Archive for the 'Objective-C' Category

Sometimes, when tracking down a bug, you’ll get a bit of console spew, an exception log, or a crash log that contains a ridiculously large number. Sometimes, that is the result of a memory smasher.

Sometimes, though, it is because of a type conversion problem.

For example, if you see a log message indicating that the value 4294967295 is causing a problem, it is probably because something archived -1 on a 32 bit system and then unarchived it on 64 bit improperly.

This has come up often enough that I like to leave the Calculator app open in Programmer Mode. Then, I can copy/paste the value into Calculator and see both the bit pattern or the hex value (which will often show patterns that base-10 does not).

An aside, I have generally tried to break myself of the habit of relying upon knowledge of magic values (like 4294967295). Sure, I’ll use ‘em as clues, but I focus much more on refining my tools to make recognition of said values unnecessary as there are a slew of different values that look non-obvious in decimal form that become darned obvious in binary or hex.

Dead obvious, I know.

Table of Contents

1. objc_msgSend() Tour Part 1: The Road Map
2. objc_msgSend() Tour Part 2: Setting the Stage
3. objc_msgSend() Tour Part 3: The Fast Path
4. objc_msgSend() Tour Part 4: Method Lookup & Some Odds and Ends

In the first three parts, I gave an overview, explained a bit of the ABI used by Objective-C, and took a near instruction by instruction tour of what happens on the fast path of Objective-C method dispatch. By fast path, I mean what happens 99.9% of the time; a very fast, no overhead, no function call, no locking, set of instructions that grabs the method implementation from the cache and does a tail-call jump to that implementation.

The slow path is used rarely. As little as once per unique selector invoked per class with a goal of filling the cache such that the slow path is never used again for that selector/class combination. A handful of operations will cause a class’s cache to be flushed; method swizzling, category loading, and the like.

Note that during +initialize, methods won’t always be cached. Yet another reason to not do any real work during +initialize! Read the rest of this entry »

Every few months, there is a discussion on cocoa-dev or a question on stackoverflow.com that basically boils down to “I have a leak or over-release and I can’t use Instruments to debug it. Help?”.

Quite often, the questioner can actually use Instruments just fine, but simply lacks the know-how or hasn’t tried in a while and doesn’t realize that Instruments has improved significantly with each release of the developer tools. No, really, Instruments is a fantastic tool and I use it whenever I can; what you see below is for the exceptional case, not the norm.

There are cases where using Instruments is either inconvenient or impractical. Namely, trying to track down an intermittent crasher or trying to gain insight into memory leaks over a long running session will create a prohibitively large dataset for Instruments to process (Instruments allows for much more detailed analysis of the object graph and this analysis loads a lot more data than the tools I’ll demonstrate below).

Thus, it is helpful to be familiar with the rather powerful set of tools available from the command line and within the debugger.

Almost always, you are going to want to enable a bit of additional data via the malloc infrastructure. Have a look at the malloc(3) man page. There is an entire section devoted to ENVIRONMENT variables and there are a handful of extremely useful variables!

First and foremost, you are almost always going to want to use MallocStackLoggingNoCompact. When enabled, malloc stack logging writes the stack trace of every allocation and free event to an efficiently compact binary file in /tmp/ (it used to be in memory and, thus, used to be a great way to exhaust heap. No longer!!). Unfortunately, it doesn’t record the retain and release events, but simply knowing where the object was allocated is generally quite useful (it is generally relatively easy to track down who retained an object once you know which object it is). Under GC, you can set the AUTO_REFERENCE_COUNT_LOGGING and CFRetain/CFRelease events will be logged to the malloc history.

You can then use the malloc_history command line tool to query for all events related to a particular address in memory.

While malloc_history requires that the process still exists, it doesn’t have to be running! If you run your app under gdb, you can still use malloc_history to query the application even when it is stopped in the debugger!

Speaking of gdb, you can use the info malloc command in gdb to query the same information. Under GC, the info gc-roots and info gc-referers commands can be used to interrogate the collector for information about the connectivity of the object graph in your running application.

If you enable zombies via setting the NSZombieEnabled environment variable to YES, the address spewed in the error message when messaging a zombie can be passed directly to malloc_history.

The leaks command line tool scans memory and detects leaks in the form of allocations of memory for which the address to that memory is not stored anywhere else within the application. The leaks tool has been vastly improved in the Snow Leopard release of the Xcode tools; it is much much faster and spits out false positives almost never. It is still possible to have a leak that leaks cannot detect, of course. And, remember, even if you can still reach memory, it is still a total waste if you never use that memory’s contents again!

So, that is a brief summary of the state of command line memory debugging on Mac OS X as of Snow Leopard. Of course, that’s just a bunch of words. How about an example?

Read the rest of this entry »

Table of Contents

1. objc_msgSend() Tour Part 1: The Road Map
2. objc_msgSend() Tour Part 2: Setting the Stage
3. objc_msgSend() Tour Part 3: The Fast Path
4. objc_msgSend() Tour Part 4: Method Lookup & Some Odds and Ends

In any case, with the foundation set — with the id of the object to be targeted in %rdi and the selector of the method to be invoked in %rsi — we can jump into objc_msgSend() and understand exactly what happens instruction by instruction. Or more specifically, the compiler issues a call into objc_msgSend() (which sets up a stackframe for objc_msgSend() which, through tail call optimization, turns into the stackframe for the called method) and the method implementation that objc_msgSend() jumps to will issue a ret instruction to unwind the stack back to the original caller’s frame.

It is pretty easy to correlate the disassembly with the comments and code in the original source file. However, if you ever need to step through the messenger (si steps by instruction in gdb), this will be easier to follow as this is closer to the reality during a debug session.

For almost all method dispatches, dispatch takes what is called the “fast path”. That is, objc_msgSend() finds the implementation in the method cache and passes control to the implementation. Since this is the most common path, it is a good opportunity to break the tour of objc_msgSend() into two parts; the fast path and the slow path (with administrivia).

Read the rest of this entry »

Table of Contents

1. objc_msgSend() Tour Part 1: The Road Map
2. objc_msgSend() Tour Part 2: Setting the Stage
3. objc_msgSend() Tour Part 3: The Fast Path
4. objc_msgSend() Tour Part 4: Method Lookup & Some Odds and Ends

Objective-C is, ultimately, a simple set of extensions to C that provide an object oriented coding and runtime model. Objective-C fully embraces C and leverages the C calling ABI throughout. Every method call is really just a C function call with objc_msgSend() acting as the preamble that figures out exactly which C function — which method — to call!

Thus, it is helpful to understand how objc_msgSend() is called and how that relates to C. That is, how does the compiler translate [someObject doSomething: 0x2a] into a call.

What follows is a bit of code that makes a [totally bogus] simple method call followed by the assembly generated.

Code:
@interface NSObject(foo)
- (id) doSomething: (NSUInteger) index;
@end
...
    NSObject *b;
    NSArray *a;
    b = [a doSomething: 0x2a]; // line 11
Assembly:
    .loc 1 11 0
    movq	-16(%rbp), %rdi
    movq	L_OBJC_SELECTOR_REFERENCES_0(%rip), %rsi
    movl	$42, %edx
    call	_objc_msgSend
    movq	%rax, -8(%rbp)

Read the rest of this entry »

What follows (across this and 3 more posts, maybe more) is a rather detailed tour of objc_msgSend() as implemented in Mac OS X 10.6.2. Rather detailed in that every instruction will be explained. Even though it is relatively few instructions, there is a considerable amount of background information that is helpful to understanding the objc_msgSend() instruction stream.

The motivation behind these posts is entirely selfish. I find the best way for me to learn something is to know it well enough to be able to explain any detail to a room full of folks in full-blown student mode.

Table of Contents

1. objc_msgSend() Tour Part 1: The Road Map
2. objc_msgSend() Tour Part 2: Setting the Stage
3. objc_msgSend() Tour Part 3: The Fast Path
4. objc_msgSend() Tour Part 4: Method Lookup & Some Odds and Ends

Read the rest of this entry »

Every six months or so, I run across a question along the lines of how do I invoke some Python code from Objective-C?

Kinda like here for which I posted the same conceptually concrete but technically vague pattern that I have posted for the last decade+.

Which led to this question.

OK — enough is enough. Here is a working example. Read the rest of this entry »

Ultimately, the whole point of resurrecting my old screensaver code was to finally port DPhyllotaxis to Snow Leopard.

Beyond being, perhaps, the most over-engineered screen saver ever for what ultimately draws colored dots, I wrote this as a sort of virtual flower for my then-girlfriend, now-wife-of-more-than-a-decade, Christine.

The underlying algorithm on this one is based entirely on phyllotaxis and the phyllotactic pattern of growth seen across so much of the plant world. The most well known example being the layout of seeds in a sunflower and that particular form of phyllotaxis is exactly what this screensaver mimics.

The color calculation in this particular screen saver is, frankly, goofy. Every floret — every dot — is actually rendered. The brightness is determined by calculating a color once, grabbing the red component and then calculating the color again using a slightly different algorithm and using the previous red value as the new brightness. Rather silly, but the results are pleasant enough.

When I originally wrote this in 1994-ish, it used Display PostScript to do all the drawing. Specifically:

/* this should not be done here */
PSarc(cp.x, cp.y, 15. + (11. * pp.r), 0, 360);
PSgsave();
PSfill();
PSgrestore();
NXSetColor(NX_COLORBLACK);
PSstroke();

I have no idea why, 15 years ago, I thought it important to note that “this should not be done here”. None. So, in the ported code, the comment is gone.

CGFloat floretDiameter = 10. + (11. * pp.r);
CGFloat floretRadius = floretDiameter / 2.;
NSRect floretRect =
    NSMakeRect(cp.x - floretRadius,
    cp.y - floretRadius,
    floretDiameter, floretDiameter);
NSBezierPath *floretPath = [NSBezierPath
    bezierPathWithOvalInRect: floretRect];

[floretPath fill];

[[NSColor blackColor] set];
[floretPath stroke];

There are, of course, many ways to make the above a ton faster. Save for reducing power usage by going a more efficient route, it just doesn’t matter for this particular use case as I already had to slow down the animation rate considerably.

Seems there has been a bit of performance jump between the 25MHZ 68040 this was originally written on and the 2GHZ Core 2 Duo machine I used for the porting work.

Code is in the same repository as the other screen savers. I also tossed a pre-built binary on the server. Only tested on 64-bit Snow Leopard, but it might should also work on 10.5 ppc/i386.

One of the features added to Objective-C 2.0 is Class Extensions.

If you have feature requests or bugs, http://bugreport.apple.com/ is your friend!

There has been a bit of confusion about their functionality and purpose.

First, a bit of background. One of the great strengths of Objective-C is the very sharp division between interface and implementation.

Not only can you declare a public interface to a class — the set of methods that clients of the class can use — but you can also declare varying degrees of private interface ranging from “stuff for my framework” through to “stuff only to be used in the implementation of this subset of methods of the class”.

Class extensions were designed to solve two problems. The first was to enable to compiler to better validate the private interfaces a class might have and the second was to solve a subtle, but gnarly, problem with properties (another feature added to Objective-C 2.0).

Read the rest of this entry »

Some confusion on StackOverflow led to a massive string of comments. This is a question that comes up often, so here is some google fodder.

In Objective-C, a class can implement +initialize. This method will be invoked the first time the class is touched, prior to any other methods (other than +load).

The documentation says:

The runtime sends initialize to each class in a program exactly one time just before the class, or any class that inherits from it, is sent its first message from within the program.

Which is exactly true. But your +initialize methods can still be executed more than once!

Specifically, if a subclass does not implement +initialize but its superclass does, then that superclass’s +initialize will be invoked once per non-implementing subclass and once for itself.

An example (Foundation Tool, Garbage Collected):

@interface Abstract: NSObject
@end
@implementation Abstract
+ (void) initialize
{
    NSLog(@"Initializing %@", self);
}

+ (void) load
{
    NSLog(@"Loading");
}
@end

@interface Sub : Abstract
@end
@implementation Sub
@end

int main (int argc, const char * argv[]) {
    [Sub class];
    return 0;
}

This will output:

ArgyBargy[3720:903] Loading
ArgyBargy[3720:903] Initializing Abstract
ArgyBargy[3720:903] Initializing Sub

Which is why most +initialize methods are implemented as:

@implementation MyClass
+ (void) initialize
{
    if (self == [MyClass class]) {
        // ... do +init stuff here ...
    }
}
...
@end

Now, categories can seriously screw things up (as usual). Namely, if you implement +initialize in a category, it will override the classes +initialize. However, a category provided +load will not; both the category’s and the class’s +load methods will be invoked.

If you were to add the following category to the Sub/Abstract/NSObject example above:

@interface Abstract(Cat)
@end
@implementation Abstract(Cat)
+ (void) load
{
    NSLog(@"Category +load");
}
+ (void) initialize
{
    NSLog(@"Category +initialize %@", self);
}
@end

The program will spew:

ArgyBargy[3919:903] Loading
ArgyBargy[3919:903] Category +load
ArgyBargy[3919:903] Category +initialize Abstract
ArgyBargy[3919:903] Category +initialize Sub

Keep in mind, as well, that the runtime sends +initialize “in a thread-safe manner”. That implies that there is a lock involved somewhere within which then also implies that you better not block on a lock in your +initialize because whoever is supposed to unlock the lock might end up blocking on +initializes lock.

Or, to put it more bluntly, do not do any heavy lifting in +initialize. Keep it super simple & fast.

For me, +initialize is to be used only as a method of last resort. Well, 2nd to last. Last resort is a constructor attributed function (or +load). Read the rest of this entry »