pythonconstruct

Dynamic length/size field in Construct

Let's say that I have a construct.Struct like this:

my_struct = construct.Struct(
    "data_length_in_bytes" / construct.Int32ub,
    "login" / construct.CString("ascii"),
    "password" / construct.CString("ascii"),
    "foo" / construct.Int32ub,
    "bar" / construct.Int32ub,
    "baz" / construct.Int8ub
    )
)

Is there a way to generate/calculate the content of the data_length_in_bytes field dynamicly based on the size of the other field (plus the 4 bytes for itself)?

Or should I use a Struct without data_length_in_bytes and then add/remove the field in after/before converting it?

Solution

  • Try using the Prefixed class. It takes two arguments. The first is a Field specifying how to store the size in bytes, and the second is what to store.

    So in this case, you'd write:

    my_struct = construct.Prefixed(construct.Int32ub, construct.Struct(
    "login" / construct.CString("ascii"),
    "password" / construct.CString("ascii"),
    "foo" / construct.Int32ub,
    "bar" / construct.Int32ub,
    "baz" / construct.Int8ub
))