How to know if a characteristic requires a long read or not?

[Andy]

Hi,

I'm developing a central application and I want my central to read the device name characteristic from 3rd-party peripherals. I do not know the length of the data in the characteristic beforehand.

If the first time I call sd_ble_gattc_read() with offset 0, how can I know whether there is more data to be read?

When I receive the BLE_GATTC_EVT_READ_RSP event I have access to a "len" and an "offset" variable. The documentation states the following:

/**@brief Event structure for @ref BLE_GATTC_EVT_READ_RSP. */
typedef struct
{
  uint16_t            handle;         /**< Attribute Handle. */
  uint16_t            offset;         /**< Offset of the attribute data. */
  uint16_t            len;            /**< Attribute data length. */
  uint8_t             data[1];        /**< Attribute data. @note This is a variable length array. The size of 1 indicated is only a placeholder for compilation.
                                           See @ref sd_ble_evt_get for more information on how to use event structures with variable length array members. */
} ble_gattc_evt_read_rsp_t;

Does len return the complete length of the attribute? Or is it just the length of the data read? If len is indeed the complete length of the attribute, how can I know the length of the data array?

Thanks for your help.

[ovrebekk]

Hi Andy

The len parameter is the number of bytes returned in this transaction, and will be limited by the MTU size. 

As an example, say you have a device name of 30 bytes and an MTU of 23 (leaving 20 bytes for data), then the len parameter will be 20 when trying to read the device name with offset 0. 

Then you should assume that there could be more data and issue another read request with offset 20. 

On the second read the len will be 10, giving you the remaining 10 bytes, and since the len is lower than the maximum you will know that there is no more data to be read. 

Best regards
Torbjørn

[Andy]

Ok, so if I understand correctly, every time I do a read where (len == ATT_MTU - 3) I need to assume that there's more data and ask for it using offset 20.

What happens if there isn't more data? Won't an error be thrown saying invalid offset or something like that? I don't see that in the documentation but somehow I recall reading something about invalid offset somewhere.

If there isn't such a thing as an invalid offset, I assume then the operation would return an event with len = 0.

[ovrebekk]

Hi Andy

That is correct. If you post a read with an offset that is too large you will get an error in return. 

Best regards
Torbjørn

[Andy]

What error? The documentation for sd_ble_gattc_read() states this:

 * @retval ::NRF_SUCCESS Successfully started or resumed the Read (Long) procedure.
 * @retval ::BLE_ERROR_INVALID_CONN_HANDLE Invalid Connection Handle.
 * @retval ::NRF_ERROR_INVALID_STATE Invalid Connection State.
 * @retval ::NRF_ERROR_BUSY Client procedure already in progress.
 * @retval ::NRF_ERROR_TIMEOUT There has been a GATT procedure timeout. No new GATT procedure can be performed without reestablishing the connection.

[ovrebekk]

Hi Andy

Sorry, I was a bit unclear in my reply. 

The call to sd_ble_gattc_read() will not cause an error. The error will be sent by the peer device after receiving the read request, as an error response, and this will be forwarded to the application as a read response event. 

When you get the read response event after issuing the read request you should check the gatt_status parameter. If it equals BLE_GATT_STATUS_ATTERR_INVALID_OFFSET it means that the offset was invalid. 

Another typical error is BLE_GATT_STATUS_ATTERR_ATTRIBUTE_NOT_LONG, which happens if you try to read a short (20 byte or less) attribute with an offset > 0.

According to the Bluetooth gurus you could also get a read response with len = 0, if you set the offset parameter equal to the length of the attribute. 

Best regards
Torbjørn

[Andy]

Hi Torbjørn,

thanks for your answer. It's clear now :)

[ovrebekk]

Hi Andy

I'm happy to help

Best regards
Torbjørn