CLR? To me, that's "Common Lisp Runtime"! (12 Mar 2008)

Alright, so I can no longer keep this to myself. I've been fantasizing about it for too long: I want a true Common Lisp implementation running on top of, and integrated with, Microsoft's CLR, and I want it badly.

It took a while, but after all those years at CoCreate (where we write a lot of Lisp code), I fell in love with the language. I want to work on projects which use Common Lisp, and I want the language to be successful and popular in lots of places - if only so that I have a choice of cool jobs should the need ever arise big grin

In other words, I want Common Lisp to become a mainstream language - which it arguably isn't, even though pretty much everybody agrees about its power and potential.

One way to acquire mainstream super-powers is to team up with one of the planet's most potent forces in both software development and marketing: Microsoft. This is the strategic reason for my proposal. Yes, I know, many Lisp gurus and geeks out couldn't care less about Microsoft and the Windows platform, or even shudder at the thought. But there are also tactical and technical reasons, so bear with me for a minute before you turn on your flamethrowers.

When I say Microsoft, I really mean .NET and its Common Language Runtime. Well, that's what they say is how to spell out CLR. But I claim that the L could just as well stand for Lisp, as the CLR, particularly in conjunction with the Dynamic Language Runtime extensions which Microsoft is working on, is a suspiciously suitable platform to build an implementation of Common Lisp upon: Not only does it provide a renowned garbage collector (designed by former Lisp guru Patrick Dussud) and a rich type system, it also has extensive reflection and code generation support, and - through the DLR - fast dynamic function calls, AST processing and compilation, debugger integration, REPL support, and all that jazz. It's no coincidence that languages such as C# and even VB.NET are picking up new dynamic language features with every new release, and that Microsoft has even added a new functional language, F#, to the set of languages which are (or will be) fully integrated into Visual Studio. The wave is coming in, and we better not miss it!

Best of all, it's not just about Windows anymore: The DLR and IronPython also run on top of Mono. Mono is available for Linux, Solaris, Mac OS X, various BSD flavors as well as for Windows, so layering Common Lisp on top of the CLR doesn't limit us to the Windows platform at all!

Note that I explicitly said "Common Lisp". I think that it's vital for an implementation on top of the CLR/DLR to be truly standards-compliant. I am not alone in this belief: In the IronPython and IronRuby projects, Microsoft went to great lengths to make sure that the implementations are true to the original language.

What would this buy us? Well, one recurring and dominant theme in discussions about the viability of Lisp as a mainstream language is the perceived or real lack of actively maintained libraries and tools. With the approach I'm outlining, we could still run all those excellent existing Common Lisp libraries and projects out there, but we'd also be able to use the huge body of code both in the .NET framework itself and in third-party .NET components. Common Lisp code could seamlessly call into a library written in, say, C#, and VB.NET programmers would be able to consume Common Lisp libraries!

Taking it a little further, we could also integrate with Visual Studio. Where I work, it would make all the difference in the world if we could edit, run and debug our Lisp code from within Visual Studio. I'm convinced that this would attract a large new group of programmers to Common Lisp. Hordes of them, in fact big grin

Yes, I know about SLIME and Dandelion and Cusp, and I'm perfectly aware that Emacs will simultaneously iron your shirts, whistle an enchanting tune, convincingly act on your behalf in today's team phone conference, and book flights to the Caribbean while compiling, debugging, refactoring and possibly even writing all your Lisp code for you in the background. Still, there's a whole caste of programmers who never felt any desire to reach beyond the confines of the Visual Studio universe, and are perfectly happy with their IDE, thank you very much. What if we could sell even those programmers on Common Lisp? (And yes, of course you and I could continue to use our beloved Emacs.)

Now, all these ideas certainly aren't original. There are a number of projects out there born out of similar motivation:

  • L Sharp .NET - a Lisp-based scripting language for .NET by Rob Blackwell
  • Yarr - Lisp-based scripting language for .NET based on L Sharp
  • dotLisp - a Lisp dialect for .NET, written by Rich Hickey (of Clojure fame)
  • Rich Hickey mentioned in a presentation that the original versions of Clojure were actually written to produce code for the CLR
  • IronLisp - Lisp on top of the DLR, initiated by Llewellyn Pritchard, who later decided to tackle IronScheme instead
  • There's a even a toy Common Lisp implementation by Microsoft which they shipped as a sample in the .NET Framework SDK (and now as part of the Rotor sources)
  • Joe Marshall has an interesting project which looks like Lisp implemented in C#.
  • LispSharp is a CLR-based Lisp compiler (Mirko Benuzzi)
  • ClearLisp is another CL dialect written in C# by Jan Tolenaar.
  • A LISP/Scheme language for .NET (Adam Milazzo)
  • CLearSharp, by Ola Bini
  • Joe Duffy's Sencha project
  • VistaSmalltalk may not sound like Lisp, but it actually contains a Lisp engine (implemented in C#), and according to the architecture notes I found, Smalltalk is implemented on top of Lisp.
  • CLinNET, by Dan Muller
  • CarbonLisp, by Eric Rochester
  • MBase, a "metaprogramming framework" providing a Lisp-like definition language
  • Sohail Somani experiments with .NET IL generation from Lispy syntax
  • RDNZL - .NET interop layer for Common Lisp (Edi Weitz)
  • FOIL - Foreign object interface for Lisp (i.e. an interop layer) on top of both the JVM and the CLR, by Rich Hickey (again!) and Eric Thorsen

Unfortunately, some of those projects are no longer actively maintained, others implement just a small subset of Common Lisp or even made design decisions which may conflict with the standard, or they are "merely" interop layers which allow Common Lisp code to call code written in managed languages, but don't provide full CLR integration. Don't get me wrong: Most of those projects produced impressive results - I don't mean to bash any of them, quite to the contrary.

What we learn from this project list is that there are quite a number of brilliant Lisp hackers out there who are both interested in such a project and capable of working on it. Most encouraging!

So maybe it isn't just me. Or am I really the only remaining Lisp programmer with such weird cravings? Be brutally honest with me: Am I a freak?

PS: I did not explicitly look for them while researching this article, but I know there are also a number of similar endeavours in the Scheme world, such as Common Larceny, Bigloo.NET, Dot-Scheme and Tachy.

PS/2: A few days after posting this article, I found that Toby Jones already coined the term "Common Lisp Runtime" three years ago...


Delegating closures to C# (26 Feb 2006)

Last time, I looked at how closures work in Lisp, and tried to mimick them in C++ (without success) using function objects.

To recap, a closure can be thought of as:

  • A function pointer referring to the code to be executed
  • A set of references to frames on the heap, namely references to all bindings of any free variables which occur in the code of the function.

C# 2.0 introduces anonymous delegates. Their implementation actually goes beyond simple delegates; they are, in fact, closures (I think). Here is an example:

class TestDelegate
{
  public delegate void MyDelegate();

  public MyDelegate GetDelegate()
  {
    string s = "Hiya";
    return delegate() { Console.WriteLine(s); }; // anon delegate
  }

  static void Main(string[] args)
  {
    TestDelegate p = new TestDelegate();
    MyDelegate anonDel = p.GetDelegate();
    anonDel();
  }
}

In the anonymous delegate, s is a free variable; the code compiles because the delegate refers to the definition of s in the surrounding code. If you run the above code, it will indeed print "Hiya", even though we are calling the delegate from Main, i.e. after we have left GetDelegate() which assigns that string to a local variable.

This is quite cool, considering that the .NET CLR uses a conventional stack and probably wasn't designed to run Lisp or Scheme all day. How do they do this?

Let's look at the disassembled code of GetDelegate() (using .NET Reflector, of course):

public TestDelegate.MyDelegate GetDelegate()
{
  TestDelegate.<>c__DisplayClass1 class1 = new TestDelegate.<>c__DisplayClass1();
  class1.s = "Hiya";
  return new TestDelegate.MyDelegate(class1.<GetDelegate>b__0);
}

So the compiler morphed our code while we were looking the other way! Instead of assigning "Hiya" to a local variable, the code instantiates a funky <>c__DisplayClass1 object, and that object apparently has a member called s which holds the string. The <>c__DisplayClass1 class also has an equivalent of the original GetDelegate function, as it seems. Hmmm.... very puzzling - let's look at the definition of that proxy class now:

[CompilerGenerated]
private sealed class <>c__DisplayClass1
{
      // Methods
      public <>c__DisplayClass1();
      public void <GetDelegate>b__0();

      // Fields
      public string s;
}

public void <GetDelegate>b__0()
{
      Console.WriteLine(this.s);
} 

Aha, now we're getting somewhere. The compiler moved the code in the anonymous delegate to the function <>c__DisplayClass1::<GetDelegate>b__0. This function has access to the field s, and that field is initialized by the compiler when the proxy object is instantiated.

So when the C# compiler encounters an anonymous delegate, it creates a proxy object which holds all "bindings" (in Lisp terminology) of free variables in the code of the delegate. That object is kept on the heap and can therefore outlive the original GetDelegate(), and that is why we can call the delegate from Main and still print the expected string instead of referring to where no pointer has gone before.

I find this quite a cool stunt; I'm impressed by how the designers of C# are adding useful abstractions to the language. Lisp isn't the only language which supports closures, and maybe wasn't even the first, but I'm pretty sure that the folks at Microsoft were probably influenced by either Lisp (or Scheme) while developing anonymous delegates. It is amazing how such an old language continues to inspire other languages to this day.

And that is, after reading a couple of good books and enlightening articles, what I understood about closures. Now, as a long-time boneheaded C++ programmer, I might have gotten it all wrong, and this blog entry is actually one way to test my assumptions; if my views are blatantly misleading, then hopefully somebody will point this out. (Well, if anybody reads this at all, of course.)

What a simple and amazing concept those closures really are! I only had to shed all my preconceptions about the supposedly one and only way to call and execute functions and how to keep their parameters and variables on a stack...

Closures are definitely very handy in all situations where callbacks are registered. Also, I already alluded to the fact that you could possibly build an object concept on top of closures in Lisp. And doesn't "snapshot of a function in execution" sound frighteningly close to "continuation" or "coroutines"? (Answer: Yes, kind of, but not quite. But that's a different story.)

I'm still trying to learn what closures do and how to best apply them in practice. But that doesn't mean they are constructs for the ivory tower: Knowing about them helped me only recently to diagnose and explain what originally looked like a memory leak in some Lisp test code that we had written. The final word of the jury is still out, but this is probably not a real leak, rather a closure which holds on to the binding of a variable, so that the garbage collector cannot simply free the resources associated with that variable.


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Blame CoCreate, for instance (14 Feb 2006)

The company I work for is called CoCreate. The name was chosen because the company's mission is all about collaboratively creating things. That's all nice and dandy, but I guess the team who picked the name didn't include a programmer, and so they overlooked something pretty obvious which causes mild confusion every now and then.

Most programmers, when confronted with our company name, think of COM. After all, one of the most important functions in all of the COM libraries prominently displays our company name: CoCreateInstance. Now, if a programmer thinks about COM (and hence software) when she hears about us, that's probably fine, because, after all, we're in the business to make and sell software.

However, customers are not necessarily that technology-savvy, nor should they have to be.

A while ago, a customer complained that our software was sloppy because it wouldn't uninstall itself properly and leave behind traces in the system. Our installer/uninstaller tests didn't seem to confirm that. So we asked the customer why he thought we were messing with his system. "Well", he said, "even after I uninstall your stuff, I still get those CoCreate error messages."

The customer sent a screenshot - it showed a message box, displayed by an application which shall remain unnamed, saying that "CoCreateInstance failed" and mumbling some COM error codes!

It took us a while to explain to the customer that no, we did not install this CoCreateInstance thing on the system, and that it is a system function, and if we actually tried to uninstall it along with our application as he requested (kind of), he wouldn't be terribly happy with his system any longer, and that the other app was actually trying to report to the customer that it had found a problem with its COM registration, and that this should be looked after, not our uninstaller. Phew.

Now if only we had the time-warping powers of the publishers of "The Hitchhiker's Guide To The Galaxy", we'd send our company marketing materials back into time before Microsoft invented COM, and then sue the living daylights out of them. Well, if we were evil, that is big grin

My memory took a little longer to swap back in, but while writing the above, it dawned on me that this incident wasn't the only one of its kind: Somebody had upgraded to a new PC and installed all applications except CoCreate's. Then, while syncing to his Palm Pilot, he got an "OLE CoCreateInstance Failed" error message, and started to search high and low on his shiny new PC for traces of CoCreate applications or components.

Puzzled, he posted to a newsgroup, and I replied with tongue-in-cheek:

Let me explain: When we kicked off CoCreate as a company, we sat together and thought about awareness strategies for the new company. So we called our buddies from Microsoft and asked them to name some API functions after us, and in exchange we would port our software to Windows NT. Neat scheme, and as you discovered on your system, the cooperation between the two companies worked just fine.

[... skipping explanation of the technical issue and hints on how to fix registry issue on the system ...]

The next step for CoCreate towards world domination will be to talk to some of our buddies in, say, Portugal, and offer them to develop a Portugese version of our application if they name their country after us.

Would I get away with a response like this if I was a support engineer? Maybe not. One more thing to like about being a software developer wink (Everybody in the newsgroup had a good chuckle back then.)


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Beware of unknown enumerators (26.1.2006)

The other day, I was testing COM clients which accessed a collection class via a COM-style enumerator (IEnumVARIANT). And those clients crashed as soon as they tried to do anything with the enumerator. Of course, the same code had worked just fine all the time before. What changed?

In COM, a collection interface often implements a function called GetEnumerator() which returns the actual enumerator interface (IEnumVARIANT), or rather, a pointer to the interface. In my case, the signature of that function was:

   HRESULT GetEnumerator(IUnknown **);

Didn't I say that GetEnumerator is supposed to return an IEnumVARIANT pointer? Yup, but for reasons which I may cover here in one of my next bonus lives, that signature was changed from IEnumVARIANT to IUnknown. This, however, is merely a syntactic change - the function actually still returned IEnumVARIANT pointers, so this alone didn't explain the crashes.

Well, I had been bitten before by smart pointers, and it happened again this time! The COM client code declared a smart pointer for the enumerator like this:

  CComPtr<IEnumVARIANT> enumerator = array->GetEnumerator();

This is perfectly correct code as far as I can tell, but it causes a fatal avalanche:

  • The compiler notices that GetEnumerator returns an IUnknown pointer. This doesn't match the constructor of this particular smart pointer which expects an argument of type IEnumVARIANT *.
  • So the compiler looks for other matching constructors.
  • It doesn't find a matching constructor in CComPtr itself, but CComPtr is derived from CComPtrBase which has an undocumented constructor CComPtrBase(int).
  • To match this constructor, the compiler converts the return value of GetEnumerator() into a bool value which compresses the 32 or 64 bits of the pointer into a single bit! (Ha! WinZip, can you beat that?)
  • The boolean value is then passed to the CComPtrBase(int) constructor.
  • To add insult to injury, this constructor doesn't even use its argument and instead resets the internally held interface pointer to 0.

Any subsequent attempt to access the interface through the smart pointer now crashes because the smart pointer tries to use its internal interface pointer - which is 0.

All this happens without a single compiler or runtime warning. Now, of course it was our own silly fault - the GetEnumerator declaration was bogus. But neither C++ nor ATL really helped to spot this issue. On the contrary, the C++ type system (and its implicit type conversions) and the design of the ATL smart pointer classes collaborated to hide the issue away from me until it was too late.


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Should Lisp programmers be slapped in public? (17.1.2006)

After fixing a nasty bug today, I let off some steam by surfing the 'net for fun stuff and new developments. For instance, Bjarne Stroustrup recently reported on the plans for C++0x. I like most of the stuff he presents, but still was left disturbingly unimpressed with it. Maybe it's just a sign of age, but somehow I am not really thrilled anymore by a programming language standard scheduled for 2008 which, for the first time in the history of the language, includes something as basic as a hashtable.

Yes, I know that pretty much all the major STL implementations already have hashtable equivalents, so it's not a real issue in practice. And yes, there are other very interesting concepts in the standard which make a lot of sense. Still - I used to be a C++ bigot, but I feel the zeal is wearing off; is that love affair over?

Confused and bewildered, I surf some other direction, but only to have Sriram Krishnan explain to me that Lisp is sin. Oh great. I happen to like Lisp a lot - do I really deserve another slap in the face on the same day?

But Sriram doesn't really flame us Lisp geeks; quite to the contrary. He is a programmer at Microsoft and obviously strongly impressed by Lisp as a language. His blog entry illustrates how Lisp influenced recent developments in C# - and looks at reasons why Lisp isn't as successful as many people think it should be.

Meanwhile, back in the C++ jungle: Those concepts are actually quite clever, and solve an important problem in using C++ templates.

In a way, C++ templates use what elsewhere is called duck typing. Why do I say this? Because the types passed to a template are checked implicitly by the template implementation rather than its declaration. If the template implementation says f = 0 and f is a template parameter, then the template assumes that f provides an assignment operator - otherwise the code simply won't compile. (The difference to duck typing in its original sense is that we're talking about compile-time checks here, not dynamic function call resolution at run-time.)

Hence, templates do not require types to derive from certain classes or interfaces, which is particularly important when using templates for primitive types (such as int or float). However, when the type check fails, you'll drown in error messages which are cryptic enough to violate the Geneva convention. To fix the error, the user of a template often has to inspect the implementation of the template to understand what's going on. Not exactly what they call encapsulation.

Generics in .NET improve on this by specifying constraints explicitly:

  static void Foobar<T>(IFun<T> fun) where T : IFunny<T>
  {
    ... function definition ...
  }

T is required to implement IFunny. If it doesn't, the compiler will tell you that T ain't funny at all, and that's that. No need to dig into the implementation details of the generic function.

C++ concepts extend this idea: You can specify pretty arbitrary restrictions on the type. An example from Stroustrup's and Dos Reis' paper:

  concept Assignable<typename T, typename U=T> {
    Var<T> a;
    Var<const U> b;
    a = b;
  };

  ;; using this in a template definition:
  template <typename T, typename U>
    where Assignable<T, U>
  ... template definition ...

So if T and U fit into the Assignable concept, the compiler will accept them as parameters of the template. This is cute: In true C++ tradition, this provides maximum flexibility and performance, but solves the original problem.

Still, that C# code is much easier on the eye...


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To cut a long filename short, I lost my mind (8.1.2006)

Yesterday, I explained how easy it is to inadvertedly load the same executable twice into the same process address space - you simply run it using its short DOS-ish filename (like Sample~1.exe) instead of its original long filename (such as SampleApplication.exe). For details, please consult the original blog entry.

I mentioned that one fine day I might report how exactly this happened to us, i.e. why in the world our app was started using its short filename. Seems like today is such a fine day smile

Said application registered itself as a COM server, and it does so using the services of the ATL Registrar. Upon calling RegisterServer, the registrar will kindly create all the required registry entries for a COM server, including the LocalServer entry which contains the path and filename of the server. Internally, this will call the following code in atlbase.h:

inline HRESULT WINAPI CComModule::UpdateRegistryFromResourceS(UINT nResID, 
  BOOL bRegister, struct _ATL_REGMAP_ENTRY* pMapEntries)
{
   USES_CONVERSION;
   ATL::CRegObject ro;
   TCHAR szModule[_MAX_PATH];
   GetModuleFileName(_pModule->GetModuleInstance(), szModule, _MAX_PATH);

   // Convert to short path to work around bug in NT4's CreateProcess
   TCHAR szModuleShort[_MAX_PATH];
   GetShortPathName(szModule, szModuleShort, _MAX_PATH);
   LPOLESTR pszModule = T2OLE(szModuleShort);
   ...

Aha! So ATL deliberately converts the module name (something like SampleApplication.exe) into its short-name equivalent (Sample~1.exe) to work around an issue in the CreateProcess implementation of Windows NT. MSKB:179690 describes this problem: CreateProcess could not always handle blanks in pathnames correctly, and so the ATL designers had to convert the path into its short-path version which converts everything into an 8+3 filename and hence guarantees that the filename contains no blanks.

Adding insult to injury, MSKB:201318 shows that this NT-specific bug fix in ATL has a bug itself... and, of course, our problem is, in fact, caused by yet another bug in the bug fix (see earlier blog entry).

For my application, the first workaround was to use a modified version of atlbase.h which checks the OS version; if it is Windows 2000 or later, no short-path conversion takes place. Under Windows NT, however, we're caught in a pickle: Either we use the original ATL version of the registration code and thus map the executable twice into the address space, or we apply the same fix as for Windows 2000, and will suffer from the bug in CreateProcess if the application is installed in a path which has blanks in the pathname.

In my case, this was not a showstopper issue because the application is targeting Windows 2000 and XP only, so I simply left it at that.

Another approach is to use the AddReplacement and ClearReplacements APIs of the ATL registrar to set our own conversion rules for the module name and thereby override ATL's own rules for the module name:

#include <atlbase.h>
#include <statreg.h>

void RegisterServer(wchar_t *widePath, bool reg)
{
  ATL::CRegObject ro;
  ro.AddReplacement(L"Module", widePath);
  reg ? ro.ResourceRegister(widePath, IDR_REGISTRY, L"REGISTRY") :
        ro.ResourceUnregister(widePath, IDR_REGISTRY, L"REGISTRY");
}


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What's my email address? (4 Jan 2006)

On various occasions, I had already tried to make sense out of directory services such as LDAP and Microsoft's ADSI. Now, while that stuff is probably not rocket science, the awkward terminology and syntax in this area have always managed to shy me away; most of the time, there was another way to accomplish the same without going through LDAP or ADSI, and so I went with that.

This time, the task was to retrieve the email address (in SMTP format) for a given user. In my first attempt, I tried to tap the Outlook object model, but then figured that a) there are a few systems in the local domain which do not have Outlook installed and b) accessing Outlook's address info causes Outlook to display warnings to the user reporting that somebody apparently is spelunking around in data which they shouldn't be accessing. Which is probably a good idea, given the overwhelming "success" of Outlook worms in the past, but not exactly helpful in my case.

However, everybody here is connected to a Windows domain server and therefore has access to its AD services, so that sounded like a more reliable approach. I googled high and low, dissected funky scripts I found out there and put bits of pieces of them together again to form this VBscript code:

  user="Claus Brod"
  context=GetObject("LDAP://rootDSE").Get("defaultNamingContext")
  ou="OU=Users,"
  Set objUser = GetObject("LDAP://CN=" & user & "," & ou & context)
  WScript.Echo(objUser.mail)

  groups=objUser.Get("memberOf")
  For Each group in groups
    WScript.Echo("  member of " & group)
  Next

This works, but the OU part of the LDAP string (the "ADsPath") depends on the local organizational structure and needs to be adapted for each particular environment; I haven't found a good way to generalize this away. Hints most welcome.

PS: For those of you on a similar mission, Richard Mueller provides some helpful scripts at http://www.rlmueller.net/freecode3.htm.


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Fingerpointing at smart pointers (31 Dec 2005)

In an ATL COM client which uses #import to generate wrapper code for objects, I recently tracked down a subtle reference-counting issue down to this single line:

  IComponentArray *compArray = app->ILoadComponents();

This code calls a method ILoadComponents on an application object which returns an array of components. Innocent-looking as it is, this one-liner caused me quite a bit of grief. If you can already explain what the reference counting issue is, you shouldn't be wasting your time reading this blog. For the rest of us, I'll try to dissect the problem.

(And for those who don't want to rely on my explanation: After I had learnt enough about the problem so that I could intelligently feed Google with search terms, I discovered a Microsoft Knowledge Base article on this very topic. However, even after reading the article, some details were still unclear to me, especially since I don't live and breathe ATL all day.)

The #import statement automatically generates COM wrapper functions. For ILoadComponents, the wrapper looks like this:

inline IComponentArrayPtr IApplication::ILoadComponents () {
    struct IComponentArray * _result = 0;
    HRESULT _hr = raw_ILoadComponents(&_result);
    if (FAILED(_hr)) _com_issue_errorex(_hr, this, __uuidof(this));
    return IComponentArrayPtr(_result, false);
}

IComponentArrayPtr is a typedef-ed template instance of _com_ptr_t. The constructor used in the code snippet above will only call AddRef on the interface pointer if its second argument is true. In our case, however, the second arg is false, so AddRef will not be called. The IComponentArrayPtr destructor, however, always calls Release().

Feeling uneasy already? Yeah, me too. But let's follow the course of action a little bit longer. When returning from the wrapper function, the copy constructor of the class will be called, and intermediate IComponentArrayPtr objects will be created. As those intermediate objects are destroyed, Release() is called.

Now let us assume that the caller looks like above, i.e. we assign the return value of the wrapper function to a CComPtr<IComponentArray> type. The sequence of events is as follows:

  • Wrapper function for ILoadComponents is called.
  • Wrapper function calls into the COM server. The server returns an interface pointer for which AddRef() was called (at least) once inside the server. The reference count is 1.
  • Wrapper function constructs an IComponentArrayPtr smart pointer object which simply copies the interface pointer value, but does not call AddRef(). The refcount is still 1.

Now we return from the wrapper function. In C++, temporary objects are destroyed at the end of the "full expression" which creates them. See also section 6.3.2 in Stroustrup's "Design and Evolution of C++". This means that the following assignment is safe:

  CComPtr<IComponentArray> components = app->ILoadComponents();

ILoadComponents returns an object of type IComponentArrayPtr. At this point, the reference count for the interface is 1 (see above). The The compiler casts IComponentArrayPtr to IComponentArray*, then calls the CComPtr assignment operator which copies the pointer and calls AddRef on it. The refcount is now 2. At the completion of the statement, the temporary IComponentArrayPtr is destroyed and calls Release on the interface. The refcount is 1. Just perfect.

Now back to the original client code:

  IComponentArray *compArray = app->ILoadComponents();

Here, we assign to a "raw" interface pointer, rather than to a CComPtr, When returning from the wrapper function, the refcount for the interface is 1. The compiler casts IComponentArrayPtr to IComponentArray* and directly assigns the pointer. At the end of the statement (i.e. the end of the "full expression"), the temporary IComponentArrayPtr is destroyed and calls Release, decrementing the refcount is 0. The object behind the interface pointer disappears, and subsequent method calls on compArray will fail miserably or crash!

So while ATL, in conjunction with the compiler's #import support, is doing its best to shield us from the perils of reference counting bugs, it won't help us if someone pulls the plug from the ATL force-field generator by incorrectly mixing smart and raw pointers.

This kind of reference counting bug would not have occurred if I had used raw interface pointers throughout; the mismatch in calls to AddRef and Release would be readily apparent in such code. However, those smart pointers are indeed really convenient in practice because they make C++ COM code so much simpler to read. However, they do not alleviate the programmer from learning about the intricacies of reference counting. You better learn your IUnknown before you do CComPtr.

This reminds me of Joel Spolsky's The Perils of JavaSchools, which is soooo 1990 (just like myself), but good fun to read.


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Should type be of type Type? (31 Dec 2005)

The other day, we were writing some .NET code for which we produced COM wrappers, including a type library (through tlbexp). One of the exported methods looked like this (in Managed C++):

  void AddButton(String *title, String *icon, ActionType type);

In a COM test client which referred to said type library, the compiler reported inexplicable errors. They hinted that the parameter named (!) type somehow was thought to be of type System.Type... but why? type is correctly declared as an ActionType, not as System::Type!

I won't tell you what other means we used to track down this issue; let me just advise you to clean those chicken blood stains as early as possible, before they stick to your keyboard just like, well, chicken blood .-) In the end, Adam Nathan's blue bible had the right hint for us: Type libraries maintain a case-insensitive identifier table. Once an identifier has been added to the table in one case, any subsequent attempts to add the identifier to the table again will simply fail, regardless of the case.

So in our example, the first "type"-ish identifier which was added to the table was System::Type. (Or maybe it was actually a parameter called type which was of type System::Type?). Later, the parameter name type was encountered, but no longer added to the table because of its unlikely relative which made it into the table first. Any subsequent references to anything called "type" or "Type" or "tYPE" would then resolve to System::Type, with the aforementioned consequences.


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.NET tut net... (13. Dezember 2004)

Auf meine alten Tage befasse ich mich doch tatsächlich auch noch mit solchen Dingen wie Microsoft .NET. Wenn man mir das während meines Studiums und meiner hyperaktiven Atari-Zeit prophezeit hätte...

Nun denn. Kommt mir neulich so ein häßlicher kleiner Käfer entgegen. Und das kam so: Wenn man Werte vom Typ bool aus "managed code" in "unmanaged code" per Marshaling überträgt, geht das in so richtig großen Stil schief.

Der "managed code" (in C++) stellt einen einfachen Aufruf zur Verfügung mit einem Rückgabewert vom Typ bool. Vereinfachter Beispielcode:

  public __gc __interface IBool {
    bool Foo(void);
  };

  public __gc class Booltest : public IBool 
  {
  public:
    Booltest() {}
    bool Foo(void) { return false; }
  };

Diesen Code übersetzt man in ein Assembly, und daraus produziert man mittels regasm eine Typenbibliothek (tlb-Datei). Gleichzeitig registriert regasm das Assembly; und hinterher werfen wir das Assembly in den Schlund des GAC. Auszug aus der erzeugten Typenbibliothek:

    interface IBool : IDispatch {
        [id(0x60020000)]
        HRESULT Foo([out, retval] unsigned char* pRetVal);
    };

Man beachte, daß der Typ des Rückgabewerts vom bool nach unsigned char abgeändert wurde. Soweit keine Überraschung, denn Nathan erwähnt in seiner COM-Interop-Bibel, daß der Marshaling-Typ für Werte vom Typ bool eben UnmanagedType::U1 ist - also unsigned char.

So richtig übel wird es aber, wenn ich nun Foo() aus einem COM-Client zu rufen versuche. Der COM-Client erzeugt sich einen "smart pointer" vom Typ IBoolPtr und ruft dann Foo():

  bool ret = pBool->Foo();

Nach diesem Aufruf ist allerdings der Stack beschädigt. Läft man im Einzelschritt durch den Code, merkt man, daß der COM-Client denkt, der Rückgabewert sei ein Byte gross; daher legt er auch nur ein Byte auf dem Stack für die Ergebnisvariable ret an. Der Marshaling-Code allerdings schreibt munter vier Bytes!

Das gleiche passiert, wenn man im "managed code" das Attribut [MarshalAs(UnmanagedType::U1)] ausdrücklich anwendet. Microsoft beschreibt im Knowledge-Base-Artikel 823071 einen möglicherweise verwandten Fehler beim Marshaling von bool-Werten, allerdings hilft der vorgeschlagene Hotfix in meinem Fall nicht. Ändere ich den Marshaling-Typ auf U2, wird's noch lustiger: Dann überschreibt der Marshaler zwar keinen Speicher mehr, räumt dafür aber den Stackpointer nach dem Aufruf nicht mehr auf!

Hat jemand Ideen?

(Siehe auch http://groups.google.de/groups?hl=de&lr=&threadm=10gd95ifu6uoe9a%40corp.supernews.com.)

PS: Einige Zeit später hat uns Microsoft bestätigt, daß das in der Tat ein Fehler in .NET 1.1 war. .NET 2.0 macht's nun richtig.



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