TweetFollow Us on Twitter

Mar 94 Top 10
Volume Number:10
Issue Number:3
Column Tag:Think Top 10

Non-interrupt
Completion Routines

By Colen Garoutte-Carson, Symantec 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. Each month we cover either a specific application of tools or a “Q&A” list.

Non-interrupt Completion Routines

If you’re using high-level Macintosh toolbox routines to manipulate files, devices, or drivers, chances are that you’re program could be executing much more efficiently.

The most efficient way to do any file or driver access is asynchronously. When you write to a file with the FSWrite routine, your application, as well as any background process, is suspended while FSWrite waits for the disk to seek to the appropriate sector. Upon reaching that sector, the drive controller issues an interrupt, which prompts the CPU to start writing to the file. The time lost while waiting for the disk to seek is time that could be better spent. When you write to a file asychronously, control returns to your program immediately. The only time taken is taken after the disk interrupt has been issued, at which time the operating system does the appropriate work, and, often, waits for yet another interrupt to be issued. The process of performing an asynchronous routine is entirely transparent to your application.

All high-level file and device IO related Macintosh toolbox routines have low-level counterparts. These routines begin with the letters “PB”, probably because they are each passed a structure called a parameter block. Rather than passing arguments necessary to complete an operation, fields of the parameter block are assigned values, and a pointer to that parameter block is passed to the PB routine. Parameter blocks are often used both to pass information to a PB routine, and return information to the calling routine.

The second, and final argument to a PB routine is a Boolean, which is set to false for synchronous, or true for asynchronous execution. All PB routines have only these two arguments.

Different PB routines require different types, and different sizes of parameter blocks. For example, the original set of Macintosh file IO routines, designed to be used on the original flat volume architecture, require a structure called a ParamBlockRec. In C and C++, a ParamBlockRec is actually a union of a number of other parameter blocks types. In Pascal, a cased record is used. Different PB routines require variables from different union members. PBRead and PBWrite require an IOParam parameter block. PBCreate and PBDelete require a FileParam parameter block. You can either use a ParamBlockRec structure for calling all of these routines, or the exact structure a particular routine requires. You can use the same ParamBlockRec for many different consecutive PB routines because it’s allocated to the size of the largest member of the union. But, if you use an IOParam, you will not be able to use it with any PB routines other than those which accept IOParam’s. For more information on the organization of the many types of Parameter Blocks, take a look at Inside Macintosh : Files, Inside Macintosh : Devices, or the Think Reference.

All parameter blocks begin with the same basic structure, a ParamBlockHeader. The first two fields of a ParamBlockHeader, qLink and qType, are also fields of a QElem structure. QElem structures are queue element structures used internally by a driver. QElem structures and parameter blocks are interchangeable. The QElem structures that a driver is using at any given moment are probably parameter blocks, passed to it by a PB routine.

Even if you are not using asynchronous IO, calling the low level PB routines instead of their high level counterparts can save time. High level routines in turn call PB routines. Filling out the parameter blocks and calling the PB routines yourself takes more code space, but saves a little bit of CPU time. Also, sometimes there can be more flexibility built into the PB version of a routine.

There are few functional differences between using PB routines synchronously and using the high level routines, but calling PB routines asynchronously adds some new considerations to your coding. The parameter block structure you pass to a PB routines must remain valid, and not move in memory until the PB routine is completed. This means that you cannot pass a pointer to a parameter block declared locally within your function if your function may be exited before the routine is completed. This also means that you cannot reuse a parameter block until the PB routine you’ve used it in last is completed. Attempting to reuse a parameter block, which is currently being used by another PB routine, would modify information which may still be in use. This could cause unpredictable results.

Once you know an asynchronous routine has completed, you can use the information it returns, if any, and reuse or deallocate the parameter block. Often it’s desirable to chain together a series of asynchronous routines. For example, you could open a file, write to the file, then close the file, all asynchronous. The completion of each of those operations could trigger the next.

There are two ways to find out if an asynchronous routine has completed. The most often used of which is a completion routine. There is a variable within the ParamBlockHeader called ioCompletion. This variable is a ProcPtr. If this variable is non-zero, it’s value will be used as an address and will be jumped to automatically when the asynchronous routine is completed.

At first glance, this appears to be the perfect solution to incorporating asynchronous routines into your program. Unfortunately, the completion routine may be executed at any time, including during interrupt time, or at a point in time when your application is not the current process your computer is handling. This means that you cannot call any routines which move memory. Well over half of the Macintosh Toolbox routines move memory, including most of the fundamental ones. This also means that, at the point your completion routine is called, your routine may not have access to your application’s global variables. There is a mechanism which allows you to gain access to your application’s global variables, but using it means you have to take yet another matter into consideration. You could potentially be modifying a global variable which another part of your code could be in the middle of using. This could be very hazardous unless you’ve written your code to take sudden changes in variables into consideration.

The second way to find out if an asynchronous routine has been completed is to poll the ioResult field of it’s parameter block. The ioResult field, like ioCompletion, is a variable within the ParamBlockHeader structure. While the asynchronous routine is executing, this variable has a positive value. When the asynchronous routine is completed, this variable holds an error code, which is a either a negative value, or zero (noErr).

Polling the ioResult field can be a lot more difficult than using a completion routine. In order to poll an ioResult field, you must have access to the parameter block. And, in order to really let the rest of your program continue to go about it’s business, you’d have to poll ioResult fields in your programs main loop, or at some other point in your code that gets executed repeatedly. In order to do this, you’d need access to all of your parameter blocks globally. And, if you did have access to all of them globally, you’d need a mechanism to find out which ones are being used, and which ones are idle. This is not very intuitive.

The best use for ioResult polling is for doing something in particular while you wait for your asynchronous routine to complete, not to make asynchronous routines transparent to the rest of your program. With ioResult polling, you could execute a routine asychronous, and use the little bit of time immediately after it’s started executing to update windows, etc, while you wait for it to complete. As far as the rest of your program is concerned, this might as well be considered sychronous execution.

For education purposes, we’re going to try to mix these two methods to develop a new means of handling asynchronous completion routines. A mechanism which we shall outline will allow you to provide a routine to be executed when the asynchronous routine is completed, yet will execute the routine during normal process time. This is done by polling the ioResult field until the asynchronous routine is complete, and then executing a specified completion routine.

This mechanism offers the best of both worlds, and the drawbacks of neither. You are not limited to non-memory moving routines, and you are able to specify an action be taken at the completion of an asynchronous routine. Because the completion routine is not executed at interrupt time, you’re free to deallocate the parameter block with DisposPtr, which would otherwise be off limits.

Our non-interrupt asynchronous completion routine mechanism will maintain a linked list of installed parameter blocks, periodically search this linked list for completed routines, and remove them from the list and execute their completion routines when completed.

First, let’s declare our linked list structure :


/* 1 */
typedef void (* CompletionProc)(ParmBlkPtr, long);

struct CompletionHandlerEntry {
 ParmBlkPtr pb;
 CompletionProc  doneProc;
 long   refCon;
 struct CompletionHandlerEntry** nextEntry;
};
typedef struct CompletionHandlerEntry CompletionHandlerEntry;

In Pascal, this would be :

/* 2 */
type
   CompletionHandlerEntryHand = ^CompletionHandlerEntryPtr;
   CompletionHandlerEntryPtr = ^CompletionHandlerEntry;
   CompletionHandlerEntry = record
      pb: ParmBlkPtr;
      doneProc: ProcPtr;
      refCon: longint;
      nextEntry: CompletionHandlerEntryHand;
   end;

Notice that in the C version of this structure we’ve declared a function pointer type. There is no equivalent to this in Pascal. We’ll discuss how the function is executed in Pascal later. Also, notice that in addition to storing the parameter block and completion routine, we are also storing a refCon variable in this structure. refCon is a commonly used name for a variable which the programmer can use for his or her own purposes. Because it’s a 4-byte value, you can allocate a pointer or a handle and safely store it in a refCon. This means that you can piggy-back any information you would like onto each parameter block in this completion routine mechanism.

Our program will have to maintain a linked list of these structures in order to implement this mechanism. The first entry in the list will have to be kept track of as a global variable, like so :

CompletionHandlerEntry **Asynchs;

In Pascal:

var
   Asynchs: CompletionHandlerEntryHand;

When the list is empty, this variable will be NULL (NIL, or zero). When there are entries in the list, this global variable will point the first entry. The list can be walked through by following the nextEntry pointer to the next entry in the list. The last entry in the list will have a nextEntry of NULL. At the start of your program you should make sure to initialize Asynchs to NULL.

Next we need to write two routines which act on these structures. One to allow you to add a parameter block to the list, and another to scan the list for completed routines. First, the routine to add the parameter block to the list :


/* 3 */
void InstallCompletion(ParmBlkPtr pb, CompletionProc theProc, 
 long refCon)
{
 CompletionHandlerEntry ** NewEntry;

 NewEntry = (CompletionHandlerEntry **)
 NewHandle(sizeof(CompletionHandlerEntry));
 if (NewEntry == NULL)
 {
 while (pb->ioParam.ioResult > 0)
 ;
 (*theProc)(pb, theProc);
 }
 else
 {
 (**NewEntry).pb = pb;
 (**NewEntry).doneProc = theProc;
 (**NewEntry).refCon = refCon;
 (**NewEntry).nextEntry = Asynchs;
 Asynchs = NewEntry;
 }
}

In Pascal :


/* 4 */
procedure InstallCompletion (pb: univ ParmBlkPtr;
   theProc: ProcPtr; refCon: longint);

var
   NewEntry: CompletionHandlerEntryHand;

begin
   Handle(NewEntry) := 
      NewHandle(sizeof(CompletionHandlerEntry));
   if NewEntry = nil then
      begin
         while pb^.ioResult > 0 do
            ;
         CallCompletion(pb, theProc);
      end
   else
      begin
         NewEntry^^.pb := pb;
         NewEntry^^.refCon := refCon;
         NewEntry^^.doneProc := theProc;
         NewEntry^^.NextOne := Asynchs;
         Asynchs := NewEntry;
      end;
end;


This routine will first allocate a new completion handler entry. If it fails, it will wait for the asynchronous routine to complete and then call the completion routine. This means the asynchronous routine will be performed pseudo-synchronously when memory is very low. If the allocation is successful, the completion handler entry is added to the beginning of our linked list.

There is room to expand upon this routine. It could be modified to use a default completion routine if a completion routine of NULL is passed to it. The default completion routine would probably only deallocate the parameter block. This routine could also be modified to save certain variables in the linked list structure to be restored when the completion routine is called. This could be very useful if you change your current resource file often. You could save the current resource fork when the asynchronous routine is installed, and restore it temporarily when the completion routine is called.

Next, we need a routine to scan this linked list for completed asynchronous routines, and execute their completion routines:


/* 4 */
void DoCompletions()
{
 CompletionHandlerEntry **curEntry = Asynchs;
 CompletionHandlerEntry **parentEntry = NULL;

 while (curEntry)
 {
 if ((**curEntry).pb->ioParam.ioResult <= 0)
 {
 if (parentEntry)
 (**parentEntry).nextEntry = (**curEntry).nextEntry;
 else
 Asynchs = (**curEntry).nextEntry;
 (*(**curEntry).doneProc) 
 ((**curEntry).pb, (**curEntry).refCon);
 DisposHandle((Handle)curEntry);
 curEntry = NULL;
 if (parentEntry)
 curEntry = (**parentEntry).nextEntry;
 else
 curEntry = Asynchs;
 }
 else
 {
 parentEntry = curEntry;
 curEntry = (**curEntry).nextEntry;
 }
 }
}

In Pascal :


/* 6 */
procedure CallDoneProc (pb: ParmBlkPtr; refCon: longint; 
   jmpAddr: ProcPtr);

inline
   $205F, $4E90;

procedure DoCompletions;

var
   curEntry: CompletionHandlerEntryHand;
   parentEntry: CompletionHandlerEntryHand;

begin
   curEntry = Asynchs;
   parentEntry = nil;
   while (curEntry <> nil) do
      begin
         if (curEntry^^.pb^.ioResult <= 0) then
            begin
               if (parentEntry <> nil) then
                  parentEntry^^.nextEntry :=
                     curEntry^^.nextEntry
               else
                  Asynchs := curEntry^^.nextEntry;
               CallDoneProc(curEntry^^.pb, curEntry^^.refCon, 
                  curEntry^^.doneProc);
               DisposHandle(Handle(curEntry));
               curEntry := nil;
               if (parentEntry <> nil) then
                  curEntry := parentEntry^^.nextEntry
               else
                  curEntry := Asynchs;
            end
         else
            begin
               parentEntry := curEntry;
               curEntry := curEntry^^.nextEntry;
            end;
      end;
end;

Notice the inline Pascal procedure. What it does is strip the last parameter off the stack, and jump to the address in memory that it represents with the rest of the parameters intact. Although this works, and is a commonly used way to execute ProcPtr’s in Pascal, the method we can use in C uses registers much more efficiently.

DoCompletions should be added to your event loop, or to some other point in your code which is executed repeatedly.

One important thing to note about this mechanism is that you cannot call InstallCompletion from within a completion routine. This is because of how the DoCompletion routine is organized. At the point your completion routine is being called, the linked list is being acted upon. Installing a new completion handler entry could cause the linked list to be incorrectly maintained. This can be fixed. Consider it an exercise to better familiarize yourself with the routines used in the mechanism. It would either be a matter of 1) keeping a list of completed asychronous routines, removing them from the linked list, then executing their completion routines, or 2) making DoCompletions execute in such a way that modifications to the linked list during the call to the completion routine are compensated for.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Macs Fan Control 1.5.14 - Monitor and co...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
VueScan 9.7.96 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
FileMaker Pro 19.6.1 - Quickly build cus...
FileMaker Pro is the tool you use to create a custom app. You also use FileMaker Pro to access your app on a computer. Start by importing data from a spreadsheet or using a built-in Starter app to... Read more
Duet 3.1.0.0 - Use your iPad as an exter...
Duet is the first app that allows you to use your iDevice as an extra display for your Mac using the Lightning or 30-pin cable. Note: This app requires a iOS companion app. Release notes were... Read more
Firefox 107.0.1 - Fast, safe Web browser...
Firefox offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and casual... Read more
War Thunder 2.21.1.91 - Multiplayer war...
In War Thunder, aircraft, attack helicopters, ground forces and naval ships collaborate in realistic competitive battles. You can choose from over 1,500 vehicles and an extensive variety of combat... Read more
Numbers 12.2.1 - Apple's spreadshee...
With Apple Numbers, sophisticated spreadsheets are just the start. The whole sheet is your canvas. Just add dramatic interactive charts, tables, and images that paint a revealing picture of your data... Read more
DEVONthink Pro 3.8.7 - Knowledge base, i...
DEVONthink is DEVONtechnologies' document and information management solution. It supports a large variety of file formats and stores them in a database enhanced by artificial intelligence (AI). Many... Read more
Drive Genius 6.2.3 - $79.00
Drive Genius features a comprehensive Malware Scan. Automate your malware protection. Protect your investment from any threat. The Malware Scan is part of the automated DrivePulse utility. DrivePulse... Read more
VLC Media Player 3.0.18 - Popular multim...
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

Latest Forum Discussions

See All

TouchArcade Game of the Week: ‘Sling Min...
The world of PC games has always blown my mind because there’s just SO MUCH stuff out there that it’s not uncommon at all for there to be a game that’s well-liked and well-reviewed, and seemingly quite popular with a solid fanbase, and have it be... | Read more »
SwitchArcade Round-Up: Reviews Featuring...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for December 2nd, 2022. So, today turned out a little quieter than the usual Friday. It was so quiet, in fact, that I decided to pen a few reviews. The Knight Witch, Railbound, and Donut... | Read more »
Blue Archive reveals its latest event st...
Nexon has announced the new update for Blue Archive, under the name of An Unconcealed Heart. Featuring a battle between two academies, the story will follow a group struggling to gain recognition, and will bring three new students to recruit. [... | Read more »
Dead Cells+ Is Out Now on Apple Arcade a...
Following the major update for Dead Cells on iOS and Android a few days ago, Playdigious has brought Dead Cells+ () to Apple Arcade. As an App Store Great, Dead Cells+ includes all prior paid DLC and content updates. It also has exclusive mobile... | Read more »
SwitchArcade Round-Up: ‘Romancing SaGa’,...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for December 1st, 2022. Wow, December. We’re already at the last month of the year? Phew. I have a lot of work to finish in the next few weeks. As for today, we’ve got a little news, a... | Read more »
‘Railbound’ Update Now Available Adding...
One of our favorite puzzlers released this year is Railbound from Afterburn Games, which hit in early September and earned our Game of the Week recognition for being an absolutely ace logic puzzler. The goal is to place rail pieces down in order to... | Read more »
The Seven Deadly Sins: Grand Cross celeb...
Netmarble Corporation has pulled out all the stops to celebrate the 3 and a half year anniversary of The Seven Deadly Sins: Grand Cross. The Grand Cross 3.5th Year Anniversary the Ultimate One, a rather wordy title, brings with it a brand new... | Read more »
‘Skullgirls Mobile’ Major Update 5.2 Out...
Developer Hidden Variable pushed out a major update for Skullgirls Mobile (Free) a few hours ago. The version 5.2 update brings in Black Dahlia (before the console and PC game), Retakes, XP Treats, free gifts, and more. Since launch, Skullgirls... | Read more »
Out Now: ‘Disgaea 4’, ‘Romancing SaGa: M...
Each and every day new mobile games are hitting the App Store, and so each week we put together a big old list of all the best new releases of the past seven days. Back in the day the App Store would showcase the same games for a week, and then... | Read more »
SwitchArcade Round-Up: ‘Elevator Action...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for November 30th, 2022. We’re finishing up the month on a quiet note, friends. There are five new releases to look at today, with a few notables amongst them. We’ve got summaries for... | Read more »

Price Scanner via MacPrices.net

New Holiday Sale: Apple retailers are offerin...
Several Apple retailers lowered prices on 10.9″ iPad Airs overnight to lows of $100 off MSRP starting at $499. Their prices are the lowest available for iPad Airs anywhere this Holiday season right... Read more
New Holiday sale at Amazon: Take $50 off Appl...
Amazon has Apple’s new 10th-generation iPads in stock and on sale, for the first time, for $50 off MSRP starting at only $399. Their discount applies to all models and all colors. With the discount,... Read more
Holiday Sale: Get an 8.3″ Apple iPad mini for...
Sams Club has 10.9″ 64GB iPad minis on Holiday sale for $80-$100 off MSRP through December 7, 2022. With their discount, prices start at $399 — the cheapest price for a new iPad mini from any of the... Read more
Sams Club Holiday December Event sale: Apple...
Apple AirPods Max headphones are on sale at Sams Club for $110 off MSRP ($439) as part of their December Event sale, ending on December 7, 2022, valid for all colors. Sale price for online orders... Read more
Apple’s 10.2″ 64GB 9th-generation iPads are o...
Sams Club has 9th-generation 64GB iPads on Holiday sale for $60 off MSRP through December 7, 2022. With their discount, prices start at $259 — the cheapest price for a new iPad from any of the Apple... Read more
11″ 128GB WiFi M2 iPad Pro on sale for $749,...
B&H Photo has the new 11″ 128GB WiFi M2-powered iPad Pro (in Space Gray or Silver) on Holiday sale for $749 including free 1-2 day shipping to most US addresses. Their price is $50 off MSRP and... Read more
Find the best Holiday sale price on an iPad u...
We’ve updated our iPad Price Trackers with the latest information on the new 10th-generation iPads, M2-powered iPad Pros, M1 iPad Airs, iPad minis, and 9th generation iPads from Apple’s authorized... Read more
Apple retailers are offering $100-$150 Holida...
Apple retailers have posted their most-recent Holiday sale prices on 13″ MacBook Airs. Take up to $150 off MSRP on M2-powered Airs with these sales with prices starting at only $1099. Free shipping... Read more
Holiday Sale: Apple’s 14″ MacBook Pros with M...
B&H Photo is offering $200-$300 discounts on Apple’s 14″ MacBook Pros with M1 Pro CPUs as part of their Holiday 2022 sale, with prices starting at $1799. Free 1-2 day shipping is available to... Read more
Deal Alert! 50% off Apple MagSafe Chargers
AT&T has Apple MagSafe Chargers on sale for 50% off MSRP as part of their Holiday sale. Service is not required to take advantage of these savings. With the discounts, their sale prices are the... Read more

Jobs Board

Support Technician II - *Apple* Support - O...
…problems and acting as a liaison between customers and resolving groups. As an Apple Technical Specialist, you will be supporting many of our popular Apple Read more
*Apple* Electronic Repair Technician - PlanI...
…a highly motivated individual to join our Production Department as an Apple Electronic Repair Technician. The computer repair technician will diagnose, assemble, Read more
Lead Developer - *Apple* tvOS - Rumble (Uni...
…earnings, and positive sentiment About the role: We are looking for a Lead Apple tvOS Developer to join our application engineering team to expand our video centric Read more
Tier 1 Endpoint Engineer - *Apple* - Red Ri...
…Desk on site, at our Client's location, with a focus on support to Apple products. This position will handle technical support requests directly from customers and Read more
Product Manager II - *Apple* - DISH (United...
…you will be doing We seek an ambitious, data-driven thinker to assist the Apple Product Development team as our new Retail Wireless division continues to grow and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.