WM_DEVICECHANGE Messages Are Not Sent to WndProc - C++ - c++

Question

My application creates a window for the purpose of handling the WM_DEVICECHANGE Windows message. WndProc does get called several times, until my application calls a function to poll for keyboard events, but for whatever reason it does not get called when I remove or insert my USB device.
This is the GUID for my USB device. I'm sure it's correct:
static const GUID _guidForCP210xDevices = {
0xA2A39220, 0x39F4, 0x4B88, 0xAE, 0xCB, 0x3D, 0x86, 0xA3, 0x5D, 0xC7, 0x48
};
This is how my window is created:
m_hInstance = ::GetModuleHandle( NULL );
if ( m_hInstance == NULL )
{
TRACE(_T("CNotifyWindow::CNotifyWindow : Failed to retrieve the module handle.\r\n\tError: %d\r\n\tFile: %s\r\n\tLine: %d\r\n"), ::GetLastError(), __WFILE__, __LINE__);
THROW(::GetLastError());
}
m_wcx.cbSize = sizeof(WNDCLASSEX); // size of structure
m_wcx.style = CS_HREDRAW | CS_VREDRAW; // initially minimized
m_wcx.lpfnWndProc = &WndProc; // points to window procedure
m_wcx.cbClsExtra = 0; // no extra class memory
m_wcx.cbWndExtra = 0; // no extra window memory
m_wcx.hInstance = m_hInstance; // handle to instance
m_wcx.hIcon = ::LoadIcon( NULL, IDI_APPLICATION ); // default app icon
m_wcx.hCursor = ::LoadCursor( NULL, IDC_ARROW ); // standard arrow cursor
m_wcx.hbrBackground = NULL; // no background to paint
m_wcx.lpszMenuName = NULL; // no menu resource
m_wcx.lpszClassName = _pwcWindowClass; // name of window class
m_wcx.hIconSm = NULL; // search system resources for sm icon
m_atom = ::RegisterClassEx( &m_wcx );
if ( m_atom == 0 )
{
TRACE(_T("CNotifyWindow::CNotifyWindow : Failed to register window class.\r\n\tError: %d\r\n\tFile: %s\r\n\tLine: %d\r\n"), ::GetLastError(), __WFILE__, __LINE__);
THROW(::GetLastError());
}
m_hWnd = ::CreateWindow(
_pwcWindowClass,
_pwcWindowName,
WS_ICONIC,
0,
0,
CW_USEDEFAULT,
0,
NULL,
NULL,
m_hInstance,
NULL
);
if ( m_hWnd == NULL )
{
TRACE(_T("CNotifyWindow::CNotifyWindow : Failed to create window.\r\n\tError: %d\r\n\tFile: %s\r\n\tLine: %d\r\n"), ::GetLastError(), __WFILE__, __LINE__);
THROW(::GetLastError());
}
::ShowWindow( m_hWnd, SW_HIDE ); // function does not fail
if ( RegisterForNotification() != ERROR_SUCCESS )
{
TRACE(_T("CNotifyWindow::CNotifyWindow : Failed to register for device notification.\r\n\tError: %d\r\n\tFile: %s\r\n\tLine: %d\r\n"), ::GetLastError(), __WFILE__, __LINE__);
THROW(::GetLastError());
}
This is how I register for device notification:
static DEV_BROADCAST_DEVICEINTERFACE dbt = {0};
ASSERT(m_hWnd != NULL);
// Populate DEV_BROADCAST_DEVICEINTERFACE structure.
dbt.dbcc_size = sizeof(DEV_BROADCAST_DEVICEINTERFACE);
dbt.dbcc_devicetype = DBT_DEVTYP_DEVICEINTERFACE;
dbt.dbcc_classguid = _guidForCP210xDevices;
// Register for HID devic notifications
m_hNotify = RegisterDeviceNotification( m_hWnd, &dbt, DEVICE_NOTIFY_WINDOW_HANDLE );
if ( m_hNotify == NULL )
{
TRACE(_T("CNotifyWindow::RegisterForNotification : Failed to register for device notification.\r\n\tError: %d\r\n\tFile: %s\r\n\tLine: %d\r\n"), ::GetLastError(), __WFILE__, __LINE__);
return ::GetLastError();
}
return ERROR_SUCCESS;
My WndProc function looks like this:
static LRESULT CALLBACK WndProc( HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam )
{
DEV_BROADCAST_HDR * pHeader = reinterpret_cast<DEV_BROADCAST_HDR *>(lParam);
switch ( uMsg )
{
case WM_DEVICECHANGE:
if ( pHeader != NULL )
{
if ( pHeader->dbch_devicetype == DBT_DEVTYP_PORT )
{
OnDeviceChange( wParam );
}
}
break;
default:
// Do nothing.
break;
}
return ::DefWindowProc( hWnd, uMsg, wParam, lParam );
}
Does anyone know what I'm doing wrong? Thanks.

Answer 1

You're missing a message pump to retrieve the notifications from the queue and dispatch them to your WndProc. The message pump is effectively a loop that checks for messages and calls the appropriate WndProc synchronously. MSDN has some good information on them. I don't know what the context of your code is, so I'm not sure if you just need to insert a pump after RegisterForNotification or if a larger architectural change is necessary.