TweetFollow Us on Twitter

Feb 97 Factory Floor

Volume Number: 13 (1997)
Issue Number: 2
Column Tag: From The Factory Floor

Mike Lockwood, Debugger

By Dave Mark, ©1996 by Metrowerks, Inc., all rights reserved.

One of the most important, yet least understood part of the Mac development process is the debugger. Sure, everyone knows how to use the debugger to step through your source code, but I've always wanted a peek beneath the hood, a chance to see how a debugger really works. This month's interview is with Mike Lockwood, debugger extraordinaire.

Dave: You worked at Apple four years before you came to Metrowerks. What did you do there?

Mike: I first started working on development tools on my own, before I joined Apple. I wrote my own MacApp debugger as a side project, and when I got it working I sold it to Apple. Apple contracted me to finish it, and it shipped on E.T.O. as SourceBug. Soon after that, I joined Apple as an employee and worked for a year on the debugger for a new integrated development environment that was to replace MPW.

After that project was canceled, I transferred to the System Software group and worked on the Finder team. At the time there was no source level debugger that could debug Finder extensions, so I kept working on the debugger from my previous project - so I could use it myself. I added support for debugging threads and shared libraries, and it was soon being used by a number of different groups within Apple. Since it was an underground, guerrilla programming effort, I decided to call it VoodooMonkey. Then when no one was looking, someone snuck it on one of the Developer CDs and VoodooMonkey shipped outside of the company. Since then, it became somewhat of a cult classic.

After working on the Finder team for awhile, I became interested in working on developer tools again and wanted to move back to the East coast. So I transferred to the Cambridge office to work on the Dylan project. There, I wrote the application framework for Apple Dylan, and served as the main guinea pig for the Dylan compiler and development environment. After that project got canceled, there was an opportunity for me at Metrowerks, and I joined them the day after I left Apple.

Dave: How did you hook up with Metrowerks?

Mike: Metrowerks first contacted me in the Fall of 1993. They needed a 68K debugger nub for CodeWarrior, Apple only had a PowerPC version of DebugServices up and running. Greg Galanos called me to see if he could use DebuggerINIT, which was the debugger nub I wrote for VoodooMonkey. At the time, I didn't know much at all about Metrowerks, other than they were working on a C/C++ compiler for PowerPC. I wanted to help them out, but I also wanted to avoid some of the political problems I ran into after VoodooMonkey shipped. So, I directed Greg to go through the proper channels in Cupertino.

I knew Greg would never be able to cut through all of the red tape, and I wasn't sure I would ever hear from him again. However, he called me two days later and said "Hey, I just licensed the sources to DebuggerINIT from Apple." Now I knew I was dealing with a force to be reckoned with.

I continued to talk to Greg and other people at Metrowerks, and provided some informal tech support for DebuggerINIT. When it was clear that the Dylan project was going to be shut down, I talked more seriously with Metrowerks about joining the company. Dan Podwall, the original author of the Metrowerks debugger, was busy working on the browser and they needed someone to take over the debugger.

Dave: What exactly is a debugger nub?

Mike: A debugger nub is a small program that controls a program that is being debugged, and is responsible for things like setting breakpoints and single stepping through code. It provides a layer between the main debugger application and the low level system stuff. The distinction is important in the case of two machine debugging, because the debugger nub runs on the machine being debugged and the debugger application runs on a different machine. MetroNub is our debugger nub for single machine MacOS and Java debugging. We have other debugger nubs for debugging code running on other platforms.

Dave: How does MetroNub interface with the Metrowerks debugger?

Mike: MetroNub is a system extension that provides an API to the Metrowerks debugger that supports debugging functionality like single stepping. MetroNub exports a structure full of mixed mode function pointers via a Gestalt selector that the debugger application can call. MetroNub also interfaces with various parts of the OS. For example, it patches the Resource Manager to track where code resources are loaded in memory, and uses an undocumented Process Manager API to suspend and resume a process that is being debugged. It also patches the 68K exception vectors and registers callback functions with the PowerPC Exception Manager and the MP kernel so it can be notified when exceptions occur.

Now let's examine what happens when you single step through a line of code. At this point, the program is already stopped, meaning that MetroNub had already told the Process Manager to suspend the program's process and make it ineligible for scheduling. The debugger application then calls the MetroNub "Step" command. When it makes this call, it would specify to MetroNub whether to step into or step over function calls. It would also specify a range of addresses for the program counter so MetroNub will not stop execution again until the PC is outside of this range. When stepping though source code, this range would correspond to a single line in the source code.

MetroNub would record the PC range specified, and would then tell the Process Manager to resume the process. The process is then eligible to be scheduled again, but will not actually resume until the debugger application calls WaitNextEvent(). After it is rescheduled, some of MetroNub's code that was left on the call chain of the program being debugged will resume execution. Before MetroNub returns control to the program being debugged it restores all the registers and sets the trace bit in the processor's status register. The trace bit tells the processor to cause a trace exception after each instruction is executed. MetroNub receives these trace exceptions, and processes them until the program counter falls out of the specified range. Then MetroNub tells the Process Manager to do an immediate context switch from the program being debugged to the debugger application. This leaves some of MetroNub's exception handling code on the call chain of the program being debugged, which will resume after the program being debugged is rescheduled by the Process Manager.

Dave: What language is MetroNub written in?

Mike: MetroNub is written almost entirely in C++, with just a small amount of 68K assembler. The assembler is used only in a few places where it is necessary to access registers directly or execute some uncommon 68K instructions. For example, the 68K exception vector patches and some of the Resource Manager patches are written in assembler. MetroNub is mostly 68K code, and only contains a small amount of PowerPC glue code for some of the callbacks it registers with the OS.

Dave: But how does the link between the source code MetroNub happen?

Mike: MetroNub doesn't know anything about the source code. The debugger application handles the mapping from source code to object code offsets, based on information in the sym file, which is generated by the compiler and linker. Suppose you want to set a breakpoint at a certain line of source code. The sym file contains a table that maps the offset in the source file to an offset in the object code for the function containing that line of code. A different table in the sym file is then used to translate the offset in the function's object code to an offset into a CFM code fragment or a code resource. The debugger then tells MetroNub to set a breakpoint at the given offset within the code fragment or code resource.

The debugger application does not have to know where the code is loaded in memory or if it is even in memory at the time the user sets the breakpoint. If it is loaded, the debugger will convert the offset to an absolute address and insert a trap instruction at the address where the breakpoint should be set. If the code is not loaded yet, MetroNub remembers the breakpoint and installs the trap instruction after the code is loaded but before the program is able to execute the code.

Dave: What changes did you have to make to your design to deal with Java?

Mike: Fortunately, no major design changes were needed to support Java. That was good news for us, because time-to-market was important. Our debugger could already debug two different processor architectures (68K and PowerPC) simultaneously, so adding a third architecture was not difficult. We just treated Java byte codes as if they were assembler instructions and the Java VM as if it was a microprocessor, and the debugger architecture we already had in place worked fine. I had to write a new implementation of the symbolics code so it could read symbolic information from Java class files and Zip files instead of a sym file. I also had to add an API to the Metrowerks Java VM so MetroNub could communicate with the Java VM and control the execution of Java code. This approach had the nice side effect that it is possible to debug both 68K and PowerPC C/C++ code and Java code simultaneously with a single debugger.

Dave: A lot of companies have said that they will no longer support the 68K. Do you think learning to debug 68K code is a worthwhile investment?

Mike: 68K is becoming less important, but a good understanding of 68K code is valuable when debugging on System 7, even if you are writing PowerPC native code. Since much of the operating system is still emulated 68K code, it is not uncommon to have to debug 68K code on a PowerMac.

Dave: What's your feeling on how people should approach debugging? Should they learn MacsBug? Should they buy a copy of Jasik's Debugger? Learn PowerPC assembler? Got any good book recommendations? Are there utilities people should definitely have on their hard drive?

Mike: The Metrowerks debugger is strictly a high level debugger. You also need to have a low level debugger like MacsBug or Jasik's debugger installed when debugging with the Metrowerks debugger. Jasik's debugger is actually both a low level and high level debugger, so if you like Jasik's debugger, you don't even need a copy of the Metrowerks debugger! Jasik's debugger is very powerful and has a lot of features that no other debugger has. But many people find the Metrowerks debugger easier to use.

Nowadays, it isn't critical that you learn PowerPC assembler, unless you need to write very low level or very optimized code. But for debugging purposes, it is often useful to have basic reading knowledge of assembler, even if you do not write it yourself (I have written a lot of 68K assembler over the years, but haven't actually had a need to write any PowerPC assembler yet). But understanding how to read and debug at the assembler level can be very useful at times.

There are a number of utilities available that can help you with debugging. You can use the Debugging Memory Manager from Apple to help catch memory problems in your application heap. Similarly, you can use the DebugNew library that comes with CodeWarrior to track down memory problems in the built-in C/C++ memory allocater. Zone Ranger, which also comes with CodeWarrior, can be helpful monitoring memory usage and watching for memory leaks. There are a number of extensions, like EvenBetterBusError, DoubleDispose, etc. that are available from Apple that watch out for common programming errors when using the Memory Manager or Resource Manager. Onyx Technology has a tool called QC that monitors all Memory Manager calls and checks for these programming errors. A demo version of QC is available on the CodeWarrior Reference CD.

 
AAPL
$100.53
Apple Inc.
+1.37
MSFT
$45.33
Microsoft Corpora
+0.22
GOOG
$586.86
Google Inc.
+4.70

MacTech Search:
Community Search:

Software Updates via MacUpdate

Chromium 36.0.1985.143 - 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. FreeSMUG-Free OpenSource Mac User Group build is... Read more
Macgo Blu-ray Player 2.10.6.1691 - Blu-r...
Macgo Mac Blu-ray Player can bring you the most unforgettable Blu-ray experience on your Mac. Overview Macgo Mac Blu-ray Player can satisfy just about every need you could possibly have in a Blu-ray... Read more
Apple Motion 5.1.2 - Create and customiz...
Apple Motion is designed for video editors, Motion 5 lets you customize Final Cut Pro titles, transitions, and effects. Or create your own dazzling animations in 2D or 3D space, with real-time... Read more
A Better Finder Rename 9.39 - File, phot...
A Better Finder Rename is the most complete renaming solution available on the market today. That's why, since 1996, tens of thousands of hobbyists, professionals and businesses depend on A Better... Read more
PopChar X 6.6 - Floating window shows av...
PopChar X helps you get the most out of your font collection. With its crystal-clear interface, PopChar X provides a frustration-free way to access any font's special characters. Expanded... Read more
MacUpdate Desktop 6.0.2 - Install Mac ap...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
PDFpenPro 6.3.2 - Advanced PDF toolkit f...
PDFpenPro allows users to edit PDF's easily. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Create... Read more
PDFpen 6.3.2 - Edit and annotate PDFs wi...
PDFpen allows users to easily edit PDF's. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Features... Read more
OS X Yosemite 10.10 DP6 - Developer Prev...
Note: This is a Developer Preview. You must be a registered Apple Mac Developer to download this update. You can also sign up for the free OS X Beta Program to download and preview public beta... Read more
Flavours 1.1.15 - Create and apply theme...
Flavours is a Mac application that allow users to create, apply and share beautifully designed themes. Classy Give your Mac a gorgeous new look by applying delicious themes! Easy Unleash your... Read more

Latest Forum Discussions

See All

Go to Bed – An Interview With Touchfight...
Touchfight Games is an exciting new indie studio that was co-formed between game journalist and author Nathan Meunier, artist Leonard Kenyon, and programmer Jon Kenyon. Their debut game Go To Bed will be released this fall, and with all the... | Read more »
Assault Vector Review
Assault Vector Review By Rob Thomas on August 19th, 2014 Our Rating: :: SIMPLE STRATEGYUniversal App - Designed for iPhone and iPad While this hex-based warp ride is fine in theory, Assault Vector is a bit too simple to be... | Read more »
Tom Hanks’ Hanx Writer has Taken the App...
Tom Hanks’ Hanx Writer has Taken the App Store by Storm Posted by Jessica Fisher on August 19th, 2014 [ permalink ] iPad Only App - Designed for the iPad | Read more »
It Came from Canada: Galaxy Dash
With its use of well-established tropes like endless flying and sci-fi space shooting, the upcoming Galaxy Dash: Race to the Outer Run most likely won’t confound expectations. However, with its robust amount of opportunities for fun player... | Read more »
PromptSmart Review
PromptSmart Review By Jennifer Allen on August 19th, 2014 Our Rating: :: SNAPPY SPEECHGIVINGUniversal App - Designed for iPhone and iPad Offering you a teleprompter in your pocket, PromptSmart is immensely helpful for those who do... | Read more »
The Firm (Games)
The Firm 1 Device: iOS iPhone Category: Games Price: $.99, Version: 1 (iTunes) Description: RELEASE SPECIAL SALE 50%! Welcome to THE FIRM, Your profil retained our full consideration and we have the pleasure to inform you that your... | Read more »
They Need To Be Fed 3 (Games)
They Need To Be Fed 3 1.0.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.0 (iTunes) Description: 360 degree gravity platforming is back! Run, jump, collect, avoid enemies and feed yourself to the monster at the... | Read more »
Who Wore it Best? Phantom Flower vs. And...
Who Wore it Best? takes a break from all the bloodshed to check out two decidedly tranquil and nature-loving puzzles games: Phantom Flower and And Then it Rained. [ Who Wore it Best? Phantom Flower vs. And Then it Rained is a post from... | Read more »
Sentinel 4: Dark Star Review
Sentinel 4: Dark Star Review By Jennifer Allen on August 19th, 2014 Our Rating: :: STURDY TOWER DEFENSEUniversal App - Designed for iPhone and iPad After 4 years, the latest entry to the Sentinel series is here. It’s not quite as... | Read more »
TrashEver2 - You can move your selected...
TrashEver2 - You can move your selected notes to trash box. 1.0.0 Device: iOS Universal Category: Productivity Price: $.99, Version: 1.0.0 (iTunes) Description: TrashEver is an application only for throwing away the note of Evernote... | Read more »

Price Scanner via MacPrices.net

Razer Taipan Mouse For Gamers And Non-Gamers...
If you’re a serious gamer on either Mac or Windows PCs, a serious gaming mouse is a necessity for first-tier performance. However, even if like me you’re not much of a gamer, there’s still a strong... Read more
15-inch 2.2GHz MacBook Pro on sale for $1899,...
Adorama has the new 15″ 2.2GHz Retina MacBook Pro on sale for $1899 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP, and it’s the lowest price available for this... Read more
Mid-Size Tablet Shootout Posted: iPad mini wi...
I ‘m curious about how many iPads Apple is actually selling these days. It’s been widely rumored and anticipated that new models with A8 SoCs, 2 GB of RAM, 8 megapixel cameras, and fingerprint... Read more
The 15 Biggest iPad Air Problems And How To A...
What’s this? Fifteen “biggest” problems with the iPad Air? Does that mean there are a lot of smaller problems as well? Say it isn’t so! My old iPad 2 has manifested no hardware problems in three... Read more
TYLT Syncable-Duo, 2-in-1 USB Cable With Appl...
TYLT has introduced the Syncable-Duo, a universal cable solution for charging and syncing data to smartphones and tablets. The Syncable-Duo eliminates the need for multiple cables by incorporating... Read more
Save up to $140 off MSRP with Apple refurbish...
Apple is offering Certified Refurbished iPad Airs for up to $140 off MSRP. Apple’s one-year warranty is included with each model, and shipping is free. Stock tends to come and go with some of these... Read more
2.5GHz Mac mini on sale for $549, save $50
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
Chromebooks Are Actually Sometimes A Better C...
Mac360′s Kate MacKenzie notes that when Steve Jobs announced the post-PC era, he was assuming that the dominant post-PC device would be the iPad he was unveiling, or another Apple product like the... Read more
Files-Finder Edition 1.0 app Brings Finder/Ex...
Appsicum’s Files Finder App is designed for iPad users who miss the functionality of the OS X Finder and Windows’ Explorer on their tablet. Files-Finder Edition App is a complete tool having rich... Read more
Apple and Samsung Both Face Growth Declines I...
As the tablet PC market matures and growth slows, the dominance of market leaders Apple and Samsung is also weakening, according to the latest NPD DisplaySearch Monthly Tablet PC Panel Shipment... Read more

Jobs Board

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
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As more and more people discover Apple , they visit our stores seeking ways to incorporate our products into their lives. It's your job, as a Store Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** Being a Business Manager at an Apple Store means you're the catalyst for businesses to discover and leverage the power, ease, and flexibility of 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.