/*------------------------------------------------------------------------------

#

#   Apple Macintosh Developer Technical Support

#

#   Exception handling for MPW Pascal, MacApp and MPW C

#

#   UFailure (aka Signals) - ÒExceptional code, with a few exceptions.Ó

#

#   UFailure.h  -   C header

#

#   Copyright © 1985-1988 Apple Computer, Inc.

#   All rights reserved.

#

#   Versions:   1.0                 11/88

#

#   Components: UFailure.p          November 1, 1988

#               UFailure.h          November 1, 1988

#               UFailure.inc1.p     November 1, 1988

#               UFailure.a          November 1, 1988

#               TestCignal.c        November 1, 1988

#               TestCignal.make     November 1, 1988

#               TestSignal.p        November 1, 1988

#               TestSignal.make     November 1, 1988

#

#   UFailure (or Signals) is a set of exception handling routines suitable for

#   use with MacApp, MPW C, and MPW Pascal. It is a jazzed-up version of the MacApp

#   UFailure unit. There is a set of C interfaces to it as well.

#

------------------------------------------------------------------------------*/

/*

Theory:

    See the Òtheory of operationÓ comments below, and the commentary in Technical

    Note #88.

    *** New ***

    The warnings in technote 88 about not using CatchSignal in an expression, etc. no

    longer apply. The exact state of the routine is now restored (all non-scratch

    registers are preserved). Also, a fixed-size nonrelocatable block is now used, so

    there is a limit on the depth of nested CatchSignals. This may be adjusted by

    changing the constant SigBlockSize in UFailure.a and rebuilding the unit. For most

    applications, though, the default depth of eight will be more than sufficient.

    Note:

    *** There is a special c version of CatchFailures called CatchCFailures. If you use it

    you will be required to call FreeSignal or Success explicitly (as with CatchFailures in

    Pascal). There is no c version of CatchFailures because c doesnÕt support the concept

    of nested procedures. CatchCFailures provides the same functionality. ***

    *** You can use either the technote mechanism or the MacApp one in your program. It

    is OK to call Signal if CatchFailures was used, and OK to call Failure if

    CatchSignal was used, i.e. the two schemes may be freely intermixed. ***

    What this version adds to the MacApp mechanism is two things:

        1.

            Exception records (FailInfo) are taken from a special heap block when

            you use CatchSignal, so you donÕt have to pass a FailInfo record as a

            parameter.

        2.

            You can take advantage of stack frames and use of Signals as described

            in Technical Note #88, without being forced to make explicit calls to

            FreeSignal. This unit is fully upwards compatible with the 1986 version

            of the technote.

    This includes all of the MacApp calls from the original UFailure, of course.

                    T H E O R Y   O F   O P E R A T I O N

    This unit implements the MacApp and Signal failure mechanisms.

    The failure mechanism is built around exception handlers.  An exception

    handler is a piece of code, generally local to some other routine, that is

    called when a failure occurs and takes action to handle the failure.

    An exception handler is of the form

        PROCEDURE ExceptionHandler (error: OSErr; message: LONGINT);

                                    or

        the handler may consist of execution returning to the point of a

        CatchSignal

    where error is the error that caused the failure, and message identifies

    the error message that may be displayed.  Consider a routine that opens

    a file, reads its contents, and closes the file.  If a failure occured

    while reading the file, an exception handler would be needed to close the

    file, as the rest of the routine would not be executed.  (See TestCignal

    and TestSignal for examples of how to use these calls.)

    References to exception handlers are kept in the FailInfo record. The

    exception handlers form a linked-list via the nextInfo field of FailInfo.

    The linked list is a stack since new exception handlers are added to the

    front of the list.

    New exception handlers are added to the stack with the CatchSignal

    function or CatchFailures procedure. They are removed from the stack

    automatically (CatchSignal) or via the Success procedure (if CatchFailures

    was used). In general you call CatchFailures/CatchSignal to post an exception

    handler when an error the application should handle might occur. You

    can then manually pop the last handler off the stack with FreeSignal or

    Success, if necessary. You may want to pop the handler (even if you used

    CatchSignal) after the possibility of a specific type of error no longer

    exists. Subsequent exceptions would then be passed to a previous (more

    general) handler.

    Any failure detected within the limits of the CatchFailures/CatchSignal call

    results in the execution of the exception handler.  (Failure does

    not have to occur in the same routine as your call to CatchFailures.

    The failure may occur in any routine called after the catch but before the

    implicit/explicit pop of the handler.)

    When MacApp (or your code) determines that a failure has occured, it

    calls Failure or Signal.  As a convenience, several procedures are provided

    to check for standard kinds of failures and call Failure if needed.

    These procedures are:

        FailNIL         Calls Failure if its parameter is NIL.

        FailOSErr       Calls Failure if its parameter is not noErr.

        FailMemError    Calls Failure if MemError returns other than noErr.

        FailResError    Calls Failure if ResError returns other than noErr.

    When the exception is raised, execution of the routine that signalled is

    terminated and the exception handler at the top of the stack is popped.

    For each routine that was called after the handler was posted

    to the stack, execution is terminated as though from an EXIT statement,

    and the exception handler is called.  It generally cleans up for the

    routine in which it is nested.  Upon completion the next exception handler

    is popped from the stack, repeating the process.

    The error causing the failure, and a message code is passed to the handler.

    MacApp Specifics

    For MacApp, the last exception handler on the stack is the one in

    TApplication.PollEvent.  It calls TApplication.ShowError, which calls

    ErrorAlert, which decodes the message and displays an alert.  Your exception

    handlers may set the message code to one more specific to your application

    by calling FailNewMessage at the end of your exception handler.

    FailNewMessage changes the message only if the current one is non-zero.

    This has the effect of allowing those exception handlers closest to the

    source of the error to set the message.

    One last note about exception handlers:  It is possible for an exception

    handler to terminate exception processing by using a non-local GOTO to

    jump back into the routine in which the exception handler is nested.  This

    is how MacApp keeps the application running when a failure occurs.  The

    last exception handler on the stack, in TApplication.PollEvent, uses a

    GOTO to continue event processing.

*/

typedef struct {

    long            regs[11];           /* D3-D7/A2-A7 */

    short           error;

    long            message;

    long            failA6;

    long            failPC;

    Ptr             nextInfo;

    /* this is needed for compatibility with MacApp debugging */

    long            whoPC;

    /* these are added for USignalFailure unit use */

    short           whatSignals;

    /* this is used to keep the old stack frame return address */

    long            sigFRet;

} FailInfo, *PFailInfo;

typedef pascal void (*HandlerFuncPtr)(short error, long message);

/* Call the following initialization routine before your other initializations (InitGraf, etc.)-

 in other words as early as you can in the application. */

extern pascal void InitUFailure();

    /* Allocates the heap block for CatchSignals and initializes the global

        variables used by the unit. C programs must use this instead of InitSignals. */

extern pascal short CatchSignal();

    /* Until the function which encloses this call returns, this will catch

        subsequent Signal calls, returning the code passed to Signal.  When

        CatchSignal is encountered initially, it returns a code of zero.  These

        calls may "nest"; i.e. you may have multiple CatchSignals in one function.

        Each nested CatchSignal call uses 72 bytes of heap space.

        If you signal with Failure and pass in a non-zero message you should use

        CatchCFailures instead so you have a way of getting at the message. */

extern pascal void FreeSignal();

    /* This undoes the effect of the last CatchSignal/CatchFailures.  A Signal will then invoke

        the CatchSignal prior to the last one. */

extern pascal void Signal(short code);

    /* Returns control to the point of the last CatchSignal/CatchFailures.  The program will

        then behave as though that CatchSignal had returned with the code parameter

        supplied to Signal. If CatchCFailures is catching, the message parameter will be 0. */

extern pascal void SignalMessage(short code, long message);

    /* Returns control to the point of the last CatchSignal. If CatchCFailures is catching,

        the message parameter will be returned. */

/*------------------------------------+

|   MacApp routines                 |

+------------------------------------*/

pascal long  BuildMessage(short lowWord, short highWord)

            = 0x2E9F;   /*  MOVE.L      (A7)+,(A7)   */

    /* Takes the 2 integers and combines them into a LONGINT.  Note that the

        low-order word is the first parameter. */

extern pascal void CatchCFailures(FailInfo *fi, HandlerFuncPtr handler);

    /* Call this to set up an exception handler. This pushes your handler onto

        a stack of exception handlers. */

extern pascal void Failure(short error, long message);

    /* Call this to signal a failure.  Control will branch to the most recent

        exception handler, which will be popped off the handler stack. */

extern pascal void FailMemError();

    /* If MemError != noErr then call Failure(MemError, 0);  If you are using

        assembler, then you should just test the return code from the Memory

        Manager in DO by calling FailOSErr. (See the discussion of MemError in

        Inside Macintosh.) */

extern pascal void FailResError();

    /* If ResError != noErr then call Failure(ResError, 0); (See Inside Macintosh.) */

extern pascal void FailNewMessage(short error, long oldMessage, long newMessage);

    /* This does:

        if (oldMessage == 0)

            Failure(error, newMessage);

        else

            Failure(error, oldMessage);

     */

extern pascal void FailNIL(Ptr p);

    /* Call this with a pointer/handle; this signals Failure(memFullErr, 0) iff

        the pointer is nil. */

extern pascal void FailOSErr(short error);

    /* Call this with an OSError; signals Failure(error, 0) iff error != noErr. */

extern pascal void Success(FailInfo *fi);

    /* Call this when you want to de-install your exception handler (pop 1

        element off the handler stack). */