TweetFollow Us on Twitter

Oct 93 Think 10
Volume Number:9
Issue Number:10
Column Tag:Think Top 10

Related Info: Event Manager

Support’s Solutions

By Kevin Irlen, THINK Technical Support, Symantec Corp.

This is a monthly column written by Symantec's Technical Support Engineers intended to provide you with information on Symantec products. This month we cover a single commonly asked question of Symantec’s THINK group.

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

A jGNE Starter Kit for THINK C

[This month’s THINK Top 10 is actually not a list of the top ten questions to Symantec. Instead, it handles a single commonly asked question of the THINK group. What we’d like to know is do you like this type of article, or would you prefer us to return to the Top 10 Q&A style? Send me your thoughts - my editorial e-mail addresses are on page 2 of this issue. - Ed.]

While all the latest and greatest stuff rushes relentlessly at us - C++, AppleScripting, Bedrock, PowerPC, etc. - there remain the numerous loose ends and open questions concerning Macintosh programming that Symantec Tech Support folks get asked for help with every day. Often, dealing with such questions is a straightforward matter of steering a programmer toward the appropriate sections of Inside Macintosh, the odd Tech Note, or to one of the programming forums online. Certain problems seem to fall between the cracks, however; too general to be solved by a couple of Toolbox calls, yet too Macintosh-specific to be readily solved with no clear documentation path to follow. The more often a given problem of this kind crops up, the more it cries out for a generic solution - a tech support solution.

Writing and installing a GetNextEvent filter is one such recurring problem. Essential to the functioning of many extensions, as well as applications, this oddity is mentioned only in a tiny Tech Note, with scant information about how actually to implement it.

From Macintosh Tech Note #85:

GetNextEvent uses a filter (GNE filter) which allows for a routine to be installed which overrides (or augments) the behavior of the system. The GNE filter is installed by pointing the low-memory global jGNEFilter (a long word at $29A) to the routine. After all other GNE processing is complete, the routine will be called with A1 pointing to the event record and D0 containing the boolean result. The filter may then modify the event record or change the function result by altering the word on the stack at 4(A7). This word will match D0 initially, of course.

Sounds good. This is exactly what you’ll need to blink the Apple menu, intercept disk insert events to check disks for viruses, trap special key sequences system-wide, etc. But this is where many a THINK user throws up his or her hands and calls us, wanting to what in the world to do with this information! How do you write such a routine? Do you have to do the whole thing in assembly? How exactly do you install it?

Clearly, given the register-based calling mechanism, “some assembly is required,” and naturally, installation will need to be done by an INIT. But why isn’t there some standard way of doing this, and shouldn’t you be able to write your filter in C? Why ask why? The jGNE Starter Kit, included in full on the companion disk, asks why, and answers by providing a jGNE installer INIT that can be used and re-used simply by replacing resources with ResEdit, along with everything else needed to develop and test custom jGNE’s. In this article, I’ll explain how the INIT works, and what it expects from the ‘CODE’ resource you drop in.

In solving the jGNE problem, there are essentially two hurdles to be cleared, the first relatively low, the second a bit higher. First, we must have a scheme for dealing with the jGNE calling mechanism described in Tech Note 85; and second, we must allow for multiple jGNE’s to coexist, i.e. to be chained. Credit for the snippet of assembly language that follows belongs, as far as I know, to Steve Stockman, a frequent contributor to the Macintosh Programming and Symantec Development Tools forums on CompuServe. It leaps the first hurdle, and alludes to clearing the second. I discovered it while browsing one day, in a message to a programmer in need, and it is what Mr. Stockman left out of his message that inspired me to create this Kit.

Anyway, here’s the only assembly necessary:

/* 1 */

  move.l   A1   ,-(SP)       ; A1    holds theEvent   
  move.w   8(SP),-(SP)       ; 8(SP) holds hasEvent 
 ; (4(SP) + 4 we just pushed)   
  jsr      myjGNEFilterFunc   
  move.w   D0,10(SP)         ; put return value back 
 ; at what will be 4(SP)   
  addq.l   #2,SP             ; pop hasEvent     (10 - 2 = 8)
  move.l   (SP)+,A1          ; pop theEvent back into A1 
 ; ( 8 - 4 = 4)
  movea.l  #0xFFFFFFFF,A0    ; this gets overwritten at 
 ; INIT time with either the   
  jmp      (A0)              ; next jGNE's address, or an 
 ; RTS if there isn't one!  

There are two essential things to note about this code. First, the GetNextEvent call mechanism has been kept completely separate from the filter function itself, which may now be a simple C function of the form:

/* 2 */
short myjGNEFilterFunc(short hasEvent,EventRecord *anEvent);  

This function must return zero if it eats the event, non-zero if the event still needs to be handled.

The second feature of this code is something that no doubt looks a bit strange. As the comment indicates: the movea.l line is not in its final form. It is going to be tampered with at INIT time! The reason for this is that you inevitably want to be able to install multiple jGNE’s, and have them chain from one to the next. The problem is that a jGNE has no way of knowing in advance whether or not it will be last in the chain; and therefore no way of knowing whether to RTS when finished (allowing GetNextEvent to return to the current application) or to JMP to the next jGNE in line. This order isn’t determined until INIT time. A jGNE could be “loaded” simply by floating its code in the system heap and storing a pointer to it in the low-memory global jGNEFilter (0x29A). But in order for multiple jGNE’s to coexist, they must cooperate at INIT time by checking to see whether or not someone has loaded before them - and if so, store the previous jGNE’s address to jump to instead of simply returning. In this way, jGNE’s can chain in last-to-load, first-to-execute order.

Now if you just want to use the jGNE Starter Kit, you don’t have to worry about any of this. All you have to do is use the above code in conjunction with the generic jGNE INIT. The latter part of this article contains a brief discussion on building your jGNE project, which should be more than enough to get you “started.” For those of you in the mood for a hack, read on!

There are any number of strategies for stashing a “special” value so that a piece of code can find it later, but the jGNE Starter Kit’s strategy here is to go to the extreme and actually overwrite the machine instruction that depends on the value of the previously loaded jGNE, once, at INIT time, and then not to worry about it again. No finding the stashed value, or testing it, or nuthin. Here’s the entire code for the INIT (excluding the sacred ShowInitIcon routine), and an explanation to follow:

/* 3 */

#define rMyINITIcon  -4094     // ID of icon to be shown 
 // at INIT time
#define rMyjGNE        128     // ID of jGNE 'CODE' resource

#define kBRAoffset  0x0018     // offset of bra to main in a 
 // std header CODE resource
#define kLoadOffset 0x0014     // offset in "main" of the 
 // line to be changed!

Ptr jGNEFilterPtr : 0x029A;   // low-memory global: 
 // jGNEfilter

OSErr ShowInitIcon(short icon_num,short move_x_by);

 Handle  myjGNE;
 Ptr     branInstruction,loadInstruction;
 OSErr   iErr;


 myjGNE = Get1Resource('CODE',rMyjGNE);

 if (myjGNE)

  branInstruction = *myjGNE         + kBRAoffset;
  loadInstruction = branInstruction + *((short *) 
 (branInstruction + 2)) +kLoadOffset;

  if (jGNEFilterPtr) *((long *) (loadInstruction + 2)) = 
 (long) jGNEFilterPtr;
  else               *loadInstruction = 0x4E75;

  jGNEFilterPtr = *myjGNE;
 else SysBeep(10);

 iErr = ShowInitIcon(rMyINITIcon,-1);

 if (iErr != noErr) SysBeep(10);

OK, this is code may be short, but it does have a couple of rather obscure lines, and a couple of unfamiliar constants. We are attempting to locate in memory, within the loaded ‘CODE’ resource that is our jGNE, the exact word where the “movea.l” instruction begins, and this depends on two things. First, since our assembly snippet comprises the main() function of the jGNE resource, one key constant is the offset of the “movea.l” instruction from the beginning of main(). Disassembly (crude, but effective) reveals that this offset is 20 words (0x0014), and we’ll name the constant kLoadOffset since it’s the instruction that loads the next jGNE address to be jumped to.

This is only halfway toward locating the instruction, though, because where, after all, is main()? The answer is that in a code resource, main() can wind up anywhere. Fortunately, the entry point to a THINK C code resource is a standard header (unless you make it otherwise), and this standard header contains an BRA.S instruction whose sole purpose is to branch to main(). Examine a code resource (using MacsBug, or a code editor), and you will see that this branch instruction is 24 words (0x0018) from the beginning (segment loader info and other no-op stuff). Hence the constant kBRAOffset.

So here’s the deal: load the code resource, dereference the handle, add kBRAOffset to its value, and you’re pointing at the BRA.S instruction. The second word of that instruction is the number of bytes to branch over to get to main(). So add kLoadOffset to that and you’re pointing at your load instruction:

/* 4 */

branInstruction = *myjGNE         + kBRAoffset;
loadInstruction = branInstruction + *((short *) 
 (branInstruction + 2)) + kLoadOffset;

Now we’re home free. All we do now is decide whether to change the second word of that load instruction to the address of the next jGNE, or just replace the instruction with an RTS (0x4E75):

/* 5 */

if (jGNEFilterPtr) *((long *) (loadInstruction + 2)) = 
 (long) jGNEFilterPtr;
else               *loadInstruction = 0x4E75;

Now we can safely replace whatever was in jGNEFilterPtr with a pointer to our jGNE filter, and chaining will take care of itself (assuming the other jGNE’s you’ve installed are just as polite).

That’s all there is to it. Although this technique may be a bit unorthodox, it provides an extremely high degree of modularity to an aspect of Mac programming that is usually shrouded in mystery - one of those tasks that is more often abandoned than solved, or else solved so painfully that one shudders at the thought of having to go through it again.

An additional windfall is that this installation procedure does not in any way depend upon being executed at INIT time. It can be run from a regular THINK C project, allowing you to install a jGNE filter on the fly, as you’re developing it. If you install a faulty filter, removing it is as easy as restoring the previous value to the low-memory global jGNEFilter. Just be sure to note the value of jGNEFilter before you do your install, then, in MacsBug for example, it’s just “SM jGNEFilter oldval” to restore it. Of course the detached resource will just lay there in your system heap, but why worry?

To wrap things up, let’s look briefly at the process of writing the jGNE filter itself. The Starter Kit comes with a generic jGNE code resource project that contains a single file, “generic jGNE.c”. If you build this project, you’ll get a resource of type ‘CODE’ and id 128 that can be dropped into the generic jGNE INIT with ResEdit (in fact, it’s already been put there, but you can do it again just to see how fun and easy it is). Once installed, this filter demonstrates the awesome power of the jGNE mechanism by beeping whenever the user types cmd-shift j, g, n, or e (author is not responsible if this interferes with the proper functioning of any other applications). Your filter will hopefully do something more useful, but the basic structure will be essentially the same. Here’s the code:

/* 6 */

  move.l   A1   ,-(SP)       ; A1    holds theEvent   
  move.w   8(SP),-(SP)       ; 8(SP) holds hasEvent 
 ; (4(SP) + 4 we just pushed)   
  jsr      myjGNEFilterFunc   
  move.w   D0,10(SP)         ; put return value back at 
 ; what will be 4(SP)   
  addq.l   #2,SP             ; pop hasEvent   
  move.l   (SP)+,A1          ; pop theEvent back into A1   
  movea.l  #0xFFFFFFFF,A0    ; this gets overwritten at INIT 
 ; time with either the   
  jmp      (A0)              ; next jGNE's address, or an 
 ; RTS if there isn't one!  

#include <SetUpA4.h>         // DO NOT move this before main! 
 // (It generates code.)

short myjGNEFilterFunc(short hasEvent,EventRecord *theEvent)
 char theChar;

 RememberA0(); SetUpA4();

 if (hasEvent)
  switch (theEvent->what)
   case keyDown:
   case autoKey:
    theChar = theEvent->message & charCodeMask;

    if  ((theEvent->modifiers & cmdKey) && 
 (theEvent->modifiers & shiftKey))
     if ((theChar == 'j') || (theChar == 'g') ||
         (theChar == 'n') || (theChar == 'e'))

      hasEvent = 0;


 return hasEvent;

Just your basic event-handling switch block, plus RememberA0, SetupA4, and RestoreA4 to allow for global and static variables. Of course, you can handle any events you like and in as sophisticated a manner as you’d like, with additional function calls, multiple source files and segments if necessary, MacTraps, ANSI-A4, etc.

With the jGNE Starter Kit, you’ll have that Apple menu blinking in no time!


Community Search:
MacTech Search:

Software Updates via MacUpdate

Path Finder 7.2 - Powerful, award-winnin...
Become a master of file management with Path Finder. Take full control over your file system. Save your time: compare and synchronize folders, view hidden files, use Dual Pane and full keyboard... Read more
Sid Meier's Civilization: Beyond Ea...
Sid Meier's Civilization: Beyond Earth is a new science-fiction-themed entry into the award-winning Civilization series. Set in the future, global events have destabilized the world leading to a... Read more
ForkLift 2.6.6 - Powerful file manager:...
ForkLift is a powerful file manager and ferociously fast FTP client clothed in a clean and versatile UI that offers the combination of absolute simplicity and raw power expected from a well-executed... Read more
Microsoft Remote Desktop 8.0.21 - Connec...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more
OnyX 3.1.0 - Maintenance and optimizatio...
OnyX is a multifunctional utility for OS X. It allows you to verify the startup disk and the structure of its System files, to run miscellaneous tasks of system maintenance, to configure the hidden... Read more
Spotify - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
ExpanDrive - Access cloud storag...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
Markly 1.5.3 - Create measurement and de...
Markly is a measurement and design-spec plugin/extension for Photoshop and Sketch. It is made for modern Web designers and app front-end developers. You can add specification marks simply by clicking... Read more
Suitcase Fusion 6 17.3.0 - Font manageme...
Suitcase Fusion 6 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
Nisus Writer Pro 2.1.2 - Multilingual wo...
Nisus Writer Pro is a powerful multilingual word processor, similar to its entry level products, but brings new features such as table of contents, indexing, bookmarks, widow and orphan control,... Read more

Balls & Holes - Tips and tricks to h...
Balls & Holes is a gorgeous new endless runner that challenges you to make it as far up a mountain as possible before inevitably being sliced into a load of gooey bits. Making this task more difficult are both the wide variety of balls that... | Read more »
Balloony Land offers a fresh twist on th...
Balloony Land by Palringo offers a fresh twist on the match three genre and is out now on iOS and Android. First-off, you'll be popping balloons instead of crushing candy and the balloons will float up and fill the empty spaces instead of dropping... | Read more »
Graphic - vector illustration and design...
Graphic - vector illustration and design 1.0 Device: iOS iPhone Category: Productivity Price: $2.99, Version: 1.0 (iTunes) Description: Autodesk Graphic is a powerful full-featured vector drawing and illustration application right in... | Read more »
Sago Mini Babies (Education)
Sago Mini Babies 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: Introducing the Sago Mini babies. Boys and girls love caring for these adorable characters. Feed Robin her favorite mush... | Read more »
PAUSE - Relaxation at your fingertip (H...
PAUSE - Relaxation at your fingertip 1.1 Device: iOS iPhone Category: Healthcare & Fitness Price: $1.99, Version: 1.1 (iTunes) Description: | Read more »
Super Sharp (Games)
Super Sharp 1.1 Device: iOS Universal Category: Games Price: $1.99, Version: 1.1 (iTunes) Description: Your finger has never been so sharp! Cut with skill to complete the 120 ingenious physics levels of Super Sharp and become a cut... | Read more »
Assembly - Graphic design for everyone...
Assembly - Graphic design for everyone 1.0 Device: iOS Universal Category: Photography Price: $2.99, Version: 1.0 (iTunes) Description: Assembly is the easiest most powerful design tool on the App Store. Create anything you can... | Read more »
Dub Dash (Games)
Dub Dash 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: ARE YOU READY FOR THE ULTIMATE CHALLENGE? UNIQUE SYMBIOSIS OF MUSIC AND GRAPHICS | Read more »
Leave Me Alone (Games)
Leave Me Alone 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: 33% off launch sale!!! Somewhere between the 1980s and 1990s there exists a world that never was. A world of skatepunks,... | Read more »
YAMGUN (Games)
YAMGUN 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: The invasion has begun! Protect the walls of the citadel against waves of enemies! But watch out, you will soon run out of ammo...... | Read more »

Price Scanner via

Apple Products Four Of 2015′s Top Ten Most Wa..., Future’s global technology brand and the UK’s biggest technology website, has unveiled results from extensive research that show • The Smart Home is what excites the public most about... Read more
13-inch 2.7GHz/128GB Retina MacBook Pro on sa...
Best Buy has the 13″ 2.7GHz/128GB Retina MacBook Pro on sale for $1174.99 for a limited time. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-... Read more
App Tamer 2.1 Makes El Capitan Cooler, Quiete...
St. Clair Software has announced the availability of a free update to its App Tamer utility for Mac OS X. App Tamer 2.1 delivers compatibility with Yosemite and improves its support for a number of... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has 2015 15″ Retina MacBook Pros on sale for up to $160 off MSRP including free shipping plus NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1849.99 $150 off - 15″ 2.5GHz Retina... Read more
12-inch Retina MacBooks on sale for $100 off...
Best Buy has 12″ Retina MacBooks available for $100 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Discounted prices available for online orders only... Read more
MacBook Airs on sale for up to $100 off MSRP
Save up to $100 on the purchase of a new 2015 13″ or 11″ 1.6GHz MacBook Air at the following resellers. Shipping is free with each model: 11" 128GB MSRP $899 11" 256GB... Read more
Save 15% with Apple refurbished Mac Pros
The Apple Store has Apple Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more
Apple restocks refurbished Mac minis for up t...
Apple has restocked Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $... Read more
TP-LINK Next-Gen Routers Support a Large Numb...
TP-LINK, specialists in consumer and business networking products, have announced the availability of Archer C2600, the company’s next-generation router featuring wireless AC, multi-user MIMO, and 4-... Read more
Apple refurbished 2014 MacBook Airs available...
Apple has Certified Refurbished 2014 MacBook Airs available starting at $679. An Apple one-year warranty is included with each MacBook, and shipping is free: - 11″ 1.4GHz/128GB MacBook Air: $679... Read more

Jobs Board

Project Manager: *Apple* Pay - Apple (Unite...
…The iOS Systems team is looking for a project manager who will manage Apple Pay server side integration project resources in a matrix organization, track tasks, prepare Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, you're Read more
Validation Engineering Manager - *Apple* Wa...
…hardware and software development to lead the validation of the next generation Apple Watch and related products. The Validation Team works closely with many Read more
Senior Payments Architect - *Apple* Pay - A...
**Job Summary** Apple , Inc. is looking for a highly motivated, innovative and hands-on senior payments architect to join the Apple Pay Engineering team. You will Read more
*Apple* Retail for Business Support Supervis...
…is looking for a motivated, outgoing, and creative individual who wants to offer Apple Business Customers an unparalleled customer experience. The Apple Retail for Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.