MmapRuby

href="https://en.wikipedia.org/wiki/Mmap"> wrapper for Ruby.

A modern fork of github.com/tenderlove/mmap.

Docs: joshuay03.github.io/mmap-ruby

Example:

# frozen_string_literal: true

require "mmap-ruby"

PAGESIZE = 4096

file = File.open("aa", "w")
file.write("\0" * PAGESIZE)
file.write("test")
file.write("\0" * PAGESIZE)
file.close

mmap = Mmap.new("aa", "rw", offset: 0)
p mmap.size == "test".size + (2 * PAGESIZE)
p mmap.scan(/[a-z.]+/) == ["test"]
p mmap.index("test") == PAGESIZE
p mmap.rindex("test") == PAGESIZE
p mmap.sub!(/[a-z.]+/, "toto") == mmap
p mmap.scan(/[a-z.]+/) == ["toto"]
begin
  mmap.sub!(/[a-z.]+/, "alpha")
  puts "not OK, must give an error"
rescue
  puts "OK: #$!"
end
mmap.munmap

mmap = Mmap.new("aa", "rw")
p mmap.index("toto") == PAGESIZE
p mmap.sub!(/([a-z.]+)/, "alpha") == mmap
p $& == "toto"
p $1 == "toto"
p mmap.index("toto") == nil
p mmap.index("alpha") == PAGESIZE
p mmap.size == 5 + 2 * PAGESIZE
mmap.gsub!(/\0/, "X")
p mmap.size == 5 + 2 * PAGESIZE

File.delete("aa")

> bundle exec ruby example.rb
true
true
true
true
true
true
OK: can't change the size of a fixed map
true
true
true
true
true
true
true
true

Benchmark (Producer-Consumer IPC with validation):

Tests scenario where one process writes 1M small messages sequentially while another process reads and validates each message as it arrives. Simulates real-time data processing where consumer must validate each item individually without bulk operations or synchronization.

# frozen_string_literal: true

require "mmap-ruby"

puts RUBY_DESCRIPTION
puts "mmap-ruby version #{MmapRuby::VERSION}"

t1 = Time.now

reader, writer = IO.pipe

fork do
  reader.close
  1_000_000.times do
    writer.write("aa\n")
  end
  writer.write("zz\n")
  writer.close
end

writer.close

line = ""
loop do
  begin
    line = reader.read_nonblock(3)
    break if line == "zz\n"
    raise "Expected 'aa\n', got '#{line}'" unless line == "aa\n"
  rescue IO::WaitReadable
  end
end

reader.close

t2 = Time.now
puts "Time taken for IO.pipe: #{t2 - t1} seconds"

file = File.open("example.txt", "w+")
file.write("\0" * 3_000_003)
file.close

t3 = Time.now

mmap = Mmap.new(file.path, "rw")

fork do
  1_000_000.times do |i|
    mmap[i * 3, 3] = "aa\n"
  end
  mmap[3_000_000, 3] = "zz\n"
end

line = ""
index = 0
loop do
  line = mmap[index, 3]
  index += 3

  if line == "\0\0\0"
    next
  end

  break if line == "zz\n"
  raise "Expected 'aa\n', got '#{line}'" unless line == "aa\n"
end

mmap.unmap

t4 = Time.now
puts "Time taken for Mmap: #{t4 - t3} seconds"

File.delete("example.txt")

> bundle exec ruby benchmark.rb
ruby 3.4.5 (2025-07-16 revision 20cda200d3) +YJIT +PRISM [arm64-darwin25]
mmap-ruby version 0.1.0
Time taken for IO.pipe: 1.336726 seconds
Time taken for Mmap: 0.15564 seconds

Installation

Install the gem and add to the application’s Gemfile by executing:

bundle add mmap-ruby

If bundler is not being used to manage dependencies, install the gem by executing:

gem install mmap-ruby

Development

After checking out the repo, run bin/setup to install dependencies. Then, run bundle exec rake to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at github.com/[joshuay03]/mmap-ruby. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the of conduct[https://github.com/[joshuay03]/mmap-ruby/blob/main/CODE_OF_CONDUCT.md].

License

The gem is available as open source under the terms of the Ruby license.

Code of Conduct

Everyone interacting in the MmapRuby project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the of conduct[https://github.com/[joshuay03]/mmap-ruby/blob/main/CODE_OF_CONDUCT.md].