Sending a byte array (type `ay`) over D-Bus using GDBus

Question

I am trying to a byte array over D-Bus using the GDBus bindings. Can you please let me know how I can achive that. I tried googling but didnt help.

Byte array contains a image file so cannot be converted to charbytearray

Any help is appriciated

Answer 1

This question has some good ideas in the answers including for passing large amounts of data by writing the data to a file and passing the filename, or using a named pipe. Writing to a file and passing the file name might be the easiest to implement.

Answer 2

I did some tests using an XML where I used the type ay. This works well with the QT binding (generated with qdbusxml2cpp) where it translates into QByteArray however it seems that it doesn't work with the glib binding (generated with gdbus-codegen) where it translates in gchar * and it seems you lose what's after \0 - because somehow it's handled as a string. However you will find that:

This automatic mapping can be turned off by using the annotation org.gtk.GDBus.C.ForceGVariant - if used then a GVariant is always exchanged instead of the corresponding native C type. This annotation may be convenient to use when using bytestrings (type-string ay) for data that could have embedded NUL bytes.

Which means according to https://developer.gnome.org/gio/stable/gdbus-codegen.html that you could handle it as GVariant. I tested this by adding the tag for annotation org.gtk.GDBus.C.ForceGVariant <annotation name="org.gtk.GDBus.C.ForceGVariant" value="true"/> on each arg and it works.

Answer 3

Actually, instead of using type "ay", you can use "a(y)". The glib binding would translate "a(y)" to GVariant*.

And then you can use "GVariant" handling to deal with the parameter.

e.g. the xml file

<method name="parsePacket">
  <arg direction="in" name="message" type="a(y)">
    <doc>
      <line>type: const Uint8 *</line>
    </doc>
  </arg>
</method>

Generated method:

gboolean (*handle_parse_packet) (
IDbusObject *object, GDBusMethodInvocation *invocation, GVariant *arg_message);

gboolean idbusobject_call_parse_packet_sync (
   IDbusObject *proxy,
   GVariant *arg_message,
   GCancellable *cancellable,
  GError **error);

You can extract and insert data using "GVariant" method.

Insert data at client side:

void parsePacket (unsigned char* arg_message, guint16 arg_length)
{
    GVariantBuilder *builder;
    GVariant *value;

    builder = g_variant_builder_new (G_VARIANT_TYPE ("a(y)"));
    for (int i = 0; i < arg_length; i++)
    {
        g_variant_builder_add (builder, "(y)", arg_message[i]);
    }
    value = g_variant_new ("a(y)", builder);
    g_variant_builder_unref (builder);

    idbusobject_call_parse_packet_sync(proxy,
        value,
        NULL,
        NULL);
}

Extract data at server side:

gboolean handleParsePacket (
    IDbusObject *object,
    GDBusMethodInvocation *invocation,
    GVariant *arg_message)
{
    unsigned char byteArray[2048];
    int actualLength = 0;

    GVariantIter *iter;
    guchar str;

    g_variant_get (arg_message, "a(y)", &iter);
    while (g_variant_iter_loop (iter, "(y)", &str))
    {
        byteArray[actualLength++] = str;
    }
    g_variant_iter_free (iter);

    idbusobject_complete_parse_packet( object, invocation);

    return (TRUE);
}

Answer 4

At the client side, you could do it easier by calling the g_variant_new_from_data() method:

GVariant* convertByteArrayToVariant(unsigned char* arg_message, guint16 arg_length)
{
    return g_variant_new_from_data(
        G_VARIANT_TYPE ("a(y)"),
        arg_message,
        arg_length,
        TRUE,
        NULL,
        NULL);
}

Or, if you have a populated a GByteArray*, you could do it like this:

GVariant* convertByteArrayToVariant(GByteArray* array)
{
    return g_variant_new_from_data(
        G_VARIANT_TYPE ("a(y)"),
        array->data,
        array->len,
        TRUE,
        NULL,
        NULL);
}