GATT ReadValue in c

GATT ReadValue in c

[Neacsu Cristian]

Hello,

I am trying to develop my GATT client using DBUS
I have the next code in order to read some data using ReadValue, but
i'm not sure that I convert properly the results into string.

If I'm using the next gdbus command:
gdbus call --system --dest org.bluez  --object-path
/org/bluez/hci1/dev_C7_5E_38_E7_67_25/service000b/char000c --method
org.bluez.GattCharacteristic1.ReadValue "{}"
([byte 0x48, 0x45, 0x4c, 0x4c, 0x4f, 0x21],) // it means HELLO!

If I use my code, is giving me nothing:

        GVariantBuilder *builder = g_variant_builder_new(
G_VARIANT_TYPE("a{sv}") );
        GVariant *options = g_variant_new ("a{sv}", builder);
        g_variant_builder_unref (builder);

        GVariant *args = g_variant_new("(@a{sv})", options);

        GVariant *result =
bluez_adapter_call_method_sync(tx_path.c_str(), "ReadValue",
"org.bluez.GattCharacteristic1", args);
        if (result) {
                GVariant *received_data = g_variant_get_child_value(result, 0);
                cout << g_variant_get_bytestring(received_data) << endl;
        }

tx_path is initialized with
/org/bluez/hci1/dev_C7_5E_38_E7_67_25/service000b/char000c

Do you have any idea what I'm doing wrong?
Thank you in advance

Cristian-Stefan

Re: GATT ReadValue in c

[Neacsu Cristian]

Hello mr Barry,

I succeeded to find out what was the problem. Actually I needed to
iterate on the list.
This is the working code:

        GVariantBuilder *builder = g_variant_builder_new(
G_VARIANT_TYPE("a{sv}") );
        GVariant *options = g_variant_new ("a{sv}", builder);
        g_variant_builder_unref (builder);

        GVariant *args = g_variant_new("(@a{sv})", options);

        GVariant *result =
bluez_adapter_call_method_sync(rx_path.c_str(), "ReadValue",
"org.bluez.GattCharacteristic1", args);
        if (result) {
                GVariant *received_data = g_variant_get_child_value(result, 0);

                GVariantIter i;
                g_variant_iter_init(&i, received_data);
                GVariant* dataByte;
                while (g_variant_iter_next(&i, "@y", &dataByte)) {
                        cout << g_variant_get_byte(dataByte) << endl;
                }
        }

Right now I'm able to send data easily, but still I am not able to read data...
Question, do you know in gdbus command line if I introspect the path:
/org/bluez/hci1/dev_C7_5E_38_E7_67_25/service000b/char000e
(C7:5E:38:E7:67:25), if it has the UUID
6e400003-b5a3-f393-e0a9-e50e24dcca9e (the regular receive UUID) and I
send the next command:
gdbus call --system --dest org.bluez  --object-path
/org/bluez/hci1/dev_C7_5E_38_E7_67_25/service000b/char000c --method
org.bluez.GattCharacteristic1.ReadValue "{}"
Why is never give me proper results?

If I write with call --system --dest org.bluez  --object-path
/org/bluez/hci1/dev_C7_5E_38_E7_67_25/service000b/char000e --method
org.bluez.GattCharacteristic1.WriteValue "(['test'], {})", the send
service with UUID 6e400002-b5a3-f393-e0a9-e50e24dcca9e, the data will
be sent properly, and if I ReadValue from the send service gdbus call
--system --dest org.bluez  --object-path
/org/bluez/hci1/dev_C7_5E_38_E7_67_25/service000b/char000e --method
org.bluez.GattCharacteristic1.ReadValue "{}", it will echo back what
it sent (string "test").

Am I doing something wrong? Shouldn't I just simply call ReadValue
from the service that has proper receiving UUID, and than that's it?
Do I need to call any NotifyAcquire or so?
Thank you and looking forward for your reply.

Cristian-Stefan

Cristian-Stefan




On Mon, Feb 20, 2023 at 9:14 AM Barry Byford <31baz66@gmail.com> wrote:
>
> Hi Cristian-Stefan,
>
> As I said, I'm not overly familiar with C so I'm not sure what the issue is with your code (or even if it is wrong). I can point a couple of areas that don't look as I would expect but I can't do more than that.
>
> The BlueZ documentation https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/doc/gatt-api.txt#n72 says that ReadValue returns array{byte} which is "ay" as a D-Bus signature. I believe that is byte[] as a C type.
>
> You seem to be using "g_variant_get_bytestring" on the result. "bytestrings" are a special concept in D-Bus as they must always end with a null termination.
>
> To simplify such use glib has helper functions like g_variant_get_bytestring() that ensures that the array is null terminated (it returns the empty string otherwise) and g_variant_new_bytestring() (creates an array of bytes including the terminating 0 from a "C string"). However, these are just a helper functions, and not something you'd use for non-bytestring byte arrays.
> https://developer-old.gnome.org/glib/stable/glib-GVariant.html#g-variant-get-bytestring
>
> As you are getting a byte array then you need to do the conversion yourself.
>
> Hope that helps.
>
> Regards,
> Barry
>
>
>
> On Sun, 19 Feb 2023 at 22:28, Neacsu Cristian <neacsu.cristianstefan@gmail.com> wrote:
>>
>> Hello,
>>
>> I am trying to develop my GATT client using DBUS
>> I have the next code in order to read some data using ReadValue, but
>> i'm not sure that I convert properly the results into string.
>>
>> If I'm using the next gdbus command:
>> gdbus call --system --dest org.bluez  --object-path
>> /org/bluez/hci1/dev_C7_5E_38_E7_67_25/service000b/char000c --method
>> org.bluez.GattCharacteristic1.ReadValue "{}"
>> ([byte 0x48, 0x45, 0x4c, 0x4c, 0x4f, 0x21],) // it means HELLO!
>>
>> If I use my code, is giving me nothing:
>>
>>         GVariantBuilder *builder = g_variant_builder_new(
>> G_VARIANT_TYPE("a{sv}") );
>>         GVariant *options = g_variant_new ("a{sv}", builder);
>>         g_variant_builder_unref (builder);
>>
>>         GVariant *args = g_variant_new("(@a{sv})", options);
>>
>>         GVariant *result =
>> bluez_adapter_call_method_sync(tx_path.c_str(), "ReadValue",
>> "org.bluez.GattCharacteristic1", args);
>>         if (result) {
>>                 GVariant *received_data = g_variant_get_child_value(result, 0);
>>                 cout << g_variant_get_bytestring(received_data) << endl;
>>         }
>>
>> tx_path is initialized with
>> /org/bluez/hci1/dev_C7_5E_38_E7_67_25/service000b/char000c
>>
>> Do you have any idea what I'm doing wrong?
>> Thank you in advance
>>
>> Cristian-Stefan