TweetFollow Us on Twitter

Threaded Apple Events

Threading Apple Events

Or: How I Learned to Stop Worrying and Love to Bomb

By Grant Neufeld, InfoDesign Corporation

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

As Jon Wiederspan points out elsewhere in this issue, threading is becoming a serious problem for webmasters intent on getting speed out of their servers. In C the problem is compounded because the AppleEvent Manager does not handle multiple simultaneous events (and Apple events are used to communicate with CGI applications). Grant Neufeld has developed a solution to this problem and distributes a CGI framework that implements it for you. In this article, he shows us how it's done.

On the Thread Manager, see also:

For Frontier as a possible solution to threading difficulties, see MacTech Magazine 12.1 (January 1966) 63-65; see also the technologies discussed by Jon in our MacWorld Expo report. On writing your CGI in C, see MacTech Magazine 11.9 (September 1995) 33-42.

The Problem of Reentrancy

As I recently discovered, trying to combine Apple events with the Thread Manager is an awkward and mind-altering experience. There are some fundamental - and obscure - requirements that are easy to miss (or so I'd like everyone to believe, so that I don't have to be so embarrassed by my own oversight).

The fundamental problem is that AEProcessAppleEvent is non-reentrant. So, if you call it, you can't call it again until the event finishes processing. I didn't know this when I originally designed the threading for my CGI framework, so I happily set about an implementation whereby I could call AEProcessAppleEvent multiple times before finishing the first call - all through the glory of the Thread Manager. My incorrect code looked like:

/* warning: bad code! Don't try this at home! */
/* Called from main event loop when a high level event arrived. */
void doHighLevelEvent ( EventRecord *theEvent )
{
 OSErr     theErr;
 ThreadID  theThread;
 
 if ( gHasThreadMgr ) {
// MyNewThreadFromPool is just a custom method to simplify
// using threads from a pool.
// Remember: this particular example of Apple event threading
// is wrong - don't do it this way!
 theErr = MyNewThreadFromPool ( doAEThread, theEvent,
 (void**)nil, &theThread );
 }
 if ( !gHasThreadMgr || (theErr != noErr) ) {
// If threading isn't available, or the attempt to thread failed,
// process the Apple event without threading.
 theErr = AEProcessAppleEvent ( theEvent );
 }
}

/* The thread entry function that was used to process the Apple event. */
pascal void * doAEThread ( void *theEvent )
{
 OSErr     theErr;
 ThreadID  currentThread;
 
 theErr = AEProcessAppleEvent ( (EventRecord *)theEvent );
 
 GetCurrentThread ( &currentThread );
 DisposeThread    ( currentThread, (void *)theErr, true );
 
 return (void *)theErr;
}

The amazing thing is that nobody - including me - caught the error until Wayne K. Walrath took a look at the code (in the eleventh release) and noticed my mistake. By then, programs were already shipping and in commercial use - including my own Random URL CGI.

So, why didn't anyone notice the code crashing? The code we all had in our threads happened to be fast enough to finish before new Apple events came in. This doesn't mean it would never crash; given a situation where two Apple events came in almost simultaneously, the code would certainly flop.

Thankfully, I had the prescience to include a suitable disclaimer and label the code "beta" and "subject to errors" to cover my legal posterior. However, my public esteem was in jeopardy! Worse still, I couldn't touch the code for a few days after receiving the bug report because of job obligations. Actually, that was a good thing because the time spent thinking about the code gave me a good basis to start from when I did actually get down to work.

Moral: Think before you code. (Carefully reading the documentation isn't a bad idea either!)

Suspending Apple Events

My original code attempted to thread every Apple event. This is not the case with the corrected code. A new thread should only be initiated within the context of an AEProcessAppleEvent call if the Apple event has been successfully suspended. Suspension of Apple events involves calls to the functions AESuspendTheCurrentEvent and AEResumeTheCurrentEvent.

If the Apple event does not suspend, you can't make any further calls to AEProcessAppleEvent, so you must not create a sub-thread unless you implement a mechanism to prevent calls to AEProcessAppleEvent until the current Apple event finishes. If you want to be creative, you can write your own Apple event dispatcher to allow reentrancy, thereby making it easier to deal with threads. People might call you crazy, but you can do it.

Which brings us to the obligatory complaint about lack of sufficient (and clear) documentation (which is part of the reason I'm writing this). Inside Macintosh: Interapplication Communication doesn't sufficiently cover the use of the necessary calls and provides no sample code for suspending Apple events (see IM:IAC 4.85-88). Hopefully, this article helps fill that gap.

The CGI Apple event is the only one I'm concerned about threading for my framework, so I didn't bother with any of the other events. I handle the Apple event arrival in the normal way with a handler installed to be called by AEProcessAppleEvent:

/* Apple event Handler for the CGI WWW sdoc event */
pascal OSErr
CGIAESearchDoc ( AppleEvent *theAppleEvent,
 AppleEvent *theReply, long Reference )
{
 OSErr     theErr;
 CGIHdl    theCGIHdl;
 ThreadID  theThread;
 
// Allocate the CGIHdl data structure - zeroing out its contents.
 theCGIHdl = (CGIHdl)
 MyNewHandleClear ( sizeof(CGIrecord), &theErr );
 if ( theCGIHdl == nil ) {
 return theErr;
 }
 
// Store references to the apple event and reply records.
 (*theCGIHdl)->appleEvent = *theAppleEvent;
 (*theCGIHdl)->replyEvent = *theReply;
 
 if ( gHasThreadMgr ) {
// It is necessary to suspend the Apple event in order to thread its processing
// because of some real weirdness with AEProcessAppleEvent not being
// "reentrant." This means you can't be processing multiple Apple events at
// the same time, so they have to be ‘suspended' if you want to deal with more
// than one (I.E., multi-threaded processing).
 theErr = AESuspendTheCurrentEvent ( theAppleEvent );
 if ( theErr == noErr) {
 (*theCGIHdl)->suspended = true;
// Apple event has been suspended, so we can spawn a thread for processing.
 theErr = MyNewThreadFromPool (
 CGIAESearchDocProcessThread, theCGIHdl,
 (void**)nil, &theThread );
 }
 }
 
 if ( !gHasThreadMgr || (theErr != noErr) ) {
// If threading isn't available, or the attempt to thread failed, or the attempt to
// suspend the Apple event failed, process the Apple event without threading.
 theErr = cgiAESearchDocProcess ( theCGIHdl );
 } else {
// We suspended the Apple event, and spawned the thread, now let's start it.
 YieldToThread ( theThread );
 }
 
 return theErr;
}

The support for threading begins in the CGIAESearchDoc function. First, I confirm the presence of the Thread Manager. If it is available, I attempt to suspend the Apple event so that it will be safe to process subsequent Apple events before finishing the current one. If the suspension is successful, I call my thread function for the event, which in turn calls the function that does the actual work of handling the event request.

/* Entry point for CGI handler thread. threadParam must not be nil. */
pascal void *
CGIAESearchDocProcessThread ( void *threadParam )
{
 OSErr     theErr;
 CGIHdl    theCGIHdl;
 ThreadID  currentThread;
 
// The threadParam is used to pass the CGIHdl.
 theCGIHdl = (CGIHdl)threadParam;
 
 theErr = cgiAESearchDocProcess ( theCGIHdl );
 
// Find the ID of current thread and use DisposeThread to dispose of it so that 
// my custom thread termination procedure will be used to recover this thread's
// allocation for the thread pool.
 GetCurrentThread ( &currentThread );
 DisposeThread ( currentThread, (void *)theErr, true );
 
// This line below is actually irrelevant, since the DisposeThread call above
// will result in the immediate termination of this thread.
// I keep it in because a return result is needed for the compiler not to issue a
// warning (and I have the "treat all warnings as errors" flag set in my
// compiler, like every programmer should).
 return (void *)theErr;
}

The basic flow of my CGI Apple event handling (if we ignore the threading) starts with CGIAESearchDoc which leads to cgiAESearchDocProcess (which calls CGIAEResumeComplete if the Apple event is suspended), finishing with cgiAEComplete. This structure allows the necessary threading and Apple event "wrapper" functions to be put around the cgiAESearchDocProcess. If the Apple event is successfully suspended, an attempt is made to create the thread, which then carries the remainder of the processing. If either the suspend or thread fails, processing will still occur - just without threading.

Resuming the Apple event

cgiAESearchDocProcess checks the suspension of the Apple event to determine how it should call the completion function. If the Apple event was suspended, it must be resumed using AEResumeTheCurrentEvent before cgiAEComplete can be called.

/* Process the CGI WWW sdoc Apple event.
    theCGIHdl must be valid (non-nil) and unlocked. */
static OSErr
cgiAESearchDocProcess ( CGIHdl theCGIHdl )
{
 OSErr       theErr;
 AppleEvent  theAppleEvent;
 
// Copy the AppleEvent record pointer into a local variable for faster access.
 theAppleEvent = (*theCGIHdl)->appleEvent;
 
// The following section (not shown in this listing) is where the parameters are
// pulled from the CGI Apple event and allocated in the CGI Handle.
// That is followed by a call to the application specific CGI handler function.
 
 if ( (*theCGIHdl)->suspended ) {
// We're in a suspended Apple event, so we'll need to resume the
// Apple event to have it complete and return the reply properly.
 theErr = AEResumeTheCurrentEvent (
 &theAppleEvent, &((*theCGIHdl)->replyEvent),
 vCGIAEResumeCompleteUPP, (long)theCGIHdl );
 } else {
// We weren't suspended, but still need to take care of the Apple event
// reply record.
 theErr = cgiAEComplete ( theCGIHdl );
 }
 
 return theErr;
}

/* Call the event completion function (cgiAEComplete) when resuming
     suspended Apple events. theReference must be a CGIHdl. */
pascal OSErr
CGIAEResumeComplete ( const AppleEvent *theAppleEvent,
 AppleEvent *theReply, long theReference )
{
 OSErr  theErr;
 
 theErr = cgiAEComplete ( (CGIHdl)theReference );
 
 return theErr;
}

/* Complete the CGI Apple event. theCGIHdl must be valid. */
static OSErr
cgiAEComplete ( CGIHdl theCGIHdl )
{
 OSErr  theErr;
    
 HLock ( (Handle)theCGIHdl );
 
 if ( (*theCGIHdl)->responseData != nil ) {
// If the user's "MyCGIProcess" function set the responseData properly, return it.
 theErr = AEPutParamPtr ( &((*theCGIHdl)->replyEvent),
 keyDirectObject, typeChar,
 (Ptr)((*theCGIHdl)->responseData),
 (*theCGIHdl)->responseSize );
 } else {
// If the user's "MyCGIProcess" failed to set the responseData properly,
// return an error header.
 theErr = AEPutParamPtr ( &((*theCGIHdl)->replyEvent),
 keyDirectObject, typeChar, (Ptr)gHTTPHeaderErr,
 gHTTPHeaderErrSize );
 }
 
 HUnlock ( (Handle)theCGIHdl );
    
 cgiDisposeHandle ( theCGIHdl );
 
 return theErr;
}

Using Preallocated Threads

One thing to be concerned about is overloading on threads. Having a handful of threads available can improve performance, but running dozens of threads can bog things down - especially in terms of memory usage, since each thread has its own stack allocated in the application heap. Because of this, creating threads can quickly eat up a significant chunk of memory. You can reduce this somewhat by calculating the maximum total space you will need for the thread stack and using that value in place of the default when allocating threads. However, memory use by threads will still be a significant matter.

A good strategy is to preallocate a limited number of threads and use only those. This has the double advantage of increasing the thread allocation speed and reducing heap fragmentation. In your application initialization sequence, you'll need to allocate a pool of threads after confirming that the Thread Manager is available.

// Pre-allocate the required number of threads.
// kStartupThreadsPreallocate is a constant defined in "MyConfiguration.h"
// which I use to define the total number of threads to preallocate
 CreateThreadPool ( kCooperativeThread,
 kStartupThreadsPreallocate, nil );

You will want to use a wrapper call to NewThread to put the current thread to sleep if there aren't any available threads in the pool. This is done so that one of the active threads may have time to complete, at which point the finishing thread will "wake up" the sleeping thread, allowing it to continue on from the point where it went to sleep, with a thread (the one that just finished) now available from the preallocated pool.

/* Allocate a new thread from the existing pool of threads.
    If there are no threads available, yield to other threads until one finishes.
    The Thread Manager must be available for this function to work. */
OSErr
MyNewThreadFromPool ( ThreadEntryProcPtr threadEntry,
 void * threadParam, void ** threadResult,
 ThreadID * threadMade )
{
 OSErr     theErr;
 short     threadsFree;
 ThreadID  currentThread;
 
 theErr = GetFreeThreadCount ( kCooperativeThread,
 &threadsFree );
 if ( theErr == noErr ) {
 theErr = GetCurrentThread ( &currentThread );
 }
 if ( (theErr == noErr) && (threadsFree == nil) ) {
// Put the current thread to sleep, to be woken up when a thread becomes available.
 gThreadSleeper = currentThread;
 theErr = SetThreadState ( currentThread,
 kStoppedThreadState, nil );
 }
 if ( theErr == noErr ) {
// Install the new thread using a premade thread from the pool.
 theErr = NewThread ( kCooperativeThread,
 threadEntry, threadParam, nil,
 kFPUNotNeeded + kUsePremadeThread,
 threadResult, threadMade );
 }
 if ( theErr == noErr ) {
// Set the termination function for the thread.
 SetThreadTerminator ( *threadMade,
 myThreadTermination, nil );
 
// Increment the total number of sub-threads.
 ++gThreadTotal;
 
// Decrease the sleep ticks so we'll take more processing time.
 gSleepTicks = kSleepTicksWhenBusy;
 }
 
 return theErr;
}

A thread termination callback procedure is used to wake up the sleeping thread when another thread is finished.

/* The Thread Manager must be available for this function to work. */
pascal void
myThreadTermination ( ThreadID threadTerminated, void
 *terminationProcParam )
{
 if ( gThreadSleeper != nil ) {
// Wake up the sleeping thread so that it may be called when other threads yield.
 theErr = SetThreadState ( gThreadSleeper,
 kReadyThreadState, nil );
 gThreadSleeper = nil;
 }
 
// Lower the count of sub-threads.
 --gThreadTotal;
 
 if ( gThreadTotal == nil ) {
// If there are no more threads, reset the sleep ticks to normal.
 gSleepTicks = kSleepTicks;
 }
}

When to Thread

After considering all this, you should ask yourself whether you really need to thread your Apple event handlers. The keys to answering this question are whether there are any points in the handler where it would be appropriate to yield the processing to other threads, and, more importantly, whether the handler is big and slow enough to see performance benefits from threading. Keep in mind that there is overhead and complexity added when you use cooperative threading.

As an example, I didn't thread my Random URL CGI because its response time (with rare exception) is under a second. To thread it would only add time to the total process without any real improvement in performance (not to mention increased memory requirements). On the other hand, a form-handling CGI I'm working on is threaded, because it will frequently have to talk (through Apple events) to other applications, which can slow things down appreciably (and waiting for a response is a great time to yield processing).

Further Considerations

If your application spawns threads from any threads apart from the main thread, you will need to maintain a queue of sleeping threads rather than just the one stored in this article's code. Another important consideration, that hasn't been covered here, is to make sure that WaitNextEvent is still periodically called if the main thread is put to sleep. You don't absolutely have to do that, but it is strongly recommended so that other applications and processes can have time to run. You may want to think about resetting the calling application's Apple event timeout timer when an event is taking a long time to process. Additionally, any CGI that wants to support the new "Send Partial" multi-stage reply mechanism in WebSTAR will need to make more advanced use of threading and Apple events.

Grant's CGI framework comes with project files for the CodeWarrior, THINK C and Symantec Project Manager environments.

Discussion mailing list: grantcgi@arpp.carleton.ca with the subject set to: help

Home page: http://arpp.carleton.ca/cgi/framework/

 
AAPL
$99.72
Apple Inc.
-1.03
MSFT
$46.26
Microsoft Corpora
-0.10
GOOG
$570.06
Google Inc.
-7.30

MacTech Search:
Community Search:

Software Updates via MacUpdate

Cocktail 8.0 Beta 2 - General maintenanc...
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
QuickBooks 2015 16.0.0.1352 R1 - Financi...
QuickBooks 2015 helps you manage your business easily and efficiently. Organize your finances all in one place, track money going in and out of your business, and spot areas where you can save.... Read more
Mac DVDRipper Pro 5.0.1 - Copy, backup,...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
Apple OS X bash Update 1.0 - Fix for sec...
The OS X bash Update fixes a security flaw in the bash UNIX shell on OS X 10.9.5 (also on OS X 10.8 and 10.7 [see Related Links below]). OS X 10.9.5 or later Downloads for OS X 10.8 and OS X 10.7 in... Read more
SyncTwoFolders 2.0.5 - Syncs two user-sp...
SyncTwoFolders simply synchronizes two folders. It supports synchronization across mounted network drives and it is a possibility to run a simulation showing in a log what will be done. Please visit... Read more
FinderPop 2.5.7 - Classic Mac utility, n...
FinderPop is a Universal preference pane that extends OS X's contextual menus using a FinderPop Items folder much as the Apple Menu Items folder used to do for the Apple menu. It has other features... Read more
VueScan 9.4.45 - 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
LibreOffice 4.3.2.2 - 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
calibre 2.4 - Complete e-library managem...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Default Folder X 4.6.9b1 - 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

Nexticy Review
Nexticy Review By Jennifer Allen on October 1st, 2014 Our Rating: :: IDEAL FORM CREATIONiPad Only App - Designed for the iPad Nexticy allows you to make your own forms for research purposes or to organize your business better. It’s... | Read more »
HeroCraft Introduces Unlimited Sequel to...
HeroCraft Introduces Unlimited Sequel to WW2: Sandbox. Strategy & Tactics Posted by Jessica Fisher on October 1st, 2014 [ permalink ] | Read more »
RGB Express Review
RGB Express Review By Jennifer Allen on October 1st, 2014 Our Rating: :: DELIGHTFUL PUZZLINGUniversal App - Designed for iPhone and iPad Guide trucks along their delivery routes in RGB Express, a testing but charming puzzle game... | Read more »
The Sagas of Fire*Wolf (Games)
The Sagas of Fire*Wolf 1.0 Device: iOS Universal Category: Games Price: $9.99, Version: 1.0 (iTunes) Description: | Read more »
BuggyFun Review
BuggyFun Review By Amy Solomon on October 1st, 2014 Our Rating: iPad Only App - Designed for the iPad BuggyFun allows children to create their own tracks for bugs to interact with for a unique open-ended experience.   | Read more »
Fold the Adventure Review
Fold the Adventure Review By Jennifer Allen on October 1st, 2014 Our Rating: :: AWKWARD FOLDSUniversal App - Designed for iPhone and iPad Fold pieces of paper to create platforms for a princely rabbit in this puzzle game; something... | Read more »
WW2: Sandbox. Strategy & Tactics (G...
WW2: Sandbox. Strategy & Tactics 1.0.0 Device: iOS Universal Category: Games Price: $9.99, Version: 1.0.0 (iTunes) Description: Sandbox is the unlimited sequel to our Strategy & Tactics: World War II. We've abandoned... | Read more »
apeFilter (Music)
apeFilter 1.0 Device: iOS Universal Category: Music Price: $6.99, Version: 1.0 (iTunes) Description: | Read more »
Shred It! Review
Shred It! Review By Jennifer Allen on September 30th, 2014 Our Rating: :: GORGEOUS BUT BASICUniversal App - Designed for iPhone and iPad It might look lovely, but Shred It! is a pretty shallow endless runner/snowboarding game.   | Read more »
Check Out the New Teaser Trailer forGAME...
Check Out the New Teaser Trailer forGAMEVIL’s Darkness Reborn Posted by Jessica Fisher on September 30th, 2014 [ permalink ] Darkness Reborn, by GAMEVIL< | Read more »

Price Scanner via MacPrices.net

Apple resting On Its iPhone Laurels? – The ‘B...
Apple calls its new iPhone 6 and 6 Plus “The Biggest Advancements in iPhone History,” but does reality live up to the hype? “Seldom have so many waited so breathlessly for so little,” tweeted veteran... Read more
Roundup of Apple Mac and iPad Education disco...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
Apple Boycotts German Magazine Computer Bild...
Apple has revoked its PR accreditation of Germany’s Computer Bild, Europe’s best-selling PC magazine, in reaction to Bild’s posting of a “#Bentgate” YouTube video. Axel Telzerow, editor in chief of... Read more
iPhone 6 & iPhone 6 Plus Available in Chi...
Apple has announced that iPhone 6 and iPhone 6 Plus will be available in China beginning Friday, October 17 from the Apple Online Store (http://www.apple.com), Apple’s retail stores, and an expansive... Read more
MacBook Airs on sale for $100 off MSRP, start...
Best Buy has the new 2014 MacBook Airs on sale for $100 off MSRP on their online store. Choose free home shipping or free local store pickup (if available). Prices valid for online orders only, in-... Read more
Apple Releases OS X Mavericks bash Update 1.0...
Apple has released a patch update for OS X Mavericks users to address the recently-detected “Shellshock” security bug in BSD UNIX’s bash shell. Apple says only a few Mac users who had manually... Read more
Pivotal Payments Ready for Apple Pay – FlexPo...
Pivotal Payments, a provider of merchant services and global payment processing solutions, has announced its proprietary FlexPoint platform will support credit and debit transactions through Apple’s... Read more
iStabilizer Announces Tabarm — First Friction...
iStabilizer, a specialist in universal lightweight compact tripods, steady cams, dollies, mounts, and remotes for smartphones, tablets, and cameras, announced today the iStabilizer tabArm, the first... Read more
IStabilizer Flex Smartphone Tripod Wins Usa T...
iStabilizer, a specialist in universal lightweight compact tripods, steady cams, and other products for smartphones, tablets, and cameras, has announced today that its iStabilizer Flex smartphone... Read more
13-inch 2.8GHz Retina MacBook Pro on sale for...
B&H Photo has the new 2014 13″ 2.8GHz Retina MacBook Pro on sale for $1699.99 including free shipping plus NY sales tax only. They’ll also include free copies of Parallels Desktop and LoJack for... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, 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
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.