TweetFollow Us on Twitter

QC Review
Volume Number:12
Issue Number:8
Column Tag:Quality Control

Debugging with QC

Stress your software, not yourself

By Jeremy Vineyard

Introduction

The Memory Manager is one of the most commonly used components of the Mac Toolbox. Most applications are continually allocating and releasing memory - loading windows, pictures, and other resources, locking them, relocating them, resizing them, and purging them when they have served their purpose.

With all of this activity in memory, it is easy for a programmer in a hurry to overlook sections of code that could cause unforeseen problems for users. Attitudes like, “This memory operation will never fail”, and, “It’s only a small memory leak”, frequently lead to software that crashes, frustrating the user to the point of switching to your competitor and advising associates not to buy your product.

Because the Memory Manager is so crucial to any Mac application, it is crucial to ensure that the use of the Memory Manager is solid. The problem is that, most of the time, the effects of memory-related bugs are seen long after the bug occurs, making tracking them down a difficult and time-consuming task.

Thankfully, there is a tool available that makes preventing and tracking down hard-to-reproduce memory-related bugs easier. This is QC (as in “Quality Control”), a low-level debugging tool for use with the Mac OS Memory Manager.

Figure 1. QC Control Panel

Setting Up and Using QC

QC requires System 7 in order to run. It is developed by Onyx Technology and costs around $100. QC is copy-protected and will allow you to run only one serialized copy at a time on a network. Additional serial numbers are available by buying additional copies of QC.

QC consists of a control panel, on-line documentation, and a set of C APIs. All interaction with QC is done in the control panel.

QC can be set up to run tests and suites of tests on a “target” or “target program”. Target programs don’t have to be applications; they can be other executable code such as control panels. Test suites include tests that validate blocks of memory, tests that detect common Memory Manager errors, and tests that stress the application heap to ensure that the application code runs properly under difficult memory conditions.

How Does It Work?

Unless you are using the API, QC performs all of its tests by patching traps. QC will only report errors related to the tests that have been enabled for the target program, and it will not report errors when it is not running. QC reports every error with a _DebugStr call which will drop into MacsBug or your high-level debugger if it is set up to do so. In addition to reporting the error, QC reports the address of the last trap called in your code before the error occurred, allowing you to trace through the code to chase down the bug.

QC can automatically launch itself along with your target. You can choose to have QC launch when the target program calls _InitGraf or when a certain resource is loaded. Making QC wait until a resource is loaded is useful for non-applications such as control panels that will not call _InitGraf.

A key combination can be set that will launch and unlaunch QC while the target program is running. QC will pick the correct set of preferences for your target program when it is launched. As soon as QC is launched, the tests it performs are in progress.

Some tests in QC cannot distinguish when an error has occurred in an application other than the program being tested. It is therefore best to test your target program with only itself and the Finder running.

If you have all of the tests enabled, the slowdown caused by QC is quite noticeable, so be prepared. However, this is to be expected from such a low-level debugging tool, and is bearable.

Memory Validation Suite

QC offers a suite of tests that validate the Memory Manager structures themselves, rather than the use of those structures. These tests are valuable for tracking down corrupted or incorrectly specified data.

The first test in the validation suite, Cross-reference Master Pointers, looks at all relocatable blocks in the heap (handles) and makes sure that they are pointed to by a master pointer within a non-relocatable block. This test can catch orphaning of relocatable blocks and corruption of the master pointer list.

The Validate Handle/Pointer test verifies that each handle/pointer is pointing to a block of memory with the correct properties (locked/unlocked for a handle, non-relocatable for a pointer) as they are passed to Memory Manager calls.

Memory Manager Misuse

QC provides the programmer with a series of tests that detect common misuses and mistakes made while using the Memory Manager. It is the commonness of these problems that makes these tests so useful in debugging code.

Activating the Detect Write to Zero test checks, on each trap call, as to whether your target program has overwritten location zero. QC cannot determine whether it was your target program or another running application that overwrote zero, so again it is best to test with as few applications open as possible.

The Dereferencing Zero test puts data into location zero that will cause a bus error when a nil pointer/handle is dereferenced by the target program (a very common mistake).

Reasonable Allocations checks to make sure that allocations of data with _NewHandle, _NewPtr, etc., are passed reasonable values for the data size. Unreasonable allocations might include a negative data size or a data size that exceeds the size of the application heap.

QC’s ability to detect when a resource is disposed of incorrectly with _DisposeHandle or a handle is disposed of incorrectly with _ReleaseResource is a very useful feature. This is probably one of the mistakes most easily made by Mac programmers when writing code that uses both handles and resources. Enabling this test almost immediately showed me places in my (untested) code where I wasn’t releasing resources with _ReleaseResource, causing memory leaks and a possibly damaged resource map.

BlockMove Checking checks the values passed to _BlockMove to make sure that all addresses and data sizes are valid. With Block Bounds Checking enabled, QC adds a small tag value to every allocated block. QC periodically checks to see if this tag has been overwritten and reports an error if so. This is useful in catching errors related to writing past the boundaries of an array or a block of data.

MemErr Detection is a warning test that reports an error every time the low-level global MemErr is set to be anything other than noErr. This does not mean that the code is bad, but it can alert the programmer to places in the code where failed memory operations are not handled correctly. Grow Lock warns about attempts to resize a locked relocatable block. Grow Non-Reloc reports attempts to change the size of a pointer. Both of these operations are very likely to fail if a locked or non-relocatable block is ahead of the block being grown larger, but, as before, this may not represent an error if it is handled correctly in the code.

Scrambled Heaps

Many times, a program is written, tested, and approved under ideal memory conditions, but when someone else uses the software with a lowered memory partition and several other applications running simultaneously, the program cannot handle the stress. It can be difficult to simulate these stressful conditions if you don’t know what to look for. Of course, you can always lower the application memory size yourself and launch several other applications every time you test your own program, but this is a difficult solution at best. QC offers a seamless method of ensuring that all of your code will continue to operate under even the most stressful memory conditions.

Scramble Heap continuously moves all relocatable blocks of memory. If you use a pointer to point to the data represented by a relocatable block and that block is moved in memory, your pointer will no longer be valid. Scrambling the heap is a good way to make sure your code handles this possibility.

Purge Heap will purge handles marked as “purgeable” whenever possible. This is a useful tool for detecting sections of your code where you did not think it possible for a handle to become purged, but it has. This is a likely situation if you have always tested your application with lots of extra memory, but the user lowers the memory partition.

Check Heap checks the structure of the heap before every Memory Manager call to ensure its reliability. Invalidate Free Memory fills all free blocks with a value that will cause a bus error if dereferenced by the target program. This will prevent you from accessing a block of data that has been released from memory.

QC API

The QC API provides a set of routines in C for accessing directly from the target program code the individual tests that QC is able to perform. This is useful for making a special “debug” build of a product that routinely checks for memory-related errors near crucial portions of code.

Any of the tests that can be enabled from the QC control panel can also be enabled with the QC API. QC must be installed for the QC API to work.

Conclusion

QC is ideal for small developers who don’t have the resources to write tools to extensively stress-test their products. Larger developers are more likely to have their own customized stress-testing tools, but QC may fill the gap for those memory-testing features that are missing.

If you are a small developer who doesn’t have the time to track down memory-related bugs after your product ships, QC is worth the money. Buy it. If your program has problems, QC will alert you to them and will protect you from being embarassed by unforeseen glitches in your code. If your program is spotless, QC will provide you with that needed bit of extra security (and extra sleep) when your product is shipped. For only $100, it is a worthwile investment in the quality of your products and the satisfaction of your customers.

MacTech’s resident programmer, Don Bresee, adds: “QC gives you the ability to consistently reproduce the nastiest of bugs, which is always the first step to finding bad code. QC immediately helped me find a bug in THINK Reference that was no less than four years old.”

 
AAPL
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

Path Finder 6.5.5 - Powerful, award-winn...
Path Finder is a file browser that combines the familiar Finder interface with the powerful utilities and innovative features. Just a small selection of the Path Finder 6 feature set: Dual pane... Read more
QuarkXPress 10.2.1 - Desktop publishing...
With QuarkXPress, you can communicate in all the ways you need to -- and always look professional -- in print and digital media, all in a single tool. Features include: Easy to Use -- QuarkXPress is... Read more
Skype 6.19.0.450 - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
VueScan 9.4.41 - 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
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - 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
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
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

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 21-inch 1.4GHz...
The Apple Store is now offering Apple Certified Refurbished 21″ 1.4GHz iMacs for $929 including free shipping plus Apple’s standard one-year warranty. Their price is $170 off the cost of new models,... Read more
Save $50 on the 2.5GHz Mac mini, on sale for...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more
Save up to $300 on an iMac with Apple refurbi...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Bad Driver Database App Allows Good Drivers t...
Bad Driver Database 1.4 by Facile Group is a new iOS and Android app that lets users instantly input and see how many times a careless, reckless or just plain stupid driver has been added to the... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. 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...
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...
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...
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
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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.