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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

FotoMagico 5.4.1 - Powerful slideshow cr...
FotoMagico lets you create professional slideshows from your photos and music with just a few, simple mouse clicks. It sports a very clean and intuitive yet powerful user interface. High image... Read more
Espresso 3.0 - Powerful HTML, XML, CSS,...
Espresso is back. For people who make delightful, innovative, and fast websites -- in an app to match. Espresso helps you write, code, design, build, and publish with flair and efficiency.... Read more
Spotify 1.0.51.693. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
Gutenprint 5.2.12 - Quality drivers for...
Gutenprint is a suite of printer drivers that may be used with most common UNIX print spooling systems, including CUPS, lpr, LPRng, or others. Gutenprint currently supports over 2000 printer models.... Read more
Firefox 52.0.2 - Fast, safe Web browser.
Firefox offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and casual... Read more
Google Chrome 57.0.2987.133 - Modern and...
Google Chrome is a Web browser by Google, created to be a modern platform for Web pages and applications. It utilizes very fast loading of Web pages and has a V8 engine, which is a custom built... Read more
RapidWeaver 7.3.3 - Create template-base...
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
Chromium 57.0.2987.133 - 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 27.0.2987.133: Note: This update has no Flash... Read more
Lyn 1.8.8 - Lightweight image browser an...
Lyn is a fast, lightweight image browser and viewer designed for photographers, graphic artists, and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it delivers... Read more
Adobe Animate CC 2017 16.2.0 - Advanced...
Animate CC 2017 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Flash Professional customer). Animate CC 2017 (was Flash CC) lets you... Read more

Dynasty Warriors: Unleashed arrives on m...
No one does mindless hack and slash quite like a Musou game. What's more fun than single-handedly chopping your way through hundreds of enemies? Now, for the first time ever, that unrivaled experience goes mobile with Dynasty Warriors: Unleashed.... | Read more »
Crash of Cars beginner's guide
Crash of Cars is a fun new entry in the realm of multiplayer beat 'em ups. It's basically bumper cars off the rails -- players drive around in vehicles of their choosing -- anything from a mini-van to the weinermobile in all of its glory --... | Read more »
Dynasty Blades new update introduces a n...
Sharpen your weapons -- Dynasty Blades is back with new and improved hack n’ slash stylings. The Romance of the Three Kingdoms-inspired action MMORPG introduces a bunch of fun new features in its latest update. For the uninitiated, Dynasty Blades... | Read more »
Meganoid(2017) (Games)
Meganoid(2017) 1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: LAUNCH DISCOUNT 20% UNTIL APRIL 2nd! Support, tip and tricks: http://www.orangepixel.net/forum/ Subscribe to our newsletter... | Read more »
Telltale's Guardians of the Galaxy...
Telltale will be releasing their rendition of Guardians of the Galaxy later this month. The first episode, Tangled Up in Blue, features familiar faces including Star-Lord, Groot, Rocket, Gamora, and Drax. If the first episode's title is any... | Read more »
Royal Dungeon (Games)
Royal Dungeon 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: The king and his queen are trapped in their castle which suddenly turned out as a very dangerous place. The goal is to escape... | Read more »
Tom Clancy's ShadowBreak is a real-...
Ubisoft is treating Tom Clancy fans to the series' very first mobile-exclusive game in ShadowBreak, a real-time, multiplayer shooter in which players snipe at enemies in fast-paced tactics-driven combat. [Read more] | Read more »
Power Rangers: Legacy Wars beginner...
Rita Repulsa is back, but this time she's invading your mobile phone in Power Rangers: Legacy Wars. What looks to be a straightforward beat 'em up is actually a tough-as-nails multiplayer strategy game that requires some deft tactical maneuvering.... | Read more »
Hearthstone celebrates the upcoming Jour...
Hearthstone gets a new expansion, Journey to Un'Goro, in a little over a week, and they'll be welcoming the Year of the Mammoth, the next season, at the same time. There's a lot to be excited about, so Blizzard is celebrating in kind. Players will... | Read more »
4 smart and stylish puzzle games like Ty...
TypeShift launched a little over a week ago, offering some puzzling new challenges for word nerds equipped with an iOS device. Created by Zach Gage, the mind behind Spelltower, TypeShift boasts, like its predecessor, a sleak design and some very... | Read more »

Price Scanner via MacPrices.net

Deal alert! 13-inch 2.0GHz MacBook Pros now f...
B&H Photo has lowered their price on 13″ Apple 2.0GHz MacBook Pros to $225 off MSRP through midnight tonight (3/30/17). Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz... Read more
1.4GHz Mac mini on sale for $399, $100 off MS...
B&H Photo has the 1.4GHz Mac mini on sale for $100 off MSRP including free shipping plus NY sales tax only: - 1.4GHz Mac mini: $399 $100 off MSRP Sale ends on March 31st. Read more
13-inch 128GB MacBook Air on sale for $849, s...
B&H Photo has lowered their price on the 13″ 1.6GHz/128GB MacBook Air to $849, or $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/128GB MacBook Air (MMGF2LL/A... Read more
Is Apple Planning An iPhone Based Modular Doc...
Today’s more powerful and larger-screened smartphones and phablets are becoming the default anchor computing device for more and more users computing devices, but even a five or six inch panel is not... Read more
Razer Launches New Razer Blade Pro World’s Fi...
Razer, the gaming and high performance hardware specialists, have announced the new Razer Blade Pro laptop — the first laptop to be qualified for THX Mobile Certification, an accreditation reserved... Read more
Gro CRM’s Apple Small Business Mac And iOS CR...
Gro Software, developers of the Mac CRM software for small business and enterprise, are included in FinancesOnline 2017 CRM Rising Stars and Great User Experience lists by business software review... Read more
Deal alert! 15-inch and 13-inch MacBook Pros...
B&H Photo has the new 2016 15″ and 13″ Apple MacBook Pros in stock today and on sale for up to $200 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
Save up to $420 on a new MacBook Pro with App...
Apple is offering Certified Refurbished 2016 15″ and 13″ MacBook Pros, including some Touch Bar models, for up to $420 off original MSRP. An Apple one-year warranty is included with each model, and... Read more
12-inch 1.2GHz Retina MacBooks on sale for $1...
B&H has 12″ 1.2GHz Retina MacBooks on sale for up to $200 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 12″ 1.2GHz Space Gray Retina MacBook: $1449 $150 off MSRP - 12″ 1.... Read more
Is A New 10.5-inch iPad Still Coming In April...
There was no sign or mention of a long-rumored and much anticipated 10.5-inch iPad Pro in Apple’s product announcements last week. The exciting iPad news was release of an upgraded iPad Air with a... Read more

Jobs Board

Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
Desktop Analyst - *Apple* Products - Montef...
…technology to improve patient care. JOB RESPONSIBILITIES: Provide day-to-day support for Apple Hardware and Software in the environment based on the team's support Read more
*Apple* Mobile Master - Best Buy (United Sta...
**493168BR** **Job Title:** Apple Mobile Master **Location Number:** 000827-Denton-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** At Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.