MACINTOSH C CARBON: A Hobbyist's Guide To Programming the Macintosh in C
Version 1.0
2001 K. J. Bricknell

CHAPTER 5

CARBON AND UNIVERSAL PROCEDURE POINTERS

Introduction

Previous demonstration programs have called a function (AEInstallEventHandler) which takes a universal procedure pointer as a parameter; however, an explanation of this term has been delayed until this chapter because the AEInstallEventHandler calls were only incidental to the main purpose of those demonstrations. The demonstration programs at Chapter 7 are the first in which calls to a system software function which takes a universal procedure pointer as a parameter are central to the demonstration. As a brief but necessary prelude for what is to come, therefore, this chapter addresses the role of universal procedure pointers in the general scheme of things, when and why they were introduced, and their relevance in the Carbon environment.

In a sense, universal procedure pointers are a piece of historical baggage dragged into Carbon by Mac OS 8/9. They was first introduced with the so-called Universal Headers which, in turn, were introduced with the Power Macintosh. They had to with the ability of the Power Macintosh to run applications that use the instruction set of the Motorola 680x0 microprocessor (used in 680x0-based Macintoshes) as well as applications that use the native instruction set of the Power Macintosh's PowerPC microprocessor.

In the Chapter 7 demonstration programs, the system software function which takes a universal procedure pointer is TrackControl. This function is called by your application when a mouse-down event occurs in a control, such as a scroll bar. Prior to the introduction of the Power Macintosh and the associated introduction of the Universal Headers, the TrackControl prototype looked like this:

     Sint16  TrackControl(TrackControl(ControlHandle theControl,Point localPoint,
                          ProcPtr actionProc);

The actionProc a parameter is simply the address of an application-defined function, called a callback procedure (or, in C terminology, a callback function) that is called repeatedly while the mouse button remains down. In other words, the TrackControl function used to take a procedure pointer (or, in C terminology, a function pointer) in its actionProc parameter.

The Universal Headers, which, amongst other things, allow you to write Classic API source code capable of being compiled as either 680x0 code or native PowerPC code, changed the prototype for TrackControl to:

     ControlPartCode  TrackControl(ControlHandle theControl,Point startPoint,
                                   ControlActionUPP actionProc);

Notice that the third parameter is now of type ControlActionUPP. This means that the actionProc parameter now takes a universal procedure pointer. This prototype has been carried through to the Carbon headers, hence the necessity to gain a basic understanding of universal procedure pointers.

The 68LC040 Emulator and the Mixed Mode Manager

The Emulator

This ability of the Mac OS 8/9 system software to execute applications that use the instruction set of the Motorola 680x0 microprocessor as well as applications that use the native instruction set of the PowerPC microprocessor is provided by an emulator (the 68LC040 Emulator). The emulator, which is essentially a 680x0 microprocessor implemented in software, provides an execution environment that is virtually identical to the execution environment found on 680x0-based Macintoshes.

One important aspect of the 68LC040 emulator is that it made it possible for parts of the system software to remain as 680x0 code while other parts were progressively re-written, primarily for reasons of speed, as native PowerPC code. In this regard, it is important to understand that some of Mac OS 8/9 still remains as 680x0 code. For example, in Mac OS 8.6, parts of the Control Manager (including, possibly, TrackControl) remain in 680x0 code. For the purposes of explanation, the following assumes that, in Mac OS 8/9, TrackControl still exists as 680x0 code.

The Mixed Mode Manager

In Mac OS 8/9, the emulator works together with a manager called the Mixed Mode Manager. The Mixed Mode Manager manages mode switches between code in different instruction set architectures (ISAs).

Mode Switches

In Mac OS 8/9, mode switches are required when an application calls a system software function (or, indeed, any other code) that exists in a different ISA. For example:

• When a PowerPC application invokes a system software function that exists only as 680x0 code, a mode switch is required to move from the native environment to the emulator environment. Then, when that system software function completes, another mode switch is required to return from the emulator to the PowerPC environment to allow the PowerPC application to continue executing.

  This situation can occur in a Carbon application running on Mac OS 8/9 because, as previously stated, not all of Mac OS 8/9 has been re-written as native PowerPC code.

• When a 680x0 application running under the emulator calls a system software function that exists as native PowerPC code, a mode switch is required to move out of the emulator and into the native PowerPC environment. Then, when that system software function completes, another mode switch is required to return to the emulator and to allow the 680x0 application to continue executing.

  This situation cannot occur in a Carbon application running on Mac OS 8/9 because all Carbon applications must be compiled as native PowerPC code.

The Mixed Mode Manager operates transparently to most applications and other types of software, meaning that most cross-mode calls (calls to code in a different ISA from the caller's ISA) are detected automatically by the Mixed Mode Manager and handled without intervention by the calling software.

Intervention in Mode Switching

Sometimes, however, executable code needs to interact directly with the Mixed Mode Manager to ensure that a mode switch occurs at the correct time. When writing native PowerPC code, you only have to intervene in the mode-switching process when you execute code whose ISA might be different from the calling code. For example, when you pass the address of your application-defined action function (native PowerPC code) to TrackControl (680x0 code), the ISA of the code whose address you are passing is different from the ISA of the function you are passing it to. In such cases, you must ensure that the Mixed Mode Manager is called to make the necessary mode switch. You do this by explicitly signalling:

• The type of code you are passing.

• The code's calling conventions.

Indicating the ISA of a Callback Function - Routine Descriptors

You create a routine descriptor for a particular function to indicate the ISA of that function (see Fig 1).The first field of a routine descriptor (goMixedModeTrap) is an executable 680x0 instruction which invokes the Mixed Mode Manager. The Mixed Mode Manager having been called, it inspects the remaining fields of the routine descriptor to determine whether a mode switch is required. The Mixed Mode Manager is particularly interested in the routineRecords field.

The routineRecords field is an array of routine structures, each element of which describes a single function. In the simplest case, the array of routine structures contains a single element.

The ISA and procInfo fields are the most important fields in a routine structure:

• ISA Field. The ISA field contains the ISA code of the function being described, and always contains one of these two constants:

       kM68kISA  = (ISAType) 0  MC680x0 architecture.
       KPowerPCISA  = (ISAType) 1  PowerPC Architecture.
  

• procInfo Field. The procInfo field contains the function's function information, including the function's calling conventions and information about the function's parameters.

Creating a Routine Descriptor For a Control Action Function

Using the function NewControlActionUPP, you can create a routine descriptor for a control action function as follows, in which myControlAction is your application-defined control action function:

     ControlActionUPP myControlActionUPP;

     myControlActionUPP = NewControlActionUPP(myControlAction);

Notice that the result returned by NewControlActionUPP is of type ControlActionUPP. The UPP stands for a universal procedure pointer, which is defined to be either a 680x0 function pointer or a pointer to a routine descriptor (hence the term "universal"). Thus, in Mac OS 8/9, the effect of the call to NewControlActionUPP depends on whether it is executed in the 680x0 environment or the PowerPC environment:

• In the 680x0 environment, NewControlActionUPP simply returns its first parameter, that is, a pointer to your application-defined control action function.

• In the PowerPC environment, NewControlActionUPP creates a routine descriptor in your application heap and returns the address of that routine descriptor.

Effect of the Routine Descriptor

Once you have created the routine descriptor, you can later call TrackControl like this:

     TrackControl(myControl,myPoint,myControlActionUPP);

In Mac OS 8/9, if your application is a PowerPC application (as will be the case in Carbon), the value passed in the myControlActionUPP parameter is not the address of your action function itself, but the address of the routine descriptor. If a 680x0 version of TrackControl executes your action function, it begins by executing the instruction in the first field of the routine descriptor. That instruction invokes the Mixed Mode Manager, which inspects the ISA of the action function (contained in the ISA field of the routine structure). Since that ISA differs from the ISA of the TrackControl function, the Mixed Mode Manager causes a mode switch. (Of course, if TrackControl existed as PowerPC code, the ISAs would be identical, and the Mixed Mode Manager would simply execute the action function without switching modes.)

In short, you solve the problem of indicating a routine's ISA by creating a routine descriptor and by using the address of that routine descriptor (that is, a universal procedure pointer) where you would have used the address of the function (that is, a procedure pointer) in the 680x0 programming environment.

Disposing of Routine Descriptors

Disposing of routine descriptors is only necessary or advisable if you know that you will not be using the descriptor any more during the execution of your application or if you allocate a routine descriptor for temporary use only.

Functions Requiring Universal Procedure Pointers

Some of the functions for which universal procedure pointers must be created are as follows:

Universal Procedure Pointers and Carbon

Carbon supports universal procedure pointers transparently. By using the system-supplied universal procedure pointer functions, your application will operate correctly in both the Mac OS 8/9 and Mac OS X environments.

On Mac OS 8/9, the universal procedure pointer creation functions allocate routine descriptors in memory just as you would expect. On Mac OS X, the implementation of universal procedure pointers depends on various factors, including the object file format you choose. Universal procedure pointers will allocate memory if your application is compiled as a CFM binary, but are likely to return a simple procedure pointer if your application is compiled as a Mach-O binary. (All demonstration programs in this book are compiled as CFM binaries so that they will run on both Mac OS 8/9 and Mac OS X. Mach-O binaries only run on Mac OS X.)

In Carbon, routine descriptors must be disposed of using the specific disposal function associated with the creation function. For example, routine descriptors created with NewControlActionUPP must be disposed of using DisposeControlActionUPP. The generic disposal function DisposeRoutineDescriptor is not supported in Carbon.

Creation/Disposal Functions Relevant to Demonstration Programs

Creating Routine Descriptors

ControlActionUPP             NewControlActionUPP(ControlActionProcPtr userRoutine);
ModalFilterUPP               NewModalFilterUPP(ModalFilterProcPtr userRoutine);
AEEventHandlerUPP            NewAEEventHandlerUPP(AEEventHandlerProcPtr userRoutine);
ControlKeyFilterUPP          NewControlKeyFilterUPP(ControlKeyFilterProcPtr userRoutine);
ControlEditTextValidationUPP NewControlEditTextValidationUPP(ControlEditTextValidationProcPtr
                             userRoutine);
ControlUserPaneDrawUPP       NewControlUserPaneDrawUPP(ControlUserPaneDrawProcPtr
                             userRoutine);
ControlUserPaneActivateUPP   NewControlUserPaneActivateUPP(ControlUserPaneActivateProcPtr
                             userRoutine);
TEClickLoopUPP               NewTEClickLoopUPP(TEClickLoopProcPtr userRoutine);
ListSearchUPP                NewListSearchUPP(ListSearchProcPtr userRoutine);
ListDefUPP                   NewListDefUPP(ListDefProcPtr  userRoutine);
DeviceLoopDrawingUPP         NewDeviceLoopDrawingUPP(DeviceLoopDrawingProcPtr userRoutine);

Disposing of Routine Descriptors

void  DisposeControlActionUPP(ControlActionUPP userUPP);
void  DisposeModalFilterUPP(ModalFilterUPP userUPP);
void  DisposeAEEventHandlerUPP(AEEventHandlerUPP userUPP);
void  DisposeControlKeyFilterUPP(ControlKeyFilterUPP userUPP);
void  DisposeControlEditTextValidationUPP(ControlEditTextValidationUPP userUPP);
void  DisposeControlUserPaneDrawUPP(ControlUserPaneDrawUPP userUPP);
void  DisposeControlUserPaneActivateUPP(ControlUserPaneActivateUPP userUPP);
void  DisposeTEClickLoopUPP(TEClickLoopUPP userUPP);
void  DisposeListSearchUPP(ListSearchUPP userUPP);
void  DisposeListDefUPP(ListDefUPP userUPP);
void  DisposeDeviceLoopDrawingUPP(DeviceLoopDrawingUPP userUPP);