#include <windows.h>

#define ID_LIST 1
#define ID_TEXT 2

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow) 
{
    static TCHAR    szAppName[] = TEXT("Environ");
    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_ARROW);
    wndclass.hbrBackground  
 
= (HBRUSH)(COLOR_WINDOW+1);
    wndclass.lpszMenuName   = NULL;
    wndclass.lpszClassName  = szAppName;

    if(!RegisterClass(&wndclass)) {
        MessageBox(NULL, TEXT("This program requires Windows NT !"), szAppName, MB_ICONERROR);
        return 0;
    }
    
    hwnd = CreateWindow(szAppName, TEXT("
 
Environment List Box"),
                        WS_OVERLAPPEDWINDOW,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        NULL, NULL, hInstance, NULL);

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

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

void FillListBox(HWND hwndList)
{
    int     iLength;
    TCHAR  *pVarBlock, *pVarBeg, *pVarEnd, *pVarName;
    
    pVarBlock = GetEnvironmentStrings();    // Get pointer to environment block
    
    while(*pVarBlock)
    {
        if(*pVarBlock != '=')               // Skip variable names beginning with '='
        {
            pVarBeg = pVarBlock;            // Beginning of variable name
            while(*pVarBlock++ != '=');     // Scan until '='
            pVarEnd = pVarBlock - 1;        // Points to '=' sign
            iLength = pVarEnd - pVarBeg;    // Length of variable name
            
            // Allocate memory for the variable name and terminating
            // zero. Copy the variable name and append a zero.
            pVarName = calloc(iLength+1, sizeof(TCHAR));
            CopyMemory(pVarName, pVarBeg, iLength*sizeof(TCHAR));
            pVarName[iLength] = '\0';
            
            // Put the variable name in the list box and free memory.
            SendMessage(hwndList, LB_ADDSTRING, 0, (LPARAM)pVarName);
            free(pVarName);
        }    
        while(*pVarBlock++ != '\0');        // Scan until terminating zero
    }
    FreeEnvironmentStrings(pVarBlock);    
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) 
{
    static HWND hwndList, hwndText;
    int         iIndex, iLength, cxChar, cyChar;
    TCHAR       *pVarName, *pVarValue;
    
    switch(message) 
    {
    case WM_CREATE:
        cxChar = LOWORD(GetDialogBaseUnits());
        cyChar = HIWORD(GetDialogBaseUnits());
        
        // Create listbox and static text windows.
        hwndList = CreateWindow(TEXT("listbox"), NULL,
                                WS_CHILD | WS_VISIBLE | LBS_STANDARD,
                                cxChar, cyChar*3,
                                cxChar*16+GetSystemMetrics(SM_CXVSCROLL),
                                cyChar*5,
                                hwnd, (HMENU)ID_LIST,
                                (HINSTANCE)GetWindowLong(hwnd, GWL_HINSTANCE),
                                NULL);
        hwndText = CreateWindow(TEXT("static"), NULL,
                                WS_CHILD | WS_VISIBLE | SS_LEFT,
                                cxChar, cyChar,
                                GetSystemMetrics(SM_CXSCREEN), cyChar,
                                hwnd, (HMENU)ID_TEXT,
                                (HINSTANCE)GetWindowLong(hwnd, GWL_HINSTANCE),
                                NULL);
        FillListBox(hwndList);
        return 0;
    
    case WM_SETFOCUS:
        SetFocus(hwndList);
        return 0;
    
    case WM_COMMAND:
        if(LOWORD(wParam) == ID_LIST && HIWORD(wParam) == LBN_SELCHANGE)
        {
            // Get current selection.
            iIndex = SendMessage(hwndList, LB_GETCURSEL, 0, 0);
            iLength = SendMessage(hwndList, LB_GETTEXTLEN, iIndex, 0) + 1;
            pVarName = calloc(iLength, sizeof(TCHAR));
            SendMessage(hwndList, LB_GETTEXT, iIndex, (LPARAM)pVarName);
            
            // Get environment string.
            iLength = GetEnvironmentVariable(pVarName, NULL, 0);
            pVarValue = calloc(iLength, sizeof(TCHAR));
            GetEnvironmentVariable(pVarName, pVarValue, iLength);
            
            // Show it in window.
            SetWindowText(hwndText, pVarValue);
            free(pVarName);
            free(pVarValue);
        }
        return 0;
        
    case WM_DESTROY: 
        PostQuitMessage(0);
        return 0;            
    }
    return DefWindowProc(hwnd, message, wParam, lParam);
}