Documentation for field-data and field-length keywords

[tsgowtham-pxl]

I'm working on a generated ASN.1/ACN file which uses the field-data and field-length keyword in it's ACN definitions. I'm unable to find any documentation around these keywords. Can someone help with this please?

Additionally, is there any document which covers the capabilities of ACN and the errors generated by asn1scc?

[maxime-esa]

There are no such keywords in ACN named "field-data" and "field-length". You can find the complete documentation for ACN here: https://gitlab.esa.int/taste/taste-setup/-/wikis/Technical_topic_ASN_1_An_introduction_to_ACN

If you upload your grammar we can try to check what's wrong.

[tsgowtham-pxl]

@maxime-esa Thanks for the quick reply!! This is a snippet from the complete file

ASN.1 snippet

ASW-PARAMETER-VALUE ::= SEQUENCE {
    field-data OCTET STRING (SIZE(1..8))
}

ACN snippet

ASW-PARAMETER-VALUE [] {
    field-length ASW-PARAMETER-VALUE-LENGTH  [size 0, encoding pos-int, mapping-function asw-mapper-20-3-parameter-value],
    field-data [size field-length]
 }

Just to clarify, I'm able to build my ASN.1 and ACN files successfully using asn1scc. My goal is to understand what these snippets mean

[maxime-esa]

Ok, I see it's a quite advanced construct. To summarize, is was designed to provide the size of the variable-size array from an external source (computed by a function named asw-mapper-20-3-parameter-value, that you provide in your C code).

Let me explain:

The ASN.1 snippet means you have single field (field-data) accessible to the end user in the data structure.

In the ACN snippet there is an added field named field-length, of type ASW-PARAMETER-VALUE-LENGTH.

The ACN parameters used for this field are the ones between brackets:

size 0 means that in practice the field will not use any bits in the final structure.
encoding pos-int is the rule for the binary encoding. Since zero bits are used, it has no effect in practice.
mapping-function indicates that you provide an external piece of code to compute the value. check the ACN documentation for details.

Then field-data [size field-length] means that the encoder and decoder expects to set/read the actual length of the field in the added field named field-length.

At runtime the encoder function will therefore encode the field-data array and call the mapping function compute the length to write to the field-length field. In practice since it is using 0 bits to encode, it will not write anything.

And the decoder function will call the mapping function to get a length, so that it will be able to decode field-data properly.

[tsgowtham-pxl]

@maxime-esa Thanks a lot for the detailed explanation! It was really helpful in understanding the ACN syntax and in clearing up my incorrect understanding of the snippet.