Print out response of Dbus Method Call in C

Question

The problem I am having is specifically printing out the response of a dbus method call in C using the low level API. I am new to C's libdbus, but have done some work in python-dbus.

• I know how to write dbus methods and method calls in python as well as the CLI
• I can find code on the internet to invoke dbus methods, but they don't return or print out the response
• I have been looking at the libdbus doxygen api, but cannot determine how to pull out the response.

The way I have my code set up, a python dbus daemon runs with methods I want to call. Some of them return a string. I want a C program to connect to the session bus, call the method, print out the reply and exit.

This is what I have currently:

#include <stdio.h>
#include <dbus/dbus.h>

static void send_dbus_message (DBusConnection *connection, const char *msg)
{
DBusMessage *message;
//initialize the message
message = dbus_message_new_signal ("/org/example/foo/bar",
                                    "org.example.foo.bar",
                                    msg);

//send the message
dbus_connection_send (connection, message, NULL);
//deallocate the message
dbus_message_unref (message);
}

int main (int argc, char **argv)
{
DBusConnection *connection;
DBusError error;

//init error message
dbus_error_init (&error);
connection = dbus_bus_get (DBUS_BUS_SESSION, &error);
if (!connection)
{
    printf ("Connection to D-BUS daemon failed: %s", error.message);

    //deallocate error message
    dbus_error_free (&error);
    return 1;
}

send_dbus_message (connection, "HelloWorld");
return 0;
}

Can be synchronous or asynchronous.

Answer 1

You can use the method mentioned in http://www.matthew.ath.cx/misc/dbus to get a method reply message.

Once you have a dbus message you can use following method to extract the data. To parse a dbus message, you need a argument iterator. Initalize it to read contents of the incoming message.

DBusMessageIter MsgIter;
dbus_message_iter_init(msg, &MsgIter);//msg is pointer to dbus message received

You have to validate the signature of the incoming message before reading it. Or you can also go for argument by argument verification. For example, if the argument type is string

if (DBUS_TYPE_STRING == dbus_message_iter_get_arg_type(&MsgIter)){
    char* str = NULL; 
    dbus_message_iter_get_basic(&MsgIter, &str);//this function is used to read basic dbus types like int, string etc. 
}

For complex types, like structures, arrays, variants and dict entries, you have to create corresponding child iterators to parse contents of each complex element. Say, for a dbus signature ofs(i{ii}i)u, the extraction is done as below

//Parsing a signature s(i{ii}i)u
DBusMessageIter rootIter;
dbus_message_iter_init(msg, &rootIter);

if (DBUS_TYPE_STRING == dbus_message_iter_get_arg_type(&rootIter))
{
    char* str = NULL; 
    dbus_message_iter_get_basic(&rootIter, &str);//this function is used to read basic dbus types like int, string etc. 
    dbus_message_iter_next(&rootIter);//Go to next argument of root iter

    //Block to enter and read structure
    if (DBUS_TYPE_STRUCT == dbus_message_iter_get_arg_type(&rootIter))
    {
    DBusMessageIter structIter;
    dbus_message_iter_recurse(&rootIter, &structIter);//Initialize iterator for struct

    //Argument 1 is int32
    if (DBUS_TYPE_INT32 == dbus_message_iter_get_arg_type(&structIter))
    {
        int a;
            dbus_message_iter_get_basic(&structIter, &a);//Read integer
        dbus_message_iter_next(&structIter);//Go to next argument of structiter

        if (DDBUS_TYPE_DICT_ENTRY == dbus_message_iter_get_arg_type(&structIter))
        {
           DBusMessageIter dictIter;           
               dbus_message_iter_recurse(&structIter, &dictIter);//Initialize iterator for dictentry
           if (DBUS_TYPE_INT32 == dbus_message_iter_get_arg_type(&dictIter))
           {
            dbus_message_iter_get_basic(&dictIter, &a);//Read integer
            dbus_message_iter_next(&dictIter);//Go to next argument of dictentry
            if (DBUS_TYPE_INT32 == dbus_message_iter_get_arg_type(&dictIter))
            {
               dbus_message_iter_get_basic(&dictIter, &a);//Read integer
            }
           }
        }
        dbus_message_iter_next(&structIter);//Go to next argument of structiter
            if (DBUS_TYPE_INT32 == dbus_message_iter_get_arg_type(&structIter))
        {
           dbus_message_iter_get_basic(&structIter, &a);//Read integer
        }
        }
    }
    dbus_message_iter_next(&rootIter);//Go to next argument of root iterator
    if (DBUS_TYPE_UINT32 == dbus_message_iter_get_arg_type(&rootIter))
    {
    uint32_t b;
    dbus_message_iter_get_basic(&rootIter, &b);//Read integer
    }
}

In above code, I used argument by argument signature check. Instead you can do a one time verfication using dbus_message_iter_get_signature. Refer to libdbus api for more info.
#
From your reply I understand that you have problems with connection setup , here is a full example, where a method call is invoked on a server and the result is printed if the first argument is a string.
#

#include <stdio.h>
#include <stdlib.h>
#include <dbus/dbus.h>
#include <assert.h>

DBusConnection* conn = NULL;

//Helper function to setup connection
void vsetupconnection();

//Send method call, Returns NULL on failure, else pointer to reply
DBusMessage* sendMethodCall(const char* objectpath, \
        const char* busname, \
        const char* interfacename, \
        const char* methodname);

#define TEST_BUS_NAME               "org.freedesktop.DBus"
#define TEST_OBJ_PATH               "/org/freedesktop/DBus"
#define TEST_INTERFACE_NAME         "org.freedesktop.DBus.Introspectable"
#define TEST_METHOD_NAME            "Introspect"

int main (int argc, char **argv)
{
    vsetupconnection();

    DBusMessage* reply = sendMethodCall(TEST_OBJ_PATH, TEST_BUS_NAME, TEST_INTERFACE_NAME, TEST_METHOD_NAME);
    if(reply != NULL)    {

        DBusMessageIter MsgIter;
        dbus_message_iter_init(reply, &MsgIter);//msg is pointer to dbus message received

        if (DBUS_TYPE_STRING == dbus_message_iter_get_arg_type(&MsgIter)){
            char* str = NULL;
            dbus_message_iter_get_basic(&MsgIter, &str);
            printf("Received string: \n %s \n",str);
        }

        dbus_message_unref(reply);//unref reply
    }
    dbus_connection_close(conn);
    return 0;
}

void vsetupconnection()
{
   DBusError err;
   // initialise the errors
   dbus_error_init(&err);
   // connect to session bus
   conn = dbus_bus_get(DBUS_BUS_SESSION, &err);
   if (dbus_error_is_set(&err)) {
      printf("Connection Error (%s)\n", err.message);
      dbus_error_free(&err);
   }
   if (NULL == conn) {
      exit(1);
   }
   else   {
       printf("Connected to session bus\n");
   }
}

DBusMessage* sendMethodCall(const char* objectpath, const char* busname, const char* interfacename, const char* methodname)
{
    assert(objectpath != NULL); assert(busname != NULL);    assert(interfacename != NULL);
    assert(methodname != NULL); assert(conn != NULL);

    DBusMessage* methodcall = dbus_message_new_method_call(busname,objectpath, interfacename, methodname);

    if (methodcall == NULL)    {
        printf("Cannot allocate DBus message!\n");
    }
    //Now do a sync call
    DBusPendingCall* pending;
    DBusMessage* reply;

    if (!dbus_connection_send_with_reply(conn, methodcall, &pending, -1))//Send and expect reply using pending call object
    {
        printf("failed to send message!\n");
    }
    dbus_connection_flush(conn);
    dbus_message_unref(methodcall);
    methodcall = NULL;

    dbus_pending_call_block(pending);//Now block on the pending call
    reply = dbus_pending_call_steal_reply(pending);//Get the reply message from the queue
    dbus_pending_call_unref(pending);//Free pending call handle
    assert(reply != NULL);

    if(dbus_message_get_type(reply) ==  DBUS_MESSAGE_TYPE_ERROR)    {
        printf("Error : %s",dbus_message_get_error_name(reply));
            dbus_message_unref(reply);
            reply = NULL;
    }

    return reply;
}