TweetFollow Us on Twitter

Filter Procs
Volume Number:1
Issue Number:10
Column Tag:Programmer's Forum

"C Glue Routines for Filter Procs"

By Van Kichline, John Pence, MacMan, Inc.

The Macintosh ROM is divided into two sections, the operating system and the programmer's toolbox. The programmer's toolbox comprises about two thirds of the ROM and is there to make it easier for the programmer to adhere to Apple's stringent but cohesive user interface guidelines. It provides easy to use routines for creating windows and text edit records, dealing with resources, conducting modal dialogs and alerts, and many, many more functions. Used with a little care, it makes your program look and perform like a commercial Macintosh product, and makes it easy and intuitive for users to use your program.

The toolbox helps the programmer do it right, but what if you want to do it just a little differently? Not many individuals would care to rewrite and debug the routines provided by the toolbox, but in many cases there are alternatives built in. Many toolbox routines include parameters for optional filter or action procedures, which can be used with a default value (usually NIL) or with a pointer to a procedure you supply. Some examples are filterProcs for SFGetFile and ModalDialog, and actionProcs for controls.

A dialog filterProc is invoked by calling ModalDialog with a procPtr to your filter procedure. It changes the way ModalDialog responds to events that take place within its domain. The object of one filter we wrote was to capture keystrokes that occurred while the command key was down, format them, and display them in a rectangle in the ModalDialog box. The filter looked at keyDown events, checked the modifiers field, changed the itemHit to 0 so that a TextEdit box in the same dialog wouldn't know about the keystroke, and then did a little string fiddling. It didn't take long to write, but it took a while to get running!

The Programmer's Toolbox expects you to be a Pascal programmer, not a C programmer, and C passes arguments quite differently than Pascal. This means that the ROM will call your function, but the way it presents its data is incompatible with Mac C. Pascal passes its parameters on the stack, and Mac C passes its parameters in registers.

Can non-Pascal programmers use filters and actionProcs at all? Is there any solution? Is this the end?

There are two solutions, actually. Assembly language routines can be used for all procedures that are called by the ROM. Assembly language is easy to mix with C programs, and allows the programmer the flexibility to deal with data in any format in which it may be presented. There is nothing at all wrong with this solution, and Inside Mac provides much valuable information for the assembly programmer. Assembly code is tight, fast, and efficient. Assembly programming, however, requires a firm grasp of the instruction set of the processor, and is more difficult and time consuming than programming in a medium level language like C.

Functions called by the ROM can be written in C with a little care, a little effort, and a little glue. The term "glue" refers to a few assembly language instructions that fasten your code and the ROM code together. A glue routine is a labeled set of instructions that interface a particular function to another, "incompatible" function. Glue routines are easy to implement , and once a glue routine for a particular case is developed, it can be readily copied to other routines of the same type with only the most trivial modifications. This allows the programmer to rapidly write the filter and action routines in C. Once debugged and tuned, the routines can be converted to assembly if required, but I haven't found a need to convert any yet.

Pascal calls FUNCTIONS and PROCEDUREs by pushing its arguments onto the stack. If the routine being called is a FUNCTION (a Pascal routine that returns a result) a place for the result is cleared on the stack, which may be two or four bytes wide. Then the parameters for the routine, which may also be two or four bytes each, are pushed on the stack in the order which they are declared in the Pascal procedure's definition. In other words, if the procedure Meza is being called, and it's defined:

PROCEDURE Meza(Homos : food ; Gyros: food ; Pita : bread) ;

Then the arguments would be pushed in the order Homos, Gyros, and Pita. When they're retrieved, they'll be popped in the order Pita, Gyros, Homos. Be careful. Think backwards. Finally, the JSR instruction that calls the procedure places the four byte return address on top of the stack, covering the parameters.

Mac C functions pass the values of the first seven arguments, assuming there are more, in the data registers D0 through D6. Excess arguments are stored on the stack, but we won't deal with the complexities of excess arguments here. The prologue code for each function defined in Mac C actually takes the arguments out of the registers and stores them on the stack in a "stack frame," but we are free to ignore what takes place once the C function is invoked. What's of importance to the writer of a glue routine is taking the Pascal parameters off the stack and placing them in the appropriate registers while preserving the return address. For Pascal FUNCTIONS, the result must also be placed in the appropriate location on the stack.

Assembling the Solution

There are several ways to construct glue routines. The way I've presented here is applicable for routines requiring up to three parameters. Another way would be to "seal" the parameters in a stack frame and extract each parameter relative to A6. (See Robert Denny's column in MT 1, 7) I've selected the more direct approach because it's easier to describe, and is sufficient for all routines I've encountered except window and menu definition procedures, which are extremely complex. Our goal is to present enough information for the reader to construct his/her own glue routines without further aid, so we've selected the direct, or "brute force" method for presentation.

Assembly language may be included in Mac C source code by bracketing the lines with the terms #asm and #endasm. What goes in between is assembly source code that's exactly like MDS assembly source. Here's the skeleton of a Pascal to Mac C glue routine:

#asm
routineName:; what you call the function 
MOVE.L  (SP)+, A0; pop return address to A0
MOVE.X  (SP)+, DX; save up to 3 params, of 2             ; or 4 bytes.
MOVEM.L A0, -(SP); return address on top of        ; stack
MOVEM.L A3-A4/D3-D7, -(SP); save the registers
JSR myFunctionInC; execute the C code
MOVEM.L (SP)+,  A3-A4/D3-D7 ; Restore registers.
MOVE.X  X0, 4(SP); if it's a function, return a          ; value
RTS
#endasm

The first line of the glue routine is the label, "routineName." This would be replaced with a unique function name in your application. The toolbox routine calls this label, not myFunctionInC. If the C function is called by the ROM instead of the glue routine, the system will crash. The label is declared as a C function at the beginning of the file. For example, if this where to be a ModalDialog filterProc, I would declare it in advance:

short routineName() ;   /* type short : returns Pascal BOOLEAN */

Thereafter, the term "routineName" represents a pointer to the function. To use it as a function for a particular modal filter, you'd use:

do{
 ModalDialog(routineName, &itemHit) ;
 switch(itemHit)
 {
 case QUIT:         (code) break 
 case CANCEL:  (code) break ;
 case CRASH:     (code) break ;
 default:                SysBeep(8) ;
 }
} while TRUE ;

Thus, the label of the glue routine is treated exactly as if it where the name of the C function it calls. The actual C function is referenced by nobody but the glue routine.

The second item in the glue routine skeleton pops the top four bytes off the stack and puts them in A0 for temporary storage. This is the return address of the routine calling our filter, pushed on the stack by a JSR in the ROM. In this case, it is ModalDialog who called, and the return address is the only way back to it.

Next is the part that does the actual gluing, and varies for different usages. Parameters, being two or four bytes in length, are popped off the stack and stored in data registers. (See the illustration "Data Configurations.") This data, remember, was pushed there by the toolbox routine before calling us and comprises the parameters our C function needs in its registers. If the data is two bytes in length MOVE.W is used, and if the data is four bytes long MOVE.L is used in place of MOVE.X. See the illustration of the stack at entry to the glue routine.

After the parameters are moved into the registers the return address, which we'd stored in A0, is placed back on top of the stack.

Next, the register set is saved. The MoveMultiple instruction saves all the registers desired in a single line. Then, the JSR instruction to the private name myFunctionInC executes the real code.

After the last parameter is moved to the appropriate data register, the stack pointer (A7) points at the place holder for the FUNCTION result if there is one, or else to "unknown territory," or other essential data that remains on the stack and must be preserved. Next, the return address is placed back on top of the stack (four bytes) followed by the registers (28 bytes.) When the JSR myFunctionInC instruction is executed, it pushes a return address to the instruction following the JSR onto the stack and puts the address of myFunctionInC in the program counter. The data the C code needs is in the registers. The C function doesn't disturb anything on the stack except the return address on the very top, which it uses to return to the glue routine with an RTS.

Once the C function returns to the glue routine that called it, the registers that the glue routine saved are restored, popping them from the stack. Directly under those registers is the return address of the caller that we were so careful to preserve earlier. If our C code was emulating a Pascal FUNCTION, the place holder for the result is directly under the return address. We must place the result of our function in this location. Mac C returns results that are values in D0, and results that are pointers in A0. So a BOOLEAN result would be in D0, and two bytes long. In this case, the last instruction before the RTS would be MOVE.W DO, 4(SP). It's always 4(SP), because the return address always four bytes long, but the instruction may move a word or a long word from D0, or a long word from A0, depending on the data type of the result. Don't put a result there if it's not required! You'll mash irreplaceable data and crash. Now a simple JSR propels us back into the ROM and the inner sanctums of ModalDialog.

Concrete Glue

Here's a concrete example. The toolbox routine TrackControl can use a pointer to an actionProc as a parameter. This actionProc represents a continuous action to be performed while the control is being tracked. Scroll bars require an actionProc in order to make the arrows and paging parts work. First, the label is declared globally.

void trackScroll() ;

The function's declared as void because it's used as a Pascal PROCEDURE, and returns no result. Then, when a mousedown occurs in a scroll bar, the application finds the controls handle and calls:

if(TestControl(controlHand, &theEvent->where) == inThumb)
 TrackControl(controlHand, &theEvent->where, NIL) ;
else
 TrackControl(controlHand, &theEvent->where, trackScroll) ;

Note that TrackControl doesn't need an actionProc for the thumb (the moving box part of the scroller), so why rewrite one?

If the else branch is taken, our glue routine is called by the ROM. An action proc for an indicator like a scroll bar receives two parameters; a ControlHandle and a short representing the partCode of the control that was activated. No result is returned. Thus the glue routine goes:

#asm
trackSrcoll:
MOVE.L  (SP)+, A0; temp storage for return addr
MOVE.W  (SP)+, D1; partCode goes in D1, 2 bytes
MOVE.L  (SP)+, D0; controlHandle goes in D0, 4 bytes
MOVE.L  A0, -(SP); push return address
MOVEM.L A3-A4/D3-D7, -(SP)       ; save regs
JSR   Cscroll  ; do the function written in C
MOVEM.L (SP)+, A3-A4/D3-D7      ; restore regs
RTS; no result. go back to TrackControl
#endasm

The labels trackScroll and Csrcoll are specific to an implementation, while the rest is constant from one actionProc to another. The size of the parameters determines which MOVE instructions to use. D1 is loaded first, then D0, because they where pushed onto the stack in order, and are popped off in reverse.

The glue routine may be contained entirely within the function called by it. This makes cutting and pasting the routine to another application easier. A complete, albeit simple example of a scrolling actionProc in C might be:

void deadCscrolls (theControl, partCode)
 ControlHandle   theControl ;
 short  partCode ;
{
 short  amount, startVal, up ;
 
 if(!partCode)
 return ;
 startVal = GetCtlValue(theControl) ;
 up = (partCode == inUpButton || 
 partCode == inPageUp) ? TRUE : FALSE;
 
 if ((up && (startVal > GetCtlMin(theControl))) ||
 (!up && (startVal<GetCtlMax(theControl))))
 {
 amount = (up) ? -1 :  1 ;
 SetCtlValue(theControl, startVal + amount) ;
 }
 return ;
 
 /* the Glue routine */
 
#asm
trackScroll:; TrackControl calls trackScroll, not        ; deadCscrolls!
 MOVE.L (SP)+, A0; save the return address
 MOVE.W (SP)+, D1; partCode to D1
 MOVE.L (SP)+, D0; theControl to D0
 MOVE.L A0, -(SP); return address goes here
 MOVEM.L  A3-A4/D3-D7, -(SP); save the registers   JSR   deadCscrolls
 ; the scroll actionproc - C
 MOVEM.L  (SP)+, A3-A4/D3-D7; restore the registers      RTS   
 ; there's nothing to return
#endasm
}

Note that every actionProc of this type uses the exact same glue routine. It may take a little while to work out the first time, but the effort doesn't need to be repeated. A small collection is all you need.

Glue routines can be used to rapidly implement toolbox modifying procedures and functions in C. Such implementation allows the programmer to write filters and actionProcs readily, and simplifies debugging and maintaining them. Such routines can be converted entirely to assembly after testing for greater efficiency, or may be left as is with little or no difference in performance.

Authors note: The techniques described here may or may not apply to other C compilers. We'd be interested in hearing.

 
AAPL
$99.00
Apple Inc.
+1.33
MSFT
$44.07
Microsoft Corpora
-0.43
GOOG
$589.79
Google Inc.
+0.77

MacTech Search:
Community Search:

Software Updates via MacUpdate

Acorn 4.4 - Bitmap image editor. (Demo)
Acorn is a new image editor built with one goal in mind - simplicity. Fast, easy, and fluid, Acorn provides the options you'll need without any overhead. Acorn feels right, and won't drain your bank... Read more
Bartender 1.2.20 - Organize your menu ba...
Bartender lets you organize your menu bar apps. Features: Lets you tidy your menu bar apps how you want. See your menu bar apps when you want. Hide the apps you need to run, but do not need to... Read more
TotalFinder 1.6.2 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Tab-based... Read more
Vienna 3.0.0 RC 2 :be5265e: - RSS and At...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more
VLC Media Player 2.1.5 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Default Folder X 4.6.7 - Enhances Open a...
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
TinkerTool 5.3 - Expanded preference set...
TinkerTool is an application that gives you access to additional preference settings Apple has built into Mac OS X. This allows to activate hidden features in the operating system and in some of the... Read more
Audio Hijack Pro 2.11.0 - 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
Intermission 1.1.1 - Pause and rewind li...
Intermission allows you to pause and rewind live audio from any application on your Mac. Intermission will buffer up to 3 hours of audio, allowing users to skip through any assortment of audio... Read more
Autopano Giga 3.6 - Stitch multiple imag...
Autopano Giga allows you to stitch 2, 20, or 2,000 images. Version 3.0 integrates impressive new features that will definitely make you adopt Autopano Pro or Autopano Giga: Choose between 9... Read more

Latest Forum Discussions

See All

Traps n’ Gemstones Review
Traps n’ Gemstones Review By Campbell Bird on July 28th, 2014 Our Rating: :: CASTLEVANIA JONESUniversal App - Designed for iPhone and iPad Fight mummies, dig tunnels, and ride a runaway minecart to discover ancient secrets in this... | Read more »
The Phantom PI Mission Apparition Review
The Phantom PI Mission Apparition Review By Jordan Minor on July 28th, 2014 Our Rating: :: GHOSTS BUSTEDUniversal App - Designed for iPhone and iPad The Phantom PI is an exceedingly clever and well-crafted adventure game.   | Read more »
More Stubies Are Coming Your Way in a Ne...
More Stubies Are Coming Your Way in a New Update Posted by Jessica Fisher on July 28th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
The Great Prank War Review
The Great Prank War Review By Nadia Oxford on July 28th, 2014 Our Rating: :: PRANKING IS SERIOUS BUSINESSUniversal App - Designed for iPhone and iPad Though short, The Great Prank War offers an interesting and fun mix of action and... | Read more »
Marvel Contest of Champions Announced at...
Marvel Contest of Champions Announced at Comic-Con Posted by Jennifer Allen on July 28th, 2014 [ permalink ] Announced over the weekend at San Diego Comic-Con was the fairly exciting looking Marvel Contest of Champions. | Read more »
Teenage Mutant Ninja Turtles Review
Teenage Mutant Ninja Turtles Review By Jennifer Allen on July 28th, 2014 Our Rating: :: DULL SWIPINGUniversal App - Designed for iPhone and iPad The pizza power is weak when it comes to this Teenage Mutant Ninja Turtles game.   | Read more »
Exploration Focused Puzzle Game Beatbudd...
Exploration Focused Puzzle Game Beatbuddy Set to Make Transition from PC to iOS this September Posted by Jennifer Allen on July 28th, 2014 [ permalink ] | Read more »
PlanetHD
PlanetHD By Nadia Oxford on July 28th, 2014 Our Rating: :: SPACE MADNESSUniversal App - Designed for iPhone and iPad PlanetHD will keep players busy for a while, though its unpredictable physics are a handful to deal with.   | Read more »
This Week at 148Apps: July 21-25, 2014
Another Week of Expert App Reviews   At 148Apps, we help you sort through the great ocean of apps to find the ones we think you’ll like and the ones you’ll need. Our top picks become Editor’s Choice, our stamp of approval for apps with that little... | Read more »
Reddme for iPhone - The Reddit Client (...
Reddme for iPhone - The Reddit Client 1.0 Device: iOS iPhone Category: News Price: $.99, Version: 1.0 (iTunes) Description: Reddme for iPhone is an iOS 7-optimized Reddit client that offers a refreshing new way to experience Reddit... | Read more »

Price Scanner via MacPrices.net

13-inch 2.5GHz MacBook Pro on sale for $1099,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $1099.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $100 off MSRP. Price is... Read more
Roundup of Apple refurbished MacBook Pros, th...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available for up to $400 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. Their prices... Read more
Record Mac Shipments In Q2/14 Confound Analys...
A Seeking Alpha Trefis commentary notes that Apple’s fiscal Q3 2014 results released July 22, beat market predictions on earnings, although revenues were slightly lower than anticipated. Apple’s Mac’... Read more
Intel To Launch Core M Silicon For Use In Not...
Digitimes’ Monica Chen and Joseph Tsai, report that Intel will launch 14nm-based Core M series processors specifically for use in fanless notebook/tablet 2-in-1 models in Q4 2014, with many models to... Read more
Apple’s 2014 Back to School promotion: $100 g...
 Apple’s 2014 Back to School promotion includes a free $100 App Store Gift Card with the purchase of any new Mac (Mac mini excluded), or a $50 Gift Card with the purchase of an iPad or iPhone,... Read more
iMacs on sale for $150 off MSRP, $250 off for...
Best Buy has iMacs on sale for up to $160 off MSRP for a limited time. Choose free home shipping or free instant local store pickup (if available). Prices are valid for online orders only, in-store... Read more
Mac minis on sale for $100 off MSRP, starting...
Best Buy has Mac minis on sale for $100 off MSRP. Choose free shipping or free instant local store pickup. Prices are for online orders only, in-store prices may vary: 2.5GHz Mac mini: $499.99 2.3GHz... Read more
Global Tablet Market Grows 11% in Q2/14 Notwi...
Worldwide tablet sales grew 11.0 percent year over year in the second quarter of 2014, with shipments reaching 49.3 million units according to preliminary data from the International Data Corporation... Read more
New iPhone 6 Models to Have Staggered Release...
Digitimes’ Cage Chao and Steve Shen report that according to unnamed sources in Apple’s upstream iPhone supply chain, the new 5.5-inch iPhone will be released several months later than the new 4.7-... Read more
New iOS App Helps People Feel Good About thei...
Mobile shoppers looking for big savings at their favorite stores can turn to the Goodshop app, a new iOS app with the latest coupons and deals at more than 5,000 online stores. In addition to being a... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Sr. Product Leader, *Apple* Store Apps - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
WW Sales Program Manager, *Apple* Online St...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.