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PPC Data Fork Tool
Volume Number:12
Issue Number:12
Column Tag:Toolbox Techniques

Leave the Data (Fork) Alone!

A developer utility to restructure fat/PowerPC applications

By Roger Smith

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


This article may change your life forever. Okay, maybe that’s a bit dramatic, but hopefully by the time you finish reading it, you will have discarded all previous knowledge on how to structure fat binaries. The utility MoveData that we will examine shortly, grew out of a need to create a fat application which stored data in its data fork. It turns out that the default model of native applications storing code in the data fork, is merely a default. Apple designed the Power Macintosh runtime architecture with sufficient flexibility to allow you to locate code fragments just about anywhere you could imagine. Bottom line, MoveData might actually save you significant amounts of development effort!


There is a certain category of applications that allow a user to create self-running and self-contained documents. For example, a graphic artist might use a multimedia presentation program to create an informative self-running presentation to give to end users without the full package. A file compression program might create an archive that when run, automatically un-archives its contents to fill your hard drive. In addition to these documents being self-running, where appropriate, it should also be possible to re-open, edit and re-save them with a minimal amount of hassle. How many times have you just peeked inside a self-extracting archive and manually extracted 1 or 2 files without un-archiving the entire contents? It would be great if these self-running documents ran native on 68K and PPC machines.

Most applications create documents where the stored information is in the data fork of the files. These applications generally use a subset of the File Manager routines for manipulating this data. Under the 68K model, this approach lends itself easily to design a self-running document. In this instance, a stripped down version of your application checks on startup to see if its data fork is empty. If it is not [empty] it assumes that it is a self running document and proceeds to load and handle the data in the data fork. The function checkforSelfContained() (see Listing 4) shows how to do this, making use of some Process Manager routines to obtain an FSSpec for the current application. It then uses this FSSpec as a basis for the subsequent calls to the File Manager to open and process the “Data” in the data fork of the application.

Developing on Power Macintosh.

The above approach breaks under the default PowerPC model since the application would ultimately try to load itself as data, (recall that by default the code for a PowerPC application resides in the data fork). If your application were attempting to interpret sound data, it could lead to some rather interesting “industrial” music. The exercise of creating a top musical hit using this approach is left as an exercise to the readers. (Hint: start with a Microsoft application!) What is a developer supposed to do?

CFRG Resource

Apple defined ‘cfrg’ resource (code fragment), which was introduced with the Power Macintosh, as the key to understanding the MoveData utility. Using ResEdit to edit this resource is tedious at best, as you will have to roll your own template to handle a resource that contains fields that vary in size. Using Resorcerer™ makes editing and changing this resource a simple task.


Figure 1. Application resource list in Resourcer.

Open the ‘cfrg’ editor by double clicking on the ‘cfrg’ resource.

Figure 2. The ‘cfrg’ resource editor in Resourcer.

Normally the various fields are filled in at link time, and you need not worry about them. The two fields that form the heart of the MoveData utility are the usage field and where does fragment start. I have defined a C structure called codeFragRecord (located in the file DSGlobals.h) that mirrors this layout.

Listing 1. Structure definition to mimic the ‘cfrg’ layout illustrated in Figure 2.

typedef struct fragDescriptors
 long   CodeType;
 long   UpdateLevel;
 long   CurrentVersion;
 long   OldestDefVersion;
 long   AppStackSize;
 short  AppLibDirectory;
 Byte   TypeOfFrag;
 Byte   LocationOfFrag;
 long   OffsetToFrag;
 long   LengthOfFrag;
 long   Reserved1;
 long   Reserved2;
 Str255 Fragname;

typedef struct fragDescriptors fragDescriptors;

typedef fragDescriptors FragArray[1];

typedef struct codeFragRecord
 long   Reserved1;
 long   Reserved2;
 long   Version;
 long   Reserved3;
 long   Reserved4;
 long   Reserved5;
 long   Reserved6;
 long   NumberofFrags;
 FragArrayfragArray; // array[0..0] of fragDescriptors;       

typedef struct codeFragRecord *codeFragRecPtr, **codeFragRecHandle;

Problem Identified

Given the task of designing a fat application that stores its data in its own data fork, you could take one of the following approaches.

(1) Write additional code to handle the case of loading and saving documents. This code would need to calculate offsets to move the file’s mark to skip over the code fragment.

(2) Compile your PowerPC application as a code resource. Write additional code compiled in the 68K version of your application that checks the environment on startup. If running on a PowerPC, load and execute the PowerPC resource (often referred to as an accelerated resource).

(3) Not to write code, instead tell the Code Fragment Manager to look in a particular resource to find the code it should load and execute.

Going back to the ‘cfrg’ editor in Resorcerer™ for a moment, we see that the usage field allows us to pick one of three options for our code fragment. If generating an application, this field is set by your development environment to indicate that the fragment is an application. Internally, the code fragment manager uses this to determine how to prepare a particular fragment and load it into memory. Refer to Inside Macintosh, PowerPC System Software if you need additional information.

Figure 3. The usage field popup in the ‘cfrg’ resource.

More importantly the where does fragment start field allows you to change from the default location in the data fork.

Figure 4. The where does fragment start popup.

To indicate to the code fragment manager that it should load the code from a resource instead of the data fork, select the segmented resource option.

Figure 5. Change the location to point to a code resource.

Enter the resource type and ID.

Figure 6. Code fragment manager set to look
for PCOD resource with ID 0 on startup.

Cut and paste the code from the data fork into the resource type you specified. On a PowerPC your fat application will load and execute from a resource, and the data fork reverts to its historical role. (Note: ‘PCOD’ is an arbitrary resource type I chose, if you need to use a different resource type, change the appropriate define in the DSGlobals.h header file.)

General Notes

You may be thinking, if I can do all of this with Resorcerer™, why do I need another developer utility? I initially took this approach. However, while I was developing my fat application I ran into the situation where copying and pasting 1MB of code out of the data fork and into a new resource required me to set Resorcerer™’s preferred memory size to >10MB. This procedure is slow and tedious if part of a regular build cycle. Using the simple AppleScript I provide automates the entire build process and reduces any chance of user error.

Code Listing

All programs were developed using CodeWarrior 7 which ships with the latest universal headers (known as the 2.1 universal interfaces). If you are using a previous version of CodeWarrior, or a different development environment, you may have to do some tweaking. For example, the GetKeys interface changed from long to unsigned long in the 2.1 headers.

There are two applications that accompany this article. The MoveData utility uses the excellent DropShell framework by Marshall Clow, Leonard Rosenthol, and Stephan Somogyi. DropShell is available on the CW7 CD and on most on-line services. Refer to its documentation for more information. To use MoveData, drag the application you wish to alter over its icon, (or select it via the Open option under the File menu). MoveData will move the PowerPC code from the data fork into a code resource, and update the ‘cfrg’ resource to point to the correct location. Should you wish to reverse the operation, hold down the option key when you drop the application onto the MoveData icon. It copies the PowerPC code from the resource back to the data fork.

The Demo program is a simple program that illustrates the technique of creating a self-running fat binary that reads and writes data from its data fork. After you build and run the program for the first time, it will prompt you to locate a text file, which it will copy into its data fork. On subsequent invocations of the demo program, it will read and display the contents of the data fork, then write the contents back to the data fork of the application reversed.

Listing 2: DSUserProcs.c
// Routine to update the ‘cfrg’ resource. Pass in the updated information and it will 
// update the ‘cfrg’ resource in the file specified by resRef. The Code fragment 
// manager checks for this resource first on startup if running on a PowerPC. 

static  OSErr UpdateCFRG(short resRef,
 Byte fragmentType,
 Byte fragmentLocation,
 long fragmentOffset,
 long   fragmentLength)
 codeFragRecHandle cfrgHndl;
 codeFragRecPtr  cfrgRecPtr;
 OSErr  theError = -1;
 cfrgHndl = (codeFragRecHandle) Get1Resource(‘cfrg’,0);
 if (cfrgHndl != NULL)
 HLock((Handle) cfrgHndl);
 (**cfrgHndl).fragArray[0].TypeOfFrag = fragmentType;
 = fragmentLocation;
 (**cfrgHndl).fragArray[0].OffsetToFrag= fragmentOffset;
 (**cfrgHndl).fragArray[0].LengthOfFrag= fragmentLength;
 HUnlock((Handle) cfrgHndl);
 ChangedResource((Handle) cfrgHndl);
 ReleaseResource((Handle) cfrgHndl);
 return noErr;

 return theError;

Listing 3: DSUserProcs.c
// This routine gets called for each item (in our case a Fat binary) that is dropped on the
// MoveData utility. We will process and update the ‘cfrg’ resource as appropriate. Notice // the extensive 
error checking as there are several stages where things could go wrong.

static OSErr ProcessItem(FSSpecPtr myFSSPtr)
 OSErr  err = noErr;
 short  refNum;
 short  resRef;
 long logEOF;
 long count;
 short  resultCode;
 short  itemHit;
// If the option key is held down when an application is dropped onto MoveData, it will 
// reverse the process, i.e., move the PowerPC code from a resource back into the data fork.

 if (gReverseOperation)
 err = FSpOpenDF(myFSSPtr, fsRdWrPerm, &refNum);         

 if (err == noErr) 
 resRef = FSpOpenResFile(myFSSPtr,fsRdWrPerm); 
 if (resRef != -1) 
 err = GetEOF(refNum,&logEOF);      // Check for PPC code
 if (logEOF == 0)
 itemHit = Alert(kNoDataFork,nil);
 if (logEOF < kMaxResourceSize)  
 POWER_PC_CODE = Get1Resource(kPowerPCCode,kPowerPCID);
 if (POWER_PC_CODE != NULL) // Check for existing resource
 POWER_PC_CODE = NewHandle(logEOF);
 POWER_PC_CODE = TempNewHandle(logEOF,&resultCode);      
 if (POWER_PC_CODE != NULL) { 
 count = logEOF;
 err = FSRead(refNum,&count,*POWER_PC_CODE);
 ”\pPPC Code”);
 err = SetEOF(refNum,0);  // set the data fork to 0;
 err = UpdateCFRG (resRef, kUsageIsApplication,                
 itemHit = Alert(kMemErrorID,NULL);
 itemHit = Alert(kTooBigID,NULL);  
 err = FSClose(refNum);
 itemHit = Alert(kDataForkError,NULL);

Listing 4: Demo.c
// Checks if this is a self running document, load and handle it if necessary.

OSErr checkforSelfContained(short *vRefNumToSearch)

 OSErr  theError;
 ProcessSerialNumber theCurrentProcess;
 ProcessInfoRec  theInfo;
 long   fileLength;
 short  refNum = 0;
 MenuHandle theMenu;

 theInfo.processName = NULL;
 theInfo.processInfoLength = sizeof(ProcessInfoRec);
 theInfo.processAppSpec = &gApplicationProcessInfo;
 theCurrentProcess.highLongOfPSN = 0;
 theCurrentProcess.lowLongOfPSN = kCurrentProcess; 
 theError = GetProcessInformation
 if (theError != noErr)
 return theError;
 theError = FSpOpenDF(&gApplicationProcessInfo,
 fsRdWrPerm, &refNum);

 if (theError == noErr)
  theError = GetEOF(refNum, &fileLength);

 if (fileLength != 0)
 theMenu = GetMHandle(FILEMENU);
 theError = processFile(&gApplicationProcessInfo,refNum,       
 if (!theError)
 theError = appendTextFileToApp(refNum);

 theError = FSClose(refNum);

 *vRefNumToSearch = refNum;

 return theError;

The best way to build the Demo program is to run the Build (Fat) Demo CW7 script. If you run the PPC project immediately after building it, the program will attempt to reverse itself, and on subsequent attempts it will fail (self-destruction). Note: You will need to edit the path in the script to get it to run on your machine.

The echoed script is here for your convenience.

Build (Fat) Demo
Listing 5:
// Simple Apple Script to coordinate the building a fat binary using the MoveData Utility.

set PATHNAME to “HD:DeskTop Folder:MacTech:”

with timeout of 60000 seconds
 tell application “CodeWarrior IDE 1.3.1”
 open {PATHNAME & “Demo68K.µ”}
 Make Project
 Close Project
 open {PATHNAME & “DemoPPC.µ”}
 Add Files PATHNAME & “Demo68K” To Segment 1
 Make Project
 Remove Files PATHNAME & “Demo68K”
 end tell
 tell application “Finder”
 select file “Demo (Fat)” of folder PATHNAME
 open selection using file “MoveData” 
 end tell
end timeout


One of the side effects of this approach is that the Finder information window for your application will not reflect that it is a PowerPC application. This is because the Virtual Memory Manager on Power Macs will only map your application into chunks that can be paged in an out, if the code resides in the data fork. By the time Copland rolls around, hopefully this issue will be moot, and the Finder will provide a better mechanism for determining if an application contains native code. With virtual memory turned on (and depending on the size of your PowerPC executable), you may notice faster application startup times since none of the virtual memory magic will apply to your application.

If you are using Jasik’s the Debugger, your debugging strategy is unaffected as the Debugger will still kick in once you launch your application. You could have several self-running documents all built using the same run time module (with different information in the data fork). Then the Debugger will kick in, provided that you name the document and system files appropriately.

Figure 7. Finder information for a PowerPC application.

Figure 8. Finder information for a PowerPC application processed using the MoveData utility.

What other usage does this technique invite? I started thinking about this one night while driving home, and it seemed like an interesting way of storing optimized versions of your application under one umbrella (i.e., self configuring software that ran at maximum speed on 601, 603 or 604 processors). It also gives the authors of virus protection programs another potential threat to consider, as this technique gives new meaning to the term Trojan program.

The Verdict

I hope the technique of moving program code out of the Data Fork and into a resource will prove useful in your own development projects. Drop me an e-mail message if you use this technique, or have any comments on this approach.


The author wishes to thank several people who contributed code to help make this article possible. In particular, I would like to thank Matthew Xavier Mora at Apple DTS for supplying the Pascal source for the original version of MoveData. I would also like to thank Lucien Dupont in the Newton Systems Group for offering his support and comments, and finally to Peter Lau (no relation to Raymond) for his suggestions.


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