Problem with union tag size between libuavcan and pyuavcan

[Juha Kuikka]

Hello,

I am sending messages between an embedded platform using libuavcan and a PC running pyuavcan.

More specifically I have a request/response that looks (paraphrasing) like this:

@union

int16 a

int16 b

int16 c

int16 d

--

@union

int16 a

int16 b

int16 c

int16 d

If I send a response of "a", with value of -1, it comes out as:

b0001 1111

b1111 1111

b1110 0000

The tag is 3 bits long, followed by 16 bits of ones for the -1, followed by padding (I have disregarded the link layer footer byte).

Looking at the generated message definition:

        typedef ::uavcan::IntegerSpec< ::uavcan::IntegerBitLen< 4 >::Result,

                                       ::uavcan::SignednessUnsigned, ::uavcan::CastModeTruncate > TagType;

The :::uavcan::IntegerBitLen< 4 >::Result comes to 3, which makes the IntegerSpec to be 3 bits long. This seems wrong as for four union members I would expect 2 bits to be sufficient. And it looks like the python side implementation agrees with that since it gets the data mangled.

Manually changing the 4 into 3 does work around the issue but that's not really a fix.

The data_type_template.tmpl could be changed have a -1 but that would break @union with only one member.

Thoughts?

 - Juha

[Pavel Kirienko]

Thank you for the report! Indeed, it is a bug; fix submitted, please review: https://github.com/UAVCAN/libuavcan/pull/128

Pavel.

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