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COM in plain C

By , 28 Mar 2006
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Prize winner in Competition "MFC/C++ Feb 2006"

Contents

Introduction

There are numerous examples that demonstrate how to use/create COM/OLE/ActiveX components. But these examples typically use Microsoft Foundation Classes (MFC), .NET, C#, WTL, or at least ATL, because those frameworks have pre-fabricated "wrappers" to give you some boilerplate code. Unfortunately, these frameworks tend to hide all of the low level details from a programmer, so you never really do learn how to use COM components per se. Rather, you learn how to use a particular framework riding on top of COM.

If you're trying to use plain C, without MFC, WTL, .NET, ATL, C#, or even any C++ code at all, then there is a dearth of examples and information on how to deal with COM objects. This is the first in a series of articles that will examine how to utilize COM in plain C, without any frameworks.

With standard Win32 controls such as a Static, Edit, Listbox, Combobox, etc., you obtain a handle to the control (i.e., an HWND) and pass messages (via SendMessage) to it in order to manipulate it. Also, the control passes messages back to you (i.e., by putting them in your own message queue, and you fetch them with GetMessage) when it wants to inform you of something or give you some data.

Not so with an OLE/COM object. You don't pass messages back and forth. Instead, the COM object gives you some pointers to certain functions that you can call to manipulate the object. For example, one of Internet Explorer's objects will give you a pointer to a function you can call to cause the browser to load and display a web page in one of your windows. One of Office's objects will give you a pointer to a function you can call to load a document. And if the COM object needs to notify you of something or pass data to you, then you will be required to write certain functions in your program, and provide (to the COM object) pointers to those functions so the object can call those functions when needed. In other words, you need to create your own COM object(s) inside your program. Most of the real hassle in C will involve defining your own COM object. To do this, you'll need to know the minute details about a COM object -- stuff that most of the pre-fabricated frameworks hide from you, but which we'll examine in this series.

In conclusion, you call functions in the COM object to manipulate it, and it calls functions in your program to notify you of things or pass you data or interact with your program in some way. This scheme is analogous to calling functions in a DLL, but as if the DLL is also able to call functions inside your C program -- sort of like with a "callback". But unlike with a DLL, you don't use LoadLibrary() and GetProcAddress() to obtain the pointers to the COM object's functions. As we'll soon discover, you instead use a different operating system function to get a pointer to an object, and then use that object to obtain pointers to its functions.

A COM object and its VTable

Before we can learn how to use a COM object, we first need to learn what it is. And the best way to do that is to create our own COM object.

But before we do that, let's examine a C struct data type. As a C programmer, you should be quite familiar with struct. Here's an example definition of a simple struct (called "IExample") that contains two members -- a DWORD (accessed via the member name "count"), and an 80 char array (accessed via the member name "buffer").

struct IExample {
   DWORD   count;
   char    buffer[80];
};

Let's use a typedef to make it easier to work with:

typedef struct {
   DWORD   count;
   char    buffer[80];
} IExample;

And here's an example of allocating an instance of the above struct (error checking omitted), and initializing its members:

IExample * example;

example = (IExample *)GlobalAlloc(GMEM_FIXED, sizeof(IExample));
example->count = 1;
example->buffer[0] = 0;

Did you know that a struct can store a pointer to some function? Hopefully, you did, but here's an example. Let's say we have a function which is passed a char pointer, and returns a long. Here's our function:

long SetString(char * str)
{
   return(0);
}

Now we want to store a pointer to this function inside IExample. Here's how we define IExample, adding a member ("SetString") to store a pointer to the above function (and I'll use a typedef to make this more readable):

typedef long SetStringPtr(char *);

typedef struct {
   SetStringPtr * SetString;
   DWORD          count;
   char           buffer[80];
} IExample;

And here's how we store a pointer to SetString inside our allocated IExample, and then call SetString using that pointer:

example->SetString = SetString;

long value = example->SetString("Some text");

OK, maybe we want to store pointers to two functions. Here's a second function:

long GetString(char *buffer, long length)
{
   return(0);
}

Let's re-define IExample, adding another member ("GetString") to store a pointer to this second function:

typedef long GetStringPtr(char *, long);

typedef struct {
   SetStringPtr * SetString;
   GetStringPtr * GetString;
   DWORD          count;
   char           buffer[80];
} IExample;

And here we initialize this member:

example->GetString = GetString;

But let's say we don't want to store the function pointers directly inside of IExample. Instead, we'd rather have an array of function pointers. For example, let's define a second struct whose sole purpose is to store our two function pointers. We'll call this a IExampleVtbl struct, and define it as so:

typedef struct {
   SetStringPtr * SetString;
   GetStringPtr * GetString;
} IExampleVtbl;

Now, we'll store a pointer to the above array inside of IExample. We'll add a new member called "lpVtbl" for that purpose (and of course, we'll remove the SetString and GetString members since they've been moved to the IExampleVtbl struct):

typedef struct {
   IExampleVtbl * lpVtbl;
   DWORD          count;
   char           buffer[80];
} IExample;

So here's an example of allocating and initializing a IExample (and of course, a IExampleVtbl):

// Since the contents of IExample_Vtbl will never change, we'll
// just declare it static and initialize it that way. It can
// be reused for lots of instances of IExample.
static const IExampleVtbl IExample_Vtbl = {SetString, GetString};

IExample * example;

// Create (allocate) a IExample struct.
example = (IExample *)GlobalAlloc(GMEM_FIXED, sizeof(IExample));

// Initialize the IExample (ie, store a pointer to
// IExample_Vtbl in it).
example->lpVtbl = &IExample_Vtbl;
example->count = 1;
example->buffer[0] = 0;

And to call our functions, we do:

char buffer[80];

example->lpVtbl->SetString("Some text");
example->lpVtbl->GetString(buffer, sizeof(buffer));

One more thing. Let's say we've decided that our functions may need to access the "count" and "buffer" members of the struct used to call them. So, what we'll do is always pass a pointer to that struct as the first argument. Let's rewrite our functions to accommodate this:

typedef long SetStringPtr(IExample *, char *);
typedef long GetStringPtr(IExample *, char *, long);

long SetString(IExample *this, char * str)
{
   DWORD  i;

   // Let's copy the passed str to IExample's buffer
   i = lstrlen(str);
   if (i > 79) i = 79;
   CopyMemory(this->buffer, str, i);
   this->buffer[i] = 0;

   return(0);
}

long GetString(IExample *this, char *buffer, long length)
{
   DWORD  i;

   // Let's copy IExample's buffer to the passed buffer
   i = lstrlen(this->buffer);
   --length;
   if (i > length) i = length;
   CopyMemory(buffer, this->buffer, i);
   buffer[i] = 0;

   return(0);
}

And let's pass a pointer to the IExample struct when calling its functions:

example->lpVtbl->SetString(example, "Some text");
example->lpVtbl->GetString(example, buffer, sizeof(buffer));

If you've ever used C++, you may be thinking "Wait a minute. This seems strangely familiar." It should. What we've done above is to recreate a C++ class, using plain C. The IExample struct is really a C++ class (one that doesn't inherit from any other class). A C++ class is really nothing more than a struct whose first member is always a pointer to an array -- an array that contains pointers to all the functions inside of that class. And the first argument passed to each function is always a pointer to the class (i.e., struct) itself. (This is referred to as the hidden "this" pointer.)

At its simplest, a COM object is really just a C++ class. You're thinking "Wow! IExample is now a COM object? That's all there is to it?? That was easy!" Hold on. IExample is getting closer, but there's much more to it. It's not that easy. If it were, this wouldn't be a "Microsoft technology", now would it?

First of all, let's introduce some COM technobabble. You see that array of pointers above -- the IExampleVtbl struct? COM documentation refers to that as an interface or VTable.

One requirement of a COM object is that the first three members of our VTable (i.e., our IExampleVtbl struct) must be called QueryInterface, AddRef, and Release. And of course, we have to write those three functions. Microsoft has already determined what arguments must be passed to these functions, what they must return, and what calling convention they use. We'll need to #include some Microsoft include files (that either ship with your C compiler, or you download the Microsoft SDK). We'll re-define our IExampleVtbl struct as so:

#include <windows.h>
#include <objbase.h>
#include <INITGUID.H>

typedef HRESULT STDMETHODCALLTYPE QueryInterfacePtr(IExample *, REFIID, void **);
typedef ULONG STDMETHODCALLTYPE AddRefPtr(IExample *);
typedef ULONG STDMETHODCALLTYPE ReleasePtr(IExample *);

typedef struct {
   // First 3 members must be called QueryInterface, AddRef, and Release
   QueryInterfacePtr  *QueryInterface;
   AddRefPtr          *AddRef;
   ReleasePtr         *Release;
   SetStringPtr       *SetString;
   GetStringPtr       *GetString;
} IExampleVtbl;

Let's examine that typedef for QueryInterface. First of all, the function returns an HRESULT. This is defined simply as a long. Next, it uses STDMETHODCALLTYPE. This means that arguments are not passed in registers, but rather, on the stack. And this also determines who does cleanup of the stack. In fact, for a COM object, we should make sure that all of our functions are declared with STDMETHODCALLTYPE, and return a long (HRESULT). The first argument passed to QueryInterface is a pointer to the object used to call the function. Aren't we turning IExample into a COM object? Yes, and that's what we're going to pass for this argument. (Remember we decided that the first argument we pass to any of our functions will be a pointer to the struct used to call that function? COM is simply enforcing, and relying upon, this design.)

Later, we'll examine what a REFIID is, and also talk about what that third argument to QueryInterface is for. But for now, note that AddRef and Release also are passed that same pointer to our struct we use to call them.

OK, before we forget, let's add HRESULT STDMETHODCALLTYPE to SetString and GetString:

typedef HRESULT STDMETHODCALLTYPE SetStringPtr(IExample *, char *);
typedef HRESULT STDMETHODCALLTYPE GetStringPtr(IExample *, char *, long);

HRESULT STDMETHODCALLTYPE SetString(IExample *this, char * str)
{
   ...

   return(0);
}

HRESULT STDMETHODCALLTYPE GetString(IExample *this, char *buffer, long value)
{
   ...

   return(0);
}

In conclusion, a COM object is basically a C++ class. A C++ class is just a struct that always starts with a pointer to its VTable (an array of function pointers). And the first three pointers in the VTable will always be named QueryInterface, AddRef, and Release. What additional functions may be in its VTable, and what the name of their pointers are, depends upon what type of object it is. (You determine what other functions you want to add to your COM object.) For example, Internet Explorer's browser object will undoubtedly have different functions than some object that plays music. But all COM objects begin with a pointer to their VTable, and the first three VTable pointers are to the object's QueryInterface, AddRef, and Release functions. The first argument passed to an object's function is a pointer to the object (struct) itself. That is the law. Obey it.

A GUID

Let's continue on our journey to make IExample a real COM object. We have yet to actually write our QueryInterface, AddRef, and Release functions. But before we can do that, we must talk about something called a Globally Universal Identifier (GUID). Ack. What's that? It's a 16 byte array that is filled in with a unique series of bytes. And when I say unique, I do mean unique. One GUID (i.e., 16 byte array) cannot have the same series of bytes as another GUID... anywhere in the world. Every GUID ever created has a unique series of 16 bytes.

And how do you create that series of 16 unique bytes? You use a Microsoft utility called GUIDGEN.EXE. It either ships with your compiler, or you get it with the SDK. Run it and you see this window:

As soon as you run GUIDGEN, it automatically generates a new GUID for you, and displays it in the Result box. Note that what you see in your Result box will be different than the above. After all, every single GUID generated will be different than any other. So you had better be seeing something different than I see. Go ahead and click on the "New GUID" button to see some different numbers appear in the Result box. Click all day and entertain yourself by seeing if you ever generate the same series of numbers more than once. You won't. And what's more, nobody else will ever generate any of those number series you generate.

You can click on the "Copy" button to transfer the text to the clipboard, and paste it somewhere else (like in your source code). Here is what I pasted when I did that:

// {0B5B3D8E-574C-4fa3-9010-25B8E4CE24C2}
DEFINE_GUID(<<name>>, 0xb5b3d8e, 0x574c, 0x4fa3, 
            0x90, 0x10, 0x25, 0xb8, 0xe4, 0xce, 0x24, 0xc2);

The above is a macro. A #define in one of the Microsoft include files allows your compiler to compile the above into a 16 byte array.

But there is one thing that we must do. We must replace <<name>> with some C variable name we want to use for this GUID. Let's call it CLSID_IExample.

// {0B5B3D8E-574C-4fa3-9010-25B8E4CE24C2}
DEFINE_GUID(CLSID_IExample, 0xb5b3d8e, 0x574c, 0x4fa3, 
      0x90, 0x10, 0x25, 0xb8, 0xe4, 0xce, 0x24, 0xc2);

Now we have a GUID we can use with IExample.

We also need a GUID for IExample's VTable ("interface"), i.e., our IExampleVtbl struct. So go ahead and click on GUIDGEN.EXE's New GUID button, and copy/paste it somewhere. This time, we're going to replace <<name>> with the C variable name IID_IExample. Here's what I pasted/edited:

// {74666CAC-C2B1-4fa8-A049-97F3214802F0}
DEFINE_GUID(IID_IExample, 0x74666cac, 0xc2b1, 0x4fa8, 
      0xa0, 0x49, 0x97, 0xf3, 0x21, 0x48, 0x2, 0xf0);

In conclusion, every COM object has its own GUID, which is an array of 16 bytes that are different from any other GUID. A GUID is created with the GUIDGEN.EXE utility. A COM object's VTable (i.e., interface) also has a GUID.

QueryInterface(), AddRef(), and Release()

Assume we want to allow another program to get hold of some IExample struct (i.e., COM object) we create/initialize, so the program can call our functions. (We won't yet examine the details of how another program gets hold of our IExample. We'll discuss that later).

Besides our own COM object, there may be lots of other COM components installed upon a given computer. (And again, we'll defer discussing how to install our COM component.) And different computers may have different COM components installed. How does that program determine if our IExample COM object is installed, and distinguish it from all of the other COM objects?

Remember that each COM object has a totally unique GUID, as does our IExample object. And our VTable for IExample has a GUID too. What we need to do is tell the developer writing that program what the GUIDs for our IExample object and its VTable are. Typically, you do that by giving him an include (.H) file with the above two GUID macros you got from GUIDGEN.EXE. OK, so the other program knows IExample's and its VTable's GUIDs. What does it do with them?

That's where our QueryInterface function comes in. Remember that every COM object must have a QueryInterface function (as well as AddRef and Release). The other program is going to pass our IExample VTable GUID to our QueryInterface function, and we're going to check it to make sure it is indeed the IExample VTable's GUID. If it is, then we'll return something to let the program know that it indeed has an IExample object. If the wrong GUID is passed, we're going to return some error that and let it know that what it has isn't an IExample object. So, all of the COM objects on the computer will return an error if their QueryInterface is passed the IExample VTable's GUID, except our own QueryInterface.

That second argument passed to QueryInterface is the GUID we need to check. The third argument is (a handle) where we will return the same object pointer passed to us, if the GUID matches the IExample VTable's GUID. If not, we'll zero out that handle. In addition, QueryInterface returns the long value NOERROR (i.e., #define'd as 0) if the GUID matches, or some non-zero error value (E_NOINTERFACE) if not. So, let's look at IExample's QueryInterface:

HRESULT STDMETHODCALLTYPE QueryInterface(IExample *this, 
                          REFIID vTableGuid, void **ppv)
{
   // Check if the GUID matches IExample
   // VTable's GUID. Remember that we gave the
   // C variable name IID_IExample to our
   // VTable GUID. We can use an OLE function called
   // IsEqualIID to do the comparison for us.
   if (!IsEqualIID(riid, &IID_IExample))
   {
      // We don't recognize the GUID passed
      // to us. Let the caller know this,
      // by clearing his handle, 
      // and returning E_NOINTERFACE.
      *ppv = 0;
      return(E_NOINTERFACE);
   }

   // It's a match!

   // First, we fill in his handle with
   // the same object pointer he passed us. That's
   // our IExample we created/initialized,
   // and he obtained from us.
   *ppv = this;

   // Now we call our own AddRef function,
   // passing the IExample. 
   this->lpVtbl->AddRef(this);

   // Let him know he indeed has a IExample. 
   return(NOERROR);
}

Now let's talk about our AddRef and Release functions. You'll notice we called AddRef in QueryInterface... if we really did have a IExample.

Remember that we're allocating the IExample on behalf of the other program. He's simply gaining access to it. And it's our responsibility to free it when the other program is done using it. How do we know when that is?

We're going to use something called "reference counting". If you look back at the definition of IExample, you'll see that I put a DWORD member in there (count). We're going to make use of this member. When we create a IExample, we'll initialize it to 0. Then, we're going to increment this member (by 1) every time AddRef is called, and decrement it by 1 every time Release is called.

So, when our IExample is passed to QueryInterface, we call AddRef to increment its count member. When the other program is done using it, the program will pass our IExample to our Release function, where we will decrement that member. And if it's 0, we'll free IExample then.

This is another important rule of COM. If you get hold of a COM object created by someone else, you must call its Release function when you're done with it. We certainly expect the other program to call our Release function when it is done with our IExample object.

Here then are our AddRef and Release functions:

ULONG STDMETHODCALLTYPE AddRef(IExample *this)
{
   // Increment the reference count (count member).
   ++this->count;

   // We're supposed to return the updated count.
   return(this->count);
}

ULONG STDMETHODCALLTYPE Release(IExample *this)
{
   // Decrement the reference count.
   --this->count;

   // If it's now zero, we can free IExample.
   if (this->count == 0)
   {
      GlobalFree(this);
      return(0);
   }

   // We're supposed to return the updated count.
   return(this->count);
}

There's one more thing we're going to do. Microsoft has defined a COM object known as an IUnknown. What's that? An IUnknown object is just like IExample, except its VTable contains only the QueryInterface, AddRef, and Release functions (i.e., it doesn't contain additional functions like our IExample VTable has SetString and GetString). In other words, an IUnknown is the bare minimum COM object. And Microsoft created a special GUID for an IUnknown object. But you know what? Our IExample object can also masquerade as an IUnknown object. After all, it has the QueryInterface, AddRef, and Release functions in it. Nobody needs to know it's really an IExample object if all they care about are just those first three functions. We're going to change one line of code so that we report success if the other program passes us either our IExample GUID or an IUnknown GUID. And by the way, Microsoft's include files give the IUnknown GUID the C variable name IID_IUnknown:

// Check if the GUID matches IExample's GUID or IUnknown's GUID.
if (!IsEqualIID(vTableGuid, &IID_IExample) && 
        !IsEqualIID(vTableGuid, &IID_IUnknown))

In conclusion, for our own COM object, we allocate it on behalf of some other program (which gains access to the object and uses it to call our functions). We're responsible for freeing the object. We use reference counting in conjunction with our AddRef and Release functions to accomplish this safely. Our QueryInterface allows other programs to verify they have the object they want, and also allows us to increment the reference count. (Actually, the QueryInterface primarily serves a different purpose that we'll examine later. But at this point, it will suffice to think of its purpose this way.)

So, is IExample now a real COM object? Yes it is! Great! Not too hard! We're done!

Wrong! We still have to package this thing into a form that another program can use (i.e., a Dynamic Link Library), and write code to do a special install routine, and examine how the other program gets hold of our IExample we create (and that will involve us writing more code).

An IClassFactory object

Now we need to look at how a program gets hold of one of our IExample objects, and ultimately, we have to write more code to realize this. Microsoft has devised a standardized method for this. It involves us putting a second COM object (and its functions) inside our DLL. This COM object is called an IClassFactory, and it has a specific set of functions already defined in Microsoft's include files. It also has its own GUID already defined, and given the C variable name of IID_IClassFactory.

Our IClassFactory's VTable has five specific functions in it, which are QueryInterface, AddRef, Release, CreateInstance, and LockServer. Notice that the IClassFactory has its own QueryInterface, AddRef, and Release functions, just like our IExample object. After all, our IClassFactory is a COM object too, and the VTable of all COM objects must start with those three functions. (But to avoid a name conflict with IExample's functions, we'll preface our IClassFactory's function names with "class", such as classQueryInterface, classAddRef, and classRelease. As long as IClassFactory's VTable defines its first three members as QueryInterface, AddRef, and Release, that's OK.)

The really important function is CreateInstance. The program calls our IClassFactory's CreateInstance whenever the program wants us to create one of our IExample objects, initialize it, and return it. In fact, if the program wants several of our IExample objects, it can call CreateInstance numerous times. OK, so that's how a program gets hold of one of our IExample objects. "But how does the program get hold of our IClassFactory object?", you may ask. We'll get to that later. For now, let's simply write our IClassFactory's five functions, and make its VTable.

Making the VTable is easy. Unlike our IExample object's IExampleVtbl, we don't have to define our IClassFactory's VTable struct. Microsoft has already done that for us by defining a IClassFactoryVtbl struct in some include file. All we need to do is declare our VTable and fill it in with pointers to our five IClassFactory functions. Let's create a static VTable using the variable name IClassFactory_Vtbl, and fill it in:

static const IClassFactoryVtbl IClassFactory_Vtbl = {classQueryInterface,
classAddRef,
classRelease,
classCreateInstance,
classLockServer};

Likewise, creating an actual IClassFactory object is easy because Microsoft has already defined that struct too. We need only one of them, so let's declare a static IClassFactory using the variable name MyIClassFactoryObj, and initialize its lpVtbl member to point to our above VTable:

static IClassFactory MyIClassFactoryObj = {&IClassFactory_Vtbl};

Now, we just need to write those above five functions. Our classAddRef and classRelease functions are trivial. Because we never actually allocate our IClassFactory (i.e., we simple declare it as a static), we don't need to free anything. So, classAddRef will simply return a 1 (to indicate that there is always one IClassFactory hanging around). And classRelease will do likewise. We don't need to do any reference counting for our IClassFactory since we don't have to free it.

ULONG STDMETHODCALLTYPE classAddRef(IClassFactory *this)
{
   return(1);
}

ULONG STDMETHODCALLTYPE classRelease(IClassFactory *this)
{
   return(1);
}

Now, let's look at our QueryInterface. It needs to check if the GUID passed to it is either an IUnknown's GUID (since our IClassFactory has the QueryInterface, AddRef, and Release functions, it too can masquerade as an IUnknown object) or an IClassFactory's GUID. Otherwise, we do the same thing as we did in IExample's QueryInterface.

HRESULT STDMETHODCALLTYPE classQueryInterface(IClassFactory *this, 
                          REFIID factoryGuid, void **ppv)
{
   // Check if the GUID matches an IClassFactory or IUnknown GUID.
   if (!IsEqualIID(factoryGuid, &IID_IUnknown) && 
       !IsEqualIID(factoryGuid, &IID_IClassFactory))
   {
      // It doesn't. Clear his handle, and return E_NOINTERFACE.
      *ppv = 0;
      return(E_NOINTERFACE);
   }

   // It's a match!

   // First, we fill in his handle with the same object pointer he passed us.
   // That's our IClassFactory (MyIClassFactoryObj) he obtained from us.
   *ppv = this;

   // Call our IClassFactory's AddRef, passing the IClassFactory. 
   this->lpVtbl->AddRef(this);

   // Let him know he indeed has an IClassFactory. 
   return(NOERROR);
}

Our IClassFactory's LockServer can be just a stub for now:

HRESULT STDMETHODCALLTYPE classLockServer(IClassFactory *this, BOOL flock)
{
   return(NOERROR);
}

There's one more function to write -- CreateInstance. This is defined as follows:

HRESULT STDMETHODCALLTYPE classCreateInstance(IClassFactory *, 
                          IUnknown *, REFIID, void **);

As usual, the first argument is going to be a pointer to our IClassFactory object (MyIClassFactoryObj) which was used to call CreateInstance.

We use the second argument only if we implement something called aggregation. We won't get into this now. If this is non-zero, then someone wants us to support aggregation, which we're not going to do, and we will indicate that by returning an error.

The third argument will be the IExample VTable's GUID (if someone indeed wants us to allocate, initialize, and return a IExample object).

The fourth argument is a handle where we'll return the IExample object we create.

So let's dive into our CreateInstance function (named classCreateInstance):

HRESULT STDMETHODCALLTYPE classCreateInstance(IClassFactory *this, 
        IUnknown *punkOuter, REFIID vTableGuid, void **ppv)
{
   HRESULT          hr;
   struct IExample *thisobj;

   // Assume an error by clearing caller's handle.
   *ppv = 0;

   // We don't support aggregation in IExample.
   if (punkOuter)
      hr = CLASS_E_NOAGGREGATION;
   else
   {
      // Create our IExample object, and initialize it.
      if (!(thisobj = GlobalAlloc(GMEM_FIXED, 
                      sizeof(struct IExample))))
         hr = E_OUTOFMEMORY;
      else
      {
         // Store IExample's VTable. We declared it
         // as a static variable IExample_Vtbl.
         thisobj->lpVtbl = &IExample_Vtbl;

         // Increment reference count so we
         // can call Release() below and it will
         // deallocate only if there
         // is an error with QueryInterface().
         thisobj->count = 1;

         // Fill in the caller's handle
         // with a pointer to the IExample we just
         // allocated above. We'll let IExample's
         // QueryInterface do that, because
         // it also checks the GUID the caller
         // passed, and also increments the
         // reference count (to 2) if all goes well.
         hr = IExample_Vtbl.QueryInterface(thisobj, vTableGuid, ppv);

         // Decrement reference count.
         // NOTE: If there was an error in QueryInterface()
         // then Release() will be decrementing
         // the count back to 0 and will free the
         // IExample for us. One error that may
         // occur is that the caller is asking for
         // some sort of object that we don't
         // support (ie, it's a GUID we don't recognize).
         IExample_Vtbl.Release(thisobj);
      }
   }

   return(hr);
}

That takes care of implementing our IClassFactory object.

Packaging into a DLL

In order to facilitate another program getting hold of our IClassFactory (and to call its CreateInstance function to obtain some IExample objects), we'll package our above source code into a Dynamic Link Library (DLL). This tutorial does not discuss how to create a DLL per se, so if you're unfamiliar with that, then you should first read a tutorial about DLLs.

Above, we've already written all the code for our IExample and IClassFactory objects. All we need to do is paste this into our source for the DLL.

But there's still more to do. Microsoft also dictates that we must add a function to our DLL called DllGetClassObject. Microsoft has already defined what arguments it is passed, what it should do, and what it should return. A program is going to call our DllGetClassObject to obtain a pointer to our IClassFactory object. (Actually, as we'll see later, the program is going to call an OLE function named CoGetClassObject, which in turn calls our DllGetClassObject.) So, this is how the program gets hold of our IClassFactory object -- by calling our DllGetClassObject. Our DllGetClassObject function must perform this job. Here's how it's defined:

HRESULT PASCAL DllGetClassObject(REFCLSID objGuid, 
        REFIID factoryGuid, void **factoryHandle);

The first argument passed is going to be the GUID for our IExample object (not its VTable's GUID). We need to check this to make sure that the caller definitely intended to call our DLL's DllGetClassObject. Note that every COM DLL has a DllGetClassObject function in it, so again, we need that GUID to distinguish our DllGetClassObject from every other COM DLL's DllGetClassObject.

The second argument is going to be the GUID of an IClassFactory.

The third argument is a handle to where the program expects us to return a pointer to our IClassFactory (if the program did indeed pass IExample's GUID, and not some other COM object's GUID).

HRESULT PASCAL DllGetClassObject(REFCLSID objGuid, 
        REFIID factoryGuid, void **factoryHandle)
{
   HRESULT  hr;

   // Check that the caller is passing
   // our IExample GUID. That's the COM
   // object our DLL implements.
   if (IsEqualCLSID(objGuid, &CLSID_IExample))
   {
      // Fill in the caller's handle
      // with a pointer to our IClassFactory object.
      // We'll let our IClassFactory's
      // QueryInterface do that, because it also
      // checks the IClassFactory GUID and does other book-keeping.
      hr = classQueryInterface(&MyIClassFactoryObj, 
                          factoryGuid, factoryHandle);
   }
   else
   {
      // We don't understand this GUID.
      // It's obviously not for our DLL.
      // Let the caller know this by
      // clearing his handle and returning
      // CLASS_E_CLASSNOTAVAILABLE.
      *factoryHandle = 0;
      hr = CLASS_E_CLASSNOTAVAILABLE;
   }

   return(hr);
}

We're almost done with what we need to create our DLL. There's just one more thing. It's not really the program that loads our DLL. Rather, the operating system does so on behalf of the program when the program calls CoGetDllClassObject (i.e., CoGetClassObject locates our DLL file, does a LoadLibrary on it, uses GetProcAddress to get our above DllGetClassObject, and calls it on behalf of the program). And unfortunately, Microsoft didn't work out any way for the program to tell the OS when the program is done using our DLL and the OS should unload (FreeLibrary) our DLL. So we have to help out the OS to let it know when it is safe to unload our DLL. We must provide a function called DllCanUnloadNow which will return S_OK if it's safe to unload our DLL, or S_FALSE if not.

And how will we know when it is safe?

We're going to have to do more reference counting. Specifically, every time we allocate an object for a program, we're going to have to increment a count. Each time the program calls that object's Release function, and we free that object, we'll decrement that same count. Only when the count is zero will we tell the OS that our DLL is safe to unload, because that's when we know for sure that the program isn't using any of our objects. So, we'll declare a static DWORD variable named OutstandingObjects to maintain this count. (And of course, when our DLL is first loaded, this needs to be initialized to 0.)

So, where is the most convenient place to increment this variable? In our IClassFactory's CreateInstance function, after we actually GlobalAlloc the object and make sure everything went OK. So, we'll add a line in that function, right after the call to Release:

static DWORD OutstandingObjects = 0;

HRESULT STDMETHODCALLTYPE classCreateInstance(IClassFactory *this, 
        IUnknown *punkOuter, REFIID vTableGuid, void **ppv)
{
   ...

         IExampleVtbl.Release(thisobj);

         // Increment our count of outstanding objects if all
         // went well.
         if (!hr) InterlockedIncrement(&OutstandingObjects);;
      }
   }

   return(hr);
}

And where is the most convenient place to decrement this variable? In our IExample's Release function, right after we GlobalFree the object. So we add a line after GlobalFree:

InterlockedDecrement(&OutstandingObjects);

But there's more. (Do the messy details never end with Microsoft?) Microsoft has decided that there should be a way for a program to lock our DLL in memory if it desires. For that purpose, it can call our IClassFactory's LockServer function, passing a 1 if it wants us to increment a count of locks on our DLL, or 0 if it wants to decrement a count of locks on our DLL. So, we also need a second static DWORD reference count which we'll call LockCount. (And of course, this also needs to be initialized to 0 when our DLL loads.) Our LockServer function now becomes:

static DWORD LockCount = 0;

HRESULT STDMETHODCALLTYPE 
        classLockServer(IClassFactory *this, BOOL flock)
{
   if (flock) InterlockedIncrement(&LockCount);
   else InterlockedDecrement(&LockCount);

   return(NOERROR);
}

Now we're ready to write our DllCanUnloadNow function:

HRESULT PASCAL DllCanUnloadNow(void)
{
   // If someone has retrieved pointers to any of our objects, and
   // not yet Release()'ed them, then we return S_FALSE to indicate
   // not to unload this DLL. Also, if someone has us locked, return
   // S_FALSE
   return((OutstandingObjects | LockCount) ? S_FALSE : S_OK);
}

If you download the example project, the source file for our DLL (IExample.c) is in the directory IExample. Also supplied are Microsoft Visual C++ project files that create a DLL (IExample.dll) from this source.

Our C++/C include file

As mentioned earlier, in order for a program written in C++/C to use our IExample DLL, we need to give that program's author our IExample's, and its VTable's, GUIDs. We'll put those GUID macros in an include (.H) file which we can distribute to others, and also include in our DLL source. We also need to put the definition of our IExampleVtbl, and IExample, structs in this include file, so the program can call our functions via the IExample we give it.

Up to now, we defined our IExampleVtbl, and IExample, structs as so:

typedef HRESULT STDMETHODCALLTYPE QueryInterfacePtr(IExample *, REFIID, void **);
typedef ULONG STDMETHODCALLTYPE AddRefPtr(IExample *);
typedef ULONG STDMETHODCALLTYPE ReleasePtr(IExample *);
typedef HRESULT STDMETHODCALLTYPE SetStringPtr(IExample *, char *);
typedef HRESULT STDMETHODCALLTYPE GetStringPtr(IExample *, char *, long);

typedef struct {
   QueryInterfacePtr  *QueryInterface;
   AddRefPtr          *AddRef;
   ReleasePtr         *Release;
   SetStringPtr       *SetString;
   GetStringPtr       *GetString;
} IExampleVtbl;

typedef struct {
   IExampleVtbl *lpVtbl;
   DWORD         count;
   char          buffer[80];
} IExample;

There is one problem with the above. We don't want to let the other program know about our "count" and "buffer" members. We want to hide them from the program. A program should never be allowed to directly access our object's data members. It should know only about the "lpVtbl" member so that it can call our functions. So, as far as the program is concerned, we want our IExample to be defined as so:

typedef struct {
   IExampleVtbl *lpVtbl;
} IExample;

Furthermore, although the typedefs for the function definitions make things easier to read, if you have a lot of functions in your object, this could get verbose and error-prone.

Finally, there is the problem that the above is a C definition. It really doesn't make things easy for a C++ program which wants to use our COM object. After all, even though we've written IExample in C, our IExample struct is really a C++ class. And it's a lot easier for a C++ program to use it defined as a C++ class than a C struct.

Instead of defining things as above, Microsoft provides a macro we can use to define our VTable and object in a way that works for both C and C++, and hides the extra data members. To use this macro, we must first define the symbol INTERFACE to the name of our object (which in this case is IExample). And prior to that, we must undef that symbol to avoid a compiler warning. Then, we use the DECLARE_INTERFACE_ macro. Inside of the macro, we list our IExample functions. Here's what it will look like:

#undef  INTERFACE
#define INTERFACE   IExample
DECLARE_INTERFACE_ (INTERFACE, IUnknown)
{
   STDMETHOD  (QueryInterface)  (THIS_ REFIID, void **) PURE;
   STDMETHOD_ (ULONG, AddRef)   (THIS) PURE;
   STDMETHOD_ (ULONG, Release)  (THIS) PURE;
   STDMETHOD  (SetString)       (THIS_ char *) PURE;
   STDMETHOD  (GetString)       (THIS_ char *, DWORD) PURE;
};

This probably looks a bit bizarre.

When defining a function, STDMETHOD is used whenever the function returns an HRESULT. Our QueryInterface, SetString, and GetString functions return an HRESULT. AddRef and Release do not. Those latter two return a ULONG. So that's why we instead use STDMETHOD_ (with an ending underscore) for those two. Then, we put the name of the function in parentheses. If the function doesn't return an HRESULT, we need to put what type it returns, and then a comma, before the function name. After the function name, we list the function's arguments in parentheses. THIS refers to a pointer to our object (i.e., IExample). If the only thing passed to the function is that pointer, then you simply put THIS in parentheses. That's the case for the AddRef and Release functions. But the other functions have additional arguments. So, we must use THIS_ (with an ending underscore). Then we list the remaining arguments. Notice that there is no comma between THIS_ and the remaining arguments. But there is a comma in between each of the remaining arguments. Finally, we put the word PURE and a semicolon.

To be sure, this is a weird macro, and it's this way mostly to define a COM object so that it works both for a plain C compiler as well as a C++ compiler.

"But where's the definition of our IExample struct?", you may ask. This macro is very weird indeed. It causes the C compiler to automatically generate the definition of a IExample struct that contains only the "lpVtbl" member. So just by defining our VTable this way, we automatically get a definition of IExample suitable for some other programmer.

Paste our two GUID macros into this include file, and we're all set. I did that to create the file IExample.h.

But as you know, our IExample really has two more data members. So what we're going to have to do is define a "variation" of our IExample, inside of our DLL source file. We'll call it a "MyRealIExample", and it will be the real definition of our IExample:

typedef struct {
   IExampleVtbl *lpVtbl;
   DWORD         count;
   char          buffer[80];
} MyRealIExample;

And we'll change a line in our IClassFactory's CreateInstance so that we allocate a MyRealIExample struct:

if (!(thisobj = GlobalAlloc(GMEM_FIXED, sizeof(struct MyRealIExample))))

The program doesn't need to know that we're actually giving it an object that has some extra data members inside it (which are for all practical purposes, hidden from that program). After all, both of these structs have the same "lpVtbl" member pointing to the same array of function pointers. But now, our DLL functions can get access to those "hidden" members just by typecasting a IExample pointer to a MyRealIExample pointer.

The Definition (DEF) file

We also need a DEF file to expose the two functions DllCanUnloadNow and DllGetClassObject. Microsoft's compiler also wants them to be defined as PRIVATE. Here's our DEF file, which must be fed to the linker:

LIBRARY IExample
EXPORTS
DllCanUnloadNow   PRIVATE
DllGetClassObject PRIVATE

Install the DLL, and register the object

We've now completed everything we need to do in order to make our IExample.dll. We can go ahead and compile IExample.dll.

But that's not the end of our job. Before any other program can use our IExample object (i.e., DLL), we need to do two things:

  1. Install our DLL somewhere that can be found by the computer running the program.
  2. Register our DLL as a COM component.

We need to create an install program that will copy IExample.DLL to a well-chosen location. For example, perhaps we'll create a "IExample" directory in the Program Files directory, and copy the DLL there. (Of course, our installer should do version checking, so that if there is a later version of our DLL already installed there, we don't overwrite it with an earlier version.)

We then need to register this DLL. This involves creating several registry keys.

We first need to create a key under HKEY_LOCAL_MACHINE\Software\Classes\CLSID. For the name of this new key, we must use our IExample object's GUID, but it must be formatted in a particular, text string format.

If you download the example project, the directory RegIExample contains an example installer for IExample.dll. The function stringFromCLSID demonstrates how to format our IExample GUID into a text string suitable for creating a registry key name with it.

Note: This example installer does not copy the DLL to some well-chosen location before registering it. Rather, it allows you to pick out wherever you've compiled IExample.dll and register it in that location. This is just for convenience in developing/testing. A production quality installer should copy the DLL to a well-chosen location, and do version checking. These needed enhancements are left for you to do with your own installer.

Under our "GUID key", we must create a subkey named InprocServer32. This subkey's default value is then set to the full path where our DLL has been installed.

We must also set a value named ThreadingModel to the string value "both", if we don't need to restrict a program to calling our DLL's functions only from a single thread. Since we don't use global data in our IExample functions, we're thread-safe.

After we run our installer, IExample.dll is now registered as a COM component on our computer, and some program can now use it.

Note: The directory UnregIExample contains an example uninstaller for IExample.dll. It essentially removes the registry keys that RegIExample created. A production quality uninstaller should also remove IExample.dll and any directories created by the installer.

An example C program

Now we're ready to write a C program that uses our IExample COM object. If you download the example project, the directory IExampleApp contains an example C program.

First of all, the C program #includes our IExample.h include file, so it can reference our IExample object's, and its VTable's, GUIDs.

Before a program can use any COM object, it must initialize COM, which is done by calling the function CoInitialize. This need be done only once, so a good place to do it is at the very start of the program.

Next, the program calls CoGetClassObject to get a pointer to IExample.dll's IClassFactory object. Note that we pass the IExample object's GUID as the first argument. We also pass a pointer to our variable classFactory which is where a pointer to the IClassFactory will be returned to us, if all goes well.

Once we have the IClassFactory object, we can call its CreateInstance function to get a IExample object. Note how we use the IClassFactory to call its CreateInstance function. We get the function via IClassFactory's VTable (i.e., its lpVtbl member). Also note that we pass the IClassFactory pointer as the first argument. Remember that this is standard COM.

Note that we pass IExample's VTable GUID as the third argument. And for the fourth argument, we pass a pointer to our variable exampleObj which is where a pointer to an IExample object will be returned to us, if all goes well.

Once we have an IExample object, we can Release the IClassFactory object. Remember that a program must call an object's Release function when done with the object. The IClassFactory is an object, just like IExample is an object. Each has its own Release function, which must be called when we're done with the object. We don't need the IClassFactory any more. We don't want to obtain any more IExample objects, nor call any of the IClassFactory's other functions. So, we can Release it now. Note that this does not affect our IExample object at all.

So next, we call the IClassFactory's Release function. Once we do this, our classFactory variable no longer contains a valid pointer to anything. It's garbage now.

But we still have our IExample pointer. We haven't yet Released that. So next, we decide to call some of IExample's functions. We call SetString. Then we follow up with a call to GetString. Note how we use the IExample pointer to call its SetString function. We get the function via IExample's VTable. And also notice that we pass the IExample pointer as the first argument. Again, standard COM.

When we're finally done with the IExample, we Release it. Once we do this, our exampleObj variable no longer contains a valid pointer to anything.

Finally, we must call CoUninitialize to allow COM to clean up some internal stuff. This needs to be done once only, so it's best to do it at the end of our program (but only if CoInitialize succeeded).

There's also a function called CoCreateInstance that can be used to replace the call to CoGetClassObject (to get the DLL's IClassFactory), and then the call to the IClassFactory's CreateInstance. CoCreateInstance itself calls CoGetClassObject, and then calls the IClassFactory's CreateInstance. CoCreateInstance directly returns our IExample, bypassing the need for us to get the IClassFactory. Here's an example use:

if ((hr = CoCreateInstance(&CLSID_IExample, 0, 
        CLSCTX_INPROC_SERVER, &IID_IExample, &exampleObj)))
   MessageBox(0, "Can't create IExample object", 
              "CoCreateInstance error", 
              MB_OK|MB_ICONEXCLAMATION);

An example C++ program

The directory IExampleCPlusApp contains an example C++ program. It does exactly what the C example does. But, you'll note some important differences. First, because the macro in IExample.h defines IExample as a C++ class (instead of a struct), and because C++ handles classes in a special way, the C++ program calls our IExample function in a different format.

In C, we get an IExample function by directly accessing the VTable (via the lpVtbl member), and we always pass the IExample as the first argument.

The C++ compiler knows that a class has a VTable as its first member, and automatically accesses its lpVtbl member to get a function in it. So, we don't have to specify the lpVtbl part. Also, the C++ compiler automatically passes the object as the first argument.

So whereas in C, we code:

classFactory->lpVtbl->CreateInstance(classFactory, 0, 
                      &IID_IExample, &exampleObj);

in C++, we instead code:

classFactory->CreateInstance(0, IID_IExample, &exampleObj);

Note: We also omit the & on the IID_IExample GUID. The GUID macro for C++ doesn't require that it be specified.

Modifying the code

To create your own object, make a copy of the IExample directory. Delete the Debug and Release sub-directories, and the following files:

IExample.dsp
IExample.dsw
IExample.ncb
IExample.opt
IExample.plg

In the remaining files (IExample.c, IExample.h, IExample.def), search and replace the string IExample with the name of your own object, for example IMyObject. Rename these files per your new object name (i.e., IMyObject.c, etc.).

Create a new Visual C++ project with your new object's name, and in this directory. For the type of project, choose "Win32 Dynamic-Link Library". Create an empty project. Then add the above three files to it.

Make sure you use GUIDGEN.EXE to generate your own GUIDs for your object and its VTable. Do not use the GUIDs that I generated. Replace the GUID macros in the .H file (and remember to replace the <<name>> part of the GUID macro too).

Remove the functions SetString and GetString in the .C and .H files, and add your own functions instead. Modify the INTERFACE macro in the .H file to define the functions you added.

Change the data members of MyRealIExample (i.e., MyRealIMyObject, whatever) to what you want.

Modify the installer to change the first three strings in the source.

In the example programs, search and replace the string IExample with the name of your object.

What's next?

Although a C or C++ program, or a program written in most compiled languages, can use our COM object, we have yet to add some support that will allow most interpreted languages to use our object, such as Visual Basic, VBscript, JScript, Python, etc. This will be the subject of Part II of this series.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Jeff Glatt

United States United States
No Biography provided

Comments and Discussions

 
Questionwhy not have function "CoFreeUnusedLibraries()" in Client Pinmemberyaxiya22-Apr-12 17:09 
GeneralMy vote of 5 Pinmembermanoj kumar choubey28-Feb-12 18:18 
QuestionSome question about com in plain c Pinmemberhash_man5-Feb-12 21:26 
AnswerRe: Some question about com in plain c Pinmembermilev yo5-Dec-12 15:24 
SuggestionFuture article...? PinmemberT800G25-Oct-11 10:07 
GeneralMy vote of 5 Pinmemberwang1986one28-Jul-11 21:09 
QuestionUsing in Windows 7 64bits PinmemberTri Cuong16-Jul-11 7:09 
AnswerRe: Using in Windows 7 64bits PinmemberJeff Glatt22-Jan-12 17:59 
Try recompiling it as a 64-bit DLL. There can be problems trying to use a 32-bit DLL on a 64-bit OS.
AnswerRe: Using in Windows 7 64bits Pinmembermilev yo5-Dec-12 15:28 
GeneralMy vote of 5 PinmemberAlenGdnt1236-Apr-11 17:46 
GeneralMy vote of 5 PinmemberLittleEd4-Feb-11 3:23 
GeneralMy vote of 5 PinmemberSam Mitchum28-Dec-10 9:39 
GeneralMy vote of 5 Pinmembera game like life5-Dec-10 20:46 
GeneralMy vote of 5 Pinmemberyao200720-Nov-10 23:55 
Generalerror on PutIBaseFilter PinmemberprojtctdMajd5-Oct-10 10:56 
QuestionRegister the object(s) Pinmembernils_a17-Aug-10 4:11 
GeneralThank you so much. Pinmemberfroma0toz92-Mar-10 17:05 
Generaldynmically loading the functions PinmemberMisterlolguy5-Feb-10 0:13 
GeneralNice Article PinmemberCybercockroach24-Sep-09 21:29 
GeneralRe: Nice Article PinmemberJeff Glatt22-Jan-12 18:22 
GeneralI like you so mush Pinmemberandy6500727-Aug-09 21:11 
GeneralUseless and obsolete Pinmemberkilt28-Jun-09 12:16 
GeneralRe: Useless and obsolete PinmemberSharjith20-Jul-09 6:13 
GeneralCOM BASICS Pinmemberjuaneric31-Mar-09 6:54 
GeneralOld book on COM with straght C Pinmemberdotmarker3-Sep-08 7:51 

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