GATT Write Event data restricted to only 1 byte

RE: GATT Write Event data restricted to only 1 byte

Maz Shar — Wed, 23 Dec 2015 16:19:40 GMT

I have discovered this is only "safe" because an extra 23 bytes is allocated for the event buffer when initialising the soft device.

See BLE_STACK_EVT_MSG_BUF_SIZE in ble_stack_handler_types.h which is allocated the size of a ble event + GATT_MTU_SIZE_DEFAULT (23 bytes).

So, the data property serves merely as a starting address. The write event secretly has an extra 23 bytes allocated to "account for" 20 bytes of data (removing headers).

This is hacky and not what your answer informed me about at all.

Would it not work to assign that GATT_MTU_SIZE_DEFAULT to data property itself i.e. data[GATT_MTU_SIZE_DEFAULT - GATT_HEADERS]? where GATT_HEADERS = 3

RE: GATT Write Event data restricted to only 1 byte

RK — Mon, 21 Dec 2015 09:21:27 GMT

The debugger just shows you what it thinks it knows which is one byte, the actual data pointed to is safely allocated to be the correct length and can be read.

RE: GATT Write Event data restricted to only 1 byte

Maz Shar — Mon, 21 Dec 2015 09:17:42 GMT

I'm still interested in a response to my above comment. Thanks!

RE: GATT Write Event data restricted to only 1 byte

Maz Shar — Fri, 18 Dec 2015 13:24:01 GMT

"You can see there's a length in there so you know there is more than one byte of data."

The fact there is a length doesn't mean we know there is more than one byte of data. That just tells us how far to read which can still be the wrong place, right?

"It just means data is a pointer to uint8_t with at least one byte in it."

This does not seem to be the case with regards to safe memory allocation. I've looked at the memory through the debugger and saw that there is 1 byte allocated in the memory directly for data in the ble_gatts_evt_write_t structure and not a 4-byte pointer which points to a value...

RE: GATT Write Event data restricted to only 1 byte

Pål Håland — Fri, 18 Dec 2015 12:11:12 GMT

Also explained in this answer: devzone.nordicsemi.com/.../

RE: GATT Write Event data restricted to only 1 byte

RK — Fri, 18 Dec 2015 11:50:53 GMT

It's a pretty standard C idiom. You can see there's a length in there so you know there is more than one byte of data, it just means data is a pointer to uint8_t with at least one byte in it, you can just use it as a pointer to uint8_t with the cast, as you did, and read up to length bytes, the actual data pointed to will contain all the 'len' bytes.