TweetFollow Us on Twitter

December 93 - KON & BAL'S PUZZLE PAGE

KON & BAL'S PUZZLE PAGE

SOUNDS LIKE TROUBLE

KONSTANTIN OTHMER AND BRUCE LEAK

[IMAGE Puzzle_Page_final_rev1.GIF]

See if you can solve this programming puzzle, presented in the form of a dialog between Konstantin Othmer (KON) and Bruce Leak (BAL) -- and a special guest, Apple summer intern Mike Dodd. The dialog gives clues to help you. Keep guessing until you're done; your score is the number to the left of the clue that gave you the correct answer. These problems are supposed to be tough. If you don't get a high score, at least you'll learn interesting Macintosh trivia.

Mike Hey, guys. I've got one for you I bet you can't get.

KON Well, I haven't been able to get you over to the poker game; maybe I can beat you here. Wanna put your summer salary on it, schoolboy?

BAL Careful, Mike. You're talking to trained professionals here.

Mike OK. We have this true multimedia application that does lots of things: plays movies and sounds and does some GWorld stuff. After a while it crashes with a corrupt heap, usually SysError 33, and sometimes with a bus error. I'm using the latest QuickTime, and I have that snazzy PowerPC QuickDraw extension that installs PowerPC native QuickDraw on 68K-class machines.

BAL Sounds like some quality software you're running there. I doubt it's QuickTime's fault.

KON Yeah, and the PowerPC stuff is pretty awesome. You wouldn't stick us with some stupid application bug, so it's probably a problem with the Sound Manager. Is this that MoveHHi Sound Manager problem? MoveHHi snags the whole stack, and when the sound interrupts come in, the stack overflows into the application space, corrupting the heap. Since the Sound Manager is at a higher interrupt level than the stack sniffer VBL, it never detects the problem. Unsolved Mysteries: Solved!

100 Mike I'm using the new Sound Manager, version 3.0.

BAL I guess Reekes did a good job with compatibility on the new Sound Manager. He even maintained all the bugs!

Mike Reekes swears there's not a line of code the same between the new and old Sound Managers. Besides, the new Sound Manager patches MoveHHi to not use as much stack.

KON Does it happen with the old Sound Manager?

90 Mike Yep. Same thing.

BAL Hmmm. So what are the circumstances around the crash? Do you have a reproducible case?

80 Mike It seems to happen fairly randomly. But it generally occurs when I push a button that plays a sound. Sometimes it happens the first time I push the button; other times I push the button over a hundred times before it crashes.

KON Turn on heap scrambling in the application and system heaps and run something like the MemHell extension, which forces a worst-case memory scenario. That should bring the problem out more frequently. Maybe you can get a reproducible case.

BAL Yeah, and turn on A-trap recording and heap checking so that we can narrow down the problem area.

70 Mike The application is running really slowly now, but the problem doesn't happen any more frequently. Every time you crash, you notice the last trap that the application called was SndNewChannel.

KON Wait a second. The application calls SndNewChannel every time it plays a sound? It should just call SndNewChannel once at startup for each channel it needs and then keep reusing those.

BAL What happens if you fix the application?

65 Mike The problem goes away. But you haven't found the bug yet. Although calling SndNewChannel all the time may slow you down, it isn't illegal and shouldn't cause heap corruption.

BAL Is anything else going on while the sound is being played?

60 Mike The problem seems to happen only while a movie is playing. The application calls SndNewChannel, SndPlay, and SndDisposeChannel every time it wants to make a sound, but why the crash?

BAL Does the movie have sound? What happens if you turn off the sound in the movie?

55 Mike The problem goes away.

KON What's the last trap called inside SndNewChannel before the crash?

50 Mike MoveHHi.

KON What if you don't play the video?

45 Mike It crashes.

BAL Change the button that's doing the SndNewChannel, SndPlay, and all that other stuff; make it so it creates a bunch of handles and calls MoveHHi on them instead.

40 Mike Now the machine crashes more frequently.

BAL So now we know that MoveHHi and playing the sound in the movie have something to do with it. Make it so that when you push the button, the movie starts playing at the beginning; then do NewHandle, MoveHHi, and DisposeHandle in a loop with a counter, and keep the loop counter at location 0 so that when you crash you can see which iteration you're on. You might have to make the size of the handles vary in case the failure depends on block size or position, since that'll help spread the allocations throughout the heap.

35 Mike It seems to happen consistently on the sixty-ninth iteration of the loop, reproducibly, if I start from launching the program.

KON Go into MacsBug and put a breakpoint in the loop when the loop counter is 69.

30 Mike You hit your breakpoint and trace over the call to MoveHHi, and it works fine. If you say go, you crash a hundred iterations later.

KON Rather than use MacsBug, change the code to break on the sixty-ninth iteration. Then what happens when you trace over MoveHHi?

25 Mike It works fine.

BAL OK. Change the code to break on the seventieth iteration.

20 Mike You get to the breakpoint at 70 and everything is fine.

KON So somehow this thing is timing sensitive. Have the program compare with a really big number and see when the heap goes bad. Then change the number so that it breaks right before the problem code.

15 Mike When you break and trace, it doesn't happen.

BAL What if I turn off interrupts during my MoveHHi loop?

10 Mike It works fine, but you only hear the first half second of sound from the movie. Since interrupts are blocked, the Sound Manager can't call back to QuickTime to get the next piece, which QuickTime has queued up in the mean time.

KON Great, so what you're saying is we can now break right before the MoveHHi that would cause it to happen if we didn't break there.

BAL Right, so go ahead and break there. Dump the state of the heap and log it to a file. Then do the same thing, this time comparing with a higher number and letting it trash itself, and dump the heap again. Compare the heaps and figure out what's going on.

[IMAGE Puzzle_Page_final_rev2.GIF]

5 Mike The interesting part of the heap before and after the MoveHHi call is shown in the figure. Before MoveHHi there was a locked block, labeled A in the figure, which is marked as relocatable afterward. The relocatable block just below locked block B is getting overwritten by the block we're calling MoveHHi on.

KON MoveHHi works by first saving the contents of the block that you're moving, then marking the block as free. Then it calls CompactMem on the heap, which bubbles all the free space up to any islands and all relocatable blocks down. Then it copies the block to the free block just before the island.

BAL And someone is coming in at interrupt time and unlocking the island, block A in the figure. Instead of remembering the location of the island, MoveHHi searches for it after the CompactMem call. Since that block was unlocked by an interrupt after CompactMem, a different block is found the second time. When MoveHHi backs up to the previous, presumably free, block and starts copying data, the heap gets trashed.

Mike Yeah, that interrupt unlocking the block was QuickTime, BAL. It turns out the Sound Manager does the same thing. Apparently the "system architects" at the time thought it was OK to call HUnlock on a locked block during an interrupt. Not! We fixed it by deferring all HUnlock calls until MoveHHi finishes. This was the cleanest fix; it keeps us from patching out huge parts of the Memory Manager. But we were stumped for quite a while.

KON Nasty.

BAL Yeah.

SCORING

  • 75-100 We'd award you a copy of Debugging Macintosh Software, but obviously you don't need it.
  • 50-70 We'd award you a copy of Debugging Macintosh Software, but then KON would miss out on his royalties.
  • 25-45 Only scores from your first reading count.
  • 5-20 Outfoxed by a summer intern. Care to join our poker game? *

KONSTANTIN OTHMER AND BRUCE LEAK have been awarded a subcontract to debug the Ada software for the cost-reduced space station backup project, code named BALKON-4. While BAL has found that rocket science makes him nauseous, KON has taken to weightlessness like a bug to code and is thinking of opening KONstellation, the first casino in space.*

Mike DODD is the official QuickTime perpetual summer hire, just finishing his fourth summer with Apple. He claims that someday he'll actually graduate from the University of Tennessee and get a full-time job. Mike spends a lot of his time inside MacsBug trying to make QuickTime crash less, or at least finding cool bugs to try to stump KON and BAL with. *

The MemHell extension is on this issue's CD. *

Thanks to Gary Davidian, Peter Hoddie, and Jim Reekes for reviewing this column. *

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Geekbench 4.4.0 - Measure processor and...
Geekbench provides a comprehensive set of benchmarks engineered to quickly and accurately measure processor and memory performance. Designed to make benchmarks easy to run and easy to understand,... Read more
CleanMyMac X 4.4.4 - Delete files that w...
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac lets you safely and intelligently scan and clean your entire system, delete large, unused files... Read more
TeamViewer 14.4.2669 - Establish remote...
TeamViewer gives you remote control of any computer or Mac over the Internet within seconds or can be used for online meetings. Find out why more than 200 million users trust TeamViewer! Free for non... Read more
Paperless 3.0.6 - $69.95
Paperless is a digital documents manager. Remember when everyone talked about how we would soon be a paperless society? Now it seems like we use paper more than ever. Let's face it - we need and we... Read more
BetterTouchTool 3.141 - Customize multi-...
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
TextMate 2.0.rc.29 - Code/markup editor...
TextMate is a versatile plain text editor with a unique and innovative feature set which caused it to win an Apple Design Award for Best Mac OS X Developer Tool in August 2006 A rapidly growing... Read more
Little Snitch 4.4.1 - Alerts you about o...
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
Little Snitch 4.4 - Alerts you about out...
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
MPlayer OSX Extended 16 - Multimedia pla...
MPlayer OSX Extended is the future of MPlayer OSX. Leveraging the power of the MPlayer and FFmpeg open source projects, MPlayer OSX Extended aims to deliver a powerful, functional and no frills video... Read more
Google Chrome 75.0.3770.142 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more

Latest Forum Discussions

See All

TEPPEN guide - Tips and tricks for new p...
TEPPEN is a wild game that nobody asked for, but I’m sure glad it exists. Who would’ve thought that a CCG featuring Capcom characters could be so cool and weird? In case you’re not completely sure what TEPPEN is, make sure to check out our review... | Read more »
Dr. Mario World guide - Other games that...
We now live in a post-Dr. Mario World world, and I gotta say, things don’t feel too different. Nintendo continues to squirt out bad games on phones, causing all but the most stalwart fans of mobile games to question why they even bother... | Read more »
Strategy RPG Brown Dust introduces its b...
Epic turn-based RPG Brown Dust is set to turn 500 days old next week, and to celebrate, Neowiz has just unveiled its biggest and most exciting update yet, offering a host of new rewards, increased gacha rates, and a brand new feature that will... | Read more »
Dr. Mario World is yet another disappoin...
As soon as I booted up Dr. Mario World, I knew I wasn’t going to have fun with it. Nintendo’s record on phones thus far has been pretty spotty, with things trending downward as of late. [Read more] | Read more »
Retro Space Shooter P.3 is now available...
Shoot-em-ups tend to be a dime a dozen on the App Store, but every so often you come across one gem that aims to shake up the genre in a unique way. Developer Devjgame’s P.3 is the latest game seeking to do so this, working as a love letter to the... | Read more »
Void Tyrant guide - Guildins guide
I’ve still been putting a lot of time into Void Tyrant since it officially released last week, and it’s surprising how much stuff there is to uncover in such a simple-looking game. Just toray, I finished spending my Guildins on all available... | Read more »
Tactical RPG Brown Dust celebrates the s...
Neowiz is set to celebrate the summer by launching a 2-month long festival in its smash-hit RPG Brown Dust. The event kicks off today, and it’s divided into 4 parts, each of which will last two weeks. Brown Dust is all about collecting, upgrading,... | Read more »
Flappy Royale is an incredibly clever ta...
I spent the better part of my weekend playing Flappy Royale. I didn’t necessarily want to. I just felt like I had to. It’s a hypnotic experience that’s way too easy to just keep playing. | Read more »
Void Tyrant guide - General tips and tri...
Void Tyrant is a card-based dungeon-crawler that doesn’t fit in the mold of other games in the genre. Between the Blackjack-style combat and strange gear system alone, you’re left to your own devices to figure out how best to use everything to your... | Read more »
Webzen’s latest RPG First Hero is offici...
You might be busy sending your hulking Dark Knight into the midst of battle in Webzen’s other recent release: the long-anticipated MU Origin 2. But for something a little different, the South Korean publisher has launched First Hero. Released today... | Read more »

Price Scanner via MacPrices.net

Amazon drops prices, now offers clearance 13″...
Amazon has new dropped prices on clearance 13″ 2.3GHz Dual-Core non-Touch Bar MacBook Pros by $200 off Apple’s original MSRP, with prices now available starting at $1099. Shipping is free. Be sure to... Read more
2018 15″ MacBook Pros now on sale for $500 of...
Amazon has dropped prices on select clearance 2018 15″ 6-Core MacBook Pros to $500 off Apple’s original MSRP. Prices now start at $1899 shipped: – 2018 15″ 2.2GHz Touch Bar MacBook Pro Silver: $1899.... Read more
Price drop! Clearance 12″ 1.2GHz Silver MacBo...
Amazon has dropped their price on the recently-discontinued 12″ 1.2GHz Silver MacBook to $849.99 shipped. That’s $450 off Apple’s original MSRP for this model, and it’s the cheapest price available... Read more
Apple’s 21″ 3.0GHz 4K iMac drops to only $936...
Abt Electronics has dropped their price on clearance, previous-generation 21″ 3.0GHz 4K iMacs to only $936 shipped. That’s $363 off Apple’s original MSRP, and it’s the cheapest price we’ve seen so... Read more
Amazon’s Prime Day savings on Apple 11″ iPad...
Amazon has new 2018 Apple 11″ iPad Pros in stock today and on sale for up to $250 off Apple’s MSRP as part of their Prime Day sale (but Prime membership is NOT required for these savings). These are... Read more
Prime Day Apple iPhone deal: $100 off all iPh...
Boost Mobile is offering Apple’s new 2018 iPhone Xr, iPhone Xs, and Xs Max for $100 off MSRP. Their discount reduces the cost of an Xs to $899 for the 64GB models and $999 for the 64GB Xs Max. Price... Read more
Clearance 13″ 2.3GHz Dual-Core MacBook Pros a...
Focus Camera has clearance 2017 13″ 2.3GHz/128GB non-Touch Bar Dual-Core MacBook Pros on sale for $169 off Apple’s original MSRP. Shipping is free. Focus charges sales tax for NY & NJ residents... Read more
Amazon Prime Day deal: 9.7″ Apple iPads for $...
Amazon is offering new 9.7″ WiFi iPads with Apple Pencil support for $80-$100 off MSRP as part of their Prime Day sale, starting at only $249. These are the same iPads found in Apple’s retail and... Read more
Amazon Prime Day deal: 10% (up to $20) off Ap...
Amazon is offering discounts on new 2019 Apple AirPods ranging up to $20 (10%) off MSRP as part of their Prime Day sales. Shipping is free: – AirPods with Charging Case: $144.99 $15 off MSRP –... Read more
Amazon Prime Day deal: $50-$80 off Apple Watc...
Amazon has Apple Watch Series 4 and Series 3 models on sale for $50-$80 off Apple’s MSRP as part of their Prime Day deals with prices starting at only $199. Choose Amazon as the seller rather than a... Read more

Jobs Board

*Apple* Systems Architect/Engineer, Vice Pre...
…its vision to be the world's most trusted financial group. **Summary:** Apple Systems Architect/Engineer with strong knowledge of products and services related to Read more
*Apple* Graders/Inspectors (Seasonal/Hourly/...
…requirements. #COVAentryleveljobs ## Minimum Qualifications Some knowledge of agricultural and/or the apple industry is helpful as well as the ability to comprehend, Read more
Best Buy *Apple* Computing Master - Best Bu...
**710003BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Store Associates **Location Number:** 000171-Winchester Road-Store **Job Description:** Read more
Best Buy *Apple* Computing Master - Best Bu...
**709786BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Sales **Location Number:** 000430-Orange Park-Store **Job Description:** **What does a Read more
Geek Squad *Apple* Master Consultation Agen...
**709918BR** **Job Title:** Geek Squad Apple Master Consultation Agent **Job Category:** Services/Installation/Repair **Location Number:** 000106-Palmdale-Store Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.