Structure and Interpretation of Computer Programmers

I make it easier and faster for you to write high-quality software.

Friday, July 31, 2009

Website relaunch!

Today I have re-launched Thaes Ofereode to focus on my new role as an independent Mac boffin. I really like the new design, which was created by the ever-delightful Freya.

edit: Gecko doesn’t understand the CSS media selector I was using to provide the iPhone CSS. I’ve therefore reverted the iPhone design until I can find a way to get Firefox to suck less.

The one thing I added to her design was a more iPhone-friendly look. For those of you without iPhones, the screenshots demonstrate how the mobile version will appear. For those of you who are CSS experts, the following will probably be rather dull but for those like me who know enough to be dangerous but no more, here’s how it’s done.

The three-column layout works really well on the desktop, but the iPhone has a tallscreen-oriented display so not much space for horizontal layout. I therefore chose to put the leftmost, menu column underneath the main content on each page, so iPhone users get to see the heading and then the meat and potatoes. If they are interested enough to get to the end, they’ll see the links to the rest of the site.

The links, btw, are just paragraphs with a border, a lot of padding and the magic -webkit-border-radius providing the roundy edges; no messing with JavaScript and funny part-circle images.

So, the third column? Well those impressive-looking widgets can’t be displayed on the phone anyway, and would be a bit out of place so they’re gone for the moment with the mobile CSS. I may code up some JavaScript replacements soon enough, but I’ll need to find somewhere else for them to go. In the meantime, I know you read my blog because you’re here, and there are many apps which can help you follow me on Twitter.

posted by Graham Lee at 19:52  

Tuesday, July 28, 2009

Next CocoaHeads Swindon meet!

So for those of you who didn’t manage to enjoy the glories to be found in the town that was the inspiration for one of Legion’s more colourful adventures,[] next Monday, the 3rd of August, offers yet another once-in-a-monthtime opportunity! As ever, the location is in (or just outside) the Glue Pot, a strong man’s stone’s throw from the Swindon train station. This month’s meeting is a recap on QTKit, to allow those who weren’t there last time due to the reschedule to catch up on integrating QuickTime into their Cocoa apps.

[] What am I doing knowing quotes like that? Well, the clue is in the user name. When I was a student my UNIX username was leeg, clearly based on my real name. In short order, I was introduced as "He is Leeg, for he are many" and thus iamleeg.

posted by Graham Lee at 23:01  

Tuesday, July 14, 2009

NSConference videos

Scotty and the gang have been getting the NSConference videos out to the public lately, and now sessions 7-9 are available including my own session on security. The videos are really high quality, I’m impressed by the postproduction that’s gone in and of course each of the sessions I watched at the conference has some great information and has been well-presented. All of the videos are available here.

I’ve also put the slides for my presentation up over on slideshare.

posted by Graham Lee at 23:20  

Saturday, July 11, 2009

Coming very shortly…

This website will be the new home for information on Cocoa and Mac OS X security. But not yet! Please check back soon; in the mean time take a look at my homepage.


posted by Graham at 02:26  

Wednesday, July 8, 2009

Refactor your code from the command-line

While the refactoring support in Xcode 3 has been something of a headline feature for the development environment, in fact there’s been a tool for doing Objective-C code refactoring in Mac OS X for a long time. Longer than it’s been called Mac OS X.

tops of the form

My knowledge of the early days is very sketchy, but I believe that tops was first introduced around the time of OPENSTEP (so 1994). Certainly its first headline use was in converting code which used the old NextStep APIs into the new, shiny OpenStep APIs. Not that this was as straightforward as replacing NX with NS in the class names. The original APIs hadn’t had much in the way of foundation classes (the Foundation Kit was part of OpenStep, but had been available on NeXTSTEP for use with EOF), so took char * strings rather than NSStrings, id[]s rather than NSArrays and so on. Also much rationalision and learning-from-mistakes was done in the Application Kit, parts of which were also pushed down into the Foundation Kit.

All of this meant that a simple search-and-replace tool was not going to cut the mustard. Instead, tops needed to be syntax aware, so that individual tokens in the source could be replaced without any (well, alright, without too much) worry that any of the surrounding expressions would be broken, without too much inappropriate substitution, and without needing to pre-empt every developer’s layout conventions.

before we continue – a warning

tops performs in-place substitution on your source code. So if you don’t like what it did and want to go back to the original… erm, tough. If you’re using SCM, there’s no problem – you can always revert its changes. If you’re not using SCM, then the first thing you absolutely need to do before attempting to try out tops on your real code is to adopt SCM. Xcode project snapshots also work.

replacing deprecated methods

Let’s imagine that, for some perverted reason, I’ve written the following tool. No, scrub that. Let’s say that I find myself having to maintain the following tool :-).

#import <Foundation/Foundation.h>

int main(int argc, char **argv, char **envp)
NSAutoreleasePool *arp = [[NSAutoreleasePool alloc] init];
NSString *firstArg = [NSString stringWithCString: argv[1]];
NSLog(@"Argument was %s", [firstArg cString]);
[arp release];
return 0;

Pleasant, non? Actually non. What happens when I compile it?

heimdall:Documents leeg$ cc -o printarg printarg.m -framework Foundation
printarg.m: In function ‘main’:
printarg.m:6: warning: ‘stringWithCString:’ is deprecated (declared at /System/Library/Frameworks/Foundation.framework/Headers/NSString.h:386)
printarg.m:7: warning: ‘cString’ is deprecated (declared at /System/Library/Frameworks/Foundation.framework/Headers/NSString.h:367)

OK so we obviously need to do something about this use of ancient NSString API. For no particular reason, let’s start with -cString:

heimdall:Documents leeg$ tops replacemethod cString with UTF8String printarg.m

So what do we have now?

#import <Foundation/Foundation.h>

int main(int argc, char **argv, char **envp)
NSAutoreleasePool *arp = [[NSAutoreleasePool alloc] init];
NSString *firstArg = [NSString stringWithCString: argv[1]];
NSLog@"Argument was %s", [firstArg UTF8String], length);
[arp release];
return 0;

Looking good. But we still need to fix the -stringWithCString:. That could be just as easy, replacemethod stringWithCString: with stringWithUTF8String: would do the trick. However let’s be a little
different here. Why don’t we use -stringWithCString:encoding:? If we do that, then we’re going to need to take a guess at the second argument, because we’ve got no idea what the encoding should be (that’s why -stringWithCString: is deprecated, after all. However if we’re happy to assume UTF8 is fine for the output, let’s do that for the input. We’d better let everyone know that’s what happened, though.

So this rule is starting to look quite complex. It says “replace -stringWithCString: with -stringWithCString:encoding:, keeping the C string argument but adding another argument, which should be NSUTF8StringEncoding. While you’re at it, warn the developer that you’ve had to make that assumption”. We also (presumably) want to combine it with the previous rule, so that if we see the original file we’ll catch both of the problems. Luckily tops lets us write scripts, which comprise of one or more rule descriptions. Here’s a script which encapsulates both our cString rules:

replacemethod "cString" with "UTF8String"
replacemethod "stringWithCString:<cString>" with "stringWithCString:<cString>encoding:<encoding>" {
replace "<encoding_arg>" with "NSUTF8StringEncoding"
} warning "Assumed input encoding is UTF8"

So why does the <encoding> token become <encoding_arg> in the sub-rule? Well that means “the thing which is passed as the encoding argument”. This avoids confusion with <encoding_param>, the parameter as declared in the class interface (yes, you can run tops on headers as well as implementations).

Now if we save this script as cStringNoMore.tops, we can run it against our source file:

heimdall:Documents leeg$ tops -scriptfile cStringNoMore.tops printarg.m

Which results in the following source:

#import <Foundation/Foundation.h>

int main(int argc, char **argv, char **envp)
NSAutoreleasePool *arp = [[NSAutoreleasePool alloc] init];
#warning Assumed input encoding is UTF8
NSString *firstArg = [NSString stringWithCString:argv[1] encoding:NSUTF8StringEncoding];
NSLog(@"Argument was %s", [firstArg UTF8String]);
[arp release];
return 0;

Now, when we compile it, we no longer get told about deprecated API. Cool! But it looks like I need to verify that the use of UTF8 is acceptable:

heimdall:Documents leeg$ cc -o printarg printarg.m -framework Foundation
printarg.m:6:2: warning: #warning Assumed input encoding is UTF8

exercises for the reader, and caveats

There’s plenty more to tops than I’ve managed to cover here. You could (and indeed Apple do) use it to 64-bit-cleanify your sources. Performing security audits is another great use – particularly using constructs such as:

replace strcpy with same error "WTF do you think you're doing?!?"

However, notice that tops is a blunter instrument than the Xcode refactoring capability. Its smallest unit of operation is the source file; refactoring only within particular methods is not quite easily achieved. Also, as I said before, remember to check your source into SCM before running a script! There is a -dont option to make tops output its proposed changes without applying them, too.

Finally tops shouldn’t be used fully automated. Always assume that you need to inspect the output carefully, don’t just Build and Go.

posted by Graham Lee at 04:33  

Monday, July 6, 2009

CocoaHeads Swindon tonight!

For those of you who’ve never explored the delights that the fine city of the Hill of Pigs has to offer, tonight offers an unparalleled opportunity. Come and sit in (or outside, weather permitting) a pub only a short distance from the railway station, and listen to Mike Abdullah speaking about WebKit. As always there’ll also be general NSDiscussion, and the occasional pint of beer. Maps etc. at our page.

posted by Graham Lee at 10:47  

Friday, July 3, 2009

Just because Brucie says it…

Bruce Schneier claims that shoulder-surfing isn’t much of a problem these days.

Plenty of people discovered “my password” at NSConference, so I disagree :-) (photo courtesy of stuff mc).

posted by Graham Lee at 23:59  

Wednesday, July 1, 2009

KVO and +initialize

Got caught by a really hard-to-diagnose issue today, so I decided to write it down in part so that you don’t get bitten by it, and partly so that next time I come across the issue, I’ll remember what it was.

I had a nasty bug in trying to add support for the AppleScript duplicate command to one of my objects. Now duplicate should, in principle, be simple: just conform to NSCopying and implement -copyWithZone:. The default implementation of NSCloneCommand should deal with everything else. But what I found was that there’s a class variable (OK, there isn’t, there’s a static in the class implementation file with accessors) with which the instances must compare some properties. And this was empty by the time the AppleScript ran. Well, that’s odd, thought I, it’s only being emptied once, and that’s when it’s created in +[MyClass initialize]. So what’s going on?

Having set a watchpoint on the static, I now know the answer: the +initialize method was being called twice. Erm, OK…why? It’s only called whenever a class is first used. It turns out that there were two classes with the same IMP for that method. The first was MyClass, and the second? NSKVONotifying_MyClass. Ah, great, Apple are adding a subclass of one of my classes for me!

It turns out that TFM has a solution:

+ (void)initialize
if (self == [MyClass class])
//real code

and I could use that solution here to fix my problem. But finding out that is the problem was a complete pig.

posted by Graham Lee at 15:35  

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