Structure and Interpretation of Computer Programmers

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Tuesday, December 24, 2013

Standing at the Crossroads

A while back I wrote Conflicts in my Mental Model of Objective-C, in which I listed a few small scale dichotomies or cognitive dissonances that plagued my notion of my work. I just worked out what the overall picture is, the jigsaw into which all of these pieces can be assembled.

And I do mean just. It’s about 1AM on Christmas Eve, but this picture hit me so hard I couldn’t stop thinking about it without writing it down and getting it out of my head. If it doesn’t explain everything, it shows me the shape of the solution at least.

A tale of two Apples

I believe that everything I wrote in the Conflicts post can be understood in terms of two different and (of course) opposed models of Apple. I also believe that the two models are irreconcilable, but that the opposition is also accidental, not essential. That by removing the supposed conflict between them, everything I thought was a problem can be resolved.

It was the best of iPads

The iPad is, I would argue (and accept that this is a subjective argument) the most tasteful application of computing technology to the world of many computer users. I would further argue, and this is perhaps on firmer ground, that the reason the iPad is so tasteful is because Apple spend a lot more time and resources on worrying about questions of taste than many of their competitors and others in the industry.

There are many visions that have combined to produce the iPad, but interestingly the one that I think is clearest is John Scully’s Knowledge Navigator. In the concept videos for Knowledge Navigator, a tablet computer with natural language speech comprehension and a multi-touch screen is able to use the many hyperlinked documents available on the Web to answer a wide range of questions, make information available to its users and even help them to plan their schedules.

This is what we have now. This is the iPad, with Siri and a host of third-party applications. Apple even used to use the slogan “there’s an app for that”. Do you have a problem? You can probably solve it with iOS and a trip to the app store.

It was the worst of iPads

OK, what do you do if your problem isn’t solved on the app store, or the available solutions aren’t satisfactory?

Well first, you’d better get yourself another computer because while the iPad is generally designed for solving problems it isn’t designed for solving general problems. You might be able to find some code editors on the iPad, but you sure aren’t going to use them to write an iPad app without external assistance.

OK, so you’ve got your computer, and you’ve learned how to do the stuff that makes iPad apps. Now you just pay a recurring fee to be allowed to put that stuff onto your iPad. And what if you want to share that with your friends? Only if it meets Apple’s approval.

If the iPad is the Knowledge Navigator, it is not the Dynabook. A Dynabook is a computer that you can use to solve your problems on, but it’s also one on which you can create solutions to your own problems.

The promise of the Dynabook is that if you understand what your problem is, you can model that problem on the Dynabook. You model it with objects-either your own or ones supplied for you. You can change and create these objects until they model the problem you have, at which point you can use them to compute a solution.

The irony is that we have all the parts needed in a Dynabook, all in the iPad. Computer so simple even children can use it? Check. Objects? Check. Repositories of objects created by other people so we don’t have to rewrite our own basic objects all the time? We call that CocoaPods (or RubyGems, or whatever the poison in your area of the world). But we just can’t put all of these things together on that computer itself.

That would be distasteful. That might let people do things that make the iPad look bad. That might mean iPads providing experiences that haven’t been vetted by the mothership.

Does using an iPad ever make you wonder how iPads work? What they can do? What you can make them do? You can answer these questions, but not using an iPad. Your Knowledge Navigator does not know the route to that particular destination.

This is what is truly meant when it is said that the iPad is not upgradeable. Forget swapping out memory chips or radio transmitters. Those are just lumps of sand inside a box made of melted sand and refined rock. The iPad is not upgradeable because you are stuck with the default experience: the out-of-the-box facilities plus those that have been approved from on high. It might be good, but it might not be good enough.

Notice that this is not an “everyone must program” position. That would be a very bad experience. The position is rather “everyone must have the facility, should they be so inclined, to make their computer better for them than the manufacturers did”.


I think that the Apple described above is not at the intersection of technology and the liberal arts. It is at the border, a self-appointed barrier of things that might flow between the two.

I believe that the two visions can be reconciled, and that a thing can be both the Knowledge Navigator and the Dynabook. I don’t believe you have to disable some experiences to provide others. I believe that Apple the champions of tasteful computing can be applauded at every turn while Apple the high priests of the church of computing can be fought tooth and nail.

Enablers? Yes, please. Arbiters? No, thanks.

posted by Graham at 01:58  

Wednesday, December 18, 2013

A sneaky preview of ClassBrowser

Let me start with a few admissions. Firstly, I have been computering for a good long time now, and I still don’t really understand compilers. Secondly, work on my GNUstep Web side-project has tailed off for a while, because I decided I wanted to try something out to learn about the compiler before carrying on with that work. This post will mostly be about that something.

My final admission: if you saw my presentation on the ObjC runtime you will have seen an app called “ClassBrowser” where I showed that some of the Foundation classes are really in the CoreFoundation library. Well, there were two halves to the ClassBrowser window, and I only showed you the top half that looked like the Smalltalk class browser. I’m sorry.

So what’s the bottom half?

This is what the bottom half gives me. It lets me go from this:

ClassBrowser before

via this:

Who are you calling a doIt?

to this:

ClassBrowser after

What just happened?

You just saw some C source being compiled to LLVM bit code, which is compiled just-in-time to native code and executed, all inside that browser app.


Well why not? Less facetiously:

  • I’m a fan of Smalltalk. I want to build a thing that’s sort of a Smalltalk, except that rather than being the Smalltalk language on the Objective-C runtime (like F-Script or objective-smalltalk), it’ll be the Objective-C(++) language on the Objective-C runtime. So really, a different way of writing Objective-C.
  • I want to know more about how clang and LLVM work, and this is as good a way as any.
  • I think, when it actually gets off the ground, this will be a faster way of writing test-first Objective-C than anything Xcode can do. I like Xcode 5, I think it’s the better Xcode 4 that I always wanted, but there are gains to be had by going in a completely different direction. I just think that whoever strikes out in such a direction should not release their research project as a new version of Xcode :-).

Where can I get me a ClassBrowser?

You can’t, yet. There’s some necessary housekeeping that needs to be done before a first release, replacing some “research” hacks with good old-fashioned tested code and ensuring GNUstep-GUI compatibility. Once that’s done, a rough-and-nearly-ready first open source release will ensue.

Then there’s more work to be done before it’s anything like useful. Particularly while it’s possible to use it to run Objective-C, it’s far from pleasant. I’ve had some great advice from LLVM IRC on how to address that and will try to get it to happen soon.

posted by Graham at 01:51  

Tuesday, December 17, 2013

By your _cmd

This post is a write-up of a talk I gave at Alt Tech Talks: London on the Objective-C runtime. Seriously though, you should’ve been there.

The Objective-C runtime?

That’s the name of the library of C functions that implement the nuts and bolts of Objective-C. Objects could just be represented as C structures, and methods could just be implemented as C functions. In fact they sort of are, but with some extra capabilities. These structures and functions are wrapped in this collection of runtime functions that allows Objective-C programs to create, inspect and modify classes, objects and methods on the fly.

It’s the Objective-C runtime library works out what methods get executed, too. The [object doSomething] syntax does not directly resolve a method and call it. Instead, a message is sent to the object (which gets called the receiver in this context). The runtime library gives objects the opportunity to look at the message and decide how to respond to it. Alan Kay repeatedly said that message-passing is the important part of Smalltalk (from which Objective-C derives), not objects:

I’m sorry that I long ago coined the term “objects” for this topic because it gets many people to focus on the lesser idea.

The big idea is “messaging” – that is what the kernal[sic] of Smalltalk/Squeak is all about (and it’s something that was never quite completed in our Xerox PARC phase). The Japanese have a small word – ma – for “that which is in between” – perhaps the nearest English equivalent is “interstitial”. The key in making great and growable systems is much more to design how its modules communicate rather than what their internal properties and behaviors should be.

Msg send

Indeed in one article describing the Smalltalk virtual machine the programming technique is called the message-passing or messaging paradigm. “Object-Oriented” is used to describe the memory management system.

Throughout this talk and post I’m talking about the ObjC runtime, but there are many. They all support an object’s introspective and message-receiving capabilities, but they have different features and work in different ways (for example, Apple’s runtime sends messages in one step, but the GNU runtime looks up messages and then invokes the discovered function in two steps). All of the discussion below relates to Apple’s most modern runtime library (the one that is delivered as part of OS X since 10.5 and iOS).

In the talk, I decided to examine a few specific areas of the runtime library’s behaviour. I looked for things that I wanted to understand better, and came up with questions I wanted to answer as part of the talk.

Dynamic class creation

Can I implement Key-Value Observing?

While I was preparing the talk, a post called KVO considered harmful started to get a lot of coverage. That post raises a lot of valid criticisms of the Key-Value Observing API, but rather than throw away the Observer pattern I wanted to explore a new implementation.

The observed (pardon the pun) behaviour of KVO is to privately subclass the observed object’s class, so that it can customise the object’s behaviour to call the KVO callback. That’s done through a function called objc_duplicateClass, unfortunately the documentation tells us that we should not call this function ourselves.

It’s still possible to implement an Observer pattern that uses the same secret-subclass behaviour, by allocating and registering a “class pair”. What’s a class pair? Well each class in Objective-C is really two classes: the class object defines the instance methods, and the “metaclass” defines the class methods. So each class is really a singleton instance of its metaclass.

The ObserverPattern implementation shows how this works. When you add an observer to an object, the receiver first works out whether it’s an instance of the observable class. If it needs to create that class, it does so: adding our own implementations of -dealloc to clean up after ourselves, and -class so that, like KVO observable objects, the generated class name doesn’t appear when you ask an observed object its type.

Having created the class, the code goes on to add a setter for the conventional Key-Value Coding selector name for the property: this setter grabs the old and new values of the property and invokes the callback which was supplied as a block object. Because we can, the block is dispatched asynchronously.

Notice that the -addObserverForKey:withBlock: method uses object_setClass() to replace the receiver’s class with the newly-constructed class. The main effect of this is to change the way messages are resolved onto methods, but you need to be careful that the original and replaced class have the same instance variable layout too. Instance variables are looked up via the runtime too, and changing the class could alter where the runtime thinks the bytes are for any given variable.

We have a little extra hurdle to overcome in storing the collection of observer tokens, because there’s nowhere to put them. Adding an instance variable to the ObserverPattern[…] class would not work, as instances of that class are never actually allocated. The objects involved have the instance variables of their initial class, which won’t include space for the observers.

The Objective-C runtime provides for this situation by giving us associated objects. Any object can have what is, conceptually, a dictionary of other objects maintained by the runtime. Observed objects can store and retrieve their observer tokens via associated references, and no extra instance variables are needed.

A little problem in the ObserverPattern implementation will become clear if you run it enough times. The observation callbacks are sent asynchronously, and can be delivered out of sequence. That means the observer can’t actually tell what the final state of the observed key is, because the “new value” received in the callback might have already been replaced. I left this fun issue in to demonstrate that KVO’s synchronous implementation is a feature, not a bug.

Creating objects

What are those extra bytes for?

When you create an Objective-C object, the runtime lets you allocate some extra storage at the end of the space reserved for its instance variables. What’s the point of that? All you can do is get a pointer to the start of the space (using object_getIndexedIvars)…hmm, indexed ivars. Well, I suppose an array is a pretty obvious use of indexed ivars…

Let’s build NSArray! There are two things to see in SimpleArray: the most obvious is the use of the class cluster pattern. The reason is that the object returned from +alloc—where we’d normally allocate space for the object—cannot know how big it’s going to be. We need to use the arguments to -initWithObjects:count: to know how many objects there are in the array. So +alloc returns a placeholder, which is then able to allocate and return the real array object.

One obvious question to ask is why we’d do this at all. Why not just use calloc() to grab an appropriately-sized buffer in which to store the object pointers? The answer is to do with a low-level performance concern called locality of reference. We know from the design of the array class that pretty much every time the array pointer is used, the buffer pointer will be used too. Putting them next to each other in RAM means we don’t have to look off at some dereferenced pointer just to find another pointer.

Message dispatch

Just how does message forwarding work?

One of the powerful features of Objective-C is that an object doesn’t have to implement a method when it’s compiled to be able to respond to messages with that selector name. It can lazily resolve the methods, or it can forward them to another object, or it can raise an error, or it can do something else. But something about this feature was bugging me: message forwarding (which happens in the runtime) calls -forwardInvocation:, passing it an NSInvocation object. But NSInvocation is defined in Foundation: does the runtime library need to “know” about Foundation to work?

I tracked down what was going on and found that no, it does not need to know about Foundation. The runtime lets applications define the forwarding function, that gets called when objc_msgSend() can’t find the implementation for a selector. On startup, CoreFoundation[+] injects the forwarding function that does -forwardInvocation:. So presumably my application can do its own thing, right?

Let’s build Ruby! OK, not all of Ruby. But Ruby has a #method_missing function that gets called when an object receives a message it doesn’t understand, which is much more similar to Smalltalk’s approach than to Objective-C’s. Using objc_setForwardHandler, it’s possible to implement methodMissing: in our Objective-C classes.


The Objective-C runtime is a powerful way to add a lot of dynamic behaviour to an application for very little work. Some developers don’t use it much beyond swizzling methods for debugging, but it has facilities that make it a powerful tool for real application code too.

[+]CoreFoundation and Foundation are really siblings, and they each expose pieces of the other’s implementation, but one has a C interface and the other an Objective-C interface. Various Objective-C classes are actually part of CoreFoundation, including NSInvocation and the related NSMethodSignature class. NSObject is not in either of these libraries: it’s now defined in the runtime itself, so that the runtime’s memory management functions know about -retain, -release and so on[++]. On the other hand, most of the *behaviour* of NSObject is implemented by categories higher up. And, of course, this is all implementation detail and the locations of these classes could be (and are) moved between versions of the frameworks.

[++]Other languages like Smalltalk and Ruby have a simple base class that does nothing except know how to be an object, called ProtoObject or BaseObject. You could imagine the runtime supplying—and being coupled to—ProtoObject, and (Core)Foundation supplying NSObject and NSProxy as subclasses of ProtoObject.

posted by Graham at 10:42  

Thursday, December 12, 2013

The Ignoble Programmer

Two programmers are taking a break from their work, relaxing on a bench in the park across from their office. As they discuss their weekend plans, a group of people jog past, each carrying their laptop in a yoke around their neck and furiously typing as they go.

“Oh, there goes the Smalltalk team,” says the senior of the two programmers on the bench. “They have to do everything at run-time.”

I love jokes. And not just because they’re sometimes funny, though that helps: I certainly find I enjoy a conversation and can relax more when at least two of the people involved are having fun. When only one person is joking, it gets awkward (particularly if everyone else is from HR). But a little levity can go a long way toward disarming an unpleasant truth so that it can be discussed openly. Political leaders through the ages have taken advantage of this by appointing jesters and fools to keep them aware of intrigues in the courts: even the authors of the American bill of rights remembered the satirist before the shooter.

I also like jokes because of the thought that goes into constructing a good (or deliberately bad) one. There’s a certain kind of creativity that goes into identifying an apparently absurd connection, exactly because of the absurdity. Being able to construct a joke, and being practised at constructing jokes, means being able to see new contexts and applications for existing ideas. Welcome to the birthplace of reuse and exploring the bounds of a construct’s application: welcome to the real home of software architecture.

But there’s a problem, or at least an opportunity (or maybe just a few thousand consulting dollars to be made and a book to be written). That problem is this: everyone else puts way more effort into their jokes than programmers do. Take this one, from the scientists:

Neural Correlates of Interspecies Perspective Taking in the Post-Mortem Atlantic Salmon

They didn’t just joke about doing a brain scan of a dead fish, they did a brain scan of a dead fish. And published the (serendipitous and unexpected) results. But they didn’t just angle for a laugh, they had a real point. The subtitle of their paper:

An Argument For Proper Multiple Comparisons Correction

And isn’t it fun that some microbiologists demonstrated that beards are significant vectors for microbial infections?

Both of these examples were lifted from the Annals of Improbable Research’s Ig Nobel Prizes, awarded for “achievements that first make people laugh, and then makes them think”. The Ig Nobels have been awarded every year since 1991, and in that time only one computer science award has been granted. That award was given to the developer behind PawSense, a utility that detects and blocks typing caused by your cat walking across your keyboard.

Jokes that first make you laugh, and then make you think, are absolutely the best jokes you can make about my work. If I conclude “you’re right, that is absurd, but what if…” then you’ve done it right. Jokes that are thought-terminating statements can make us laugh, and maybe make us feel good about what we’re doing, but cannot make us any better at it because they don’t give us the impetus to reflect on our craft. Rather, they make us feel smug about knowing better than the poor sap who’s the butt of the joke. They confirm that we’ve nothing to learn, which is never the correct outlook.

We need more Ig Nobel-quality achievements in computing. Disarming the absurd and the downright broken in programming and presenting them as jokes can first make us laugh, and then make us think.

N.B. My complete connection to the Annals of Improbable Research is that I helped out on the AV desk at a couple of their talks. At their talk in Oxford in 2006 I was inducted into the Luxuriant and Flowing Hair Club for Scientists.

posted by Graham at 00:06  

Friday, December 6, 2013

Agile application security

There’s a post by clever security guy Jim Bird on Appsec’s Agile Problem: how can security experts participate in fast-moving agile (or Agile™) projects without either falling behind or dragging the work to a halt?

I’ve been the Appsec person on such projects, so hopefully I’m in a position to provide at least a slight answer :-).

On the team where I did this work, projects began with the elevator pitch, application definition statement, whatever you want to call it. “We want to build X to let Ys do Z”. That, often with a straw man box-and-line system diagram, is enough to begin a conversation between the developers and other stakeholders (including deployment people, marketing people, legal people) about the security posture.

How will people interact with this system? How will it interact with our other systems? What data will be touched or created? How will that be used? What regulations, policies or other constraints are relevant? How will customers be made aware of relevant protections? How can they communicate with us to identify problems or to raise their concerns? How will we answer them?

Even this one conversation has achieved a lot: everybody is aware of the project and of its security risks. People who will make and support the system once it’s live know the concerns of all involved, and that’s enough to remove a lot of anxiety over the security of the system. It also introduces awareness while we’re working of what we should be watching out for. A lot of the suggestions made at this point will, for better or worse, be boilerplate: the system must make no more use of personal information than existing systems. There must be an issue tracker that customers can confidentially participate in.

But all talk and no trouser will not make a secure system. As we go through the iterations, acceptance tests (whether manually run, automated, or results of analysis tools) determine whether the agreed risk profile is being satisfied.

Should there be any large deviations from the straw man design, the external stakeholders are notified and we track any changes to the risk/threat model arising from the new information. Regular informal lunch sessions give them the opportunity to tell our team about changes in the rest of the company, the legal landscape, the risk appetite and so on.

Ultimately this is all about culture. The developers need to trust the security experts to make their expertise available and help out with making it relevant to their problems. The security people need to trust the developers to be trying to do the right thing, and to be humble enough to seek help where needed.

This cultural outlook enables quick reaction to security problems detected in the field. Where the implementors are trusted, the team can operate a “break glass in emergency” mode where solving problems and escalation can occur simultaneously. Yes, it’s appropriate to do some root cause analysis and design issues out of the system so they won’t recur. But it’s also appropriate to address problems in the field quickly and professionally. There’s a time to write a memo to the shipyard suggesting they use thicker steel next time, and there’s a time to put your finger over the hole.

If there’s a problem with agile application security, then, it’s a problem of trust: security professionals, testers, developers and other interested parties[*] must be able to work together on a self-organising team, and that means they must exercise knowledge where they have it and humility where they need it.

[*] This usually includes lawyers. You may scoff at the idea of agile lawyers, but I have worked with some very pragmatic, responsive, kind and trustworthy legal experts.

posted by Graham at 21:27  

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