TweetFollow Us on Twitter

Feb 91 Letters
Volume Number:7
Issue Number:2
Column Tag:Letters

Enough Bus Errors

By Kirk Chase, Editor

Mr. Bus Error

David Dunham

Seattle, WA

Aargh! I’ve had it! Enough bus errors! As a developer, I like to reduce my number of bugs, so I keep Mr. Bus Error running (more later). As a user, I like to run programs. I don’t like them generating bus errors, having to reboot without Mr. Bus Error and rerunning the program. I’ve had to do this with software from Aldus, Apple, Ashton-Tate, and Microsoft (to name just a few large companies).

With the Macintosh’s double-pointer memory handles, it’s easy to dereference a NIL handle. Consider this code:

/* 1 */

h = NewHandle(sizeof(dataStruct));
(**h).field = 0;

The problem, as should be obvious, is that NewHandle() may fail if the heap is too full or fragmented. Before we use a handle, we should test it:

/* 2 */

if (h != NIL)  /* Is it valid? */

But we may have a lot of NewHandle()s (or GetResource()s) in our program. It’s easy to miss one. What happens if we do? Both NewHandle() and GetResource() return NIL (0L) if they can’t return a valid handle. We then try to double-dereference this. *h is *(0L), the contents of location 0. We have no way of knowing what’s there (it’s usually set by Finder when an application starts, but of course the Macintosh is a multi-tasking machine, with other applications, drivers, and GetNextEvent() patches, any of which could change it on purpose or through their own bug). **h is **(0L), or whatever location 0 points to. We really have no idea what location 0 points to. So (**h).field = 0; has just trashed some unknown address. This may never hurt us (since there are so many possible addresses), which is why so many errors of this type are released. But this is just gambling that location 0 will always contain some benevolent value. I wouldn’t bet my data on that.

How can we catch dereferencing a NIL handle? We end up using the contents of location 0 as an address, so the simple solution is to put an invalid address value at location 0. Mr. Bus Error is a fine INIT that does just this (once per VBL interrupt, if my memory’s correct). The value it uses is 0xF0F0F1. If used as an address, this generates a bus error on 68020 and 68030 machines. On a 68000, it generates an odd address exception. Simple. Easy. No work on your part (other than to locate Mr. Bus Error; it’s on any developer CD-ROM from Apple and no doubt many other places). No more will you forget to test for NIL handles and make my life miserable (and your own, as you try to find those intermittent crashes -- the ones from f = (**h).field are really fun).

Peter Polash came up with a technique that’s even better, though it does involve actual work. Each time through your event loop, test what’s at location 0. If it’s not what you set it to, you know you’ve changed it. Otherwise, you set it to a value like 0xF0F0F1. This gives you exactly the same error-finding functionality as Mr. Bus Error, with the added bonus of being able to tell you if you ever set something in a purged handle ((**h).field = 0; since *h is NIL for a purged handle) or NIL pointer, and (to within one event) when you set it. You may have minor compatibility problems during testing with this technique, since you’ll probably uncover other tasks changing location 0 on you.

Please use NIL handle discipline (which brings up the point that many debuggers can help you, too). Other error schemes aren’t sufficient (one unrunnable program was written with MacApp).

A Small Error

Allen Stenger

Gardena, CA

Thank you for publishing the interesting and clearly-written article by Dennis R. Cohen on LZW compression (October 1990). MacTutor needs more short articles like this one, that illustrate a technique without trying to develop a full-blown application.

I would like to point out a small error in the code which may trouble your readers: the compression program produces incorrect results if it overwrites an existing file. The format of the output file is a 56-byte header, followed by the compressed data. When opening a new output file, the routine GetOutputFile writes a dummy block for the header, which will later be filled in with the real header data; in the meantime the compressed data will be written following the dummy header. But when opening an existing file, no dummy header is written, so the compressed data is written from the very beginning of the file and then the first part is overwritten with the header.

This may touch on a question of style. The compression and decompression programs use an “economical” approach of reusing an existing file, rather than the “brute force” approach of deleting an existing file and always starting from scratch. Which is better? Because the economical approach splits the processing into two paths, it may be more error-prone (as shown by the error that did occur, above; also note that the file creator and file type of the existing file are left unchanged, rather than being set to the values for new files). Does the economical approach have any advantages?

Revised DrawMyStuff.c

Chip Zempel

Fair Oaks, CA

Note: Source code files accompanying this letter are located on MacTech CD-ROM or source code disks.

Thanks for Mark B. Kauffman’s introduction to using the THINK C Class Library in the September ‘90 issue. It finally made me get off my butt and get my feet wet in OOP, if I can mix my metaphors a bit.

I made some changes to his code that illustrate one of OOP’s best features -- polymorphism, or the ability of a subclass to override an ancestor’s methods -- and I thought you might like to share them with your other readers who are just learning OOP.

Kauffman’s DrawMyStuff() sets up a line, an array of four rectangles, and an array of four ovals. Then he sends them various messages to set their locations and draw and erase themselves. In a “real world” drawing program, of course, you wouldn’t know beforehand what type of objects the user would want to create. Your program would probably allocate a big array of pointers to CShapes (the parent class) and then stuff in whatever specific shapes the user selected at run time.

My revised DrawMyStuff() has just one big generic array of CShapes, and sends them all generic locate, draw, and erase messages. Note that the only part of my code that “knows” whether each shape is a rectangle, line, or oval is the section that initializes each object. (The user would do this in a real program.) Everything else -- setting locations, drawing, erasing, and deleting -- is done “blind.” My version doesn’t know what object is there, and it doesn’t need to. It just says “draw yourself,” or whatever, and the object responds to the message it receives with the method implemented for its subclass.

The final result looks identical to the original when the screen is drawn. (Note however, that I drew and erased the line object in a different order, so that I could send all nine objects their Draw() message in the same “for” loop.) Here’s the code (and please keep running more THINK Class Library tutorials!):

/* 3 */

/*
 * Source  - DrawMyStuff.c
 * based on Mark B. Kauffman’s article
 * in the 9/90 MacTutor
 *
 * revised to demonstrate polymorphism
 * -- the way a subclass can override
 * its ancestor’s methods  
 */
#include “oops.h”
#include “CShape.h”

void DrawMyStuff()
{
   int  i;
 
   /* set up a big generic array */
   CShape *myShape[9];
   
   /* let’s create 4 rectangles... */
   for (i=0; i<4; i++)
   myShape[i] = new(CRectangle);
   /* ... one line...     */
   myShape[4] = new(CLine);
   /* ... and 4 ovals     */
   for (i=5; i<9; i++)
   myShape[i] = new(COval);

   /* place the rectangles  */
   myShape[0]->SetShapeLoc(200,100,300,200);
   myShape[1]->SetShapeLoc(210,110,290,190);
   myShape[2]->SetShapeLoc(220,120,280,180);
   myShape[3]->SetShapeLoc(230,130,270,170);

   /* place the line */
   myShape[4]->SetShapeLoc(100,50,300,50);

   /* place the ovals     */
   myShape[5]->SetShapeLoc(100,100,200,200);
   myShape[6]->SetShapeLoc(110,110,190,190);
   myShape[7]->SetShapeLoc(120,120,180,185);
   myShape[8]->SetShapeLoc(130,130,170,170);

   /* here’s the fun part! send each shape */
   /* a Draw() message, generically         */
   for (i=0; i<9; i++)
   myShape[i]->Draw();
   
   /* relocate the line and Erase() */
   myShape[4]->SetShapeLoc(210,110,290,110);
   myShape[4]->Erase();
   
   /* delete everything, again generically */
   for (i=0; i<9; i++)
   {
   delete(myShape[i]);
   }
}
/* end of listing - DrawMyStuff.c */

P.S. I found a terrific book on OOP -- Object Oriented Program Design, With Examples in C++ by Mark Mullin, from Addison-Wesley. The reader gets to look over his shoulder as he develops a inventory-personnel-accounting database application using object-oriented techniques. He explains why he makes the choices he makes, presents the special features of OOP languages (such as abstraction, encapsulation, and polymorphism) as they become useful, and lays out general rules (and some exceptions!) for good OOP practice. Definitely worth a look!

HyperDA II and HyperEngine 2.0

Symmetry Software

SCOTTSDALE, AZ

HyperDA II , Symmetry Software’s HyperCard stack access desk accessory, begins shipping in December. HyperDA II will support major HyperCard™ 2.0 features such as stylized text, multiple, scrollable, resizable windows, “Hot Text”, visual effects and sound. Support for selection tools that allow users to copy graphics and text to the clipboard are built in also. HyperDA II will support either HyperCard 1.x or 2.0 file formats, and you can even have both running in seperate windows on the desktop at the same time.

Using HyperDA II, the enduser gains immediate access to stacks via the Apple DA menu, while bypassing the large memory requirements of MultiFinder and HyperCard. Users enjoy the same browse, edit and print tools found in HyperCard.

Registered users of HyperDA version 1.x can obtain an update for $29 (includes s/h) by calling Symmetry’s Customer Service department. The upgrade is FREE (with proof of purchase plus $5 shipping), if HDA 1.x was purchased on or after November 10, 1990. HyperDA II’s manufacturer’s suggested retail is $129.

Supporting the enhanced HyperCard™ 2.0 command set, HyperEngine 2.0, Symmetry Software’s stack access development tool, is due for release Q1’91. HyperEngine will support major features such as stylized text, multiple, scrollable, resizable windows, “Hot Text”, visual effects and sound. Support for selection tools that allow users to copy graphics and text to the clipboard are built in also. The technology is based on Symmetry’s HyperDA II Software and has been licensed to other developers since 1988.

Supporting MPW, Think C and Assembly languages, HyperEngine allows the developer to build stack access into their applications. The enduser gains immediate access to stacks while bypassing the large memory requirements of MultiFinder and HyperCard.

A HyperEngine developer’s tool kit is sold by Symmetry that provides developers with the resources to add stack access in their own applications. Help, tutorial or reference stacks can be designed with the HyperCard environment, saving developers the need to use valuable programming resources. You can obtain the developer kit for $125 USD, plus shipping and handling direct from Symmetry Software.

Symmetry Software Corporation can be reached at 8603 East Royal Palm Road, Ste 110, Scottsdale, Arizona, 85258. 602-998-9106.

TextWare Technology Now Available

TextWare Corporation

P.O. Box 3267

Park City, UT 84060

(801) 645-9600

(801) 645-9610 (FAX)

TextWare Corporation has now released its technology for the Macintosh. TextWare brings full text indexing and retrieval capabilities to the Macintosh. It is fully compatible with TextWare for PC and LAN platforms for those working in a mixed environment. TextWare is ideal for indexing large volumes of information such as those found on servers or CD-ROMs. The TextWare Toolkit is available to developers for those wishing to add TextWare capability to their products. Contact TextWare for more information.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

How to win big in Slots Deluxe
Cheating while gambling is illegal and morally wrong, and in some parts of the world it leads to men with names like Vinnie "Six Knuckles" Manchenzo beating you to a pulp in a dark alley. [Read more] | Read more »
How to take over the world in Dictator 2
Running a country isn't easy - especially when you're a dictator who wants to take over the world and crush everyone in your path while you do it. [Read more] | Read more »
Tips and tricks to get a higher score in...
Tank.iois - you guessed it! - another multiplayer arena battler likeAgar.io and Slither.io. It does differentiate itself by putting you in a tiny tank though, so it's not exactly the same. To help you get that all-important high score, we've got a... | Read more »
How to unlock characters in One Tap Tenn...
As the title suggests, One Tap Tennis requires only a single tap to play its particular brand of tennis, and rewards you with a ton of unlockable characters if you perform well. Fortunately for you, we at 148Apps have got a few tips and tricks to... | Read more »
Grab it now: Game Craft’s Legend of War...
The real time strategy game is now available for you to sink your teeth into, through the App Store and Google Play. Combining elements of skill, strategy and empire building, Legend of War is a real gamers’ game. [Read more] | Read more »
Skateboard Party 3 ft. Greg Lutzka (Gam...
Skateboard Party 3 ft. Greg Lutzka 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Skateboard Party is back! This third edition of the popular sports franchise features professional skater... | Read more »
Cubious (Games)
Cubious 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Cubious – How smart are you? How high is your IQube? Solve the impossible puzzles to find out, and help a lost little cube find his... | Read more »
Goat Simulator Waste of Space (Games)
Goat Simulator Waste of Space 1.1 Device: iOS Universal Category: Games Price: $4.99, Version: 1.1 (iTunes) Description: ** IMPORTANT - SUPPORTED DEVICESiPhone 4S, iPad 2, iPod Touch 5 or better.** | Read more »
Wildfulness - Unwind in nature and calm...
Wildfulness - Unwind in nature and calm your mind with nature sounds and illustrations 1.0 Device: iOS Universal Category: Healthcare & Fitness Price: $1.99, Version: 1.0 (iTunes) Description: Spending time in nature helps you to... | Read more »
Dr. Panda Racers (Education)
Dr. Panda Racers 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: STEP ON THE GAS, RACE AND WIN!Fasten your seat belts and get ready to race! Speed your way to the finish line while doing... | Read more »

Price Scanner via MacPrices.net

Enterprise Workers Pick Technology Over Perks...
New Adobe study shows surprising attitudes about office jobs and where the future of work is heading. Adobe has released survey findings revealing that a surprising 70 percent of U.S. office workers... Read more
Goal Zero and OtterBox Partner to Expand iPh...
Goal Zero, specialists in portable power, have announced a partnership with OtterBox, brand smartphone case protection, to offer the Slide and Slide Plus Batteries as modules compatible with the new... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Clearance 2015 13-inch MacBook Airs available...
B&H Photo has clearance 2015 13″ MacBook Airs available for $250 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799... Read more
Apple refurbished Apple TVs available for up...
Apple has Certified Refurbished 32GB and 64GB Apple TVs available for up to $30 off the cost of new models. Apple’s standard one-year warranty is included with each model, and shipping is free: -... Read more
21-inch iMacs on sale for up to $120 off MSRP
B&H Photo has 21″ iMacs on sale for up to $120 off MSRP including free shipping plus NY sales tax only: - 21″ 3.1GHz iMac 4K: $1379.99 $120 off MSRP - 21″ 2.8GHz iMac: $1189 $110 off MSRP - 21″ 1... Read more
Kanex Introduces GoPower USB-C Rechargeable B...
Kanex has announced its GoPower USB-C portable battery for the USB-C MacBook, featuring the new industry standard connector and cable used for connectivity and power. Providing users with a new... Read more
Convertible and Detachable Devices Winning Ov...
According to the latest figures published by International Data Corporation (IDC), Western European shipments of ultraslim convertibles and detachables posted positive growth (44.7%) to account for... Read more
New MacBook Pros And Will MacBook Air Be Upgr...
With my mid-2013 13-inch MacBook Air closing on its third anniversary come November, I’m in system upgrade mode. Actually the Haswell CPU equipped Air is still doing a fine job, but my good wife is... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more

Jobs Board

*Apple* Project Engineer - Smart Source Inc...
SmartSource is in need of an Apple Project Engineer for a 12 month contract opportunity in Pittsburg, PA. Role: Apple Project Engineer Location: Pittsburg, PA Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
Service Assistant - *Apple* Chevrolet *App...
Apple Automotive is one of the fastest growing dealer...and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive, we Read more
Editor, *Apple* News - APPLE (United States...
Job Summary The Apple News team is looking for a passionate and knowledgeable editor with experience covering entertainment/pop culture and experience running social Read more
*Apple* Nissan Service Technicians - Apple A...
Apple Automotive is one of the fastest growing dealer...and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.