Попытка добавить текст в элемент управления редактирования в диалоговом окне. Я не могу заставить _tcscat_s правильно добавить. Он аварийно завершает работу и говорит что-то о том, что буфер слишком мал или что-то о пустой строке.

int WINAPI WinMain( HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow )
{
    return DialogBox( hInstance, MAKEINTRESOURCE( IDD_MAIN ), NULL, DlgProc );
}

BOOL CALLBACK DlgProc( HWND hwnd, UINT Message, WPARAM wParam, LPARAM lParam ) 
{
    switch( Message )
    {
        case WM_INITDIALOG:
            OpenAndReadFile( hwnd );
            return TRUE;
        case WM_COMMAND:
            switch( LOWORD( wParam ) )
            {
                case IDSTART:
                    EndDialog( hwnd, IDSTART );
                    break;
                case IDQUIT:
                    EndDialog( hwnd, IDQUIT );
                    break;
            }
            break;
        case WM_CLOSE:
            EndDialog( hwnd, 0 );
            break;
        default:
            return FALSE;
    }
    return TRUE;
}

BOOL OpenAndReadFile( const HWND &hwnd ) 
{   
    // Open the file

    HANDLE hFile;
    hFile = CreateFile( TEXT( "sites.txt" ),    // file to open
                        GENERIC_READ,           // open for reading
                        FILE_SHARE_READ,        // share for reading
                        NULL,                   // default security
                        OPEN_EXISTING,          // existing file only
                        FILE_ATTRIBUTE_NORMAL,  // normal file
                        NULL );                 // no attr. template

    if ( hFile == INVALID_HANDLE_VALUE )
    {
        SetDlgItemText( hwnd, IDC_OUTPUT, TEXT( "Error: File could not be opened\r\n" ) );
        return FALSE;
    }
    else
        SetDlgItemText( hwnd, IDC_OUTPUT, TEXT( "sites.txt opened\r\n" ) );

    AppendText( hwnd, TEXT("TEXT") );

    // Read data from file

    const DWORD BUFFERSIZE = GetFileSize( hFile, NULL );
    char *ReadBuffer = new char [BUFFERSIZE]();
    DWORD dwBytesRead = 0;

    // read one character less than the buffer size to save room for the
    // terminate NULL character.
    //if ( FALSE == ReadFile( hFile, ReadBuffer, BUFFERSIZE - 1, &dwBytesRead, NULL ) )
    {

    }

    return TRUE;
}

void AppendText( const HWND &hwnd, TCHAR *newText )
{
    // get size to determine buffer size
    int outLength = GetWindowTextLength( GetDlgItem( hwnd, IDC_OUTPUT ) );

    // create buffer to hold current text in edit control
    TCHAR * buf = ( TCHAR * ) GlobalAlloc( GPTR, outLength + 1 );

    // get existing text from edit control and put into buffer
    GetDlgItemText( hwnd, IDC_OUTPUT, buf, outLength + 1 );

    // append the newText to the buffer
    _tcscat_s( buf, outLength + 1, newText );

    // Set the text in the dialog
    SetDlgItemText( hwnd, IDC_OUTPUT, buf );
}