How do I call Windows DLL functions from Ruby?

There is the open source win32-api "drop-in replacement for Win32API" by Hiroshi Hatake and Daniel Berger. It works with Ruby 1.8, 1.9, and 2.X.

I think you can also use ruby/dl http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/95a483230caf3d39

or ffi makes it easier and more cross VM friendly:

https://github.com/ffi/ffi/wiki/Windows-Examples

You can use Fiddle: http://ruby-doc.org/stdlib-2.0.0/libdoc/fiddle/rdoc/Fiddle.html

Fiddle is a little-known module that was added to Ruby's standard library in 1.9.x. It allow you to interact directly with C libraries from Ruby.

It works by wrapping libffi, a popular C library that allows code written in one language to call methods written in another. In case you haven't heard of it, "ffi" stands for "foreign function interface." And you're not just limited to C. Once you learn Fiddle, you can use libraries written in Rust and other languages that support it.

http://blog.honeybadger.io/use-any-c-library-from-ruby-via-fiddle-the-ruby-standard-librarys-best-kept-secret/

require 'fiddle'

libm = Fiddle.dlopen('/lib/libm.so.6')

floor = Fiddle::Function.new(
  libm['floor'],
  [Fiddle::TYPE_DOUBLE],
  Fiddle::TYPE_DOUBLE
)

puts floor.call(3.14159) #=> 3.0

or

require 'fiddle'
require 'fiddle/import'

module Logs
  extend Fiddle::Importer
  dlload '/usr/lib/libSystem.dylib'
  extern 'double log(double)'
  extern 'double log10(double)'
  extern 'double log2(double)'
end

# We can call the external functions as if they were ruby methods!
puts Logs.log(10)   # 2.302585092994046
puts Logs.log10(10) # 1.0
puts Logs.log2(10)  # 3.321928094887362

Have a look at Win32API stdlib. It's a fairly easy (but arcane) interface to the Windows 32 API, or DLLs.

Documentation is here, some examples here. To give you a taste:

require "Win32API"    
def get_computer_name
  name = " " * 128
  size = "128"
  Win32API.new('kernel32', 'GetComputerName', ['P', 'P'], 'I').call(name, size)  
  name.unpack("A*")  
end