TweetFollow Us on Twitter

Spotlight-Illuminate Bugs

Volume Number: 13 (1997)
Issue Number: 8
Column Tag: Tools Of The Trade

Spotlight: Illuminate Your Bugs

by Paul Robichaux

Memory protection, trap parameter checking, and leak checking

Introduction

Generally, Macintosh development tools stand up very well to their counterparts on other platforms -- with one exception. Until recently, the Mac lacked a comprehensive debugging tool to compare with BoundsChecker for Windows or Purify for Unix. These products -- much beloved by developers on other platforms -- generally offer three classes of services: memory protection, parameter checking for system APIs, and leak detection. While their implementations differ, they generally don't require any changes to the source code and can be used throughout the development process to trap offending code before it is referred to QA or user testing.

Of course, the Mac toolbox presents a number of opportunities for doing Bad Things in your code: writing in other apps' heaps, dereferencing a handle after it's been moved, and forgetting to dispose of dynamically allocated resources are three perennial favorites. A tool which could detect these, and other, errors would be valuable indeed. While there are products on the Mac which offer one or two of these services, no one has yet combined all three into a single, easy-to-use package. Spotlight, called by Onyx Technologies an "automatic memory debugger," provides all three services in a robust, easy-to-use package.

What Spotlight Does

Spotlight is designed to be run as part of the development process. When you run it, it loads your executable, makes a copy of it, and patches the copy to insert its watchdog code. You don't have to modify your source code (except in rare cases as described below) or even relink! The watchdog code that Spotlight inserts will cause an exception to occur when your app does something it's not supposed to; Spotlight will catch the exception and show you a window detailing the offense (see Figure 1.) In this case, I deliberately called InsetRect() with a nil pointer. Notice how Spotlight not only flagged the error, but told me what I'd done wrong.

Figure 1. Spotlight's error reporting window.

Spotlight offers three types of debugging services: memory protection, leak detection, and Toolbox call validation. Let's see how Spotlight provides each class of service.

Memory Protection

Unix and Windows NT both offer per-process memory protection. If a process tries to access memory outside its own address range, an exception occurs. The operating system traps these exceptions and stops the offending process. While not perfect, per-process protection usually keeps one ill-behaved application from trashing others -- as frequently happens on the Mac. This same sort of coarse-grained protection was planned for MacOS 8; in the meantime, though, System 7 doesn't offer any OS-level memory protection. Spotlight checks every read and write instruction before executing it. If your application tries to access memory in a heap owned by the system, or another application, Spotlight stops it and displays the offending source line and address, as well as the contents of memory at that address.

In addition to interprocess memory protection, Spotlight also watches memory inside your application's heap. If you try to use a handle block after freeing it, or write past the end of a stack variable, Spotlight will catch it and warn you immediately.

Leak Detection

If Isaac Newton had lived in the time of digital computers, his First Law might well have said "For every allocation, there must be an equal and opposite deallocation." Unfortunately, this law is often recalled in the breach -- it's all too easy to forget that call to delete, DisposeHandle, or ReleaseResource. When you allocate something and forget to free it, that's a leak. Resources, handles, Macintosh or C pointer blocks, and C++ objects can all be leaked. Spotlight will detect leaked blocks, handles, resources, and objects and display them in a summary which lists the block size and a stack trace of the code which allocated it.

Toolbox Call Validation

In an ideal world, your code could pass any kind of garbage to a Toolbox routine and get a useful error code or exception in return. In practice, passing invalid parameters to Toolbox routines is easy to do but hard to detect -- most often, these bad parameters result in buried misbehavior that surfaces, like a great white shark, to bite you when you least expect it. Spotlight does a pre-flight check on arguments to about 400 different Toolbox routines; if you pass in an invalid parameter, you'll get a warning indicating which parameters and which Toolbox call are suspect.

Spotlight also checks for resource-handling errors by checking the value of ResError after each Resource Manager call; I consider this as extra Toolbox validation, so I won't discuss it separately.

At this point, you might be wondering how Spotlight differs from Onyx's QC; after all, they're both debugging tools and seem to have a lot in common. The two products complement each other. QC runs on 68K machines; Spotlight doesn't. Spotlight can detect memory and resource leaks; QC can't. Both offer some memory protection and Toolbox call validation features; Spotlight's implementation is more complete. QC was designed as a stress-testing tool -- besides its debugging features, it can automatically scramble, purge, and free memory and resources to help you find subtle errors which only show up under low-memory conditions. Spotlight is designed as a day-in-and-day-out debugging tool that you use in parallel with your compiler, linker, and source-level debugger.

Plugging in the Light

Spotlight DR1 ships as a single PowerPC binary; it only runs on PowerPC machines. Besides the Spotlight application itself, Onyx includes a copy of Apple's PowerMac Debug Services debug nub and a 6-page electronic manual in self-reading document (SRD) format. During installation, you have to provide the serial number included with the package; once you've done that, you'll be able to run Spotlight on your applications.

To use Spotlight, just launch it and open an .xSYM file; you don't have to recompile or relink your application unless you need to use Spotlight's API routines, which let you turn Spotlight on and off in critical sections of your code. The API itself is very simple -- only 5 routines -- but most applications won't need it.

Spotlight copies your executable and patches in its own routines for inspecting memory allocation, watching for leaks, and validating Toolbox parameters. This patching process takes a noticeable amount of time (about 6 seconds for my app on an 8100/100 with 32Mb of RAM), but you can speed it up by first copying your app and .xSYM file to a RAM disk.

Once Spotlight is finished patching your application, it will launch and run normally -- albeit slower than usual. You can use your app just as you would while running in a source- or low-level debugger; when Spotlight detects an illegal access in your code, it will display its reporting window so you can see what's wrong and choose what to do about it. In addition to highlighting the offending instruction, Spotlight will also tell you what fault it detected. (Note that Spotlight doesn't stop for leaks, since it can't tell whether an allocation is a leak or not until you stop the program.)

When the reporting window appears, you can use commands in the Debug menu to control what Spotlight does:

  • Ignore (cmd-I) will cause Spotlight to pretend that it didn't see anything amiss and continue executing your program. If Spotlight catches something that's not really an error (say, when you call Get1Resource() to see if something exists), you can use this button to skip over it. If the error is real, you ignore it at your peril, because your app can still crash while Spotlight's running.
  • Ignore Address (cmd-A) works like Ignore, but it tells Spotlight to ignore any error generated by the source code address being reported. This is useful if you're doing something in a loop or a frequently called function that's not really an error.
  • Log (cmd-L) tells Spotlight to log the error to its log file, which is named "Spotlight Log" and lives in the same folder as the application and symbol file you're debugging. Each log entry contains the cause of the failure and a stack trace pointing to the offending code. When you use the Log command, Spotlight continues executing your program -- if you log an error that can cause your app to crash, it will crash after the log entry's written.
  • Kill (cmd-K) stops your application. This is often the safest course when Spotlight catches your code writing trash in another process's space.
  • Display Memory and Display Variables show the Memory and Variables windows, which display what you'd expect them to. One nice touch is that Spotlight draws suspect memory addresses in the Memory window in gray, so you can easily spot them.
  • Reset Leaks resets the internal leak counters; you can use this to make Spotlight forget about leaked objects from a previous set of operations (for example, if you're testing leaks when opening documents.)
  • Dump Leaks saves the current leak report to a log file; in conjunction with Reset Leaks, you can easily get leak data for one particular command or action in your application.
  • Debugger (cmd-D) drops you into whatever low-level debugger you have installed. From Macsbug, the G command will return you to Spotlight.

When you're done running your application (either because you quit it or because you used Spotlight's Kill command), Spotlight balances its records of the memory and resource allocations and deallocations your application's made; it will then present a window showing you what it logged. See Figure 2 for an example log window.

Figure 2. The Spotlight log window.

The log file is written into the folder which contains the application and .xSYM file. The first time you run Spotlight on an application, the log file will be named "Spotlight Log"; subsequent logs in the same folder get a number at the end of the filename, so you can separate log files from different runs. In a welcome concession to editor religion, you can specify the type and creator used for log files so they will automatically open in your preferred editor.

Once you’ve reviewed the log file contents, you must quit Spotlight before you can run it again; there’s no way to re-run a program, whether or not it’s been relinked.

Working With Spotlight

For the most part, Spotlight is an extremely unobtrusive addition to the development process. You don’t have to link with any special libraries or make any changes to your source code (except as noted below.) You must run it in place of a source-level debugger (it might work with Jasik’s debugger, but I don’t have it and didn’t test it), so it’s not the best tool to use when you’re looking for control-flow or processing errors. I found that the most productive way for me to use Spotlight in my workflow was to run it after I was satisfied with the overall behavior and performance of my code; it serves as a last hurdle (along with Onyx’s QC Pro) that my code has to pass before it advances to QA.

You have some control over what Spotlight checks; the Options dialog, shown in Figure 3, allows you to turn each of the four primary test categories on or off. At present, there’s no way to turn individual tests on or off; for example, you can’t tell Spotlight to check allocations made with MacOS calls but to not check allocations made with malloc() or operator new.

Figure 3. Spotlight's Options dialog.

Because of the way Spotlight patches your application, you may have to make some minor changes to keep things running smoothly. The documentation warns that Spotlight will crash the machine when it catches an error in an interrupt handler routine unless you've bracketed the routine with calls to the SLEnterInterrupt() and SLLeaveInterrupt() routines. Spotlight also offers two routines to programmatically turn it on or off. You can use SLEnable() and SLDisable() to bracket code sections which trigger false alarms in Spotlight, or where performance is critical.

These interfaces are provided as C and Pascal header files, so you can call them from C, C++, Pascal, Object Pascal, or any other language which can import either type of declaration.

Documentation and Support

Spotlight is delivered electronically; unlike its sister product QC, there's no current way to get a physical copy of the product. Onyx will ship physical product free of charge to all purchasers of the DR releases once the final version ships. The distribution archive comes with a 6-page self-reading document. The bad news is that this document is very short and contains relatively little information on how to use Spotlight. The good news is that Spotlight is easy enough to use that the skimpy docs aren't a hindrance. Better still, Onyx promises a more substantial printed manual for the final release. Having said that, one item that I particularly missed was a list of which Toolbox routines Spotlight can do parameter checking on.

Onyx provides technical support to users via e-mail and fax; they also maintain WWW and FTP sites with updates and patches. Onyx has earned a reputation for excellent support of their QC product, including providing free patches and updaters on their web site. The questions and comments I sent in during my evaluation were promptly and courteously answered. One particularly nice touch that other vendors should emulate is Onyx's maintenance of a mailing list for upgrade and release announcements. Onyx also maintains an active presence on Usenet.

Many development tool vendors have moved away from irregularly scheduled point releases and to a subscription model. Onyx has adopted this approach for Spotlight. If you buy Spotlight now, you'll get whichever developer release is currently shipping. As of this writing, that would be DR1, but by the time this reaches print DR2 should be shipping instead. When Onyx releases DR3 and the final 1.0 version, anyone who purchased DR1 or DR2 will get them automatically.

Spotlight lists for US$199, but it's available for $149 to QC owners, or you can buy the two products together for $248. Because the physical version isn't shipping yet, it's only available directly from Onyx, although other distributors (notably DevDepot) will be selling the release version when available.

Spotlight Limitations

The DR1 release does have a few rough edges and limitations. Let's deal with the limitations first: most significantly, you can only use Spotlight on PowerPC applications. If you're writing code for the 68K, or writing code resources or shared libraries like Component Manager components or Photoshop plugins, Spotlight won't help you at present. Spotlight requires a .SYM-format symbol file, so your development environment must be able to generate one.

Spotlight doesn't integrate with the Metrowerks or Symantec debuggers, so you can either debug your code or Spotlight it at any one time. Onyx has promised better integration with MWDebug as their #1 future priority; the two companies did a very nice job of integrating Onyx's QC and MWDebug, so let's hope they follow through on this as well.

As might be expected with a DR1 product, the user interface is not as polished as a final release product; you can't cut or copy text from the log window, and the Close command doesn't close the log window. Some little touches would ease the daily process of using Spotlight. For example, it would be handy to open a source file in its creating application by double-clicking it in Spotlight's stack trace pane, and it would be useful to be able to attach a note in the log file when you use the Log command.

Is It Magic?

The first few times Spotlight flushed out camouflaged defects in my code, I couldn't do anything but smile. Every flaw that a tool like Spotlight finds is one flaw that my QA team, beta testers, and end users won't have to deal with. In that sense, it really is like magic.

The magic comes at a reasonable cost, too. You don't have to learn any arcane commands to use Spotlight, and the majority of applications won't require any changes to take advantage of its defect-finding abilities. Despite the fact that it can't debug anything but PowerPC applications, Spotlight is a valuable tool which I highly recommend. At US$199, it's a bargain compared to the amount of time you can waste hunting for bugs which can be Spotlighted in minutes.

Product Reviewed in this Article

Spotlight DR1, Onyx Technology. 7811 27th Avenue West, Bradenton, FL 34209. (941) 795-7801.

Useful URLs

http://www.onyx-tech.com - Home page for Onyx Technology.


Paul Robichaux must be the luckiest guy alive -- he gets paid to write Macintosh cryptography and security software by day and still manages to have free time to enjoy his family. He welcomes your comments via e-mail to paulr@hiwaay.net.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

CleanMyMac 3.9.2 - $39.95
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac lets you safely and intelligently scan and clean your entire system, delete large, unused files... Read more
Printopia 3.0.4 - Share Mac printers wit...
Run Printopia on your Mac to share its printers to any capable iPhone, iPad, or iPod Touch. Printopia will also add virtual printers, allowing you to save print-outs to your Mac and send to apps.... Read more
Tinderbox 7.3.1 - Store and organize you...
Tinderbox is a personal content management assistant. It stores your notes, ideas, and plans. It can help you organize and understand them. And Tinderbox helps you share ideas through Web journals... Read more
ExpanDrive 6.1.6 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
Printopia 3.0.4 - Share Mac printers wit...
Run Printopia on your Mac to share its printers to any capable iPhone, iPad, or iPod Touch. Printopia will also add virtual printers, allowing you to save print-outs to your Mac and send to apps.... Read more
Tinderbox 7.3.1 - Store and organize you...
Tinderbox is a personal content management assistant. It stores your notes, ideas, and plans. It can help you organize and understand them. And Tinderbox helps you share ideas through Web journals... Read more
ExpanDrive 6.1.6 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
VOX 3.0.1 - Music player that supports m...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
Merlin Project 4.3.3 - $289.00
Merlin Project is the leading professional project management software for OS X. If you plan complex projects on your Mac, you won’t get far with a simple list of tasks. Good planning raises... Read more
Mac DVDRipper Pro 7.1 - Copy, backup, an...
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

Latest Forum Discussions

See All

Animal Crossing: Pocket Camp mid-game gu...
By this point, you've probably hit or surpassed mid-way inAnimal Crossing: Pocket Camp.The game changes quite a bit as your focus slowly shifts from endlessly fulfilling requests for campsite buddies to improving your amenities. Here are a few tips... | Read more »
Mille Bornes (Games)
Mille Bornes 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: | Read more »
Blob gets a new look in Give It Up! 3
Blob makes his triumphant return, as Yoozoo Games and Invictus Gaming have joined forces to create Give It Up! 3, the third in a series of delightful action-adventure games featuring our wobbly friend Blob. In this newest adventure, you’ll get to... | Read more »
148Apps' Ultimate Guide to Black Fr...
Black Friday is here, and there are a whole lot of discounts running right now for folks on the lookout for new mobile devices, accessories, and yes, even games. Here's a helpful rundown of what you'll find both in stores and online. Happy... | Read more »
The best Black Friday mobile game deals
Black Friday's upon us, and if you've happened to nab a fancy new phone during the week's big savings, you might be searching for some new games to fill up space on your new gadget. There are a lot of great games on sale right now for Black Friday... | Read more »
The best mobile games to play while your...
Thanksgiving is a time to reconnect with loved ones, eat lots of food, and all of that jazz, but once the festivities start to wind down, folks tend to head to the couch to watch whatever football is happening for Turkey Day. | Read more »
The best Black Friday deals for Apple ga...
Black Friday is hours away at this point, but many popular retailers are getting a jump on things with plenty of pre-Black Friday sales already available. Many of those early bird sales including some sharp discounts on the latest Apple phones... | Read more »
The Inner World 2 (Games)
The Inner World 2 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Solve mind-bending puzzles in a world full of mystery and save the family of the flute-noses! Their dynasty has been... | Read more »
warbot.io wants you for the robot wars
Fans of epic gundam-style battles will find a lot to love in warbot.io, the first game for up and coming developer Wondersquad. The game saw a lot of success when it first launched for browsers and Facebook, and now even more people are getting the... | Read more »
Uncover alien mysteries in cross-genre s...
If the Alien franchise taught us anything, it’s that landing on a strange planet at the behest of a faceless corporation is probably asking for trouble. And Eldritch Game’s Deliria doesn’t prove otherwise. In 2107, Dimension LG7 is rich with... | Read more »

Price Scanner via MacPrices.net

Apple 9″ 32GB iPad for $80 off on Walmart onl...
Snag a 9.7″ 32GB WiFi iPad for $249 on Walmart’s online store as part of their Black Friday sale. That’s $80 off MSRP, and it’s the lowest price available for an iPad so far this season. Sale price... Read more
Apple Black Friday sale for 2017: $150 Apple...
BLACK FRIDAY Apple has posted their Black Friday deals for 2017. Receive a $150 Apple gift card with the purchase of select Macs and up to $100 with various iPads, iPhones, and Apple Watches. The... Read more
Black Friday 2017: Where to find the best dea...
B&H Photo has 15″ and 13″ MacBook Pros on sale for up to $200 off MSRP as part of the Black Friday and Holiday sale. Shipping is free, and B&H charges sales tax for NY & NJ residents only... Read more
Black Friday 2017: Where to find the best dea...
B&H Photo has 12″ MacBooks on sale for $150 off MSRP as part of the Black Friday and Holiday sale. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 12″ 1.2GHz... Read more
Black Friday 2017: Where to find the best dea...
B&H Photo has 10.5″ iPad Pros in stock today and on sale for up to $130 off MSRP. Each iPad includes free shipping, and B&H charges sales tax in NY & NJ only: – 10.5″ 64GB WiFi iPad Pro... Read more
Black Friday 2017: Where to find the best dea...
B&H Photo has 13″ MacBook Airs on sale for up to $150 off MSRP as part of the Black Friday and Holiday sale. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13″... Read more
Black Friday 2017: Where to find the best dea...
B&H Photo has 27″ and 21″ iMacs on sale for up to $200 off MSRP as part of the Black Friday and Holiday sale. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 27... Read more
Black Friday 2017: Where to find the best dea...
B&H Photo has Mac minis on sale for $100 off MSRP as part of their Black Friday sale, each including free shipping plus NY & NJ sales tax only: – 1.4GHz Mac mini: $399 $100 off MSRP – 2.6GHz... Read more
Black Friday 2017: Find the best deals and lo...
Scan our exclusive price trackers for the latest Black Friday 2017 sales & deals and the lowest prices available on Apple Macs, iPads, and gear from Apple’s authorized resellers. We update the... Read more
Black Friday: 27″ 3.4GHz iMac for $1599, save...
Amazon has the 27″ 3.4GHz Apple iMac on sale for $1599.99 as part of their Black Friday sale. That’s $200 off MSRP, and shipping is free. Their price is currently the lowest price available for this... Read more

Jobs Board

Information Systems Engineer, *Apple* Retai...
# Information Systems Engineer, Apple Retail Engineering Job Number: 112895982 Santa Clara Valley, California, United States Posted: 02-Oct-2017 Weekly Hours: 40.00 Read more
Commerce Engineer, *Apple* Media Products (...
# Commerce Engineer, Apple Media Products (New York City) Job Number: 113028813 New York City, New York, United States Posted: 20-Sep-2017 Weekly Hours: 40.00 **Job Read more
Business Development Manager, *Apple* Pay -...
# Business Development Manager, Apple Pay Job Number: 112919084 Santa Clara Valley, California, United States Posted: 18-Aug-2017 Weekly Hours: 40.00 **Job Summary** Read more
Digital Marketing Media Planner, *Apple* Se...
# Digital Marketing Media Planner, Apple Services Job Number: 113080212 Culver City, California, United States Posted: 03-Oct-2017 Weekly Hours: **Job Summary** Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.