TweetFollow Us on Twitter





[IMAGE Huxham_Marriott_rev1.GIF]

This is a supplement to the article "Macintosh Debugging: A Weird Journey Into the Belly of the Beast" in Issue 8 of develop. It presents a few debugging tools that were discussed at Apple's Worldwide Developers Conference in May 1992. Like those discussed in the previous article, these tools are designed to help you force the nasty, subtle bugs in your code to show their hideous little faces immediately, rather than lying in wait and biting you when you least expect it.

People often ask us, "How can I be a totally awesome, godlike debugging stud [or studette] like you?" Unfortunately, the big truth from the Issue 8 debugging article is just as true now as it was then: debugging is hard. That's just the way it is. The only way to get better at it is to practice. Now that we've got that straight and before we get into describing the new debugging tools, here are three pearls of wisdom to guide you in your practice.

First of all, it helps to know a lot about the operating system. The better sense you have of how the Macintosh works, the better off you'll be trying to track down a nasty bug. Dare to delve into the bowels of the OS. Read and rereadInside Macintosh; take it with you to bed, to the bathroom, out to dinner, and on dates. (You might want to invest in a sturdy wheelbarrow, especially with the new Inside Macintosh volumes proliferating like rabbits.) For that matter, read every Macintosh programming book ever written (especially those listed at the end of this article) and every Technical Note, Snippet, piece of Sample Code, and issue ofdevelop, as well as every word on the AppleLink Discussion boards. Also, spend lots of time in debuggers, watching the system do its thing. If you're not dreaming in hex, you're not spending enough time in MacsBug.

Second, get slammed a lot. The people who are the best at debugging are usually the ones who've had to track down the most bugs and therefore have an encyclopedic knowledge of them. If you have a really nasty bug in your code that crashes the machine on a seemingly random basis and takes you three days to find and squash, then by jove you'll remember that bug the next time you see it. Simply put, the more bugs you find, the better you'll be at finding bugs.

Last, use good tools, and use them all. Reread the Issue 8 article. Turn on those tools and stress your code. Bend, fold, staple, and mutilate it. Show no mercy.

These things will help you on your way to becoming a primo bug stomper, but debugging is like any complex skill in that it can't really be taught past a certain point. You simply have to do it a lot, andover time you'll get better. Tools and techniques such as the ones presented here can help enormously, especially by forcing hidden bugs to the surface, but they can never do the whole job for you.

This time there are only four new tools to talk about -- Double Trouble, Dispose Resource, Blat, and Smart Friends -- so this article is much shorter than the last one. The tools are available on theDeveloper CD Seriesdisc, as well as on AppleLink and elsewhere. We're doing this backward from the last time: first we'll present a buggy code sample, then we'll talk about the tool that would find the bug.


Can you find the bug in this code sample?
myHandle = NewHandle(100);
if (myHandle) {
    AddResource(myHandle, 'dumb', 10, "\p");
    if (resError()) HandleTheError();

OK, time's up. This one's not too hard. The problem is that during CloseResFile the Resource Manager disposes of all the resources in memory. The DisposeHandle call afterward is unnecessary and is actually potentially disastrous. Normally you'll just get an error and DisposeHandle will do nothing, but occasionally the data structures in the Memory Manager will conspire to really screw you.

Here's how: Master pointers are allocated in clumps called master pointer blocks, which are nonrelocatable blocks in your application's heap. The master pointers that are currently free for use are kept in a linked list by the Memory Manager. The list is LIFO, like a stack: when you allocate a new handle, the Memory Manager uses the first master pointer in the free list, and when you dispose of a handle the freed master pointer is returned to the beginning of the list.

Now the plot thickens. If the first master pointer in the free list also happens to be the first master pointer in its master pointer block (so that the master pointer and the master pointer block have the same address) and then you dispose of a handle twice by mistake,very bad thingswill happen. On the first dispose, everything is fine: the Memory Manager frees the block the master pointer points to and returns the master pointer to the start of the free list. At this time, the master pointerstill points to a valid block of memory, but now it's the master pointer block itself! So on the second, unintentional dispose, when the Memory Manager dutifully frees the block for reuse, you're set up for disaster. Subsequent memory use will likely result in writing over many master pointers, which will of course trash you one way or another.

Figure 1 illustrates this scenario. On the left is the top part of a master pointer block that resides in the heap at address 80. The heap's free list is a standard linked list (each entry contains the next entry's address) beginning at hFstFree. Note that the first entry in the heap's free list is also the first master pointer in the block. This is the first step to trouble.

[IMAGE Huxham_Marriott_rev2.GIF]

Figure 1 How Disposing of the Same Handle Twice Can Spell Disaster

Now we call DisposeHandle on the master pointer at 81. DisposeHandle looks at the block pointed to by the master pointer (in this case the block at 144, not shown), determines that it is indeed a valid block, marks it as free for reuse, and adds the newly freed master pointer to the front of the free list. So far so good. Now the master pointer block looks like the one on the right in the figure.

Then we call DisposeHandle on 81 again by mistake. DisposeHandle looks at the block pointed to by the master pointer (now it's the block at 80, our master pointer block!), determines that it is indeed a valid block (uh oh), marks it as free for reuse (yikes!), and adds the newly freed master pointer to the front of the free list -- and the heap is now hosed for good. This Memory Manager bug is subtle and rare, but oh so nasty.

Even if you're lucky enough to avoid this particular sequence of events, a double disposal is definitely a bug. Double Trouble is a system extension that watches calls to DisposeHandle to make sure it's not being called on something in the free list. If it is, Double Trouble drops into the debugger with a suitable warning.

We'll be the first to admit that Double Trouble is far from perfect. It infers the existence of heap zones by watching InitZone and then trying to figure out when a heap isn't a heap anymore. The possibility exists that it will guess wrong and cause a bus error when trying to walk a free list that's no longer a free list. Furthermore, in some cases Double Trouble can noticeably slow down parts of the system. (After playing a long QuickTime movie, for instance, the machine may freeze for almost a minute.)

But despite Double Trouble's shortcomings, we do still recommend running it all the time. Just try to remember that it's running so you don't chase your tail trying to find the cause of occasional mysterious slowdowns.


Here's the code. What's the bug?
myPicture = GetPicture(kPicID);
if (myPicture) {
    DrawPicture(myPicture, &myRect);

That's right, you should never call DisposeHandle on a resource handle. If you do, the Memory Manager will free it just fine, but the Resource Manager has another reference to it, stored in the resource map, that will be left dangling. Later on, since the Resource Manager doesn't know the handle was disposed of, it may try some manipulation with the handle. The results may not crash you immediately, or at all -- it depends on what the operation is and what's in the handle -- but they're certainly not what was intended. Instead of DisposeHandle, you should always call ReleaseResource on resource handles. ReleaseResource will properly dispose of the handleandwill update the resource map. (Note that KillPicture won't do the right thing here either; it's intended for pictures created via OpenPicture, not for PICT resources.)

Dispose Resource is another extension a lot like Double Trouble. It also watches DisposeHandle calls, this time looking to see if the handle being disposed of is a resource handle. If so, you'll drop into the debugger with a suitable warning.

Dispose Resource has one idiosyncrasy you should know about: it's been known to indicate "false positives." Some parts of the system (we haven't been able to track down which ones yet) seem to save a resource handle's state, detach the resource, and then restore the state of the handle (restoring the resource bit!). Use Dispose Resource. It will ensure that you don't make the same mistake.


This time the code's in assembler:
; Offset the rect by 128 pixels in each direction.
PEA         theRect(A6)
MOVE.W      $0080, -(SP)
MOVE.W      $0080, -(SP)

If you have "iron man" syndrome and insist on programming in assembly language, this can happen to you. We forgot to type a # in front of each $0080. As a result, instead of moving the number $0080 (128) onto the stack twice in preparation for the OffsetRect call, we're moving the contents of memory location $0080. Often this kind of bug is immediately obvious, but not always. If you're moving a Boolean, for instance, you have a fifty-fifty chance of getting the right value, even though you're getting it from some random spot in memory. It's those cases that will give you debugging headaches.

One easy (and recommended) way to avoid the problem in this example is to write in a higher-level language. But we realize that's not always possible, and besides, this is really a wholeclassof problems: reads and writes from places in memory you didn't intend. The best way to catch this wild memory reference kind of problem is, naturally, with memory protection, something that -- sadly -- the Macintosh normally lacks. In the last article we mentioned Jasik's implementation, but now there's something else you should know about. Bo3b Johnson has written a dcmd called Blat that uses the MMU to protect memory locations 0-255 from both reads and writes.

Blat has been tested and works well on the Macintosh IIfx, IIx, and SE/30. Because its operation is so hardware dependent, it's hard to predict whether it will work on a given machine. Some basic guidelines are that it requires an MMU and won't work with 68040 machines or with most configurations of machines with the IIci ROM (IIci, IIsi, LC). For further details, see the release notes and the source code, thoughtfully provided by Bo3b along with the dcmd itself.


This bug is subtle, so pay close attention:
#pragma parameter __d0 GetA0
Ptr GetA0(void) = {0x2008};  // MOVE.l A0,D0

void MyCompletionRoutine()
    long        saveA5;
    HooHahPtr   myHooHah;
    myHooHah = (HooHahPtr)GetA0();
    saveA5 = SetA5(myHooHah->myA5);
    gSomething[0].flag = true; // Set a flag in a global array. 

This code really tries hard to do everything right. As the name implies, it's a completion routine, so it could be called at interrupt time. First a pointer to the data is retrieved from A0, and then A5 is set to a previously saved value, thus allowing the routine to access its global variables. Once A5 is set up, the global reference can be made safely. Finally, A5 is restored to its previous value to clean up. Sounds great, right? The only problem is, it doesn't work.

Here's why: the MPW C compiler will actually set up the global referencebeforethe SetA5 call, so accessing the global accesses some unknown part of memory. This is legal compiler optimization behavior! If GetA0 and SetA5 were functions or traps, the bug would disappear, but since they're declared inline the compiler doesn't feel compelled to delay the evaluation of the global array reference. The solution is to set up A5, then call a different routine that does the global reference.

Now in this case, how do you think we -- the debugging gods -- figured out the bug? We tried the first few things we could think of; but then when we weren't making headway after a few probes, we didn't just sit there and suffer in silence, banging our heads against the proverbial wall. We called in some Smart Friends! The veil of illusion was torn from our eyes, and we were shown the heart of the truth (in other words, one of them had seen this bug before). The point is that in debugging, two (or more) heads are far, far better than one. Bugs are not like germs: when you share them, everyone benefits. Maybe your very own Smart Friends have had a similar bug before, so they'll recognize immediately what's going on. Or maybe they'll think of something different to try. At the very least, they'll temporarily divert you from your frustration, maybe make you feel less stupid, and then you can all go out for pie together.


Add these tools to your arsenal of bug sprays and foggers. Use them all and use them well, and you, your code, and your customers will be far better off.


Bedside books for the serious student of debugging:

  • How to Write Macintosh Software, 3rd ed., by Scott Knaster and Keith Rollin (Addison-Wesley, 1992).
  • Macintosh Programming Secrets, 2nd ed., by Scott Knaster and Keith Rollin (Addison-Wesley, 1992).
  • Debugging Macintosh Software with MacsBug by Konstantin Othmer and Jim Straus (Addison-Wesley, 1991).
  • MC68000 Family Programmer's Reference Manual (Motorola, Inc.).

FRED HUXHAM (AppleLink FRED) was born and raised in California. He used to be a tremendous athlete, know bazillions of babes, and go to wild parties in New York and California with people like Andy Warhol and Keith Haring. Now he's 15 pounds heavier, knows only one babe (his wife), and thinks a day spent sitting on his roof deck watching boats go through the Golden Gate is really exciting.*

GREG MARRIOTT (AppleLink GREG) is a SWM, 28, 6'0", 195 lbs., brown hair and eyes, sincere, hardworking, good sense of humor. Enjoys music, romantic walks, quiet evenings, and good books. Seeks nice woman for friendship and more. Send photo.*

In Bo3b's name, the "3" is silent. *



Community Search:
MacTech Search:

Software Updates via MacUpdate

Apple Safari 10.0.1 - Apple's Web b...
Note: The direct download link is currently unavailable. It is available in the OS X 10.11.6 release, as well as in the Apple Security Updates. Apple Safari is Apple's web browser that comes with OS... Read more
Apple macOS Sierra 10.12.1 - The latest...
With Apple macOS Sierra, Siri makes its debut on Mac, with new features designed just for the desktop. Your Mac works with iCloud and your Apple devices in smart new ways, and intelligent... Read more
Apple iOS 10.1 - The latest version of A...
iOS 10 is the biggest release of iOS ever. A massive update to Messages brings the power of the App Store to your conversations and makes messaging more personal than ever. Find your route with... Read more
Hazel 4.0.7 - Create rules for organizin...
Hazel is your personal housekeeper, organizing and cleaning folders based on rules you define. Hazel can also manage your trash and uninstall your applications. Organize your files using a familiar... Read more
Opera 40.0.2308.90 - High-performance We...
Opera is a fast and secure browser trusted by millions of users. With the intuitive interface, Speed Dial and visual bookmarks for organizing favorite sites, news feature with fresh, relevant content... Read more
BetterTouchTool 1.93 - Customize Multi-T...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
Backblaze - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more
Postbox 5.0.5 - Powerful and flexible em...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
Coda 2.5.19 - One-window Web development...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few surprises... Read more
Toast Titanium 15.1 - $99.99
Roxio Toast 15 Titanium, the leading DVD burner for Mac, makes burning even better, adding Roxio Secure Burn to protect your files on disc and USB in Mac- or Windows-compatible formats. Get more... Read more

Latest Forum Discussions

See All

WitchSpring2 (Games)
WitchSpring2 1.27 Device: iOS Universal Category: Games Price: $3.99, Version: 1.27 (iTunes) Description: This is the story of Luna, the Moonlight Witch as she sets out into the world. This is a sequel to Witch Spring. Witch Spring 2... | Read more »
4 popular apps getting a Halloween makeo...
'Tis the season for all things spooky. So much, so, in fact, that even apps are getting into the spirt of things, dressing up in costume and spreading jack o' lanterns all about the place. These updates bring frightening new character skins, scary... | Read more »
Pokémon GO celebrates Halloween with can...
The folks behind Pokémon GO have some exciting things planned for their Halloween celebration, the first in-game event since it launched back in July. Starting October 26 and ending on November 1, trainers will be running into large numbers of... | Read more »
Best Fiends Forever Guide: How to collec...
The fiendship in Seriously's hit Best Fiends has been upgraded this time around in Best Fiends Forever. It’s a fast-paced clicker with lots of color and style--kind of reminiscent of a ‘90s animal mascot game like Crash Bandicoot. The game... | Read more »
5 apps for the budding mixologist
Creating your own cocktails is something of an art form, requiring a knack for unique tastes and devising interesting combinations. It's easy to get started right in your own kitchen, though, even if you're a complete beginner. Try using one of... | Read more »
5 mobile strategy games to try when you...
Strategy enthusiasts everywhere are celebrating the release of Civilization VI this week, and so far everyone seems pretty satisfied with the first full release in the series since 2010. The series has always been about ultra-addictive gameplay... | Read more »
Popclaire talk to us about why The Virus...
Humanity has succumbed to a virus that’s spread throughout the world. Now the dead have risen with a hunger for human flesh, and all that remain are a few survivors. One of those survivors has just called you for help. That’s the plot in POPCLAIRE’... | Read more »
Oceans & Empires preview build sets...
Hugely ambitious sea battler Oceans & Empires is available to play in preview form now on Google Play - but download it quickly, as it’s setting sail away in just a few days. [Read more] | Read more »
Rusty Lake: Roots (Games)
Rusty Lake: Roots 1.1.4 Device: iOS Universal Category: Games Price: $2.99, Version: 1.1.4 (iTunes) Description: James Vanderboom's life drastically changes when he plants a special seed in the garden of the house he has inherited.... | Read more »
Flippy Bottle Extreme! and 3 other physi...
Flippy Bottle Extreme! takes on the bottle flipping craze with a bunch of increasingly tricky physics platforming puzzles. It's difficult and highly frustrating, but also addictive. When you begin to master the game, the sense of achievement is... | Read more »

Price Scanner via

Apple’s Thursday “Hello Again” Event A Largel...
KGI Securities analyst Ming-Chi Kuo, who has a strong record of Apple hardware prediction accuracy, forecasts in a new note to investors released late last week that a long-overdue redo of the... Read more
12-inch Retina MacBooks on sale for $100 off...
Amazon has 2016 12″ Apple Retina MacBooks on sale for $100 off MSRP. Shipping is free: - 12″ 1.1GHz Silver Retina MacBook: $1199.99 $100 off MSRP - 12″ 1.1GHz Gold Retina MacBook: $1199.99 $100 off... Read more
Save up to $600 with Apple refurbished Mac Pr...
Apple has Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The following... Read more
PixelStyle Inexpensive Photo Editor For Mac W...
PixelStyle is an all-in-one Mac Photo Editor with a huge range of high-end filters including lighting, blurs, distortions, tilt-shift, shadows, glows and so forth. PixelStyle Photo Editor for Mac... Read more
13-inch MacBook Airs on sale for $100-$140 of...
B&H has 13″ MacBook Airs on sale for $100-$140 off MSRP for a limited time. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/128GB MacBook Air (sku MMGF2LL/A): $899 $100 off... Read more
2.8GHz Mac mini available for $988, includes...
Adorama has the 2.8GHz Mac mini available for $988, $11 off MSRP, including a free copy of Apple’s 3-Year AppleCare Protection Plan. Shipping is free, and Adorama charges sales tax in NY & NJ... Read more
21-inch 3.1GHz 4K on sale for $1379, $120 off...
Adorama has the 21″ 3.1GHz 4K iMac on sale $1379.99. Shipping is free, and Adorama charges NY & NJ sales tax only. Their price is $120 off MSRP. To purchase an iMac at this price, you must first... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more
Apple, Samsung, Lead J.D. Power Smartphone Sa...
Customer satisfaction is much higher among smartphone owners currently subscribing to full-service wireless carriers, compared with those purchasing service through a non-contract carrier, according... Read more
Select 9-inch Apple WiFi iPad Pros on sale fo...
B&H Photo has select 9.7″ Apple WiFi iPad Pros on sale for up to $50 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 9″ Space Gray 256GB WiFi iPad Pro: $799 $0 off... Read more

Jobs Board

Software Engineering Intern: Integration / QA...
Job Summary Apple is currently seeking enthusiastic interns who can work full-time for a minimum of 12-weeks between Fall 2015 and Summer 2016. Our software Read more
Software Engineering Intern: Frameworks at *...
Job Summary Apple is currently seeking enthusiastic interns who can work full-time for a minimum of 12-weeks between Fall 2015 and Summer 2016. Our software Read more
*Apple* Retail - Multiple Positions- Nashua,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions- Napervi...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Security Data Analyst - *Apple* Information...
…data sources need to be collected to allow Information Security to better protect Apple employees and customers from a wide range of threats.Act as the subject Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.