消息框错误:外部导入不安全

Question

import Graphics.Win32
import System.Win32.DLL
import Control.Exception (bracket)
import Foreign
import System.Exit
main :: IO ()
main = do
    mainInstance <- getModuleHandle Nothing
    hwnd <- createWindow_ 200 200 wndProc mainInstance
    createButton_ hwnd mainInstance
    messagePump hwnd
wndProc :: HWND -> WindowMessage -> WPARAM -> LPARAM -> IO LRESULT
wndProc hwnd wmsg wParam lParam
    | wmsg == wM_DESTROY = do
        sendMessage hwnd wM_QUIT 1 0
        return 0
    | wmsg == wM_COMMAND && wParam == 1 = do
        messageBox nullPtr "Yahoo!!" "Message box" 0 -- Error! Why? :(
        return 0
    | otherwise = defWindowProc (Just hwnd) wmsg wParam lParam
createWindow_ :: Int -> Int -> WindowClosure -> HINSTANCE -> IO HWND
createWindow_ width height wndProc mainInstance = do
    let winClass = mkClassName "ButtonExampleWindow"
    icon <- loadIcon Nothing iDI_APPLICATION
    cursor <- loadCursor Nothing iDC_ARROW
    bgBrush <- createSolidBrush (rgb 240 240 240)
    registerClass (cS_VREDRAW + cS_HREDRAW, mainInstance, Just icon, Just cursor, Just bgBrush, Nothing, winClass)
    w <- createWindow winClass "Button example" wS_OVERLAPPEDWINDOW Nothing Nothing (Just width) (Just height) Nothing Nothing mainInstance wndProc
    showWindow w sW_SHOWNORMAL
    updateWindow w
    return w
createButton_ :: HWND -> HINSTANCE -> IO ()
createButton_ hwnd mainInstance = do
    hBtn <- createButton "Press me" wS_EX_CLIENTEDGE (bS_PUSHBUTTON + wS_VISIBLE + wS_CHILD) (Just 50) (Just 80) (Just 80) (Just 20) (Just hwnd) (Just (castUINTToPtr 1)) mainInstance
    return ()
messagePump :: HWND -> IO ()
messagePump hwnd = allocaMessage $  msg ->
    let pump = do
        getMessage msg (Just hwnd) `catch`  _ -> exitWith ExitSuccess
        translateMessage msg
        dispatchMessage msg
        pump
    in pump

这是一个简单的win32 gui应用程序与一个按钮，但当我点击按钮必须有一个消息框(22行)，但有错误:

button .exe: schedule:重新输入不安全。也许是一个"外国人"不安全的进口应该是"安全的"?

Answer 1

就像Daniel Wagner评论的那样，这是Win32包中的一个bug。MessageBoxW必须安全进口，因为它有许多副作用。

messageBox函数是"不安全"导入的MessageBoxW函数的包装器。当一个不安全导入的函数函数被不安全导入时，Haskell假定线程在返回之前不会调用任何Haskell代码。但是，如果您调用MessageBoxW, Windows将向您在第30行中创建的窗口抛出相当多的窗口消息，因此当您在不安全的外部函数中时，Haskell代码将运行。这也是为什么对messageBox的调用在窗口创建之前将工作的原因。

一个可能的解决方法是自己简单地纠正函数。首先,改变

import Graphics.Win32

import Graphics.Win32 hiding (messageBox, c_MessageBox)

然后，从模块Graphics.Win32.Misc中复制messageBox和c_MessageBox的定义，删除unsafe和/或添加safe:

messageBox :: HWND -> String -> String -> MBStyle -> IO MBStatus
messageBox wnd text caption style =
  withTString text $  c_text ->
  withTString caption $  c_caption ->
  failIfZero "MessageBox" $ c_MessageBox wnd c_text c_caption style
foreign import stdcall safe "windows.h MessageBoxW"
  c_MessageBox :: HWND -> LPCTSTR -> LPCTSTR -> MBStyle -> IO MBStatus