Wrong union tag size

Applications & Usage Libcanard
1

I do not know if my topic below is related to Custom message signature fail
but, here I go:

I have a DSDL definition that looks like:
myDSDL.uavcan

@union
uint8 value1
uint8 value2
uint8 value3
uint8 value4

When compiled with libcanard dsdl_compiler, I can see in the _myDSDL_encode_internal(…) function that tag size is 3 bits, I expected 2…:

uint32_t test_MyDSDL_encode_internal(test_MyDSDL* source,
  void* msg_buf,
  uint32_t offset,
  uint8_t CANARD_MAYBE_UNUSED(root_item))
{
    // Max Union Tag Value
    CANARD_ASSERT(source->union_tag <= 3);

    // Union Tag 3 bits
    canardEncodeScalar(msg_buf, offset, 3, (void*)&source->union_tag); // 3 bits
    offset += 3;

    if (source->union_tag == 0) {
    canardEncodeScalar(msg_buf, offset, 8, (void*)&source->value1); // 255
    offset += 8;

    }
    else if (source->union_tag == 1) {
    canardEncodeScalar(msg_buf, offset, 8, (void*)&source->value2); // 255
    offset += 8;

    }
    else if (source->union_tag == 2) {
    canardEncodeScalar(msg_buf, offset, 8, (void*)&source->value3); // 255
    offset += 8;

    }
    else if (source->union_tag == 3) {
    canardEncodeScalar(msg_buf, offset, 8, (void*)&source->value4); // 255
    offset += 8;

    }

    return offset;
}

libuavcan compiler generates tag size = 2.

What is wrong?

Regards

2

The DSDL compiler in Libcanard was broken so we removed it. If you check the upstream, there is no compiler there. Please don’t use it.

3

If compiling libcanard for v0, what compiler shall I use?

4

If I am not completely wrong, is this the place were union is calculated?

In init.py:

    if t.kind == t.KIND_MESSAGE:
        t.has_array = inject_cpp_types(t.fields)
        t.has_float16 = has_float16(t.fields)
        inject_cpp_types(t.constants)
        t.all_attributes = t.fields + t.constants
        t.union = t.union and len(t.fields)
        if t.union:
            t.union = len(t.fields).bit_length()
    else:
        t.request_has_array = inject_cpp_types(t.request_fields)
        t.request_has_float16 = has_float16(t.request_fields)
        inject_cpp_types(t.request_constants)
        t.response_has_array = inject_cpp_types(t.response_fields)
        t.response_has_float16 = has_float16(t.response_fields)
        inject_cpp_types(t.response_constants)
        t.all_attributes = t.request_fields + t.request_constants + t.response_fields + t.response_constants
        t.request_union = t.request_union and len(t.request_fields)
        t.response_union = t.response_union and len(t.response_fields)
        if t.request_union:
            t.request_union = len(t.request_fields).bit_length()
        if t.response_union:
            t.response_union = len(t.response_fields).bit_length()

If so, to just fix my issue with wrong union tag size, can some of the above rows be changed to work properly?

5

Currently, there is no DSDL compiler for libcanard. We may develop one later for v1.

I suppose you should subtract one from len(...).bit_length().

6

Correct.
(len(...)-1).bit_length()
solved it.

1 Like