Length of sd_ble_gatts_sys_attr_get

[Marcel Mulder]

According to the device manager the length in bytes returned by sd_ble_gatts_sys_attr_get() is: ((ATTR_SIZE * (CCCD_COUNT + 1) + CRC) * 4)

When I call sd_ble_gatts_sys_attr_get with p_sys_attr_data = NULL the returned length is 92.

How can the above formula give 92 as a result knowing that ATTR_SIZE = 6 and CRC = 2

I am using SD 7.1.0.

[Håkon Alseth]

I think you've misread the defines. The first define is to sum the size of a system attribute, and the next one is for word-aligning the buffer. There is no "* 4" in the calculation of the byte size, only for word-alignment.

This is the formula taken from SDK v7.2.0:

(DM_GATT_ATTR_SIZE * (DM_GATT_CCCD_COUNT+1 )) + DM_GATT_CRC_SIZE

Where the defines are:

#define DM_GATT_ATTR_SIZE            6
#define DM_GATT_CRC_SIZE             2

You get back 92 bytes from sd_ble_gatts_sys_attr_get(). Strip away the CRC:

DM_GATT_ATTR_SIZE * (CCCD_COUNT+1) = 92 - DM_GATT_CRC_SIZE = 90 bytes

Now calculate the rest:

(CCCD_COUNT + 1) = 90 / 6 = 15

Your CCCD_COUNT is 14.

What (most likely) has confused you is this calculation:

#define DM_GATT_SERVER_ATTR_MAX_SIZE sizeof(uint32_t) *  CEIL_DIV(DM_GATT_ATTR_TOTAL_SIZE, sizeof(uint32_t))

This sequence is just a rounding operation to ensure that the buffer is word-aligned. Input 22, and you get 24 back for instance.

Cheers, Håkon

[Marcel Mulder]

Sorry for not letting know that I already answered my own question. The misunderstanding was indeed in the DM_GATT_SERVER_ATTR_MAX_SIZE part due to the sizeof(uint32_t) multiply part. But it was divided also :-)

Thanks anyway.

Cheers, Marcel

[chd]

Anyone happens to know why we have to add 1 to the CCCD_COUNT?

[Marcel Mulder]

The 1 which is added to CCCD_COUNT is de CCCD of the services changed characteristic.