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

iWatermark Pro 1.72 - Easily add waterma...
iWatermark Pro is the essential watermarking app for professional, business, and personal use. iWatermark, is the number 1 and only watermarking tool available for all 4 platforms iPhone/iPad, Mac,... Read more
RapidWeaver 6.3 - Create template-based...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
Adobe Photoshop CC 2015 16.0.1 - Profess...
Photoshop CC 2015 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Photoshop customer). Photoshop CS6 is still available for purchase (... Read more
Together 3.4.6 - Store and organize all...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Smart storage. With simple drag-and-drop functionality,... Read more
Monosnap 3.1.0 - Versatile screenshot ut...
Monosnap lets you capture screenshots, share files, and record video and .gifs! Capture: Capture full screen, just part of the screen, or a selected window Make your crop area pixel perfect with... Read more
Cocktail 8.5.1 - General maintenance and...
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
Vienna 3.0.6 :5eaf312: - RSS and Atom ne...
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
Kodi 15.1.rc1 - Powerful media center to...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
Bookends 12.5.8 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Access the power of Bookends directly from Mellel, Nisus Writer Pro, or MS Word (... Read more
Chromium 44.0.2403.125 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 44.0.2403.125: This release contains a number... Read more

This Week at 148Apps: July 27-31, 2015
Winding Down July With 148Apps How do you know what apps are worth your time and money? Just look to the review team at 148Apps. We sort through the chaos and find the apps you're looking for. The ones we love become Editor’s Choice, standing out... | Read more »
You'll Want to Keep an Eye Out for...
If you're the kind of person who had fun hunting down and completing all the codex puzzles in Assassin's Creed 2, then are you ever in for a treat. The Guides looks like it's going to be a very robust collection of similarily odd, seemingly... | Read more »
Vivid Games has Announced Real Boxing 2...
The original Real Boxing was a pretty impressive bit of fisticuffs, but if the trailer Vivid Games is showing off for GamesCom is any indication Real Boxing 2 is going to be even better. [Read more] | Read more »
PAC-MAN Championship Edition DX - Tips,...
[Read more] | Read more »
Card King: Dragon Wars - Tips, Tricks an...
[Read more] | Read more »
Pac-Man Championship Edition DX has brou...
Bandai Namco has released Pac-Man Championship Edition DX on iOS and Android, which features the classic arcade gameplay that we've all grown to love. Pac-Man Championship Edition DX can be enjoyed in much shorter bursts than the arcade versions... | Read more »
Cosmonautica (Games)
Cosmonautica 1.1 Device: iOS Universal Category: Games Price: $6.99, Version: 1.1 (iTunes) Description: Cast off! Are you ready for some hilarious adventures in outer space? | Read more »
Rescue humanity from a Demon horde in An...
Angel Stone is Fincon's follow up to the massively successful Hello Hero and is out now on iOS and Android. You play as a member of The Resistance, a group of mighty human warriors who have risen up in defiance of the Demon horde threatening to... | Read more »
Gallery Doctor (Photography)
Gallery Doctor 1.0 Device: iOS iPhone Category: Photography Price: $2.99, Version: 1.0 (iTunes) Description: Free up valuable iCloud and iPhone storage with Gallery Doctor, the only iPhone cleaner that automatically identifies the... | Read more »
You Against Me (Games)
You Against Me 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: A simple game… You. Me. Claim, steal, lock, score, win! | Read more »

Price Scanner via MacPrices.net

PDF Element Tool Kit For PDF For Windows 10,...
South Surrey, British Columbia based software developer Wondershare has posted an interesting infographic tracking the development of Microsoft’s flagship Windows operating system over the years,... Read more
27-inch 3.5GHz 5K iMac on sale for $81 off MS...
Adorama has the 27″ 3.5GHz 5K iMac on sale for $2218, $81 off MSRP, including a free copy of Apple’s 3-Year AppleCare Protection Plan. Shipping is free, and Adorama charges sales tax in NY & NJ... Read more
Back-to-School with Tablet and Smartphone Acc...
Belkin helps you prepare for the coming school year with a wide variety of the latest mobile and tablet accessories to outfit both grade school and college students. The line-up includes charging... Read more
11-inch MacBook Airs on sale for $100 off MSR...
Best Buy has 11-inch MacBook Airs on sale for $100 off MSRP. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: - 11″ 1.6GHz... Read more
iPad Air 2 on sale for up to $100 off MSRP
Best Buy has iPad Air 2s on sale for up to $100 off MSRP on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale prices available for online... Read more
Sale! 13-inch MacBook Pros on sale for $100 o...
B&H Photo has 13″ MacBook Pros on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.5GHz/500GB MacBook Pro: $999.99 save $100 - 13″ 2.7GHz/128GB Retina... Read more
Sale! Save $100 on 13-inch MacBook Airs this...
B&H Photo has the 13″ 1.6GHz/128GB MacBook Air on sale for $899.99 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price available for this model.... Read more
Worldwide Tablet Market Decline Continues, Ap...
The worldwide tablet market declined -7.0% year-over-year in the second quarter of 2015 (2Q15) with shipments totaling 44.7 million units according to preliminary data from the International Data... Read more
TP-LINK TL-PA8030P KIT Powerline Featuring Ho...
Consumer and business networking products provider TP-LINK is now shipping its TL-PA8030P KIT AV1200 3-Port Gigabit Passthrough Powerline Starter Kit that expands your home’s network over its... Read more
Apple refurbished iPad Air 2s available for u...
The Apple Store has Apple Certified Refurbished iPad Air 2s available for up to $140 off the price of new models. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB... 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
Infrastructure Engineer - *Apple* /Mac - Hil...
Infrastructure Engineer - Apple /Mac Job Code: 1608 # of openings: 1 Description Our fortune 500 client is looking to hire an experienced Infrastructure Engineer to join Read more
Executive Administrative Assistant, *Apple*...
…supporting presentation development for senior leadership. * User experience with Apple hardware and software is preferred. Additional Requirements The following list Read more
*Apple* Bus Company is now hirin - Apple Bus...
Apple Bus Company is now hiring school bus drivers in the Pettis County area. Class B CDL preferred. Free training provided. No nights or weekends required. Flexible Read more
*Apple* Certified Mac Technician - Updated 6...
…and friendly, hands-on technical support to customers troubleshooting and repairing Apple /Mac products with courtesy, speed and skill. Use your problem-solving skills Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.