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

VueScan 9.5.75 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Opera 44.0.2510.1449 - High-performance...
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
Opera 44.0.2510.1449 - High-performance...
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
Skim 1.4.29 - PDF reader and note-taker...
Skim is a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF, but is also great for viewing any PDF file. Skim includes many features and has a... Read more
FontExplorer X Pro 6.0.2 - Font manageme...
FontExplorer X Pro is optimized for professional use; it's the solution that gives you the power you need to manage all your fonts. Now you can more easily manage, activate and organize your... Read more
1Password 6.7.1 - Powerful password mana...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
Vivaldi 1.9.818.44 - An advanced browser...
Vivaldi is a browser for our friends. In 1994, two programmers started working on a web browser. Our idea was to make a really fast browser, capable of running on limited hardware, keeping in mind... Read more
Vivaldi 1.9.818.44 - An advanced browser...
Vivaldi is a browser for our friends. In 1994, two programmers started working on a web browser. Our idea was to make a really fast browser, capable of running on limited hardware, keeping in mind... Read more
Skim 1.4.29 - PDF reader and note-taker...
Skim is a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF, but is also great for viewing any PDF file. Skim includes many features and has a... Read more
1Password 6.7.1 - Powerful password mana...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more

Latest Forum Discussions

See All

Fire Emblem Heroes event announces new m...
As reported yesterday, Nintendo was gearing up a live press event for their popular mobile game,Fire Emblem Heroes. While the stream revealed a lot of new things, the event was entirely in Japanese. Luckily we have a rundown of what was announced... | Read more »
Best games we played this week
Another week, another slate of new mobile games. Although there weren't as many big name releases as last week, there were plenty of unique video game titles that came out that's sure to keep you interested over the weekend. Everything from classic... | Read more »
Olli by Tinrocket (Photography)
Olli by Tinrocket 1.0 Device: iOS iPhone Category: Photography Price: $2.99, Version: 1.0 (iTunes) Description: Get drawn in with Olli by TinrocketOlli instantly turns your everyday moments into hand-drawn art and animations. • Watch... | Read more »
Penarium (Games)
Penarium 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: | Read more »
Fire Emblem Heroes is way more profitabl...
Profits for Nintendo's mobile game Fire Emblem Heroes are apparently impressive enough to beat out other Nintendo titles likeSuper Mario Run, despite having 10 times fewer downloads. [Read more] | Read more »
Classic series Robot Unicorn Attack 3 no...
The classic Adult Swim browser game, Robot Unicorn Attack, branched off into a series of popular mobile games. Now, the latest entry into the series, Robot Unicorn Attack 3, is available for iOS and Android mobile devices. [Read more] | Read more »
Sudoku Sweeper (Games)
Sudoku Sweeper 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: A minimalist mashup of Minesweeper and Sudoku. Logic puzzle perfection. Every row, column and zone contains a bomb and one of... | Read more »
Under Leaves (Games)
Under Leaves 1.0.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.0 (iTunes) Description: Journey into the forest, the jungle or the depths of the deep blue sea. Find chestnuts for the pigs, a caterpillar for the... | Read more »
Ninja Pizza Girl (Games)
Ninja Pizza Girl 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: In the not-so-distant future, rampart traffic congestion has resulted in only one way to deliver pizzas across town in thirty... | Read more »
SCRAP (Games)
SCRAP 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: That day, for no apparent reason, SCRAP decided to wake up and run. He had to, because his activation was a mistake the "Factory" could... | Read more »

Price Scanner via MacPrices.net

13-inch 2.7GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 13″ 2.7GHz/128GB Retina MacBook Pros available for $200 off MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz/128GB... Read more
13-inch Gray 2.9GHz/512GB Touch Bar MacBook P...
Amazon has the 13″ Space Gray 2.9GHz/512GB Touch Bar MacBook Pro (model MNQF2LL/A) in stock today and on sale for $150 off MSRP. Shipping is free: - 13″ 2.9GHz/512GB Touch Bar MacBook Pro Space Gray... Read more
15-inch 2.7GHz Space Gray Touch Bar MacBook P...
B&H Photo has the 15″ 2.7GHz Space Gray Touch Bar MacBook Pro in stock today and on sale for $2599…$200 off MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: - 15″ 2.7GHz... Read more
13-inch 2.9GHz/256GB Space Gray Touch Bar Mac...
B&H Photo has the 13″ 2.9GHz/256GB Space Gray Touch Bar MacBook Pro in stock today and on sale for $150 off MSRP including free shipping plus NY & NJ sales tax only: - 13″ 2.9GHz/256GB Touch... Read more
21-inch iMacs on sale for up to $151 off MSRP
B&H Photo has 21″ iMacs on sale for up to $151 off MSRP, each including free shipping plus NY sales tax only: - 21″ 3.1GHz iMac 4K: $1348 $151 off MSRP - 21″ 2.8GHz iMac: $1199.99 $100 off MSRP... Read more
Weekend deal: Up to $420 off new MacBook Pros...
Apple has Certified Refurbished 2016 15″ and 13″ MacBook Pros available for $230 to $420 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2.6GHz... Read more
Price drop: 15-inch 2.2GHz Retina MacBook Pro...
Amazon has dropped their price on 15″ 2.2GHz Retina MacBook Pros (MJLQ2LL/A) to $1709.99 including free shipping. Their price is $290 off MSRP for this model. Note that stock may sell out quickly at... Read more
2.8GHz Mac mini on sale for $899, save $100
B&H Photo has the 2.8GHz Mac mini (model number MGEQ2LL/A) on sale for $899 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP. 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
New System Clock for macOS by B-Eng Now Avail...
Fehraltorf, Switzerland based B-Eng has announced the release and immediate availability of System Clock, the company’s new system monitor and information app developed exclusively for macOS. System... Read more

Jobs Board

Product Manager, *Apple* Platforms - Viacom...
…Product Manager to drive the execution of its iOS and AppleTV experiences. The Apple Platform Product Manager will be a leader in our Agile/Scrum environment and Read more
*Apple* Mobile Master - Best Buy (United Sta...
**493714BR** **Job Title:** Apple Mobile Master **Location Number:** 001024-Weatherford-Store **Job Description:** **What does a Best Buy Apple Mobile Master Read more
*Apple* OS X Server Administrator (Active Se...
** Apple OS X Server Administrator \(Active Secret Clearance\)** **Description** Come be a part of a top notch team, apply today\!\! Tuva TUVA provides turnkey Read more
*Apple* Mac Computer Technician - GeekHampto...
…complex computer issues over the phone and in person? GeekHampton, Long Island's Apple Premium Service Provider, is looking for you! Come work with our crew Read more
Best Buy *Apple* Computing Master - Best Bu...
**501846BR** **Job Title:** Best Buy Apple Computing Master **Location Number:** 001126-South Bay Center-Store **Job Description:** **What does a Best Buy Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.