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Guy
Guy

Reputation: 137

Ruby - DOS (Win32) path name to NT (native) path name

I need to get the NT (native) path of a file from the DOS (Win32) path in Ruby (1.9.3).

Meaning, I have the string:

dos_path = "C:\Windows\regedit.exe"

but I need:

nt_path = "\Device\HarddiskVolume1\Windows\regedit.exe"

Is there any way to do so? Thanks!

Upvotes: 4

Views: 492

Answers (1)

cremno
cremno

Reputation: 4927

A MS-DOS device name can be converted to a NT path with the QueryDosDevice function. Calling such a function from Ruby can be done with Fiddle. It is part of Ruby's standard library since 1.9.3. However the following example will only work with 2.0.0 or newer.

require 'fiddle/import'
require 'fiddle/types'

module DOS2NT
  extend Fiddle::Importer # makes function importing easier
  dlload 'kernel32.dll'
  include Fiddle::Win32Types # so DWORD can be used instead of unsigned long
  extern 'DWORD QueryDosDeviceW(void*, void*, DWORD)', :stdcall
  extern 'DWORD GetLastError()', :stdcall
  ERROR_INSUFFICIENT_BUFFER = 122

  SIZEOF_WCHAR = 2 # sizeof(WCHAR) on Windows

  # a generic error class
  class Error < StandardError
  end

  def self.dos_device_name_to_path(devicename)
    initial_len = 256
    grow_factor = 2
    # we care about Unicode (Windows uses UTF-16LE)
    devicename.encode!(Encoding::UTF_16LE)
    # create buffer
    buf = "\0\0".force_encoding(Encoding::UTF_16LE) * initial_len
    # call QueryDosDeviceW until the call was successful
    while (written_chars = QueryDosDeviceW(devicename, buf, buf.length)) == 0
      # it wasn't
      case (error = GetLastError())
      # resize buffer as it was too short
      when ERROR_INSUFFICIENT_BUFFER
        buf *= grow_factor
      # other errors like ERROR_FILE_NOT_FOUND (2)
      else
        raise Error, "QueryDosDeviceW failed (GetLastError returned #{error})"
      end
    end
    # truncate buffer (including the null character)
    path = buf[0, written_chars - SIZEOF_WCHAR]
    # transcode the path to UTF-8 as that's usually more useful
    path.encode!(Encoding::UTF_8)
  end
end
# example strings from the question
dos_path = 'C:\Windows\regedit.exe'
nt_path = '\Device\HarddiskVolume1\Windows\regedit.exe'
# convert and print inspected result
p dos_path.sub(/\A[A-Z]:/i) { |m| DOS2NT.dos_device_name_to_path(m) } # => "\\Device\\HarddiskVolume1\\Windows\\regedit.exe"

Upvotes: 2

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