The BLE chararacteristic gives junc information when the length is varied.

[Boris]

Hi everyone, I make the project whith nrf51822 BLE module and need to configure the bluetooth service characteristic as input/output for reading and writing the string-information with the variable length. For this purpose I configure the characteristic so:

ble_gatts_attr_t    attr_char_value;
...
 memset(&attr_char_value, 0, sizeof(attr_char_value));
attr_char_value.p_uuid       = &ble_uuid;
  attr_char_value.p_attr_md    = &attr_md;
  attr_char_value.init_len     = DataLen;  //   DataLen ≤ 21
  attr_char_value.init_offs    = 0;
  attr_char_value.max_len      = DEV_NAME_DATA_MAXLEN;  // DEV_NAME_DATA_MAXLEN = 21
  attr_char_value.p_value      = data;

sd_ble_gatts_characteristic_add(p_nms->service_handle,
                                         &char_md,
                                         &attr_char_value,
                                         &p_nms->devname_char_handles);

When I try to read the n-lentgh-data by the client device from this characteristic the data contains the maximal length (21 characters) when the first n characters contain the required data, but in addition to them the characteristic gives 21-n unneeded junc-characters when the returned length is given as maximal (21) instead beeng given n, the actual size of the returned data.

How is it possible to fix this problem?

[Petter Myhre]

What is the length of the characteristic value when you read it? I can see your initial length and max length, but what is your actual length when you read it? How are you setting it? With sd_ble_gatts_value_set()?

[Boris]

Yes, The first that I tried to do it was to set the string by this "sd_ble_gatts_value_set()" function after the initialization. And exacly here I had seen this problem so I decided to move the setting the start value right in the initialisation "sd_ble_gatts_characteristic_add()" function. But in this case the problem repeated too. For example if I need to set the string "Hello" with 5 characters to this characteristic with 21-characteristic-maximal-length the read value via bluetooth willbe something as it "Hello\0fdsj;lkrjf47g83". The output data contains the exactly 21 chars equal to the maximal length (attr_char_value.max_len) although the actual length (attr_char_value.init_len) is given 5 chars only.

[Petter Myhre]

Do you have attr_md.vlen = 1; ? Or 0?

Which SDK and SoftDevice are you using?

[Boris]

I have attr_md.vlen=0;

A part of code .... BLE_GAP_CONN_SEC_MODE_SET_OPEN(&attr_md.read_perm); BLE_GAP_CONN_SEC_MODE_SET_OPEN(&attr_md.write_perm); attr_md.vloc = BLE_GATTS_VLOC_STACK; attr_md.rd_auth = 0; attr_md.wr_auth = 0; attr_md.vlen = 0; ....

[Petter Myhre]

Please try setting attr_md.vlen=1; and see if that changes anything. If attr_md.vlen=0; the attribute length is fixed. It seems you want variable length.