Thread: conversion from 'WPARAM' to 'int' ???

I thought wparam was an int? I get the following warning from MSVC.net:

...cpp(127): warning C4244: 'return' : conversion from 'WPARAM' to 'int', possible loss of data

Here is a snippet of my code. What am I doing wrong?

Code:

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
                    PSTR lpszCmdLine, int iCmdShow)
{
    HWND     hwnd;
    MSG      msg ;
    WNDCLASS wndclass ;
    wndclass.style           = CS_HREDRAW | CS_VREDRAW ;
    wndclass.lpfnWndProc   = WndProc ;
    wndclass.cbClsExtra    = 0 ;
    wndclass.cbWndExtra    = 0 ;
    wndclass.hInstance     = hInstance ;
    wndclass.hIcon         = LoadIcon (NULL, IDI_APPLICATION);
    wndclass.hCursor       = LoadCursor (NULL, IDC_CROSS);
    wndclass.hbrBackground = (HBRUSH) GetStockObject (WHITE_BRUSH);
    wndclass.lpszMenuName  = szAppName ;
    wndclass.lpszClassName = szAppName ;
    RegisterClass (&wndclass);
    hMenu = LoadMenu (hInstance, MAKEINTRESOURCE(IDR_MENU1));
    
    ///////////////////////////////////////////////////////////////////////////////////////////////////
    //    Allow only one instance of program!                        
    void*    hMapping = CreateFileMapping(INVALID_HANDLE_VALUE, NULL,PAGE_READONLY, 0, 32, szAppName);
    if (hMapping)// Check to see if app is already running                                             
    {                                                                                                 
        if (GetLastError() == ERROR_ALREADY_EXISTS)                                                     
        {                                                                                             
        SetForegroundWindow(FindWindow(szAppName, NULL));                                             
           {                                                                                         
               CloseHandle(hMapping);                                                                 
               ExitProcess(1);                                                                         
           }                                                                                         
        }                                                                                             
    }                                                                                                 
    else                                                                                             
    {                                                                                                 
        MessageBox(NULL, "Error creating file mapping.", "Error",MB_ICONERROR| MB_OK);            
        ExitProcess(1);                                                                                
    }                                                                                                
    // End allow only one instance of program.                                                        
    ///////////////////////////////////////////////////////////////////////////////////////////////////

    hwnd = CreateWindow (szAppName, szAppName, WS_OVERLAPPEDWINDOW, CW_USEDEFAULT
        , CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, hMenu, hInstance, NULL);
    if (!hwnd) 
    {
        MessageBox(NULL, "failed window creation", "error", NULL);
        return 0;
    }

    ShowWindow (hwnd, iCmdShow);
    UpdateWindow (hwnd);

    while (GetMessage (&msg, NULL, 0, 0))
    {
         TranslateMessage (&msg);
         DispatchMessage (&msg);
    }
    return msg.wParam ;
}

On Win64 a WPARAM will be 64 bits while an int will remain at 32 bits. MSVC gives this warning to forewarn you of possible problems when you try to compile your code for Win64. In this case you can use a cast and ignore the warning. More information.

On another note, unless you need a file mapping, the preferred function to make sure your application is single-instance is CreateMutex. Also, you appear to be leaking the hMapping handle.

I will look into CreateMutex. I just remember seeing the file mapping example here on cprogramming.com. Thank you for pointing out my hMapping leak also.
I thought of casting to avoid the warning but I wanted to be sure I wasn't doing something stupid.

Thanks for the quick replies!

>>I just remember seeing the file mapping example here on cprogramming.com.

Might be mine. Looked at other options and found that file mapping worked well enough. Never managed to make it fail so kept it.

I wanted to say that it came from you novacain but I couldn't remember for sure!