How do I find out which process has a file open?

Classically, there was no way to find out which process has a file open. A file object has a reference count, and when the reference count drops to zero, the file is closed. But there's nobody keeping track of which processes own how many references. (And that's ignoring the case that the reference is not coming from a process in the first place; maybe it's coming from a kernel driver, or maybe it came from a process that no longer exists but whose reference is being kept alive by a kernel driver that captured the object reference.)

This falls into the category of not keeping track of information you don't need. The file system doesn't care who has the reference to the file object. Its job is to close the file when the last reference goes away.

You do the same thing with your COM object reference counts. All you care about is whether your reference count has reached zero (at which point it's time to destroy the object). If you later discover an object leak in your process, you don't have a magic query "Show me all the people who called AddRef on my object" because you never kept track of all the people who called AddRef on your object. Or even, "Here's an object I want to destroy. Show me all the people who called AddRef on it so I can destroy them and get them to call Release."

At least that was the story under the classical model.

Enter the Restart Manager.

The official goal of the Restart Manager is to help make it possible to shut down and restart applications which are using a file you want to update. In order to do that, it needs to keep track of which processes are holding references to which files. And it's that database that is of use here. (Why is the kernel keeping track of which processes have a file open? Because it's the converse of the principle of not keeping track of information you don't need: Now it needs the information!)

Here's a simple program which takes a file name on the command line and shows which processes have the file open.

#include <windows.h>
#include <RestartManager.h>
#include <stdio.h>

int __cdecl wmain(int argc, WCHAR **argv)
{
 DWORD dwSession;
 WCHAR szSessionKey[CCH_RM_SESSION_KEY+1] = { 0 };
 DWORD dwError = RmStartSession(&dwSession, 0, szSessionKey);
 wprintf(L"RmStartSession returned %d\n", dwError);
 if (dwError == ERROR_SUCCESS) {
   PCWSTR pszFile = argv[1];
   dwError = RmRegisterResources(dwSession, 1, &pszFile,
                                 0, NULL, 0, NULL);
   wprintf(L"RmRegisterResources(%ls) returned %d\n",
           pszFile, dwError);
  if (dwError == ERROR_SUCCESS) {
   DWORD dwReason;
   UINT i;
   UINT nProcInfoNeeded;
   UINT nProcInfo = 10;
   RM_PROCESS_INFO rgpi[10];
   dwError = RmGetList(dwSession, &nProcInfoNeeded,
                       &nProcInfo, rgpi, &dwReason);
   wprintf(L"RmGetList returned %d\n", dwError);
...
...

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