TweetFollow Us on Twitter

Feb 90 Letters
Volume Number:6
Issue Number:2
Column Tag:Letters

CheckAbort() Revised

By David E. Smith, Editor & Publisher, MacTutor

Let’s Doodat!

Greg King, Ph.D.

SCIEX

Ontario, Canada

This letter is to point out 2 bugs in Lee Neuse’s “check_abort” routine (Three Doodats, Sept. 89).

The first bug occurs when the command period event is the last event in the queue. Mr. Neuse’s code removes the event and then tests to see if it was the last event. This will never succeed because the event has been removed. The result is usually a bus error because the qLink field of the last event contains a -1. The funny thing is that the routine will not crash if the command-period is the only event in the queue.

The second bug is more subtle. Dequeue does not de-allocate the memory for the event entry being removed from the queue, this is the responsibility of the program calling it (IM2-383). Mr. Neuse doesn’t do this so every time an event is removed it hangs around the heap.

Below is my version of a command-period handler that addresses the above stated problems. It has been tested for the past week in the print and update loops of a 400K application. It is a scientific application that often has to plot many thousands of data points on the screen, and it is often desirable to be able to both abort long updates and queue up multiple keystroke commands. Please excuse my style, I was originally a Pascal programmer.

/* 1 */

Boolean CmdPeriod()
{
EvQElPtreq_p, theEntry;
Boolean f_found, last, dispose_entry;

f_found = FALSE;
last = FALSE;

/* Start at head of internal queue. */
eq_p = (EvQElPtr) (EventQueue.qHead);

do {
 /* Test for end of queue first. */
 if (eq_p == (EvQElPtr) EventQueue.qTail)
 last = TRUE;

 /* Is this a cmd-period event. */
 if ((eq_p->evtQWhat == keyDown 
 || eq_p->evtQWhat == autoKey) 
 && (eq_p->evtQModifiers & cmdKey) 
 && (eq_p->evtQMessage & charCodeMask) == ‘.’)
 {
 /* Remove the event from the queue. */
 Dequeue((QElemPtr) eq_p, &EventQueue);
 f_found = TRUE;
 /* Setup for disposal */
 dispose_entry = TRUE;
 /* Point to the beginning of entry. */
 theEntry = (EvQElPtr) ((long) eq_p - 4);
 }
 else
 dispose_entry = FALSE;
 
 if (!last)
 /* Continue with next queue entry. */
 eq_p = (EvQElPtr) (eq_p->qLink);

 /* Dequeue doesn’t deallocate entry */
 if (dispose_entry)
 DisposPtr((Ptr) theEntry);
 }
while (!last);

return (f_found);
}

On the whole, I found Mr. Neuse’s article well written and informative.

SCIEX is a Canadian company that produces high end mass spectrometers for environmental, security, biotech, and research applications. Our latest product uses a Macintosh II for data aquisition and processing. We are dedicated to using the Macintosh for scientific applications.

Let’s Doodat Again

J. Peter Alfke

Tucson, AZ

The idea of “doodats” as presented by Lee A. Neuse (“Three Doodats”, September 1989, p. 82) is a good one. Unfortunately, the checkAbort() doodat (which checks for Cmd-”.”) has at least two bugs, one apparently harmless, the other fatal.

The harmless bug concerns the most common case: an empty event. As written, checkAbort() charges ahead and examines the event-queue element pointed to by EventQueue.qTail. Unfortunately, if the event queue is empty this value is NIL. Fortunately, whatever data is at NIL doesn’t usually look like a Cmd-”.” keypress, and the routine exits with false.

But consider another case: there are at least two entries in the queue, the last of which is a bona-fide Cmd-”.”. The last queue entry gets dequeued and then checked to see if it matches EventQueue.qTail. Of course, it can never match since it was just removed from the queue. So the routine always goes around again to the next queue element. But the next element is reference through the old one’s qLink field, which is NIL since it was the last entry in the queue.

Now we are looking at the queue entry at NIL, just like in the other bug, But this time EventQueue.qTail isn’t conveniently NIL (it’s pointing to the last remaining event) so we jump to the next entry instead of exiting. This involves dereferencing NIL and we are on a runaway journey to Bus Error Land.

After sitting in the THINK C debugger for a while figuring all this out. I came up with a revised version, as shown below:

/* 2 */

bool
checkAbort(void)
{
 EvQElPtr q, nextq;
 bool last;
 bool found = false;

 q = (EvQElPtr) EventQueue.qHead;
 if (q) /* Ignore empty event queue */
 do {
 last = (q == (EvQElPtr) EventQueue.qTail); /* Last event? */
 nextq = (EvQEl*) q->qLink; /* the next one */
 if ((q->evtQWhat == keyDown || q-> evtQWhat == autoKey) && q->evtQModifiers 
& cmdKey 
 && (q->evtQModifiers & shiftKey) == 0
 && (q->evtQMessage & charCodeMask) == ‘.’) {
 Dequeue(q, &EventQueue);
 found = true;
 }
 q = nextq;
 } while (!last);
 
 return found;
}

My routine also ignores Cmd-Shift-”.” (i.e. Cmd-”>”), which many applications (such as mine, along with Microsoft Word) uses as an Increment-Point-Size command.

Doodat One More Time

Joe Rice

Doodat #1 in your C Workshop feature in the September issue encouraged me to implement a solution to a problem we were having in the application on which I’m currently working. In some areas of our application, we’re running real-time loops that cannot afford to be interrupted by Multifinder yet need to be able to test for several events, not just Cmd-Period. While the sample code provided an excellent starting, it had a few bugs.

First, it doesn’t check for an empty queue before plunging into the loop.

Second, it retrieves a value (qLink) from the queue element pointed to by eq_p after the element has potentially been dequeued.

Third, dequeuing elements from the EventQueue confuses the Event Manager big time. My guess is that when the Event Manager dequeues an element, it probably enqueues that element outside onto a queue of “free” elements so that it can be reused. Dequeuing the element outside of the Event Manager appears to shorten the EventQueue permanently causing the application to grind to a halt within short order. The solution is to change the event to a null event rather than dequeuing it.

The code that we implemented is below. It’s organized somewhat differently than the example code in MacTutor so that it would be a bit more general.

/* 3 */

/**********************
Name:  CheckForEvent
Purpose:  check the event queue for an event without calling Get/WaitNextEvent. 
 Note that this routine removes all events which pass the test from the 
Event queue, not just the first.

TestMDown and TestCmdPeriod are two TestProcs for CheckForEvent.

CheckForEvent((ProcPtr) TestMDown) will return TRUE if the mouse has 
been clicked.

CheckForEvent((ProcPtr) TestCmdPeriod) will return TRUE if a Cmd-Period 
has been typed.
**********************/

Boolean CheckForEvent(TestProc)
ProcPtr TestProc;
{
EvQElPtreqPtr, nextEqPter;
Boolean found;
OSErr status;

found = FALSE;

eqPtr = (EvQElPtr) (EventQueue.qHead);
while (eqPtr != 0) {
 if ((*TestProc) (eqPtr)) {
 eqPtr->evtQWhat = nullEvent;
 found = TRUE;
 }

 if (eqPtr == (EvQElPtr) EventQueue.qTail)
 break;
 
 eqPtr = (EvQEl *) (eqPtr->qLink);
 }
return found;
}

Boolean TestCmdPeriod (eqPtr)
EvQElPtreqPtr;
{
if ((eqPtr->evtQWhat == keyDown 
 || eqPtr->evtQWhat == autoKey) 
 && (eqPtr->evtQModifiers & cmdKey) 
 && (eqPtr->evtQMessage & charCodeMask) == ‘.’)
 return TRUE;

return FALSE;
}

Boolean TestMDown (eqPtr)
EvQElPtreqPtr;
{
if ((eqPtr->evtQWhat == mouseDown)
 return TRUE;

return FALSE;
}

[Although each of the three previous letters deal with the bug in the original article. I thought their solutions were interesting in their own right to be printed here. The first letter dealt with deallocation of memory. The second deals with Cmd-’.’ over Cmd-’>’. The third takes a unique approach by setting the event type to null rather than removing the event. It also gives a mouse down event remover.-ed]

Ken Manly

Buffalo Chip Software

Buffalo, NY

Your readers may be amused by an ‘undocumented feature’ that Forrest Tanaka (MacDTS) and I discovered. If you create an Apple menu using ResEdit, there are two ways you can type the apple symbol into the menu title field. If you type control-T, the value stored will be $14, and all will be well.

If you type shift-command-K (which is what Key Caps suggests) you will get a menu which looks and acts like an Apple menu, but it is not recognized by Notification Manager routines (in System 6.x). The first symptom is that when your alarm clock goes off while your application is running, the alarm icon (which should alternate with the apple) never appears. Thanks to Forrest for guessing there was something wrong with my apple, although he had no way of knowing what.

ADB??

Kirk Chase

Anaheim CA

It seems someone did not know what the acronym “ADB” meant in a recent article. “ADB” stands for “Apple’s Desktop Bus”. It was introduced with the new keyboard. It allows the chaining of serial devices such as keyboards, mice, tablets, and so on to the Mac.

If you would like more information on ADB, there is an article by Dave Kelly and David Smith in the March '89 issue. It explains some of the concepts. It also references Tech Note #206. You can also find information on the Apple Desktop Bus in Inside Macintosh Vol. V.

MacFortran subroutines from MPW Assembler

Bob Robinson

Plainfield NJ 07060

One of the first things I needed after buying MPW was a way to get the files from the assembler into a format that could be used as a subroutine by Absoft MacFortran (chemists aren’t instructed in ‘C’ in college or grad school.) MacFortran subroutines can be speeded up dramatically (also true of other languages) by hand-tuning the compiler’s assembly language output. The Fortran program listed below strips out the unneeded bytes from the MPW assembler object file. The resulting file is callable from MacFortran as a subroutine, which can be loaded dynamically or linked in with the main program. Also listed below is a short assembly language Fortran function for compilation by MPW. The function locks down a Mac memory manager handle and returns a pointer. MacFortran expects function results to be returned in register D0, so the subroutine must save the result in D0 before exiting (the toolbox traps used here happen to use D0.) If it’s to be loaded dynamically by MacFortran, the subroutine must preserve A0. Note: makesub displays the subroutine name in the menu bar, to avoid the Fortran TTY window (compile with ‘O’ option.)

*4

*****************************************************************************************
 program makesub
* R.S. Robinson 6/12/89
* Converts MPW Asm ‘.a.o’ files into MacFortran ‘.sub’ files.
* File name is obtained from clipboard.
* Removes 1st 36 bytes, and last 8 bytes (44 bytes smaller)
 implicit none   ! always a good idea
 include toolbx.par! MacFortran toolbox definitions
 integer i,j,toolbx,htoptr,length,scrap_h,scrapptr
 integer*1 subr(16384)  ! can make bigger if needed
 character*6 fname ! subroutine names always ¾ 6 chars long plus ‘.sub’
 logical*4 exists! error checking
 scrap_h=toolbx(NEWHANDLE,0)! needs a dummy handle
 length=toolbx(GETSCRAP,scrap_h,”TEXT”,i)    ! we don’t use i
* funct ‘htoptr’ locks handle & returns 32bit comptble ptr;
* or use toolbox calls: call toolbx(HLOCK,scrap_h);scrapptr=LONG(scrap_h)
 scrapptr=htoptr(scrap_h)
 fname=’’;if (length>6) length=6
 do (i=1,length);fname(i:i)=CHAR(BYTE(scrapptr+i-1));repeat
 call toolbx(HUNLOCK,scrap_h) ! finished with the scrap, release the 
handle
 inquire (file=TRIM(fname)//’.a.o’,exist=exists)
 if (.NOT.exists) stop
 call toolbx(INSERTMENU,toolbx(NEWMENU,20,char(length)//fname),0)
 call toolbx(DRAWMENUBAR) ! show that we found the file
 open(20,file=fname//’.a.o’,form=’unformatted’,recl=1)
 do (i=1,36);read(20,end=100) subr(i);repeat ! skip 1st 36 bytes
 do (i=1,16384);read(20,end=100) subr(i);repeat ! read the ‘.a.o’ file
100close(20);i=i-9 ! ignore last 8 bytes plus loop overrun
 open(20,file=fname//’.sub’,status=’new’,form=’unformatted’,recl=1)
 do (j=1,i);write(20) subr(j);repeat;close(20) ! write ‘.sub’ file
 end

*****************************************************************************************
* Assemble the code below with MPW Asm, then run ‘makesub’ after copying 
‘htoptr’
* to the clipboard in MPW. The ‘makesub’ program will produce a MacFortran-compatible
* subroutine from the MPW ‘a.o’ file. As set up here, ‘makesub’ must 
be in the same
* folder as the ‘.a.o’ file. The MPW command sequence is:
*
*(copy ‘htoptr’ to clipboard, then)
*Asm [pathname:]htoptr.a
*[pathname:]makesub (must remove ‘ apl’ extension from Fortran program)
*
;integer*4 function htoptr(handle)
; R.S. Robinson 6/12/89
; Takes ‘handle’ as argument, returns locked pointer as function result.
; Function results are obtained by MacFortran from register D0.
;
 INCLUDE ‘Traps.a’ ; MPW equates
Start PROC; needed for MPW
HTOPTR: MOVE.L A0,A2 ; preserve A0 for MacFortran
 MOVEA.L4(A7),A0 ; load pointer to ‘handle’ argument
 MOVE.L (A0),A0  ; load ‘handle’
 _MoveHHi ; move handle to top of heap zone
 _HLock ; lock it
 MOVE.L (A0),D0  ; convert to pointer, ready to strip
 _StripAddress   ; it’s now ’32-bit clean;’ result is in D0
 MOVE.L A2,A0    ; restore A0
 RTS    ; all done; return to Fortran
 END

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

TunnelBear 4.0.2 - Subscription-based pr...
TunnelBear is a subscription-based virtual private network (VPN) service and companion app, enabling you to browse the internet privately and securely. Features Browse privately - Secure your data... Read more
Kodi 18.9 - Powerful media center tool f...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
Iridient Developer 3.4.1 - Powerful imag...
Iridient Developer (was RAW Developer) is a powerful image-conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more
Vallum 3.3.4 - $15.00
Vallum is a little tool that helps you monitor and block apps connections and throttle apps bandwidth. It is able to intercept connections at the application layer, and hold them while you decide... Read more
DiskCatalogMaker 8.2.6 - Catalog your di...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast Finder-like intuitive look and feel Super-fast search algorithm Can compress catalog data for... Read more
SoftRAID 5.8.4 - High-quality RAID manag...
SoftRAID allows you to create and manage disk arrays to increase performance and reliability. SoftRAID allows the user to create and manage RAID 4 and 5 volumes, RAID 1+0, and RAID 1 (Mirror) and... Read more
Audio Hijack 3.7.3 - Record and enhance...
Audio Hijack (was Audio Hijack Pro) drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio... Read more
CleanMyMac X 4.6.15 - Delete files that...
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
Suitcase Fusion 21.2.1 - Font management...
Suitcase Fusion is the creative professional's font manager. Every professional font manager should deliver the basics: spectacular previews, powerful search tools, and efficient font organization.... Read more
Civilization VI 1.3.6 - Next iteration o...
Civilization® VI is the award-winning experience. Expand your empire across the map, advance your culture, and compete against history’s greatest leaders to build a civilization that will stand the... Read more

Latest Forum Discussions

See All

Mulled 2 is a refreshingly simple ball-p...
Some of the greatest mobile games are actually the simplest ones, and it’s this philosophy that Mulled 2 was built on. Out now for iOS and Android, the puzzle sequel takes the accessible mechanics of its predecessor and builds on them with enticing... | Read more »
Physics-based tower defense game Goblin...
Indie developer Arif Games has released Goblin Raiders on iOS and Android. [Read more] | Read more »
PUBG Mobile has provided yet another upd...
PUBG Mobile has been making a point of publicly mentioning all of their ongoing efforts to vanquish cheating from the popular battle royale. Today two teams within the company have provided updates on their progress. [Read more] | Read more »
Zombieland: AFK Survival is celebrating...
Zombieland: AFK Survival is currently celebrating its one-year anniversary. If you don't quite recognise the name that's because it initially launched as Zombieland: Double Tapper. Anyway, the game is celebrating turning one with two Halloween-... | Read more »
Distract Yourself With These Great Mobil...
There’s a lot going on right now, and I don’t really feel like trying to write some kind of pithy intro for it. All I’ll say is lots of people have been coming together and helping each other in small ways, and I’m choosing to focus on that as I... | Read more »
Genshin Impact Guide - Gacha Strategy: W...
If you're playing Genshin Impact without spending money, you'll always need to be looking for ways to optimize your play to maximize rewards without getting stuck in a position where you're tempted to spend. The most obvious trap here is the game'... | Read more »
Genshin Impact Adventurer's Guide
Hello and well met, fellow adventurers of Teyvat! Check out our all-in-one resource for all things Genshin Impact. We'll be sure to add more as we keep playing the game, so be sure to come back here to check for updates! [Read more] | Read more »
Genshin Impact Currency Guide - What...
Genshin Impact is great fun, but make no mistake: this is a gacha game. It is designed specifically to suck away time and money from you, and one of the ways the game does this is by offering a drip-feed of currencies you will feel compelled to... | Read more »
XCOM 2 Collection on iOS now available f...
The XCOM 2 Collection, which was recently announced to be coming to iOS in November, is now available to pre-order on the App Store. [Read more] | Read more »
Presidents Run has returned for the 2020...
IKIN's popular endless runner Presidents Run has returned to iOS and Android just in time for the 2020 election season. It will see players choosing their favourite candidate and guiding them on a literal run for presidency to gather as many votes... | Read more »

Price Scanner via MacPrices.net

Amazon issues $50-$71 instant discount on all...
Amazon has new 2020 Apple 11″ iPad Pros on sale for $50-$70 off Apple’s MSRP. These are the same iPad Pros sold by Apple in its retail and online stores. Be sure to select Amazon as the seller,... Read more
B&H Photo offers early Black Friday prici...
B&H Photo has Apple AirPods on sale for as little as $129 and up to $50 off MSRP for a limited time. Expedited shipping is free to many addresses in the US: – AirPods with Charging Case: $129.99... Read more
Get a 13″ 2.0GHz MacBook Pro for $300 off MSR...
Apple has a full line of Certified Refurbished 2020 13″ 2.0GHz 4-Core Touch Bar MacBook Pros, with 10th generation Intel processors, available starting at $1529 and up to $300 off MSRP. Apple’s one-... Read more
Apple’s 16″ MacBook Pros are back in stock st...
Apple has Certified Refurbished 2019 16″ MacBook Pros back in stock for up to $420 off the cost of new models, starting at $2039. Each model features a new outer case, shipping is free, and an Apple... Read more
Use our exclusive iPhone Price Trackers to fi...
Looking for a new Apple iPhone 12 or 12 Pro? Perhaps a deal on last year’s iPhone 11? Check out our iPhone Price Tracker here at MacPrices.net. We track new and clearance iPhone prices from Apple as... Read more
Weekend deal: $100 off 13″ MacBook Airs at Am...
Amazon has new 2020 13″ MacBook Airs on sale for $100 off Apple’s MSRP, starting at only $899. Their prices are the lowest available for new MacBooks from any Apple resellers. These are the same 13″... Read more
New 10.9″ 64GB Apple iPad Air on sale for $55...
Amazon has Apple’s new 2020 10.9″ 64GB WiFi iPad Air on sale today for $549.99 shipped. That’s $40 off MSRP. Pre-orders are available today at this discounted price, and Amazon states that the iPad... Read more
Get a clearance 2019 27″ 5K iMac for up to $5...
Apple has Certified Refurbished 2019 27″ 5K iMacs available starting at $1439 and up to $520 off their original MSRP. Apple’s one-year warranty is standard and shipping is free. The following... Read more
AT&T offers the Apple iPhone 11 for $10/m...
AT&T is offering Apple’s 64GB iPhone 11 for $10 per month, for customers opening a new line of service, no trade-in required. Discount is applied via monthly bill credits over a 30 month period.... Read more
Apple’s 2020 11″ iPad Pros on sale today for...
Apple reseller Expercom has new 2020 11″ Apple iPad Pros on sale for $50-$75 off MSRP, with prices starting at $749. These are the same iPad Pros sold by Apple in their retail and online stores: – 11... Read more

Jobs Board

Geek Squad *Apple* Consultation Professiona...
**789134BR** **Job Title:** Geek Squad Apple Consultation Professional **Job Category:** Store Associates **Store Number or Department:** 000554-Boca Raton-Store Read more
*Apple* Mobility Specialist - Best Buy (Unit...
**787854BR** **Job Title:** Apple Mobility Specialist **Job Category:** Store Associates **Store Number or Department:** 001764-Downingtown-Store **Job Read more
*Apple* Mac, Windows & Mobile Device Sup...
Apple Mac, Windows & Mobile Device Support Engineer at created 30-Sep-2020"/> Skip to content **Xerox** at https://xerox.avature.net/en_US/careers Menu ** Apple Read more
Freelance Features Writer ( *Apple* Products...
…interested in iOS, watchOS, iPadOS, macOS? Do you like to explain and teach Apple products to other people with less understanding? If these things sound like you, Read more
Geek Squad *Apple* Consultation Professiona...
**788452BR** **Job Title:** Geek Squad Apple Consultation Professional **Job Category:** Store Associates **Store Number or Department:** 000401-Southland Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.