1);
   }

   // loop, testing for new messages
   while (true) {
      // non blocking read of the next available message
      dbus_connection_read_write(conn, 0);
      msg = dbus_connection_pop_message(conn);

      // loop again if we haven't got a message
      if (NULL == msg) {
         sleep(1);
         continue;
      }
     
      // check this is a method call for the right interface & method
      if (dbus_message_is_method_call(msg, "test.method.Type", "Method"))
         reply_to_method_call(msg, conn);

      // free the message
      dbus_message_unref(msg);
   }

   // close the connection
   dbus_connection_close(conn);
}

/**
 * Listens for signals on the bus
 */
void receive()
{
   DBusMessage* msg;
   DBusMessageIter args;
   DBusConnection* conn;
   DBusError err;
   int ret;
   char* sigvalue;

   printf("Listening for signals\n");

   // initialise the errors
   dbus_error_init(&err);
  
   // connect to the 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);
   }
  
   // request our name on the bus and check for errors
   ret = dbus_bus_request_name(conn, "test.signal.sink", DBUS_NAME_FLAG_REPLACE_EXISTING , &err);
   if (dbus_error_is_set(&err)) {
      fprintf(stderr, "Name Error (%s)\n", err.message);
      dbus_error_free(&err);
   }
   if (DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER != ret) {
      exit(1);
   }

   // add a rule for which messages we want to see
   dbus_bus_add_match(conn, "type='signal',interface='test.signal.Type'", &err); // see signals from the given interface
   dbus_connection_flush(conn);
   if (dbus_error_is_set(&err)) {
      fprintf(stderr, "Match Error (%s)\n", err.message);
      exit(1);
   }
   printf("Match rule sent\n");

   // loop listening for signals being emmitted
   while (true) {

      // non blocking read of the next available message
      dbus_connection_read_write(conn, 0);
      msg = dbus_connection_pop_message(conn);

      // loop again if we haven't read a message
      if (NULL == msg) {
         sleep(1);
         continue;
      }

      // check if the message is a signal from the correct interface and with the correct name
      if (dbus_message_is_signal(msg, "test.signal.Type", "Test")) {
        
         // read the parameters
         if (!dbus_message_iter_init(msg, &args))
            fprintf(stderr, "Message Has No Parameters\n");
         else if (DBUS_TYPE_STRING != dbus_message_iter_get_arg_type(&args))
            fprintf(stderr, "Argument is not string!\n");
         else
            dbus_message_iter_get_basic(&args, &sigvalue);
        
         printf("Got Signal with value %s\n", sigvalue);
      }

      // free the message
      dbus_message_unref(msg);
   }
   // close the connection
   dbus_connection_close(conn);
}

int main(int argc, char** argv)
{
   if (2 > argc) {
      printf ("Syntax: dbus-example [send|receive|listen|query] [<param>]\n");
      return 1;
   }
   char* param = "no param";
   if (3 >= argc && NULL != argv[2]) param = argv[2];
   if (0 == strcmp(argv[1], "send"))
      sendsignal(param);
   else if (0 == strcmp(argv[1], "receive"))
      receive();
   else if (0 == strcmp(argv[1], "listen"))
      listen();
   else if (0 == strcmp(argv[1], "query"))
      query(param);
   else {
      printf ("Syntax: dbus-example [send|receive|listen|query] [<param>]\n");
      return 1;
   }
   return 0;
}





If you get following Error


Listening for method calls
Name Error (Connection ":1.48" is not allowed to own the service "test.method.server" due to security policies in the configuration file)
Not Primary Owner (-1)





How to solve.

i have gone thru the "/etc/dbus-1/system.conf" and found these lines
<!-- Config files are placed here that among other things, punch
holes in the above policy for specific services. -->
<includedir>system.d</includedir>

<!-- This is included last so local configuration can override what's
in this standard file -->
<include ignore_missing="yes">system-local.conf</include>

::remove this line from config file
<include ignore_missing="yes">system-local.conf</include>



Then create
/etc/dbus-1/system.d/system-local.conf

<!-- This configuration file specifies the required security policies
     for Bluetooth core daemon to work. -->

<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>

  <!-- ../system.conf have denied everything, so we just punch some holes -->

<policy context="default">
<!-- Deny everything then punch holes -->
<allow send_interface="*"/>
<allow receive_interface="*"/>
<allow own="*"/>
<!-- But allow all users to connect -->
<allow user="*"/>
<!-- Allow anyone to talk to the message bus -->
<!-- FIXME I think currently these allow rules are always implicit
even if they aren't in here -->
<allow send_destination="org.freedesktop.DBus"/>
<allow receive_sender="org.freedesktop.DBus"/>
<!-- valid replies are always allowed -->
<allow send_requested_reply="true"/>
<allow receive_requested_reply="true"/>
</policy>

</busconfig>


Now should not receive above error :-D