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.”

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Latest Forum Discussions

See All

Top Mobile Game Discounts
Every day, we pick out a curated list of the best mobile discounts on the App Store and post them here. This list won't be comprehensive, but it every game on it is recommended. Feel free to check out the coverage we did on them in the links... | Read more »
Price of Glory unleashes its 1.4 Alpha u...
As much as we all probably dislike Maths as a subject, we do have to hand it to geometry for giving us the good old Hexgrid, home of some of the best strategy games. One such example, Price of Glory, has dropped its 1.4 Alpha update, stocked full... | Read more »
The SLC 2025 kicks off this month to cro...
Ever since the Solo Leveling: Arise Championship 2025 was announced, I have been looking forward to it. The promotional clip they released a month or two back showed crowds going absolutely nuts for the previous competitions, so imagine the... | Read more »
Dive into some early Magicpunk fun as Cr...
Excellent news for fans of steampunk and magic; the Precursor Test for Magicpunk MMORPG Crystal of Atlan opens today. This rather fancy way of saying beta test will remain open until March 5th and is available for PC - boo - and Android devices -... | Read more »
Prepare to get your mind melted as Evang...
If you are a fan of sci-fi shooters and incredibly weird, mind-bending anime series, then you are in for a treat, as Goddess of Victory: Nikke is gearing up for its second collaboration with Evangelion. We were also treated to an upcoming... | Read more »
Square Enix gives with one hand and slap...
We have something of a mixed bag coming over from Square Enix HQ today. Two of their mobile games are revelling in life with new events keeping them alive, whilst another has been thrown onto the ever-growing discard pile Square is building. I... | Read more »
Let the world burn as you have some fest...
It is time to leave the world burning once again as you take a much-needed break from that whole “hero” lark and enjoy some celebrations in Genshin Impact. Version 5.4, Moonlight Amidst Dreams, will see you in Inazuma to attend the Mikawa Flower... | Read more »
Full Moon Over the Abyssal Sea lands on...
Aether Gazer has announced its latest major update, and it is one of the loveliest event names I have ever heard. Full Moon Over the Abyssal Sea is an amazing name, and it comes loaded with two side stories, a new S-grade Modifier, and some fancy... | Read more »
Open your own eatery for all the forest...
Very important question; when you read the title Zoo Restaurant, do you also immediately think of running a restaurant in which you cook Zoo animals as the course? I will just assume yes. Anyway, come June 23rd we will all be able to start up our... | Read more »
Crystal of Atlan opens registration for...
Nuverse was prominently featured in the last month for all the wrong reasons with the USA TikTok debacle, but now it is putting all that behind it and preparing for the Crystal of Atlan beta test. Taking place between February 18th and March 5th,... | Read more »

Price Scanner via MacPrices.net

AT&T is offering a 65% discount on the ne...
AT&T is offering the new iPhone 16e for up to 65% off their monthly finance fee with 36-months of service. No trade-in is required. Discount is applied via monthly bill credits over the 36 month... Read more
Use this code to get a free iPhone 13 at Visi...
For a limited time, use code SWEETDEAL to get a free 128GB iPhone 13 Visible, Verizon’s low-cost wireless cell service, Visible. Deal is valid when you purchase the Visible+ annual plan. Free... Read more
M4 Mac minis on sale for $50-$80 off MSRP at...
B&H Photo has M4 Mac minis in stock and on sale right now for $50 to $80 off Apple’s MSRP, each including free 1-2 day shipping to most US addresses: – M4 Mac mini (16GB/256GB): $549, $50 off... Read more
Buy an iPhone 16 at Boost Mobile and get one...
Boost Mobile, an MVNO using AT&T and T-Mobile’s networks, is offering one year of free Unlimited service with the purchase of any iPhone 16. Purchase the iPhone at standard MSRP, and then choose... Read more
Get an iPhone 15 for only $299 at Boost Mobil...
Boost Mobile, an MVNO using AT&T and T-Mobile’s networks, is offering the 128GB iPhone 15 for $299.99 including service with their Unlimited Premium plan (50GB of premium data, $60/month), or $20... Read more
Unreal Mobile is offering $100 off any new iP...
Unreal Mobile, an MVNO using AT&T and T-Mobile’s networks, is offering a $100 discount on any new iPhone with service. This includes new iPhone 16 models as well as iPhone 15, 14, 13, and SE... Read more
Apple drops prices on clearance iPhone 14 mod...
With today’s introduction of the new iPhone 16e, Apple has discontinued the iPhone 14, 14 Pro, and SE. In response, Apple has dropped prices on unlocked, Certified Refurbished, iPhone 14 models to a... Read more
B&H has 16-inch M4 Max MacBook Pros on sa...
B&H Photo is offering a $360-$410 discount on new 16-inch MacBook Pros with M4 Max CPUs right now. B&H offers free 1-2 day shipping to most US addresses: – 16″ M4 Max MacBook Pro (36GB/1TB/... Read more
Amazon is offering a $100 discount on the M4...
Amazon has the M4 Pro Mac mini discounted $100 off MSRP right now. Shipping is free. Their price is the lowest currently available for this popular mini: – Mac mini M4 Pro (24GB/512GB): $1299, $100... Read more
B&H continues to offer $150-$220 discount...
B&H Photo has 14-inch M4 MacBook Pros on sale for $150-$220 off MSRP. B&H offers free 1-2 day shipping to most US addresses: – 14″ M4 MacBook Pro (16GB/512GB): $1449, $150 off MSRP – 14″ M4... Read more

Jobs Board

All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.