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

Question

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 · Accepted Answer

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

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

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