Thursday, November 18, 2010

DBus - How to pass dict as parameter

This tutorial is designed for those ones that need DBus but suffer a lot to find documentation even about simple things, such as how to pass a dictionary as parameter.

Initially, I had to invoke a Bluez method that needs a dictionary as parameter. But how could I do it? It not easy at all to find a detailed documentation about it and I had to look for a solution at BlueZ source code.

In this case, I'm using the newest BlueZ Health API (support for HDP/MCAP). The following piece of code shows

static char *start_health_session(DBusConnection *conn)

DBusMessage *msg, *reply;
DBusMessageIter args;
DBusError err;
const char *reply_path;
char *path;

msg = dbus_message_new_method_call("org.bluez",

if (!msg) {
printf(" network:dbus Can't allocate new method call\n");
return NULL;

// append arguments

dbus_message_iter_init_append(msg, &args);

if ( !iter_append_dictionary(&args, DATA_TYPE_VALUE,
printf(" network:dbus Can't append parameters\n");
return NULL;



A DBus dict type needs a message iterator, which is properly initialised before it is used.

Once the message iterator is properly created, let's open it and add tuples to it.

static int iter_append_dictionary(DBusMessageIter *iter,
dbus_uint16_t dataType,
const char *role,
const char *description,
const char *channelType)
DBusMessageIter dict;

dbus_message_iter_open_container(iter, DBUS_TYPE_ARRAY,

dict_append_entry(&dict, "DataType", DBUS_TYPE_UINT16, &dataType);

dict_append_entry(&dict, "Role", DBUS_TYPE_STRING, &role);

dict_append_entry(&dict, "Description", DBUS_TYPE_STRING, &description);

dict_append_entry(&dict, "ChannelType", DBUS_TYPE_STRING, &channelType);

dbus_message_iter_close_container(iter, &dict);

At first, you have to open the container and specify the data type of each tuple. In this case, the dictionary consists of tuples <"DataType",uint16>, <"Role",string>, <"Description",string>, and <"ChannelType",string>. Once the value data type for each tuple varies (uint16 or string), we declare it as a variant. Therefore, the dictionary data type definition is:


Finally, you simply add the basic data type to message iterator (the dictionary itself).

static void append_variant(DBusMessageIter *iter, int type, void *val)
DBusMessageIter value;
char sig[2] = { type, '\0' };

dbus_message_iter_open_container(iter, DBUS_TYPE_VARIANT, sig, &value);

dbus_message_iter_append_basic(&value, type, val);

dbus_message_iter_close_container(iter, &value);

static void dict_append_entry(DBusMessageIter *dict,
const char *key, int type, void *val)
DBusMessageIter entry;

if (type == DBUS_TYPE_STRING) {
const char *str = *((const char **) val);
if (str == NULL)

dbus_message_iter_open_container(dict, DBUS_TYPE_DICT_ENTRY,
NULL, &entry);

dbus_message_iter_append_basic(&entry, DBUS_TYPE_STRING, &key);

append_variant(&entry, type, val);

dbus_message_iter_close_container(dict, &entry);