string content as parameter

[schiebel]

Hello, this could be user error... I am trying to follow the documentation, but I cannot get parameters to work for string content. I have a lot of data blocks like:

      - id: name_len
        type: u4
      - id: name
        contents: ["TableRecord"]

so I would like to be able to use parameters like:

label:
    params:
      - id: value
         type: str
    seq:
      - id: name_len
         type: u4
      - id: name
         contents: value

I tried label( ["TableRecord"] ), label( "TableRecord" ) and label( [0x54, 0x61, 0x62, 0x6c, 0x65, 0x52, 0x65, 0x63, 0x6f, 0x72, 0x64] ), I put the square brackets in the parameterized struct, I tried removing the type from value but nothing seems to work.

Is it possible to parameterize string content? Thanks in advance for any help...

[generalmimon]

@schiebel The contents key does not support expressions. The string value is always treated literally, so if you write

      - id: magic
        contents: value

it literally expects the following bytes:

Offset(h) 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f

00000000  76 61 6c 75 65                                   value

Since the .ksy language is based on YAML, quoting has no effect (it will be processed by the YAML parser and KSC only receives the 5-character string "value" again):

      - id: magic
        contents: 'value'

If you want to use expressions, you have to use the valid key (sorry, this feature still doesn't have any proper documentation even though it was introduced in 0.9, the issue ref is the best I can give you) - see description in #435 (comment).

As suggested in https://doc.kaitai.io/user_guide.html#param-types, you can declare:

1. a byte array parameter:

   label:
     params:
       - id: value
         type: bytes
     seq:
       - id: name_len
         type: u4
       - id: name
         size: value.length
         valid:
           eq: value

   Note: valid/eq also has a shorthand notation:

           - id: name
             size: value.length
   -         valid:
   -           eq: value
   +         valid: value

   Note that with this approach, you must really pass a byte array in it. For the ASCII bytes "Rec", that would be

     seq:
       - id: foo
         type: label([0x52, 0x65, 0x63])

   You cannot pass a string literal - that won't work because strings and byte arrays are incompatible. This is wrong (the parameter expects a byte array):

   -  seq:
   -    - id: foo
   -      type: label("Rec")

   The reason why you can give either a byte array or a UTF-8 string literal to contents (or some mix of bytes and strings, which will be concatenated) is that contents is a very special key that encodes the strings in UTF-8 to bytes (for convenience). This behavior has nothing to do with the expression language.

2. a string parameter - you have to specify an encoding when parsing. The following relies on a single-byte encoding such as ASCII (for a multi-byte encoding, it won't work in general):

   label:
     params:
       - id: value
         type: str
     seq:
       - id: name_len
         type: u4
       - id: name
         size: value.length # `str.length` gives the number of *characters*, not bytes as we would need here; but in ASCII they're equal
         type: str
         encoding: ASCII
         valid:
           eq: value

   Now you can pass a string:

     seq:
       - id: foo
         type: label("Rec")

[schiebel]

Thanks very much for explaining the issues @generalmimon