[dbus] The notes for DBus APIs

The previous post: [dbus] The useful information and example of D-Bus has introduced the concept and an example. This post is focused on its API.

DBusConnection:
http://dbus.freedesktop.org/doc/api/html/group__DBusConnection.html#details
The keypoint that is worth mentioning is about dealing with asynchronous message. 
The function dbus_connection_read_write_dispatch() for example does all three of these things, offering a simple alternative to a main loop. If you don't need to be asynchronous, you can ignore DBusWatch, DBusTimeout, and dbus_connection_dispatch(). Instead, dbus_connection_read_write_dispatch() can be used.

Here is an example of how to use dbus watch and dbus timeout:
http://stackoverflow.com/questions/9378593/dbuswatch-and-dbustimeout-examples

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#include <dbus/dbus.h>

struct dbus_ctx {
    DBusConnection *conn;
    struct event_base *evbase;
    struct event dispatch_ev;
    void *extra;
};

static void dispatch(int fd, short ev, void *x)
{
    struct dbus_ctx *ctx = x;
    DBusConnection *c = ctx->conn;

    logger(LOG_DEBUG "dispatching\n");

    while (dbus_connection_get_dispatch_status(c) == DBUS_DISPATCH_DATA_REMAINS)
        dbus_connection_dispatch(c);
}

static void handle_dispatch_status(DBusConnection *c,
                                   DBusDispatchStatus status, void *data)
{
    struct dbus_ctx *ctx = data;

    logger(LOG_DEBUG "new dbus dispatch status: %d\n", status);

    if (status == DBUS_DISPATCH_DATA_REMAINS) {
        struct timeval tv = {
            .tv_sec = 0,
            .tv_usec = 0,
        };
        event_add(&ctx->dispatch_ev, &tv);
    }
}

static void handle_watch(int fd, short events, void *x)
{
    struct dbus_ctx *ctx = x;
    struct DBusWatch *watch = ctx->extra;

    unsigned int flags = 0;
    if (events & EV_READ)
        flags |= DBUS_WATCH_READABLE;
    if (events & EV_WRITE)
        flags |= DBUS_WATCH_WRITABLE;
    /*if (events & HUP)
        flags |= DBUS_WATCH_HANGUP;
    if (events & ERR)
        flags |= DBUS_WATCH_ERROR;*/

    logger(LOG_DEBUG "got dbus watch event fd=%d watch=%p ev=%d\n",
           fd, watch, events);
    if (dbus_watch_handle(watch, flags) == FALSE)
        logger(LOG_ERROR "dbus_watch_handle() failed\n");

    handle_dispatch_status(ctx->conn, DBUS_DISPATCH_DATA_REMAINS, ctx);
}

static dbus_bool_t add_watch(DBusWatch *w, void *data)
{
    if (!dbus_watch_get_enabled(w))
        return TRUE;

    struct dbus_ctx *ctx = data;
    ctx->extra = w;

    int fd = dbus_watch_get_unix_fd(w);
    unsigned int flags = dbus_watch_get_flags(w);
    short cond = EV_PERSIST;
    if (flags & DBUS_WATCH_READABLE)
        cond |= EV_READ;
    if (flags & DBUS_WATCH_WRITABLE)
        cond |= EV_WRITE;

    struct event *event = event_new(ctx->evbase, fd, cond, handle_watch, ctx);
    if (!event)
        return FALSE;

    event_add(event, NULL);

    dbus_watch_set_data(w, event, NULL);

    logger(LOG_DEBUG "added dbus watch fd=%d watch=%p cond=%d\n", fd, w, cond);
    return TRUE;
}

static void remove_watch(DBusWatch *w, void *data)
{
    struct event *event = dbus_watch_get_data(w);

    if (event)
        event_free(event);

    dbus_watch_set_data(w, NULL, NULL);

    logger(LOG_DEBUG "removed dbus watch watch=%p\n", w);
}

static void toggle_watch(DBusWatch *w, void *data)
{
    logger(LOG_DEBUG "toggling dbus watch watch=%p\n", w);

    if (dbus_watch_get_enabled(w))
        add_watch(w, data);
    else
        remove_watch(w, data);
}

static void handle_timeout(int fd, short ev, void *x)
{
    struct dbus_ctx *ctx = x;
    DBusTimeout *t = ctx->extra;

    logger(LOG_DEBUG "got dbus handle timeout event %p\n", t);

    dbus_timeout_handle(t);
}

static dbus_bool_t add_timeout(DBusTimeout *t, void *data)
{
    struct dbus_ctx *ctx = data;

    if (!dbus_timeout_get_enabled(t))
        return TRUE;

    logger(LOG_DEBUG "adding timeout %p\n", t);

    struct event *event = event_new(ctx->evbase, -1, EV_TIMEOUT|EV_PERSIST,
                                    handle_timeout, t);
    if (!event) {
        logger(LOG_ERROR "failed to allocate new event for timeout\n");
        return FALSE;
    }

    int ms = dbus_timeout_get_interval(t);
    struct timeval tv = {
        .tv_sec = ms / 1000,
        .tv_usec = (ms % 1000) * 1000,
    };
    event_add(event, &tv);

    dbus_timeout_set_data(t, event, NULL);

    return TRUE;
}

static void remove_timeout(DBusTimeout *t, void *data)
{
    struct event *event = dbus_timeout_get_data(t);

    logger(LOG_DEBUG "removing timeout %p\n", t);

    event_free(event);

    dbus_timeout_set_data(t, NULL, NULL);
}

static void toggle_timeout(DBusTimeout *t, void *data)
{
    logger(LOG_DEBUG "toggling timeout %p\n", t);

    if (dbus_timeout_get_enabled(t))
        add_timeout(t, data);
    else
        remove_timeout(t, data);
}

static DBusHandlerResult handle_nameownerchanged(DBusMessage *message,
                                                 void *data)
{
    struct dbus_ctx *ctx = data;
    char *name, *old, *new;
    if (dbus_message_get_args(message, NULL,
                              DBUS_TYPE_STRING, &name,
                              DBUS_TYPE_STRING, &old,
                              DBUS_TYPE_STRING, &new,
                              DBUS_TYPE_INVALID) == FALSE) {
        logger(LOG_ERROR "spurious NameOwnerChanged signal\n");
        return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
    }
    logger(LOG_DEBUG "dbus NameOwnerChanged %s -> %s\n", old, new);

    if (new[0] != '\0')
        return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;

    /* XXX handle disconnecting clients */

    return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}

static DBusHandlerResult msg_filter(DBusConnection *connection,
                                    DBusMessage *message, void *data)
{
    if (dbus_message_is_signal(message, DBUS_INTERFACE_DBUS,
                               "NameOwnerChanged"))
        return handle_nameownerchanged(message, data);

    logger(LOG_DEBUG "got dbus message %d %s -> %s %s/%s/%s %s\n",
           dbus_message_get_type(message),
           dbus_message_get_sender(message),
           dbus_message_get_destination(message),
           dbus_message_get_path(message),
           dbus_message_get_interface(message),
           dbus_message_get_member(message),
           dbus_message_get_type(message) == DBUS_MESSAGE_TYPE_ERROR ?
           dbus_message_get_error_name(message) : "");

    return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}

static void unregister_func(DBusConnection *connection, void *data)
{
}

static DBusHandlerResult message_func(DBusConnection *connection,
                                      DBusMessage *message, void *data)
{
    struct dbus_ctx *ctx = data;

    logger(LOG_DEBUG "got dbus message sent to %s %s %s\n",
           dbus_message_get_destination(message),
           dbus_message_get_interface(message),
           dbus_message_get_path(message));

    /* XXX handle DBus message */

    return DBUS_HANDLER_RESULT_HANDLED;
}

static DBusObjectPathVTable dbus_vtable = {
    .unregister_function = unregister_func,
    .message_function = message_func,
};

struct dbus_ctx *dbus_init(struct event_base *eb)
{
    DBusConnection *conn = NULL;
    struct dbus_ctx *ctx = calloc(1, sizeof(struct dbus_ctx));
    if (!ctx) {
        logger_perror("can't allocate dbus_ctx\n");
        goto out;
    }

    conn = dbus_bus_get_private(DBUS_BUS_SESSION, NULL);
    if (conn == NULL) {
        logger(LOG_ERROR "failed to get bus\n");
        goto out;
    }

    dbus_connection_set_exit_on_disconnect(conn, FALSE);

    ctx->conn = conn;
    ctx->evbase = eb;
    event_assign(&ctx->dispatch_ev, eb, -1, EV_TIMEOUT, dispatch, ctx);

    if (!dbus_connection_set_watch_functions(conn, add_watch, remove_watch,
                                             toggle_watch, ctx, NULL)) {
        logger(LOG_ERROR "dbus_connection_set_watch_functions() failed\n");
        goto out;
    }

    if (!dbus_connection_set_timeout_functions(conn, add_timeout,
                                               remove_timeout, toggle_timeout,
                                               ctx, NULL)) {
        logger(LOG_ERROR "dbus_connection_set_timeout_functions() failed\n");
        goto out;
    }

    if (dbus_connection_add_filter(conn, msg_filter, ctx, NULL) == FALSE) {
        logger(LOG_ERROR "dbus_connection_add_filter() failed\n");
        goto out;
    }

    dbus_connection_set_dispatch_status_function(conn, handle_dispatch_status,
                                                 ctx, NULL);

    char match[256];
    snprintf(match,
             sizeof(match),
             "type='signal',interface='%s',member='NameOwnerChanged'",
             DBUS_INTERFACE_DBUS);
    DBusError error;
    dbus_error_init(&error);
    dbus_bus_add_match(conn, match, &error);
    if (dbus_error_is_set(&error)) {
        logger(LOG_ERROR "dbus_bus_add_match() %s failed: %s\n",
               "NameOwnerChanged", error.message);
        dbus_error_free(&error);
        goto out;
    }

    snprintf(match,
             sizeof(match),
             "type='signal',interface='%s',member='%s'",
             GNP_IPC_INTERFACE, GNP_IPC_SIGNAL_DELIVER_SA);
    dbus_error_init(&error);
    dbus_bus_add_match(conn, match, &error);
    if (dbus_error_is_set(&error)) {
        logger(LOG_ERROR "dbus_bus_add_match() %s failed: %s\n",
               GNP_IPC_SIGNAL_DELIVER_SA, error.message);
        dbus_error_free(&error);
        goto out;
    }

    if (dbus_connection_register_object_path(conn, GNP_IPC_PATH, &dbus_vtable,
                                             ctx) != TRUE) {
        logger(LOG_ERROR "failed to register object path\n");
        goto out;
    }

    return ctx;

out:
    if (conn) {
        dbus_connection_close(conn);
        dbus_connection_unref(conn);
    }
    if (ctx)
        free(ctx);
    return NULL;
}

void dbus_close(struct dbus_ctx *ctx)
{
    if (ctx && ctx->conn) {
        dbus_connection_flush(ctx->conn);
        dbus_connection_close(ctx->conn);
        dbus_connection_unref(ctx->conn);
        event_del(&ctx->dispatch_ev);
    }
    if (ctx)
        free(ctx);
}

DBusMessage:
http://dbus.freedesktop.org/doc/api/html/group__DBusMessage.html
This module is to deal with messages.

DBus Message Bus API:
http://dbus.freedesktop.org/doc/api/html/group__DBusBus.html
This module is the only one in libdbus that's specific to communicating with the message bus daemon. The rest of the API can also be used for connecting to another application directly.

DBusPendingCall API:
http://dbus.freedesktop.org/doc/api/html/group__DBusPendingCall.html


An example to look over names on the message (bus driver) by using proxy object:
http://stackoverflow.com/questions/14263390/how-to-compile-a-basic-d-bus-glib-example

#include <stdlib.h>          // for exit()   
#include <dbus/dbus.h>       // for dbus_*   
#include <dbus/dbus-glib.h>  // for dbus_g_*

int
main (int argc, char **argv)
{
  DBusGConnection *connection;
  GError *error;
  DBusGProxy *proxy;
  char **name_list;
  char **name_list_ptr;

  g_type_init ();

  error = NULL;
  connection = dbus_g_bus_get (DBUS_BUS_SESSION,
                           &error);
  if (connection == NULL)
    {
      g_printerr ("Failed to open connection to bus: %s\n",
              error->message);
      g_error_free (error);
      exit (1);
    }

  /* Create a proxy object for the "bus driver" (name "org.freedesktop.DBus") */

  proxy = dbus_g_proxy_new_for_name (connection,
                                 DBUS_SERVICE_DBUS,
                                 DBUS_PATH_DBUS,
                                 DBUS_INTERFACE_DBUS);

  /* Call ListNames method, wait for reply */
  error = NULL;
  if (!dbus_g_proxy_call (proxy, "ListNames", &error, G_TYPE_INVALID,
                      G_TYPE_STRV, &name_list, G_TYPE_INVALID))
    {
      /* Just do demonstrate remote exceptions versus regular GError */
      if (error->domain == DBUS_GERROR && error->code == DBUS_GERROR_REMOTE_EXCEPTION)
    g_printerr ("Caught remote method exception %s: %s",
            dbus_g_error_get_name (error),
            error->message);
      else
    g_printerr ("Error: %s\n", error->message);
      g_error_free (error);
      exit (1);
    }

  /* Print the results */

  g_print ("Names on the message bus:\n");

  for (name_list_ptr = name_list; *name_list_ptr; name_list_ptr++)
    {
      g_print ("  %s\n", *name_list_ptr);
    }
  g_strfreev (name_list);

  g_object_unref (proxy);

  return 0;
}

The example of dbus_connection_add_filter() and handler method:
http://lists.freedesktop.org/archives/dbus/2003-September/000468.html

[dbus] The useful information and example of D-Bus

D-Bus Main Page: http://www.freedesktop.org/wiki/Software/dbus/#index5h1
D-Bus Basic Overview: http://pythonhosted.org/txdbus/dbus_overview.html
The Installation Guide: http://www.linuxfromscratch.org/blfs/view/svn/general/dbus.html
The Introduction to D-Bus ( more details ) : http://www.freedesktop.org/wiki/IntroductionToDBus/
The D-Bus Tutorial: http://dbus.freedesktop.org/doc/dbus-tutorial.html
D-Bus APIs: http://dbus.freedesktop.org/doc/api/html/


Example Source: http://www.matthew.ath.cx/misc/dbus
I use the example: dbus-example.c from the above source and modify a little bit for bug fixed. And also, here is my test conf file for the permission of using the system bus.

• dbus-example.c

#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;
   dbus_bool_t stat = FALSE;
   dbus_uint32_t level = 0;

   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;
   dbus_bool_t 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;
}

• mytesting.conf

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

  <!-- Only root or user testing can own the testing service -->
  <policy user="liudanny">
    <allow own="test.signal.source"/>
    <allow own="test.signal.sink"/>
    <allow own="test.method.server"/>
<allow own="test.method.caller"/>
  </policy>
  <policy user="root">
    <allow own="test.signal.source"/>
<allow own="test.signal.sink"/>
<allow own="test.method.server"/>
<allow own="test.method.caller"/>
  </policy>

  <!-- Allow anyone to invoke methods on testing server -->
  <policy context="default">
    <allow send_destination="test.signal.source"/>
    <allow receive_sender="test.signal.source"/>
  </policy>
   <policy context="default">
    <allow send_destination="test.signal.sink"/>
    <allow receive_sender="test.signal.sink"/>
  </policy>
  <policy context="default">
    <allow send_destination="test.method.server"/>
    <allow receive_sender="test.method.server"/>
  </policy>
  <policy context="default">
    <allow send_destination="test.method.caller"/>
    <allow receive_sender="test.method.caller"/>
  </policy>

  <!-- Allow everything, including access to SetHostName to users of the group "liudanny" -->
  <policy group="liudanny">
    <allow send_destination="test.signal.source"/>
    <allow receive_sender="test.signal.source"/>
  </policy>
  <policy user="root">
    <allow send_destination="test.signal.source"/>
    <allow receive_sender="test.signal.source"/>
  </policy>
  <policy group="liudanny">
    <allow send_destination="test.signal.sink"/>
    <allow receive_sender="test.signal.sink"/>
  </policy>
  <policy user="root">
    <allow send_destination="test.signal.sink"/>
    <allow receive_sender="test.signal.sink"/>
  </policy>
    <policy group="liudanny">
    <allow send_destination="test.method.server"/>
    <allow receive_sender="test.method.server"/>
  </policy>
  <policy user="root">
    <allow send_destination="test.method.server"/>
    <allow receive_sender="test.method.server"/>
  </policy>
    <policy group="liudanny">
    <allow send_destination="test.method.caller"/>
    <allow receive_sender="test.method.caller"/>
  </policy>
  <policy user="root">
    <allow send_destination="test.method.caller"/>
    <allow receive_sender="test.method.caller"/>
  </policy>


</busconfig>

[vMotion] The reason of sending RARP packet after vMotion finished

Recently I read the article about How vMotion impacts the forwarding table, I found I didn't quite get understood the following sentence:
"The following article mentioned "When the VM-MAC1 is moved from Host 1 to Host 2 through vMotion process a Reverse ARP (RARP) is issued by Host2’s kernel module on behalf of the VM-MAC1."

After searching RARP and vMotion information, I know why RARP is used here. The answer is quoted from here:
"The goal for sending these frames is to make sure the physical switches in the network learns the location of the Virtual Machines. A physical switch does this learning by observing each incoming frame and make a note of the field called Source MAC Address. Based on that information the switches build tables with mappings between MAC addresses and the switch port where this address could be found."

So, in sum, physical switches and vSphere will update their forwarding table by receiving the broadcast address of RARP packet.

[ONIE] The related URL of Open Network Installation Environment

http://onie.github.io/onie/docs/index.html
https://github.com/onie/onie
http://onie.github.io/onie/

[VMware] NSX Topologies with Traffic Flows

http://packetpushers.net/wp-content/uploads/2013/09/PPP-VMware-NSX-Topologies-Traffic-Flows.pdf

[U-Boot] Use U-Boot command to reload image file and reboot

The following step is about the U-Boot to reboot your machine:

U-Boot>printenv

• You can find the image address and bootm address.

U-Boot>setenv ipaddr 192.168.4.40
U-Boot>setenv serverip 192.168.4.26
U-Boot>setenv ethaddr 00:01:02:03:04:05
U-Boot>setenv netmask 255.255.255.0
U-Boot>saveenv
U-Boot>tftpboot 0xA800000040000000 <<your image file>>
U-Boot>bootm 0xA800000040000074

[BPDU] To understand BPDU Filtering an BPDU Guard

Quote from http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_55_se/configuration/guide/swstpopt.html#wp1046220

Understanding BPDU Guard

The BPDU guard feature can be globally enabled on the switch or can be enabled per port, but the feature operates with some differences.

At the global level, you enable BPDU guard on Port Fast-enabled ports by using the spanning-tree portfast bpduguard default global configuration command. Spanning tree shuts down ports that are in a Port Fast-operational state if any BPDU is received on them. In a valid configuration, Port Fast-enabled ports do not receive BPDUs. Receiving a BPDU on a Port Fast-enabled port means an invalid configuration, such as the connection of an unauthorized device, and the BPDU guard feature puts the port in the error-disabled state. When this happens, the switch shuts down the entire port on which the violation occurred.

To prevent the port from shutting down, you can use the errdisable detect cause bpduguard shutdown vlan global configuration command to shut down just the offending VLAN on the port where the violation occurred.

At the interface level, you enable BPDU guard on any port by using the spanning-tree bpduguard enable interface configuration command without also enabling the Port Fast feature. When the port receives a BPDU, it is put in the error-disabled state.

The BPDU guard feature provides a secure response to invalid configurations because you must manually put the interface back in service. Use the BPDU guard feature in a service-provider network to prevent an access port from participating in the spanning tree.

Understanding BPDU Filtering

The BPDU filtering feature can be globally enabled on the switch or can be enabled per interface, but the feature operates with some differences.

At the global level, you can enable BPDU filtering on Port Fast-enabled interfaces by using the spanning-tree portfast bpdufilter default global configuration command. This command prevents interfaces that are in a Port Fast-operational state from sending or receiving BPDUs. The interfaces still send a few BPDUs at link-up before the switch begins to filter outbound BPDUs. You should globally enable BPDU filtering on a switch so that hosts connected to these interfaces do not receive BPDUs. If a BPDU is received on a Port Fast-enabled interface, the interface loses its Port Fast-operational status, and BPDU filtering is disabled.

At the interface level, you can enable BPDU filtering on any interface by using the spanning-tree bpdufilter enable interface configuration command without also enabling the Port Fast feature. This command prevents the interface from sending or receiving BPDUs.