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 */

main()
{  
 asm  
 {   
  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);

main()
{
 Handle  myjGNE;
 Ptr     branInstruction,loadInstruction;
 OSErr   iErr;

 SetZone(SysZone);

 myjGNE = Get1Resource('CODE',rMyjGNE);

 if (myjGNE)
 {
  DetachResource(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 */

main()
{  
 asm  
 {   
  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'))
     {
      SysBeep(10);

      hasEvent = 0;
     }

   default:
    break;
  }
 }
 RestoreA4();

 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!

 
AAPL
$106.98
Apple Inc.
-0.36
MSFT
$46.05
Microsoft Corpora
-0.57
GOOG
$550.31
Google Inc.
+0.98

MacTech Search:
Community Search:

Software Updates via MacUpdate

Cocktail 8.0.1 - General maintenance and...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LibreOffice 4.3.3.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
VMware Fusion 7.0.1 - Run Windows apps a...
VMware Fusion allows you to create a Virtual Machine on your Mac and run Windows (including Windows 8.1) and Windows software on your Mac. Run your favorite Windows applications alongside Mac... Read more
OneNote 15.3.2 - Free digital notebook f...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
Audio Hijack Pro 2.11.4 - Record and enh...
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 with Audio Hijack... Read more
Iridient Developer 3.0.0 beta 3 - Powerf...
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
TextWrangler 4.5.11 - Free general purpo...
TextWrangler is the powerful general purpose text editor, and Unix and server administrator's tool. Oh, and also, like the best things in life, it's free. TextWrangler is the "little brother" to... Read more
NeoFinder 6.6 - Catalog your external me...
NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs all your data, so you stay in control of your data archive or disk... Read more
Chromium 38.0.2125.111 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
Default Folder X 4.6.11 - Enhances Open...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click... Read more

Latest Forum Discussions

See All

Audio Defence : Zombie Arena (Games)
Audio Defence : Zombie Arena 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A zombie shooter audio game. Made from gut-wrenching 3D binaural sound, for a new kind of weird immersion. You... | Read more »
RPG Asdivine Hearts (Games)
RPG Asdivine Hearts 1.1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.1.0 (iTunes) Description: SPECIAL PRICE50% OFF (USD 7.99 -> USD 3.99)!!! Travel alongside four companions and a cat in the adventure of a... | Read more »
Haunt the House: Terrortown (Games)
Haunt the House: Terrortown 1.0.1 Device: iOS Universal Category: Games Price: $.99, Version: 1.0.1 (iTunes) Description: 66.6% OFF! SPECIAL SPOOKY HALLOWEEN LAUNCH PRICE! 66.6% OFF! ...What was that sound? Is somebody there? | Read more »
SAS: Zombie Assault 4 Review
SAS: Zombie Assault 4 Review By Jennifer Allen on October 30th, 2014 Our Rating: :: FLAWED SHOOTERUniversal App - Designed for iPhone and iPad Shoot everything that moves in this fun, if flawed, twin-stick shooter.   | Read more »
Naailde the Witch Review
Naailde the Witch Review By Amy Solomon on October 30th, 2014 Our Rating: :: PITCH-PERFECT STORYTELLINGUniversal App - Designed for iPhone and iPad Marvelous storytelling, narration, and moving illustrations make this storybook... | Read more »
1st & Goal Review
1st & Goal Review By Andrew Fisher on October 30th, 2014 Our Rating: :: FOR THE D&D LOVING QBUniversal App - Designed for iPhone and iPad 1st & Goal is a board gamer’s football game, a football fan’s board game, and... | Read more »
French Developer Pated Unveils Seashine
French Developer Pated Unveils Seashine Posted by Ellis Spice on October 30th, 2014 [ permalink ] French one-man studio Pated has unveiled Seashine, “a poetic journey into the abyss.” Players take on the role of a jellyfish strugglin | Read more »
Agents of Storm: Tips, Tricks, and Strat...
Calling all agents: Would you like to see what we thought of this rather pretty base builder? Check out our Agents of Storm review! Have you downloaded Agents of Storm, been bowled over by the graphics, and aren’t quite sure what to do next? Never... | Read more »
Any.DO 2.0 Hopes to Help Manage Producti...
Any.DO 2.0 Hopes to Help Manage Productivity Posted by Ellis Spice on October 30th, 2014 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
Base Busters Review
Base Busters Review By Jennifer Allen on October 30th, 2014 Our Rating: :: FUN BUT RESTRICTED MIXUniversal App - Designed for iPhone and iPad Mixing up two forms of tower defense gaming and collectible cards, Base Busters is a fun... | Read more »

Price Scanner via MacPrices.net

Apple Regains Momentum As Windows Stutters An...
The latest smartphone sales data from Kantar Worldpanel ComTech, for the three months to March 2014, shows Apple performing strongly in the first quarter of the year, with sales bouncing back in... Read more
Worldwide Smartphone Shipments Increase 25.2%...
New smartphone releases and an increased emphasis on emerging markets drove global smartphone shipments above 300 million units for the second consecutive quarter, according to preliminary data from... Read more
Apple now offering refurbished 2014 15-inch M...
The Apple Store is now offering Apple Certified Refurbished 2014 15″ Retina MacBook Pros for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple drops prices on refurbished 2013 Retina...
The Apple Store has dropped prices on 2013 Apple Certified Refurbished 13″ and 15″ Retina MacBook Pros, with Retina models now available starting at $999. Apple’s one-year warranty is standard, and... Read more
New 2.8GHz Mac mini on sale for $949, save $5...
Abt Electronics has the new 2.8GHz Mac mini in stock and on sale for $949.05 including free shipping. Their price is $50 off MSRP, and it’s the lowest price available for this model from any reseller... Read more
Sale! 3.7GHz Quad Core Mac Pro available for...
 B&H Photo has the 3.7GHz Quad Core Mac Pro on sale for $2649 including free shipping plus NY sales tax only. Their price is $350 off MSRP, and it’s the lowest price for this model from any... Read more
Mujjo Steps Up The Game With Refined Touchscr...
Netherlands based Mujjo have just launched their Refined Touchscreen Gloves, stepping up their game. The gloves feature a updated elegant design that takes these knitted gloves to the next level. A... Read more
Sale! Preorder the new 27-inch 5K iMac for $2...
 Abt Electronics has the new 27″ 3.5GHz 5K iMac on sale and available for preorder for $2374.05 including free shipping. Their price is $125 off MSRP, and it’s the lowest price available for this... Read more
Simplex Solutions Inc. Brings Secure Web Surf...
New York based Simplex Solutions Inc. has announced the release and immediate availability of Private Browser 1.0, its revolutionary new secure web browser developed for iPhone, iPad and iPod touch... Read more
Save up to $180 off MSRP with an Apple refurb...
The Apple Store has Apple Certified Refurbished 2014 MacBook Airs available for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is free.... Read more

Jobs Board

Position Opening at *Apple* - Apple (United...
**Job Summary** Every day, business customers come to the Apple Store to discover what powerful, easy-to-use Apple products can do for them. As a Business Leader, Read more
Sr. Manager, *Apple* Deployment Programs fo...
**Job Summary** Apple is seeking candidates for a new position on the Education Content and Technology team. iPad and Mac is in the hands of millions of teachers and Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
*Apple* Solutions Consultant (ASC) - Apple I...
…important role that the ASC serves is that of providing an excellent Apple Customer Experience. Responsibilities include: * Promoting Apple products and solutions Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.