Does COM activation of LocalServer32 EXE from the same user account share an existing process or not?

6
votes

I have a COM server LocalServer32 EXE started when a client application calls c_com_ptr::CreateInstance (using ATL wrappers.)

On Windows 7, when a second client application running under the same user account also calls c_com_ptr::CreateInstance, a second copy of the EXE is launched running under the same user account. I was under the impression, from a past life, that the second client would share the first EXE.

Is the LocalServer32 process shared, or not? When, or when not? Googling for an answer gives me a huge noise to signal ratio and I can't find the answer.

My CLSID registry key has the LocalServer32 value giving the EXE path, ProgID, Programmable (empty string), TypeLib (GUID), and a VersionIndependentProgId. I have an AppID key.

I do not want to run the EXE as a service, and I don't mind that the process is not shared. I just want to know the rules so I know what to expect (on Windows Server 2003 onwards.)

EDIT: Following Chris' answer below, I examined the CoRegisterClassObject call in my server. I'm using ATL, and I overrode MyServer::RegisterClassObjects to hook into the calling chain to CAtlExeModuleT::RegisterClassObjects and see that ATL is using CLSCTX_LOCAL_SERVER and REGCLS_MULTIPLEUSE.

Changing this to CLSCTX_LOCAL_SERVER and REGCLS_SINGLEUSE causes more processes to be started, depending on the number of COM objects created by the client, as expected.

Still, going back to REGCLS_MULTIPLEUSE, I get one COM server process per COM client process, each server process containing all of the COM objects for its client, as expected, except that if two COM clients are running under the same user account, they each get their own server which is not how I understood REGCLS_MULTIPLEUSE.

Could the difference be that the clients themselves are actually Windows services? (They are.) When a Windows service process running as a user account creates a COM object under REGCLS_MULTIPLEUSE, is this treated differently, causing the observed behavior? Why am I getting more than one process? (And just to clarify, I do not want my COM server to run as a Windows service, but the clients that use it do run as Windows services.)

Also, running the clients as either Local System, or Network Service, REGCLS_MULTIPLEUSE works as I would have expected: only a single COM server EXE process is started. The multiple processes are started when the COM clients are Windows services running under user accounts.

1
windowscomatlout-of-process

1 Answers

7
votes

The routing of out-of-process activation requests is controlled by the registration of class objects with the COM Service Control Manager. If the SCM has a usable registered class object, that will be used to service the request. If it doesn't, it will start an exe process instance of the COM server to get one. Whether multiple activation requests are routed to a single COM server exe process therefore depends on the following factors at least (I'm not sure if this is a complete list):

  • the activation flags specified by the COM server when it calls CoRegisterClassObject to register with the SCM can cause future activation requests to result in a new exe process instance being started, the simplest and commonest case being the use of the flag REGCLS_SINGLEUSE, which allows the registered class object to be used for a single activation only.
  • Depending on how the class is registered, activation requests from different security contexts may be serviced by different COM server exe instances (it seems this won't apply in your scenario as your client applications run under the same security contexts).