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!

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Drop to GIF 1.28 - Drag-and-drop creatio...
Drag to GIF means zero-click movie-to-GIF conversion. Select a folder to watch and every movie saved or moved into that folder will be converted to an animated GIF. Features Dragging - Drag a movie... Read more
BetterTouchTool 3.158 - Customize multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom)... Read more
NeoFinder 7.4.1 - Catalog your external...
NeoFinder (formerly CDFinder) rapidly organizes your data, either on external or internal disks, or any other volumes. It catalogs and manages all your data, so you stay in control of your data... Read more
Tunnelblick 3.8.0 - 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
Tweetbot 3 for Twitter 3.3.1 - Popular T...
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
Monosnap 3.5.9 - Versatile screenshot ut...
Monosnap lets you capture screenshots, share files, and record video and .gifs. Features Capture Capture full screen, just part of the screen, or a selected window Make your crop area pixel... Read more
Monosnap 3.5.9 - Versatile screenshot ut...
Monosnap lets you capture screenshots, share files, and record video and .gifs. Features Capture Capture full screen, just part of the screen, or a selected window Make your crop area pixel... Read more
Tweetbot 3 for Twitter 3.3.1 - Popular T...
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
Tunnelblick 3.8.0 - 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
Bookends 13.2.5 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more

Latest Forum Discussions

See All

Void Tyrant guide - Tips and tricks for...
Void Tyrant continues to get a lot of play in these parts. Probably because the game is just so deep and varied. The next stop on our guide series for Void Tyrant is class-specific guides. First up is the Knight, as it’s the first class anyone has... | Read more »
Summon beasts and battle evil in epic re...
Imagine a tale of conlict between factions of good and evil, where rogueish heroes summon beasts to aid them in them in warfare and courageously battle dragons over fields of scorched earth and brimstone - that's exactly the essence of epic fantasy... | Read more »
Upcoming visual novel Arranged shines a...
If you’re in the market for a new type of visual novel designed to inform and make you think deeply about its subject matter, then Arranged by Kabuk Games could be exactly what you’re looking for. It’s a wholly unique take on marital traditions in... | Read more »
TEPPEN guide - The three best decks in T...
TEPPEN’s unique take on the collectible card game genre is exciting. It’s just over a week old, but that isn’t stopping lots of folks from speculating about the long-term viability of the game, as well as changes and additions that will happen over... | Read more »
Intergalactic puzzler Silly Memory serve...
Recently released matching puzzler Silly Memory is helping its fans with their intergalactic journeys this month with some very special offers on in-app purchases. In case you missed it, Silly Memory is the debut title of French based indie... | Read more »
TEPPEN guide - Tips and tricks for new p...
TEPPEN is a wild game that nobody asked for, but I’m sure glad it exists. Who would’ve thought that a CCG featuring Capcom characters could be so cool and weird? In case you’re not completely sure what TEPPEN is, make sure to check out our review... | Read more »
Dr. Mario World guide - Other games that...
We now live in a post-Dr. Mario World world, and I gotta say, things don’t feel too different. Nintendo continues to squirt out bad games on phones, causing all but the most stalwart fans of mobile games to question why they even bother... | Read more »
Strategy RPG Brown Dust introduces its b...
Epic turn-based RPG Brown Dust is set to turn 500 days old next week, and to celebrate, Neowiz has just unveiled its biggest and most exciting update yet, offering a host of new rewards, increased gacha rates, and a brand new feature that will... | Read more »
Dr. Mario World is yet another disappoin...
As soon as I booted up Dr. Mario World, I knew I wasn’t going to have fun with it. Nintendo’s record on phones thus far has been pretty spotty, with things trending downward as of late. [Read more] | Read more »
Retro Space Shooter P.3 is now available...
Shoot-em-ups tend to be a dime a dozen on the App Store, but every so often you come across one gem that aims to shake up the genre in a unique way. Developer Devjgame’s P.3 is the latest game seeking to do so this, working as a love letter to the... | Read more »

Price Scanner via MacPrices.net

13″ 1.8GHz MacBook Air on sale again for only...
Amazon has the 2017 13″ 1.8GHz/128GB MacBook Air on sale today for only $749.99 shipped. That’s $250 off Apple’s original MSRP for this model and the cheapest MacBook Air available from any Apple... Read more
Apple’s $1489 clearance price on refurbished...
Apple has Certified Refurbished 2018 13″ 2.3GHz 4-Core Touch Bar MacBook Pros available starting at $1489. Apple’s one-year warranty is included, shipping is free, and each MacBook has a new outer... Read more
New 2019 13″ 2.4GHz 4-Core MacBook Pros on sa...
Apple resellers B&H Photo and Amazon are offering the new 2019 13″ 2.4GHz 4-Core Touch Bar MacBook Pros for $150 off Apple’s MSRP. These are the same MacBook Pros sold by Apple in its retail and... Read more
B&H drops prices another $50 on clearance...
B&H Photo has dropped prices on clearance 2018 13″ MacBook Airs by a further $50 with models now available for $250 off Apple’s original MSRP. Overnight shipping, or expedited shipping, is free... Read more
Find the best sales & lowest prices on Ap...
Our Apple award-winning price trackers are the best place to look for the best sales and lowest prices on Apple gear. Scan our price trackers for the latest information on sales, bundles, and... Read more
Apple has clearance 2018 13″ MacBook Airs now...
Apple has Certified Refurbished 2018 13″ MacBook Airs available starting at only $849. Each MacBook features a new outer case, comes with a standard Apple one-year warranty, and is shipped free. The... Read more
Save $400 on the 8-Core iMac Pro today at Ama...
Amazon has the base 8-core iMac Pro on sale today for $4599 including free shipping. Their price is $400 off Apple’s MSRP, and it’s the currently lowest price available for an iMac Pro. For the... Read more
Flash sale! New 11″ 1TB WiFi iPad Pros for th...
Amazon has the 11″ 1TB WiFi iPad Pro on sale today for only $1199.99 including free shipping. Their price is $350 off Apple’s MSRP for this model, and it’s the lowest price ever for a 1TB 11″ iPad... Read more
Weekend Deal: 2018 13″ MacBook Airs starting...
B&H Photo has clearance 2018 13″ MacBook Airs available starting at only $999 with all models now available for $200 off Apple’s original MSRP. Overnight shipping, or expedited shipping, is free... Read more
Apple has clearance 10.5″ iPad Pros available...
Apple has Certified Refurbished 2017 10.5″ iPad Pros available starting at $469. An Apple one-year warranty is included with each iPad, outer shells are new, and shipping is free: – 64GB 10″ iPad Pro... Read more

Jobs Board

Best Buy *Apple* Computing Master - Best Bu...
**711495BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Store Associates **Location Number:** 000882-Waterfront-Store **Job Description:** The Read more
Best Buy *Apple* Computing Master - Best Bu...
**711597BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Sales **Location Number:** 000873-Colma CA-Store **Job Description:** **What does a Read more
Best Buy *Apple* Computing Master - Best Bu...
**711346BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Store Associates **Location Number:** 001095-Chesterfield-Store **Job Description:** Read more
Best Buy *Apple* Computing Master - Best Bu...
**704899BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Sales **Location Number:** 000135-Pleasant Hill-Store **Job Description:** **What does Read more
Best Buy *Apple* Computing Master - Best Bu...
**707083BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Sales **Location Number:** 000045-Rockford-Store **Job Description:** **What does a Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.