Callback to WinAPI

windows
ccall

#1

Hi,
I’m trying to rewrite the following code from
https://sjohannes.wordpress.com/2012/03/23/win32-python-getting-all-window-titles/
from Python to Julia

import ctypes

EnumWindows = ctypes.windll.user32.EnumWindows
EnumWindowsProc = ctypes.WINFUNCTYPE(ctypes.c_bool, ctypes.POINTER(ctypes.c_int), ctypes.POINTER(ctypes.c_int))
GetWindowText = ctypes.windll.user32.GetWindowTextW
GetWindowTextLength = ctypes.windll.user32.GetWindowTextLengthW
IsWindowVisible = ctypes.windll.user32.IsWindowVisible

titles = []
def foreach_window(hwnd, lParam):
    if IsWindowVisible(hwnd):
        length = GetWindowTextLength(hwnd)
        buff = ctypes.create_unicode_buffer(length + 1)
        GetWindowText(hwnd, buff, length + 1)
        titles.append(buff.value)
    return True
EnumWindows(EnumWindowsProc(foreach_window), 0)
print(titles)

The code lists titles of all currently visible windows.
I’m struggling with how to specify the stdcall calling convention for the callback function. In a simple ccall one can specify the calling convention

tickCount = ccall( (:GetTickCount, "kernel32"), stdcall, UInt32, () )

but for callback, the function pointer created by cfunction has no calling convention property. Is such functionality available?

Cheers,
Petr


#2

No, calling convention specification is not supported for cfunction. But calling conventions are unified on 64-bit Windows so this won’t matter unless you are on 32-bit Windows.

see also: https://github.com/JuliaLang/julia/issues/5613


#3

@ihnorton Thank you for the explanation. Still my attempt crashes julia:

function foreach_window{T}(hwnd::T, lParam::T)
   return convert(Cint, 1)::Cint
end
const windowEnumCallback = cfunction(foreach_window, Cint, (Ref{Cint}, Ref{Cint}))
ccall((:EnumWindows, "User32"), stdcall, Cint, (Ptr{Void}, Ptr{Cint}), windowEnumCallback, Ref{Cint}(0))

the signatures are
https://msdn.microsoft.com/en-us/library/windows/desktop/ms633497(v=vs.85).aspx

BOOL WINAPI EnumWindows(
  _In_ WNDENUMPROC lpEnumFunc,
  _In_ LPARAM      lParam
);
BOOL CALLBACK EnumWindowsProc(
  _In_ HWND   hwnd,
  _In_ LPARAM lParam
);

#4

I’m also interested.


#5

Please review the C interface section of the manual. There are a few problems, such as:

  • the cfunction signature must match the function declaration
  • Ref is Julia-owned memory, so it doesn’t make sense in the cfunction signature
  • foreach_window should not be parameterized by type ({T})
  • passing a Ref{Cint}(0) to EnumWindows doesn’t make sense – it’s just a long.

The code below works; hopefully enough to get you going:

typealias HANDLE Ptr{Void}
typealias LPARAM Clong

handles = HANDLE[]

function foreach_window(hwnd::HANDLE, lParam::LPARAM)
    push!(handles, hwnd)
    return convert(Cint, 1)::Cint
end
const windowEnumCallback = cfunction(foreach_window, Cint, (HANDLE, LPARAM))

ccall((:EnumWindows, "User32"), stdcall, Cint, (Ptr{Void}, LPARAM), windowEnumCallback, 0)

(I have some old demo code here that might be helpful – although I have no idea if it works at all in current Julia)


#6

Works great, thank you Isaiah. Making clarification in manual.