Exception Handling? Impossible!

[richw at ada-uts.UUCP]

I think I've figured out how to improve upon my first try (the original
note) based on Robert Stroud's mentioning a stack for setjmp/longjmp
environments.  ALL of the problems (except for the fairly trivial #5)
which I mentioned in the first note seem to be taken care of now.
I also fixed a problem not realized originally (i.e. why a stack
is needed instead of a single environment...)

IN ANY CASE, my new test files (test.c and add.c) and the files
which encapsulate the exception handling mechanism (exceptions.{c,h})
follow.  I'm not claiming the mechanism is bug-free!!  I still haven't
read (or found) the papers suggested, so I'm still kinda "wingin'-it"...
I DO know that THIS test case seems to work.

Again, I'd love comments...

-- Rich Wagner

P.S.  Because I use a funny terminal, I can't directly type left
and right-square-brackets.  In the following code, these are
represented by [ and ], respectively.

P.P.S.  Many thanks to Robert Stroud !!

-------------------------------------------------------------------------
/*********  exceptions.h  *********/

#include <setjmp.h>
#include <stdio.h>

typedef char *Exception;

extern jmp_buf _env_stack[];
extern int _env_index, _bad_index;
extern Exception _exc;

#define Begin { if (_env_index == _bad_index) {\
                     fflush(stdout);\
		     fprintf(stderr, "Handlers nested to deeply\n");\
		     exit(1);}\
		if (!setjmp(_env_stack[_env_index++])) {

#define Except      _env_index--;

#define When(e) } else if (_exc == e) {\
		    _env_index--;

#define Others  } else if (1) {\
                    _env_index--;
			
#define End     } else {\
                    fflush(stdout);\
		    fprintf(stderr,"Unhandled Exception: %s\n",_exc);\
		    exit(1);\
		} }

#define raise(e) {_exc = e; longjmp(_env_stack[_env_index-1],1); }

-------------------------------------------------------------------------
/*********  exceptions.c  *********/

#include <setjmp.h>

#define STACK_SIZE 50

jmp_buf _env_stack[STACK_SIZE];
int _env_index = 0;
int _bad_index = STACK_SIZE;
char *_exc;

-------------------------------------------------------------------------
/*********  test.c  *********/

#include <stdio.h>
#include "exceptions.h"

extern Exception overflow;
extern Exception underflow;

main()
{
    int x;

    Begin
	fprintf(stderr, "Calling add(4,1)\n");
	x = add(4,1);
	fprintf(stderr, "After call to add(4,1)\n");
    Except
        When(overflow)
	    fprintf(stderr, "Overflow caught but not expected\n");
    End;

    Begin
	fprintf(stderr, "Calling add(4,7) -- expect overflow\n");
	x = add(4,7);
	fprintf(stderr, "After call to add(4,7)\n");
	fprintf(stderr, "Expected exception not raised");
    Except
        When(overflow)
	    fprintf(stderr, "Overflow caught\n");
    End;

    Begin
	fprintf(stderr, "Calling add(-4,-7) -- expect underflow\n");
	x = add(-4,-7);
	fprintf(stderr, "After call to add(-4,-7)\n");
	fprintf(stderr, "Expected exception not raised");
    Except
        When(overflow)
	    fprintf(stderr, "Overflow caught but not expected\n");
    End;
    
    fprintf(stderr, "Last line of main\n");
}

-------------------------------------------------------------------------
/*********  add.c  *********/

#include "exceptions.h"

Exception overflow  = "Overflow";
Exception underflow = "Underflow";

static my_add(a, b)
int a, b;
{
    a += b;
    if (a >= 10)  raise(overflow);
    if (a <= -10) raise(underflow);
    return (a);
}

add(a, b)
int a, b;
{
    Begin
        return my_add(a,b);
    Except
        When(overflow)
            raise(overflow);
        When(underflow)
            raise(underflow);
    End;
}