TweetFollow Us on Twitter

Universal Headers Gotcha
Volume Number:11
Issue Number:5
Column Tag:Compiler Settings

The Devil’s In The Details

Don’t get surprised by the Universal Headers

By Bill Karsh, billKarsh@aol.com

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

There is a particularly insidious problem in Apple’s Universal Headers. Whether you are affected at all, and how devastating it is, depends upon how you use their headers in your projects. Fortunately, the problem only affects 68K code generation. Here are the details, and what you can do as a work-around until new headers become available.

Which Header Versions?

The problem appears in the headers I got with Think C/C++ version 7, although these headers themselves do not seem to have a version number. The problem persists through the latest headers I got with MetroWerks CodeWarrior 5. These are version 2.0a3. Undoubtedly, you are using an affected version (I think they all are).

Structure Alignment Problem

The problem is a structure alignment conflict. Most compilers today let you apply some options regarding how structures are defined, throughout the whole of a project, or while certain pragmas are in effect. For example, you find a pop-up menu in CodeWarrior’s Processor Preferences panel, that let’s you select options called “68K,” “68K 4-byte,” or “PowerPC.” You can also use pragma statements like these:

#pragma options align=mac68k
#pragma options align=mac68k4byte
#pragma options align=native
#pragma options align=power
#pragma options align=reset

These can be used to override a globally set option. I believe that the last option stays in effect until either the next alignment statement, or the end-of-file is encountered.

In Symantec C++, the Compiler Settings page has a radio button group called “Struct Field Alignment,” that sets alignment options for a project.

Generally, the chosen alignment determines how much padding (extra space) is inserted into structures between fields, and how much padding is appended to the end of structures. Deciding which option you would want is a matter of trading-off between compactness of the data (less space) and higher performance (fields aligned on natural addresses for the field’s data-type). We won’t go into those decisions here. What is immediately essential is understanding two things.

First, these options are available to you (settable in an options page or by pragma statements) independently of whether you are generating PowerPC (ppc) code or 68K code.

Second, structures defined by the Mac OS, that is, defined in Apple’s headers, must have no extra padding in them. When the compiler comes across the definition of any Mac OS structure, like GrafPort, BitMap, or whatever, the current alignment option must be mac68k. If the current setting were anything else, your code, and the system software, would disagree on the offsets to various fields in a GrafPort, for example. That’s a disaster.

Inside Mac Headers

If you look inside virtually any Apple header, you will see one of the following examples near the top of the file:

#if defined(powerc) || defined (__powerc)
#pragma options align=mac68k
#endif

or

#if GENERATINGPOWERPC
#pragma options align=mac68k
#endif

Near the bottom you will see one of:

#if defined(powerc) || defined (__powerc)
#pragma options align=reset
#endif

or

#if GENERATINGPOWERPC
#pragma options align=reset
#endif

The intent here is to force the alignment type to mac68k while any Mac OS structures are defined. Then, align=reset puts the alignment back to whatever was in effect before. Can you see the problem? The pragma statements are made conditional upon what type of code is being generated, ppc or 68K. If you are generating ppc code only, there is no problem. If you are generating 68K code, the pragmas are not executed. You’ve got a bad problem in 68K code if, when the headers are compiled, the current alignment option is not mac68k. The pragma statements in the Apple headers were not supposed to be conditional upon anything. I’m guessing here, but it looks like a clear case of macro-mania-a-go-go.

Why Didn’t I See That?

I know, right now you are saying “how can any of my code work at all if this is true? Why didn’t I catch this immediately!?” There are several sneaky ways this problem might have escaped your notice thus far. For one thing, 68K compilers typically ship with mac68k as the default alignment option. If you never changed that, you would have been spared. Another possibility involves precompiled headers.

Your environment was shipped with precompiled headers. That is, many, but not all, of the Apple headers were precompiled at the factory into a giant header called MacHeaders68K (CW) or MacHeaders (Think), or something similar. Such a file is usually included in a global preferences dialog. That would be the Language panel in CW, or the Prefix page in Think’s dialog. There may be separate precompiled headers for C and C++. Again, chances are that you never removed it. Back at the factory, they compiled these precompiled files using the mac68k setting. Therefore, all the structures defined in there are fine. However, not all of the Apple headers are typically included in these things, only the most commonly used. This keeps symbol table sizes down, and speeds compile times.

Any time you find a need to explicitly include an Apple header in your sources, and it’s not among those precompiled at the factory, you are letting yourself in for it. You have to do something to make sure that when the compiler reads that included header, the alignment is mac68k. You have to take these steps yourself because the provision to do this automatically in the headers themselves is incorrectly coded.

What Do I Do?

If you understand what’s happening, you can choose whatever method you like to adjust for this; whatever works and is convenient. Let’s suppose you need Timer.h, and it’s not currently precompiled. Here are some possibilities:

Re-precompile MacHeaders68K, or whichever variant your project needs. Uncomment Timer.h in the source file that generates the precompiled header. Make sure the project’s global option is “68K” (CW) or “Align to 1 byte Boundary” (Think). Precompile a new MacHeaders, and include it in your working project’s preferences dialog.

Include Timer.h in a source file like this:

#pragma options align=mac68k
#include <Timer.h>
#pragma options align=reset

Repair the Apple header directly. Change this:

#if GENERATINGPOWERPC
#pragma options align=mac68k
#endif

to this:

#pragma options align=mac68k

Make the alignment options unconditional, as they were supposed to be. Do this for the reset lines too!

I know this will save you days of debugging and thousands of dollars of development costs. Could you send me just one dollar as a thank you? Worth a try.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Is there cross-platform play in slither....
So you've sunken plenty of hours into crawling around in slither.io on your iPhone or iPad. You've got your stories of tragedy and triumph, the times you coiled four snakes at one time balanced out by the others when you had a length of more than... | Read more »
Rodeo Stampede guide to running a better...
In Rodeo Stampede, honing your skills so you can jump from animal to animal and outrun the herd as long as possible is only half the fun. Once you've tamed a few animals, you can bring them home with you. [Read more] | Read more »
VoxSyn (Music)
VoxSyn 1.0 Device: iOS Universal Category: Music Price: $6.99, Version: 1.0 (iTunes) Description: VoxSyn turns your voice into the most flexible vocal sound generator ever. Instantly following even subtle modulations of pitch and... | Read more »
Catch Battleplans on Google Play from Ju...
Real-time strategy title Battleplans is due for release on Google Play on June 30th, following its release for iOS systems last month. With its simple interface and pretty graphics, the crowd-pleaser brings a formerly overlooked genre out for the... | Read more »
iDoyle: The interactive Adventures of Sh...
iDoyle: The interactive Adventures of Sherlock Holmes - A Scandal in Bohemia 1.0 Device: iOS Universal Category: Books Price: $1.99, Version: 1.0 (iTunes) Description: Special Release Price $1.99 (Normally $3.99) | Read more »
Five popular free apps to help you slim...
Thanks to retail and advertising, we're used to thinking one season ahead. Here we are just a week into the summer and we're conditioned to start thinking about the fall. [Read more] | Read more »
How to ride longer and tame more animals...
It's hard to accurately describe Rodeo Stampede to people who haven't seen it yet. It's like if someone took Crossy Roadand Disco Zoo and put them in a blender, yet with a unique game mechanic that's still simple and fun for anyone. [Read more] | Read more »
Teeny Titans - A Teen Titans Go! Figure...
Teeny Titans - A Teen Titans Go! Figure Battling Game 1.0.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.0 (iTunes) Description: Teeny Titans, GO! Join Robin for a figure battling RPG of epic proportions! TEENY... | Read more »
NinjAwesome: Tips and tricks to be a mor...
Sorry about that headline, but I'm going to go ahead and assume that GameResort would not have named its game NinjAwesome without expecting some of that. It is, in fact, pretty awesome the way it combines an endless runner and old school arcade... | Read more »
Into Mirror (Games)
Into Mirror 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: "Is all that we see or seem, but a dream within a dream?"- Edgar Allan Poe New game by Lemon Jam Studio, the team behind Pursuit... | Read more »

Price Scanner via MacPrices.net

12-inch 32GB and 128GB WiFi iPad Pros on sale...
B&H Photo has 12″ 32GB & 128GB WiFi iPad Pros on sale for up to $80 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $749 $50... Read more
6-core Mac Pro available for $3799, save $200
B&H Photo has the 6-core 3.5GHz Mac Pro on sale for $200 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.5GHz 6-core Mac Pro (sku MD878LL/A): $3799.99, $200 off MSRP Read more
Apple refurbished Apple TVs available for up...
Apple has Certified Refurbished 32GB and 64GB Apple TVs available for up to $30 off the cost of new models. Apple’s standard one-year warranty is included with each model, and shipping is free: -... Read more
13-inch Retina MacBook Pros on sale for up to...
B&H Photo has 13″ Retina MacBook Pros on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1179 $120 off MSRP - 13″ 2.7GHz/... Read more
Apple refurbished Mac minis available for up...
Apple has Certified Refurbished Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $80 off MSRP - 2.6GHz Mac... 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 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Apple refurbished iMacs available for up to $...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
15-inch Retina MacBook Pros on sale for $200-...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799.99 $200 off MSRP - 15″ 2.5GHz... Read more
Mac minis on sale for up to $100 off MSRP
B&H Photo has Mac minis on sale for up to $100 off MSRP including free shipping plus NY sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $649 $50 off MSRP - 2.8GHz Mac mini... Read more
Clearance 2015 13-inch MacBook Airs available...
B&H Photo has clearance 2015 13″ MacBook Airs available for $300 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799.... Read more

Jobs Board

*Apple* iPhone 6s and New Products Tester Ne...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, Read more
*Apple* Retail - Multiple Positions, Towson...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* iPhone 6s and New Products Tester Ne...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, Read more
Music Marketing Lead, iTunes & *Apple*...
…Music Marketing Lead is responsible for developing robust marketing campaigns and programs for Apple Music and iTunes across the whole of Apple ecosystem. This Read more
*Apple* Valley Medical Clinic is Hiring - AP...
Apple Valley Medical Clinic is Hiring! Apple Valley Medical Clinic is an independently owned practice operating a Family Medicine Clinic, a 24/7 Urgent Care, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.