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[mLib] / exc.h

/* -*-c-*-
 *
 * $Id: exc.h,v 1.1 1998/06/17 23:44:42 mdw Exp $
 *
 * Structured exception handling in C
 *
 * (c) 1998 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of the mLib utilities library.
 *
 * mLib is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * mLib is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with mLib; if not, write to the Free Software Foundation,
 * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */

/*----- Revision history --------------------------------------------------*
 *
 * $Log: exc.h,v $
 * Revision 1.1  1998/06/17 23:44:42  mdw
 * Initial revision
 *
 */

#ifndef EXC_H
#define EXC_H

#ifdef __cplusplus
  extern "C" {
#endif

#include <setjmp.h>

/*----- Quick documentation -----------------------------------------------*
 *
 * This header file provides some exception handling facilities in C
 * programs.  It modifies the syntax of the language slightly, using the
 * preprocessor.
 *
 * The `throw' expression returns no value.  It has the syntax:
 *
 *   THROW ( expr , expr )
 *
 * The first expression must have type compatible with unsigned integer; it
 * identifies an `exception type'.  The second must have type compatible
 * with pointer to void; it contains the `exception data'.  Control is
 * passed to the current exception handler.
 *
 * The `RETHROW' expression, valid only within an exception handler, causes
 * the current exception to be thrown again.
 *
 * A `try' statement has the syntax:
 *
 *   TRY stat CATCH stat END_TRY;
 *
 * The first statement is called the `test'; the second is the `handler'.
 * During execution of the test, the handler is added to a stack of
 * active exception handlers; the topmost handler on this stack is called
 * the `current' handler.  When execution of the test completes, the
 * corresponding handler is removed from the stack.
 *
 * The test statement may complete in one of these ways:
 *
 *   * Normal completion -- control reaches the end of the statement
 *     normally.
 *
 *   * Throwing an exception -- an exception is thrown when the handler is
 *     the current exception handler.
 *
 *   * By executing a `break' statement.
 *
 *   * By executing the expression `EXIT_TRY' and transferring control to
 *     a point outside the entire `try' statement (e.g., executing a `goto'
 *     or `return' statement).
 *
 * Any other attempt to leave the test causes undefined behaviour.
 *
 * If an exception is thrown while the handler is the current exception
 * handler, it is given control.  The variables `exc_type' and `exc_val'
 * denote the exception type and value respectively -- they are passed
 * unchanged from the `throw' expression which caused the exception.
 * A handler is deactivated before it is invoked; if it causes an
 * exception to be thrown (and does not contain a nested `try' statement)
 * control will be passed to an earlier active handler.
 *
 * Control is passed to handlers using the `longjmp' function.
 *
 * Example:
 *
 *   TRY {
 *     ... something dangerous ...
 *   } CATCH switch (exc_type) {
 *     case EXC_INTERESTING:
 *   	 ... handle exception ...
 *   	 break;
 *     default:
 *   	 ... do tidying up ...
 *   	 RETHROW;
 *   } END_TRY;
 */

/*----- Exception type allocation -----------------------------------------*
 *
 * Nobody allocates exception types, so we'll just have to try to get along
 * without too many collisions.  An exception type is an unsigned long,
 * which gives us four bytes.  The top two bytes identify the library which
 * `owns' the exception, with special values zero meaning `defined as part
 * of the system' and 0xFFFF providing a shared space of types which can
 * be used by anyone as long as they don't get seen by anyone else.
 *
 * The lower byte pair encodes a type number, and a value which defines
 * the type of the value field (see below).
 */

/* --- Type type of an exception --- */

typedef unsigned long exc_extype;

/* --- Build a byte pair from two characters --- *
 *
 * Note the icky casting to handle signed chars.
 */

#define EXC_PAIR(x, y) (((unsigned long)(unsigned char)(x) << 8) |	\
			(unsigned long)(unsigned char)(y))

/* --- Allocate an exception number --- */

#define EXC_ALLOC(owner, type) (((unsigned long)(owner) << 16) |	\
				(unsigned long)(type))

/* --- Special owner codes --- */

#define EXC_GLOBAL 0u			/* The global space defined here */
#define EXC_SHARED 0xFFFFu		/* The shared space for everyone */

/*----- Exception values --------------------------------------------------*
 *
 * Exception values can have several different types.  This is a mess, and
 * C doesn't handle it too well, but we can try.  I'll encode the value type
 * as part of the exception type, in the top bits of the bottom byte.  Messy?
 * You betcha.
 */

/* --- Encoding a value type in an extype --- */

#define EXC_TYPECODE(t, w) (((w) & ~0xC0u) | ((t) & 0xC0u))

/* --- The various value types --- */

#define EXC_NOVAL 0x00u			/* No interesting value */
#define EXC_INTVAL 0x40u		/* Integer value */
#define EXC_PTRVAL 0x80u		/* Arbitrary pointer value */
#define EXC_STRVAL 0xC0u		/* Pointer to character string */

/* --- Allocating exceptions with appropriate types --- */

#define EXC_ALLOCN(o, t) EXC_TYPECODE(EXC_NOVAL,  EXC_ALLOC(o, t))
#define EXC_ALLOCI(o, t) EXC_TYPECODE(EXC_INTVAL, EXC_ALLOC(o, t))
#define EXC_ALLOCP(o, t) EXC_TYPECODE(EXC_PTRVAL, EXC_ALLOC(o, t))
#define EXC_ALLOCS(o, t) EXC_TYPECODE(EXC_STRVAL, EXC_ALLOC(o, t))

/* --- A union representing the type --- */

typedef union exc_exval {
  int i;
  void *p;
  char *s;
} exc_exval;

/*----- Predefined exceptions ---------------------------------------------*/

/* --- @EXC_NOMEM@ --- *
 *
 * Value:	---
 *
 * Meaning:	An attempt to allocate memory failed.
 */

#define EXC_NOMEM EXC_ALLOCN(EXC_GLOBAL, 0u)

/* --- @EXC_ERRNO@ --- *
 *
 * Value:	@int errno@ = the error raised
 *
 * Meaning:	Some kind of OS error occurred.
 */

#define EXC_ERRNO EXC_ALLOCI(EXC_GLOBAL, 1u)

/* --- @EXC_OSERROR@ --- *
 *
 * Value:	@os_error *e@ = pointer to error block
 *
 * Meaning:	For RISC OS programmers only: alternative way of propagating
 *		errors.
 */

#define EXC_OSERROR EXC_ALLOCP(EXC_GLOBAL, 1u)

/* --- @EXC_SIGNAL@ --- *
 *
 * Value:	@int sig@ = signal number
 *
 * Meaning:	Report the raising of a signal.
 */

#define EXC_SIGNAL EXC_ALLOCI(EXC_GLOBAL, 2u)

/* --- @EXC_FAIL@ --- *
 *
 * Value:	@const char *p@ = pointer to expanatory string
 *
 * Meaning:	Miscellaneous error.
 */

#define EXC_FAIL EXC_ALLOCS(EXC_GLOBAL, 0xFFu)

/*----- An exception handler block ----------------------------------------*/

/* --- Try to think of this as being opaque --- */

typedef struct __exc_hnd {
  struct __exc_hnd *next;		/* Pointer to next record down */
  exc_extype type;			/* Type of this exception */
  exc_exval val;			/* Value of this exception */
  jmp_buf buf;				/* Jump buffer when exceptions hit */
} __exc_hnd;

/*----- Global variables --------------------------------------------------*/

extern __exc_hnd *__exc_list;		/* List of active handlers */

/*----- Macros ------------------------------------------------------------*/

/* --- References to current exception type and value --- */

#define exc_type (__exc_ec.type)
#define exc_val (__exc_ec.val)
#define exc_i (__exc_ec.val.i)
#define exc_p (__exc_ec.val.p)
#define exc_s (__exc_ec.val.s)

/* --- How it actually works --- *
 *
 * A `try' block is contained within a block which provides an exception
 * handler buffer in automatic storage.  This block is a loop, to allow
 * `break' to escape from it.  It adds the handler buffer to the top of a
 * list, and does a `setjmp' to allow a return here following an exception.
 * The `setjmp' returns zero for the `try' section, and nonzero if there's
 * an exception to `catch'.  It looks a little like this:
 *
 *   do {
 *     __exc_hnd h;
 *     add_handler(&h);
 *     if (!setjmp(h.buf)) {
 *  	 do <try code> while (0);
 *  	 remove_handler(&h);
 *     } else
 *  	 <catch code>
 *   } while (0)
 *
 * Everything else is ugly hacking to make things work.
 */

/* --- Trying things which may cause exceptions --- */

#define TRY do {							\
  volatile __exc_hnd __exc_ec;						\
  __exc_ec.next = __exc_list;						\
  __exc_list = (__exc_hnd *)&__exc_ec;					\
  if (!setjmp(*(jmp_buf *)&__exc_ec.buf /* very nasty! */ )) { do

#define EXIT_TRY do __exc_list = __exc_ec.next; while (0)
#define CATCH while (0); EXIT_TRY; } else

#define END_TRY } while (0)

/* --- Raising exceptions --- */

#define THROW __exc_throw
#define RETHROW __exc_rethrow(__exc_ec.type, __exc_ec.val)

/*----- Functions ---------------------------------------------------------*/

/* --- @exc_uncaught@ --- *
 *
 * Arguments:	@void (*proc)(exc_extype type, exc_exval val) = new handler
 *
 * Returns:	Pointer to the old handler value.
 *
 * Use:		Sets the handler for uncaught exceptions.
 */

typedef void (*exc__uncaught)(exc_extype /*type*/, exc_exval /*val*/);
extern exc__uncaught exc_uncaught(exc__uncaught /*proc*/);

/* --- @__exc_throw@ --- *
 *
 * Arguments:	@exc_extype type@ = type of exception to throw
 *
 * Returns:	Doesn't
 *
 * Use:		NOT FOR USER CONSUMPTION.  Reads an appropriate exception
 *		value and throws an exception.
 */

extern void __exc_throw(exc_extype /*type*/, ...);

/* --- @__exc_rethrow@ --- *
 *
 * Arguments:	@exc_extype type@ = type of exception to throw
 *		@exc_exval val@ = value of exception to throw
 *
 * Returns:	Doesn't
 *
 * Use:		NOT FOR USER CONSUMPTION.  Does the donkey-work of raising
 *		an exception.
 */

extern void __exc_rethrow(exc_extype /*type*/, exc_exval /*val*/);

/*----- That's all, folks -------------------------------------------------*/

#ifdef __cplusplus
  }
#endif

#endif