Variable length attribute

[oneguy]

I wasn't able to figure this one out from the documentation:

when do I set attr_md.vlen to 1 or 0? What are the implications?

Thanks M

[Sigurd]

The vlen field can be set to mark a characteristic as variable length. A characteristic can either be of constant length (vlen = 0), or of variable length (vlen = 1).

E.g. for the UART characteristics, it makes sense to have vlen = 1, since you may want to transmit any size of data from 1 to 20 bytes. But, e.g. for the Temperature Type, which is a fixed length characteristic (it has 8 bits and every bit is fixed),vlen should be 0.

[oneguy]

Thank you!

[Moritz]

What happens when vlen is set to 0, but the received data length is shorter or longer than specified?

Use case: Today I specify a characteristic that uses only 1 byte. But later on I realize I want to send more (optional) data, that should be ignored by devices that don't understand it.

[Sigurd]

Hi Moritz. It will then read the length of the whole characteristic. For the UART example it will therefore read 20 bytes, even though you maybe just sent 5 bytes. The 15 other bytes will be garbage values. If you want to send more data, you should instead create a new characteristic.