ffi-bitfield

Bit field for Ruby-FFI

Installation

gem install ffi-bitfield

Usage

Classes

• class BitStruct < FFI::Struct

• class ManagedBitStruct < FFI::ManagedStruct

Loading

require 'ffi/bit_struct'
require 'ffi/managed_bit_struct'

Define your struct

class Struct1 < FFI::BitStruct
  layout \
    :a, :uint8,
    :b, :uint8

  bit_fields :a,
    :u, 2,
    :v, 2,
    :w, 1,
    :x, 1,
    :y, 1,
    :z, 1
end

Reading

s = Struct1.new

s[:a] = 63
p s[:u] # 3
p s[:v] # 3
p s[:w] # 1
p s[:x] # 1
p s[:y] # 0
p s[:z] # 0

Writing

s = Struct1.new

s[:u] = 0
s[:v] = 0
s[:w] = 0
s[:x] = 0
s[:y] = 1
p s[:a] # 64

Typed Bit Fields

You can use the bit_fields_typed method to define bit fields with type information. This method accepts a hash where keys are field names and values are arrays containing [width, type]. For boolean fields (width of 1 with :bool type), it automatically generates a ? helper method:

class AccessFlags < FFI::BitStruct
  layout \
    :flags, :uint8

  bit_fields_typed :flags,
    revoked: [1, :bool],       # Creates revoked? method
    expired: [1, :bool],       # Creates expired? method
    some_string: [4, :string],
    reserved: [2, :int]
end

flags = AccessFlags.new
flags[:revoked] = 1
flags[:expired] = 0

p flags.revoked?  # => true
p flags.expired?  # => false

flags[:expired] = 1
p flags.expired?  # => true

The ? methods are only generated for fields that are 1 bit wide and have type :bool. Other field types can be used for documentation purposes and future functionality.

Inspecting Bit Fields

You can use the bit_field_members method to get a hash of bit fields grouped by parent field:

class Flags < FFI::BitStruct
  layout \
    :value, :uint8

  bit_fields :value,
    :read,    1,
    :write,   1,
    :execute, 1,
    :unused,  5
end

p Flags.bit_field_members
# => {:value=>[:read, :write, :execute, :unused]}

For more detailed information, you can use the bit_field_layout method:

p Flags.bit_field_layout
# => {
#      :value => {
#        :read    => { :start => 0, :width => 1 },
#        :write   => { :start => 1, :width => 1 },
#        :execute => { :start => 2, :width => 1 },
#        :unused  => { :start => 3, :width => 5 }
#      }
#    }

These methods are useful for custom pretty printing or introspection of your bit struct classes.

Loading

require 'ffi/bit_field'

The above is the same as below.

require 'ffi/bit_struct'
require 'ffi/managed_bit_struct'

Development

git clone https://github.com/kojix2/ffi-bitfield
cd ffi-bitfield
bundle install
bundle exec rake test

• ffi-bitfield - read/write bit fields with Ruby-FFI

Contributing

Your feedback is important.

ffi-bitfield is a library under development, so even small improvements like typofix are welcome! Please feel free to send us your pull requests. Bug reports and pull requests are welcome on GitHub at github.com/kojix2/ffi-bitfield.

Do you need commit rights to my repository?
Do you want to get admin rights and take over the project?
If so, please feel free to contact me @kojix2.

License

The gem is available as open source under the terms of the MIT License.