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ffi-bitfield - read/write bit fields with Ruby-FFI
However, as of July 2021, Ruby-FFI does not support struct bit field. The fact that bit field is not supported is stated in the Wiki. When I contacted the developers, they replied that bit field is still not supported. I was taught how to add a custom method instead.
If only a few bit fields are used, I think it is enough to create some custom methods to read or wite bit fields. However, if a large number of bit fields are provided, it is necessary to create many custom methods. This is a lot of work and increases the probability of mistakes.
I'm working on a bioinformatics-related binding called ruby-htslib. htslib makes heavy use of bit fields throughout the library, so supporting bit fields is inevitable.
I would like to create a gem to solve this problem at once.
So I created a gem to read or write bit fields in Ruby/FFI.
Actually, I didn't know anything about bitwise operations, so I managed to understand the elementary part after thinking about it for several hours.
Even so, I couldn't write efficient code by myself, so I asked other people to write better code for me on Ruby-JP slack and Stack Overflow. At times like this, the Internet community is really helpful.
gem install ffi-bitfieldloading
require 'ffi/bit_struct'
# FFI::BitStruct
require 'ffi/managed_bit_struct'
# FFI::ManagedBitStruct
require 'ffi/bit_structs'
# Both FFI::BitStruct and FFI::ManagedBitStructStructures and bitfields are defined as follows.
require 'ffi/bit_struct'
class Struct1 < FFI::BitStruct
layout \
:a, :uint8,
:b, :uint8
bit_fields :a,
:a0, 1,
:a1, 1,
:a2, 1,
:a3, 1,
:a4, 1,
:a5, 1,
:a6, 1,
:a7, 1
bit_fields :b,
:b0, 1,
:b1, 1,
:b2, 2,
:b3, 4
endbit_field is an alias for bit_fileds. You can use either of them.Same as Ruby-FFI.
a = Struct1.newSame as Ruby-FFI.
p a[:a0] # 0Same as Ruby-FFI.
a[:a7] = 1
p a[:0] # 128It ’s very easy, isn’t it?
I wrote a test to check that it works. In many cases, it will work fine, I think.
However, there may still be bugs in the corner case. If you find any strange behavior, please report it to the github issue. I would be very grateful.
Ruby allows you to call bits with
Integer#[]. Did you know that?128[0] # 0
128[7] # 1
128[6..7] # 2
128[6, 2] # 2In order to take advantage of this, ffi-bitfield only supports newer versions of Ruby.
Thank you for reading. Have a nice day!
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