Why only one WM_DEVICECHANGE message when a multi-volume USB Device is removed?

usb, usb-mass-storage, winapi

Solution

Ok, so I was able to answer one of my own questions: Is there a way to make Windows notify me of both volume removals?

Yes - even though windows sends only one `DBT_DEVTYP_VOLUME` `WM_DEVICECHANGE` message, you actually do get notified of both volume removals - but, as always, the answer lies deep down buried in MSDN:

Although the dbcv_unitmask member may specify more than one volume in any message, this does not guarantee that only one message is generated for a specified event. Multiple system components may independently generate messages for logical volumes at the same time.

So, all I had to do was ignore the example function that Microsoft gives in one of their samples,

char FirstDriveFromMask (ULONG unitmask)
{
   char i;

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

   return (i + 'A');
}

And replace it with a function that interprets the mask for all drives affected. So the one message I was getting was indeed for both volumes, and both volume drive letters were available in the mask.

// [IN] ULONG unitmask
// [IN/OUT] char* outDriveLetters  - an array of characters to be passed in
//                                   that is filled out with the drive letters
//                                   in the mask (this must be 26 bytes to be safe)
// RETURNS the number of drive letters in the mask
int MaskToDriveLetters (ULONG unitmask, char* outDriveLetters)
{
   int cnt = 0;
   for (i = 0; i < 26; ++i)
   {
      if (unitmask & 0x1)
      {
         outDriveLetters[cnt++] = 'A' + i;
         cnt++;
      }
      unitmask = unitmask >> 1;
   }

   outDriveLetters[cnt] = 0; // set the last character to \0 (optional)
   return cnt;  // the number of drives that were set in the mask
}

I still have the other question to answer though - how can the two messages (`DBT_DEVTYP_DEVICEINTERFACE` and `DBT_DEVTYP_VOLUME`) be correlated?

Problem

I'm writing an application that detects when a certain USB Mass Storage Device is plugged-in, and when it is unplugged - by listening for WM_DEVICECHANGE messages. I've also registered my application to listen for `WM_DEVICECHANGE` messages for `DBT_DEVTYP_DEVICEINTERFACE` (using the RegisterDeviceNotification API call) and I get both the `DBT_DEVICEARRIVAL` and `DBT_DEVICEREMOVECOMPLETE` messages when a USB Mass Storage Device is plugged-in or unplugged. Now, the problem occurs when a USB device that has multiple volumes is plugged-in, and then unplugged. I get the following messages when the device is plugged in: - `WM_DEVICECHANGE` (`DBT_DEVICEARRIVAL` of type `DBT_DEVTYP_DEVICEINTERFACE`) - `WM_DEVICECHANGE` (`DBT_DEVICEARRIVAL` of type `DBT_DEVTYP_VOLUME`) - `WM_DEVICECHANGE` (`DBT_DEVICEARRIVAL` of type `DBT_DEVTYP_VOLUME`) And the following messages when it is plugged out: - `WM_DEVICECHANGE` (`DBT_DEVICEREMOVECOMPLETE` of type `DBT_DEVTYP_VOLUME`) - `WM_DEVICECHANGE` (`DBT_DEVICEREMOVECOMPLETE` of type `DBT_DEVTYP_DEVICEINTERFACE`) So, only one remove message even though there are two volumes. Why?? I have two questions: - How can I correlate `DBT_DEVTYP_DEVICEINTERFACE` messages with `DBT_DEVTYP_VOLUME` messages (essentially, how do I know which VOLUME message corresponds to which DEVICEINTERFACE message - since I get them both for the device)? - Is there a way to make Windows notify me of both volume removals?

Original source