Synopsis: Adding magic numbers to structures for a safer act(). December 96
From: erwin@pip.dknet.dk
To: MERC/Envy Mailing List <merc-l@webnexus.com>
Subject: Adding magic numbers to CHAR_DATA/OBJ_DATA

You might have already run into this problem with act: the 2 void* 
parameters make it impossible to do proper checking on the passed parameters.

If you pass act() an object as the second void parameter (which would be 
used as $P in the string) but use $N mistakenly, act() will cast the 
object to a char_data - resulting in most likely a crash, when it tries 
to call can_see() with it as parameter.

It is not too hard to avoid this when programming. However, if you are
using MobProgs and act triggers, a mob prog which uses $o (equivalent of
$P in act()) but triggers via an action that uses $N will crash the MUD. 

In his improved version of MobProgs, Markuu 'Newt' Nyylander suggests to 
extend act so that it takes typed parameters - however, it is somewhat 
cumbersome to rewrite the many act() calls so they adhere to a new standard.

I've implemented another solution, which is somewhat easier and may have 
other applications. Simply add a common magic header to both the 
CHAR_DATA and OBJ_DATA structure:

struct char data
{
	char magic; /* Magic header */
	...
};

This magic header must be filled by create_object, create_mobile and 
load_char_obj with a value that is different for char_data and obj_data.

In act() as well as expand_arg() and cmd_eval() in mob_prog.c you will 
need to add checks that what supposedly points to a char_data has the 
magic number is its first byte, e.g.:

                    case 'p':
                        if (!obj1 || obj1->magic != MAGIC_OBJ)
                        {
				/* Report a bad act string */

Note that while as char magic; is only one byte, becuase of alignment
you'll waste 4 bytes on it. I've chosen not to make it an int because of
strings: an act() could be passed a one-character string - reading 4 bytes
from that location might cause a segmentation fault. 

That you risk getting a string passed as parameter also means that you 
should choose magic numbers that are unlikely to show up in a string.

==============================================================================
Erwin Andreasen  Viby J, Denmark   Computer Science Student at Aarhus Business
erwin@pip.dknet.dk        <URL:http://pip.dknet.dk/~pip1773>           College
==============================================================================