Ruby 1.9.2 DL nested structs

dl_readfile.rb

require 'dl/import'
require 'dl/types'

module Win32FileAPI
  extend DL::Importer

  dlload 'kernel32.dll', 'user32.dll'

  include DL::Win32Types

  # structs
  SECURITY_ATTRIBUTES = struct [
    'DWORD nLength',
    'PVOID lpSecurityDescriptor',
    'BOOL  bInheritHandle',
  ]
  typealias 'PSECURITY_ATTRIBUTES', 'SECURITY_ATTRIBUTES*'

  # FIXME these 3 are wrong
  OL_OFFSET = struct [
    'DWORD Offset',
    'DWORD OffsetHigh',
  ]
  OL_UNION = union [
    'OL_OFFSET _offset',
    'PVOID Pointer',
  ]
  OVERLAPPED = struct [
    'PDWORD Internal',
    'PDWORD InternalHigh',
    'OL_UNION _internal',
    'HANDLE hEvent',
  ]
  typealias 'POVERLAPPED', 'OVERLAPPED*'

  # user32.dll functions
  extern 'int MessageBoxA(HWND, LPCSTR, LPCSTR, UINT)', :stdcall

  # kernel32.dll functions
  extern 'HANDLE CreateFileA(LPCSTR, DWORD, DWORD, PSECURITY_ATTRIBUTES, DWORD, DWORD, HANDLE)', :stdcall
  # XXX cheat? - BOOL ReadFile(HANDLE, PVOID, DWORD, PDWORD, char*)
  #     really cheat? - BOOL ReadFile(HANDLE, PVOID, DWORD, PDWORD, PVOID)
  extern 'BOOL ReadFile(HANDLE, PVOID, DWORD, PDWORD, POVERLAPPED)', :stdcall
end

rv = Win32FileAPI::MessageBoxA(DL::NULL, 'Hello from Ruby 1.9 DL!', 'DL Info', 0x40 | 0x3)
puts rv

Union means that members are overlaped - you can use either DWORDs Offset and OffsetHigh, or Pointer.

And MSDN states that Pointer should not be used, so I think you should try to directly delare Offset and OffsetHigh like

  OVERLAPPED = struct [
    'PDWORD Internal',
    'PDWORD InternalHigh',
    'DWORD Offset',
    'DWORD OffsetHigh',
    #'OL_UNION _internal', # skip since Offsets are declared directly
    'HANDLE hEvent',
  ]

And then OL_OFFSET and OL_UNION are not needed.

re: unions, understood. I overlooked Pointer's doc "Reserved for system use; do not use after initialization to zero."

I think you're right that it all collapses down to your struct example...thanks...back to looking at porting something like http://msdn.microsoft.com/en-us/library/aa365592(v=vs.85).aspx to ruby.