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.


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);
 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 */

 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)
 eqPtr = (EvQEl *) (eqPtr->qLink);
return found;

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

return FALSE;

Boolean TestMDown (eqPtr)
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.


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.)


 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)
 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 
 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
 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
 do (j=1,i);write(20) subr(j);repeat;close(20) ! write ‘.sub’ file

* Assemble the code below with MPW Asm, then run ‘makesub’ after copying 
* 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


Community Search:
MacTech Search:

Software Updates via MacUpdate

Tidy Up (Five Users) 4.1.5 - Find duplic...
Tidy Up is a complete duplicate finder and disk-tidiness utility. With Tidy Up you can search for duplicate files and packages by the owner application, content, type, creator, extension, time... Read more
Mellel 3.4.3 - The word processor of cho...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
Skype - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
Bookends 12.6.0 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Access the power of Bookends directly from Mellel, Nisus Writer Pro, or MS Word (... Read more
Apple iBooks Author 2.4 - Create and pub...
Apple iBooks Author helps you create and publish amazing Multi-Touch books for iPad. Now anyone can create stunning iBooks textbooks, cookbooks, history books, picture books, and more for iPad. All... Read more
Web Snapper 3.3.9 - Capture entire Web p...
Web Snapper lets you capture Web pages exactly as they appear in your browser. You can send them to a file as images or vector-based, multi-page PDFs. It captures the whole Web page - eliminating the... Read more
Tunnelblick 3.6beta10 - GUI for OpenVPN...
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
EtreCheck 2.5.1 - For troubleshooting yo...
EtreCheck is a simple little app to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support... Read more
Paragraphs 1.0.4 - Writing tool just for...
Paragraphs is an app just for writers. It was built for one thing and one thing only: writing. It gives you everything you need to create brilliant prose and does away with the rest. Everything in... Read more
Things 2.8 - Elegant personal task manag...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more

Camel Up (Games)
Camel Up 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: | Read more »
The Martian: Bring Him Home (Games)
The Martian: Bring Him Home 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Based on the best selling novel and critically acclaimed film, THE MARTIAN tells the story of Astronaut Mark... | Read more »
This Week at 148Apps: September 21-30, 2...
Leap Into Fall With 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice, standing out above... | Read more »
Tweetbot 4 for Twitter (Social Networki...
Tweetbot 4 for Twitter 4.0 Device: iOS Universal Category: Social Networking Price: $4.99, Version: 4.0 (iTunes) Description: *** 50% off for a limited time. *** | Read more »
Mori (Games)
Mori 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Stop, rewind and unwind with Mori. Time is always running, take a moment to take control. Mori is an action puzzle game about infinitely... | Read more »
100 Years' War (Games)
100 Years' War 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Tower in the Sky (Games)
Tower in the Sky 0.0.60 Device: iOS Universal Category: Games Price: $1.99, Version: 0.0.60 (iTunes) Description: | Read more »
hocus. (Games)
hocus. 1.0.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0.0 (iTunes) Description: New, polished, mind-bending, minimal puzzle game with dozens of levels and extra-ordinary design Features:- Beautifully crafted... | Read more »
Mos Speedrun 2 (Games)
Mos Speedrun 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Mos is back, in her biggest and most exciting adventure ever! Wall-jump to victory through 30 mysterious, action packed levels... | Read more »
3D Touch could be a game-changer, but it...
Were you one of the lucky/financially secure enough ones to buy a new iPhone 6s or iPhone 6s Plus over the weekend? Yup, me too (I’m not convinced I was either of those two things, but let’s go with lucky for now), so I thought I’d delve into just... | Read more »

Price Scanner via

Apple refurbished iPad Air 2s available for u...
Apple has Certified Refurbished iPad Air 2s available for up to $140 off the price of new models. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB Wi-Fi iPad Air 2... Read more
Save up to $100 on Mac AppleCare Protection P...
Adorama has 3-Year AppleCare Warranties on sale for up to $100 off MSRP including free shipping plus NY & NJ sales tax only: - Mac Laptops 15″ and Above: $249 $100 off MSRP - Mac Laptops 13″ and... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
12-inch MacBooks in stock for up to $120 off,...
Adorama has 12″ Retina MacBooks in stock for up to $120 off MSRP including free shipping plus NY & NJ sales tax only. For a limited time, Adorama will include a free Apple USB-C to USB Adapter,... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for... has the 15″ 2.2GHz Retina MacBook Pro on sale for $1799 including free shipping. Their price is $200 off MSRP, and it’s the lowest price available for this model (except for Apple’s $1699... Read more
iPhone 6s and 6s Plus Feature Improved Durabi...
Upgraded components in the new iPhone 6s Plus cost $16 more than the components in the earlier iPhone 6 Plus according to a preliminary estimate from IHS Inc. The bill of materials (BOM) for an... Read more
13-inch Retina MacBook Pros on sale for up to...
Adorama has 13″ Retina MacBook Pros on sale for up to $130 off MSRP. Shipping is free, and Adorama charges sales tax for NY & NJ residents only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1199.99 $... Read more
Apple refurbished 2014 13-inch Retina MacBook...
Apple has Certified Refurbished 2014 13″ 2.6GHz/128GB SSD Retina MacBook Pros available $979, $320 off original MSRP. An Apple one-year warranty is included, and shipping is free: - 13″ 2.6GHz/128GB... Read more
iOS 9 Reflections Ten Days In – The ‘Book Mys...
I’ve never been much of an early adopter by philosophy or temperament, although I did buy the iPad Air 2 I’m typing this column on last fall only about a month after Apple unveiled it. However, my... Read more
Apple refurbished Time Capsules available for...
Apple has certified refurbished Time Capsules available for $120 off MSRP. Apple’s one-year warranty is included with each Time Capsule, and shipping is free: - 2TB Time Capsule: $179, $120 off - 3TB... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Site Security Manager - Apple (Unite...
**Job Summary** The Site Security Manager is a high-profile security position at Apple . The Site Manager is the face of Apple Global Security (GS) and primary point Read more
*Apple* Fulfillment Operations Execution Ana...
**Job Summary** The AMR Apple Fulfillment Operations Team is seeking a talented team player to drive the Apple Online Store (AOS) fulfillment performance to ensure a Read more
*Apple* Distinguished Educator (ADE) Communi...
**Job Summary** Apple is seeking candidates for a new position on the Apple Distinguished Educator (ADE) Program team as ADE Community Support Manager. Join a team Read more
iOS Passbook & *Apple* Pay Engineer - A...
…place. New in iOS 8, you can now add credit and debit cards to Passbook using Apple Pay. You can use Apple Pay in stores accepting contactless payments and in apps Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.