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- Automatic Struct Layout
- Basic Usage
- Binary data
- Core Concepts
- Features at a Glance
- Functions with multiple signatures
- Loading Libraries
- Projects Using FFI
- Proposal for standard 1.0 API
- Using Multiple and Alternate Libraries
- Using ruby ffi on OsX with mixed architectures, universal binaries.
- Why use FFI
- Windows Examples
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require 'ffi' module Win VK_VOLUME_DOWN = 0xAE; VK_VOLUME_UP = 0xAF; VK_VOLUME_MUTE = 0xAD; KEYEVENTF_KEYUP = 2 extend FFI::Library ffi_lib 'user32' ffi_convention :stdcall attach_function :keybd_event, [ :uchar, :uchar, :int, :pointer ], :void # simulate pressing the mute key on the keyboard keybd_event(VK_VOLUME_MUTE, 0, 0, nil); keybd_event(VK_VOLUME_MUTE, 0, KEYEVENTF_KEYUP, nil); end
This example shows the common task of calling a native function with a pointer to a new struct, then using that same struct once it has been populated and returned by the native function.
require 'ffi' module Win extend FFI::Library class SystemTime < FFI::Struct layout :year, :ushort, :month, :ushort, :day_of_week, :ushort, :day, :ushort, :hour, :ushort, :minute, :ushort, :second, :ushort, :millis, :ushort end ffi_lib 'kernel32' ffi_convention :stdcall attach_function :GetLocalTime, [ :pointer ], :void end mytime = Win::SystemTime.new Win.GetLocalTime(mytime) args = [ mytime[:month], mytime[:day], mytime[:year], mytime[:hour], mytime[:minute], mytime[:second] ] puts "Date: %u/%u/%u\nTime: %02u:%02u:%02u" % args
<notextile> module Win extend FFI::Library ffi_lib 'kernel32' ffi_convention :stdcall =begin DWORD WINAPI GetShortPathName( __in LPCTSTR lpszLongPath, __out LPTSTR lpszShortPath, __in DWORD cchBuffer ); =end attach_function :path_to_8_3, :GetShortPathNameA, [:pointer, :pointer, :uint], :uint end out = FFI::MemoryPointer.new 256 # bytes Win.path_to_8_3("c:\\program files", out, out.size) p out.get_string # be careful, the path/file you convert to 8.3 must exist or this will be empty
This example shows how to use a native function that takes a callback specifying simple input parameters. The callback itself calls a native function which returns a string via its pointer to a character buffer parameter.
require 'ffi' module Win extend FFI::Library ffi_lib 'user32' ffi_convention :stdcall # BOOL CALLBACK EnumWindowProc(HWND hwnd, LPARAM lParam) callback :enum_callback, [ :pointer, :long ], :bool # BOOL WINAPI EnumDesktopWindows(HDESK hDesktop, WNDENUMPROC lpfn, LPARAM lParam) attach_function :enum_desktop_windows, :EnumDesktopWindows, [ :pointer, :enum_callback, :long ], :bool # int GetWindowTextA(HWND hWnd, LPTSTR lpString, int nMaxCount) attach_function :get_window_text, :GetWindowTextA, [ :pointer, :pointer, :int ], :int end win_count = 0 title = FFI::MemoryPointer.new :char, 512 Win::EnumWindowCallback = Proc.new do |wnd, param| title.clear Win.get_window_text(wnd, title, title.size) puts "[%03i] Found '%s'" % [ win_count += 1, title.get_string(0) ] true end if not Win.enum_desktop_windows(nil, Win::EnumWindowCallback, 0) puts 'Unable to enumerate current desktop\'s top-level windows' end
Note that the param you can pass “through” to the enum call is basically a pointer. An example of using this pointer for multiple objects can be found in the win32screenshot gem.
This example shows how to use Unicode text from Ruby with the Unicode version of the
MessageBox Windows API function.
require "ffi" module Win extend FFI::Library ffi_lib "user32" ffi_convention :stdcall # use MessageBoxA if you want to pass it strings with ASCII encoding attach_function :message_box, :MessageBoxW, [ :pointer, :buffer_in, :buffer_in, :int ], :int end # MSFT uses UTF-16 little endian and expects double NULL # at the end of Unicode strings. msg = "Test with umlaut: \u00e4\0".encode("UTF-16LE") Win.message_box(nil, msg, msg, 0)
There are three crucial points shown in the Ruby FFI code:
- The Windows API Unicode function version (
MessageBoxW) is explicitly specified to FFI’s
- Rather than using
:stringparameter types (as would normally be used with the
:buffer_inparameter types are used for Unicode strings. In Ruby-FFI
:stringmeans “null terminated C string” where UTF-16 is closer to a binary blob of data that can contain NULL bytes. NOTE: currently Ruby-FFI checks for NULL chars in
:stringparameters to help avoid NULL byte poisoning attacks from outside string sources.
- The Ruby string needs to be encoded to UTF-16LE and have a Unicode string terminator (double NULL)
This example shows how to interface with a native function that takes a pointer to a struct which contains an embedded union, both of which are populated before being provided to the native function.
<notextile> require 'ffi' module Win extend FFI::Library ffi_lib 'user32' ffi_convention :stdcall MOUSEEVENTF_MOVE = 1 INPUT_MOUSE = 0 class MouseInput < FFI::Struct layout :dx, :long, :dy, :long, :mouse_data, :ulong, :flags, :ulong, :time, :ulong, :extra, :ulong end class InputEvent < FFI::Union layout :mi, MouseInput end class Input < FFI::Struct layout :type, :ulong, :evt, InputEvent end # UINT SendInput(UINT nInputs, LPINPUT pInputs, int cbSize); attach_function :SendInput, [ :uint, :pointer, :int ], :uint end myinput = Win::Input.new myinput[:type] = Win::INPUT_MOUSE in_evt = myinput[:evt][:mi] in_evt[:dx] = ARGV.to_i in_evt[:dy] = ARGV.to_i in_evt[:mouse_data] = 0 in_evt[:flags] = Win::MOUSEEVENTF_MOVE in_evt[:time] = 0 in_evt[:extra] = 0 Win.SendInput(1, myinput, Win::Input.size)
The above FFI code takes a shortcut in the name of saving wiki page space. The actual Windows
INPUT struct contains an anonymous union member with
HARDWAREINPUT members. As the
MOUSEINPUT struct is the largest of these members, we can get away with the hacky
InputEvent FFI union definition for example purposes.
TODO explain FFI syntax for embedded struct members which is opposite of typical C usage.
Bringing windows to the foreground is tricky, since no single call seems to always force it to the foreground.
http://betterlogic.com/roger/?p=2950 describes how.
Note that for windows’ core working you use a @ ffi_convention :stdcall@ but with normal DLL’s it appears you do not see here
- For all functions that take string arguments, the Windows API provides “short name” macros that expand to function names with a suffix indicating ASCII or Unicode. ASCII versions are suffixed with a “A”, and Unicode versions are suffixed with a “W”. For example, the Windows API
FindWindowfunction gets defined as either
DWORD appears to be an :uint (32 bits that is, so :int should work well
- NB that MSDN says it is a long-but for MS, long is 32 bits, even in 64 bit architecture)
- HWND appears to be a :pointer see this thread for why you should not actually read from the value it points to. You may as well just use a :long or :ulong.
- LPDWORD is a pointer (the P standing for pointer)
- LPARAM appears to be a long (hence the L)
- WPARAM appears to be a long (i.e. 64 bits on 64bit OS).
- BOOL is an :int
- HANDLE is a :pointer apparently
another good list (the VB list at the bottom is especially helpful)
A few gems use FFI with the windows API.
- http://github.com/jarmo/win32screenshot Actually has a lot of windows-examination/use/enumeration/minimize etc. methods fleshed out.
- http://github.com/arvicco/win A ruby-esque wrapper for many windows API’s (general use).
- http://github.com/rdp/sensible-cinema Uses ffi for a few things like moving mouse.
class ScreenTracker extend FFI::Library ffi_lib 'user32' # second parameter, pointer, LPRECT is FFI::MemoryPointer.new(:long, 4) # read it like rect.read_array_of_long(4) attach_function :GetWindowRect, [:long, :pointer], :int # returns a BOOL end def get_coords_of_window_on_display out = FFI::MemoryPointer.new(:long, 4) ScreenTracker.GetWindowRect @hwnd, out out.read_array_of_long(4) end end