HOWTO: Getting Notification of CD-ROM Insertion or Removal

• Microsoft Win32 Application Programming Interface (API) included with: - Microsoft Windows NT version 4.0 - Microsoft Windows 95

SUMMARY

Some Win32 applications need to know when the user inserts or removes a compact disc from a CD-ROM drive without polling for media changes. Windows 95 and Windows NT provide a way to notify these applications through the WM_DEVICECHANGE message. This article explains how to handle the WM_DEVICECHANGE message to detect CD-ROM media changes.

MORE INFORMATION

Windows 95 and Windows NT send all top-level windows a WM_DEVICECHANGE message when new devices or media are added and become available, and when existing devices or media are removed. Each WM_DEVICECHANGE message has an event associated with it to describe the change, and a structure that provides detailed information about the change.

The structure consists of an event-independent header followed by an event- dependent structure. The event-dependent part of the structure describes which device the event applies to. To use this structure, applications must first determine the event type and the device type. Then, they can use the correct structure to take appropriate action.

When the user inserts a new compact disc into a drive, applications get a WM_DEVICECHANCE message with a DBT_DEVICEARRIVAL event. The application must check the event to make sure that the type of device arriving is a volume (DBT_DEVTYP_VOLUME) and that the event's media flag (DBTF_MEDIA) is set.

When the user removes a compact disc from a CD-ROM drive, applications will get a WM_DEVICECHANCE message with a DBT_DEVICEREMOVECOMPLETE event. As with DBT_DEVICEARRIVAL above, the application must check the event to make sure that the device being removed is a volume and that the event's media flag is set.

Sample Code

The following code demonstrates how to use the WM_DEVICECHANGE message to check for compact disc insertion or removal.

   #include <windows.h>
   #include <dbt.h>

   char FirstDriveFromMask (ULONG unitmask);  //prototype


   /*----------------------------------------------------------------------
   Main_OnDeviceChange (hwnd, wParam, lParam)

   Description
      Handles WM_DEVICECHANGE messages sent to the application's
      top-level window.
   ----------------------------------------------------------------------*/

   void Main_OnDeviceChange (HWND hwnd, WPARAM wParam, LPARAM lParam)
   {

      PDEV_BROADCAST_HDR lpdb = (PDEV_BROADCAST_HDR)lParam;
      char szMsg[80];


      switch(wParam)
      {
      case DBT_DEVICEARRIVAL:
         // See if a CD-ROM was inserted into a drive.
         if (lpdb -> dbch_devicetype == DBT_DEVTYP_VOLUME)
         {
            PDEV_BROADCAST_VOLUME lpdbv = (PDEV_BROADCAST_VOLUME)lpdb;

            if (lpdbv -> dbcv_flags & DBTF_MEDIA)
            {
               wsprintf (szMsg, "Drive %c: arrived\n",
                         FirstDriveFromMask(lpdbv ->dbcv_unitmask));

               MessageBox (hwnd, szMsg, "WM_DEVICECHANGE", MB_OK);
            }
         }
        break;


     case DBT_DEVICEREMOVECOMPLETE:
        // See if a CD-ROM was removed from a drive.
        if (lpdb -> dbch_devicetype == DBT_DEVTYP_VOLUME)
        {
           PDEV_BROADCAST_VOLUME lpdbv = (PDEV_BROADCAST_VOLUME)lpdb;

           if (lpdbv -> dbcv_flags & DBTF_MEDIA)
           {
              wsprintf (szMsg, "Drive %c: was removed\n",
                        FirstDriveFromMask(lpdbv ->dbcv_unitmask));

              MessageBox (hwnd, szMsg, "WM_DEVICECHANGE", MB_OK);
           }
        }
        break;

     default:
        /*
           Other WM_DEVICECHANGE notifications get sent for other devices
           or reasons; we don't care about them here.  If they were
           important, we would check for them and act accordingly.
        */
         ;
      }
   }


   /*----------------------------------------------------------------------
   FirstDriveFromMask (unitmask)

      Finds the first valid drive letter from a mask of drive letters. The
      mask must be in the format bit 0 = A, bit 1 = B, bit 3 = C, etc.

      A valid drive letter is defined when the corresponding bit is set to
      1.

      Returns the drive letter that was first found.
   ----------------------------------------------------------------------*/
   char FirstDriveFromMask (ULONG unitmask)
   {
      char i;

      for (i = 0; i < 26; ++i)
      {
         if (unitmask & 0x1)
            break;
         unitmask = unitmask >> 1;
      }

      return (i + 'A');
   }

REFERENCES

Microsoft Win32 SDK documentation, "System Messages" chapter