How do I start reading at an offset in binary data?

Question

I have a C-like struct like this:

SomeStruct << BinData::Record
endian :little

uint32 :offsetOfName
uint32 :offsetOfLastname
#...
uint32 :lenVars
struct :Person, :length => :lenVars
    string :name
    string :lname
    #...
end

I have a bunch of offsets and lengths before :Person. All the offsets and lengths describe the data within the :Person struct.

How can I start reading data at a specified offset, for the given length, or until the next offset?

Answer 1

Seek to offset 1234 and then read 32 bytes into String s:

open 'some-binary-file', 'r' do |f|
  f.seek 1234
  s = f.read 32
  # tho in your case, something like:
  o = aBinData_object.read f
  p s
end

Update: It looks like BinData understands records that encode the lengths of their own fields, but I doubt if there is any way to make it seek for you, unless you are willing to essentially encode dummy fields the size of the seeked-over space, and then ignore forever the data that it's skipping.

I suspect that a good solution will involve an explicit seek and then someBinDataObject.read(f) to get the record.

Answer 2

BinData has two options related to offsets - :check_offset and :adjust_offset. These are not documented in the manual, but are documented in bindata/offset.rb

Here's how they'd work into your example.

class SomeStruct < BinData::Record
  endian :little

  uint32 :offsetOfName
  uint32 :offsetOfLastname
  #...
  uint32 :lenVars

  struct :person do
    string :name,  :adjust_offset => :offsetOfName,
                   :read_length => lambda { offsetOfLastName - offsetOfName }
    string :lname, :adjust_offset => :offsetOfLastName,
                   :read_length => ...
    #...
  end
end

Answer 3

Docs for BinData: http://bindata.rubyforge.org/#nested_records

Not familiar with BinData so maybe I'm off base here but the examples seem to define a class for the outermost structure:

class SomeStruct < BinData::Record
...

Then it talks about nesting anonymous structs within that class:

  struct :person do
  ...

Also, looks like you're giving your inner struct :person a length. My guess is that length doesn't apply here.