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➜ SMAUG
➜ SMAUG coding
➜ Query about MAX_BITS
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| Posted by
| Nick Gammon
Australia (23,158 posts) Bio
Forum Administrator |
| Date
| Wed 18 Jul 2007 05:51 AM (UTC) |
| Message
| I am working my way through the SMAUG FUSS code to adapt it to the Lua interface, and I have struck one strange thing. Perhaps someone can clear it up for me?
Look at the define for MAX_BITS:
/*
* Defines for extended bitvectors
*/
#ifndef INTBITS
#define INTBITS 32
#endif
#define XBM 31 /* extended bitmask ( INTBITS - 1 ) */
#define RSV 5 /* right-shift value ( sqrt(XBM+1) ) */
#define XBI 4 /* integers in an extended bitvector */
#define MAX_BITS XBI * INTBITS
OK, we can see from the above that MAX_BITS is 128 (4 * 32).
However, the first bit you can set is bit zero, right? So the highest bit you can set is MAX_BITS - 1 (127) not MAX_BITS. Agreed?
There seem to be heaps of places in the code where you are allowed to set up to, and including MAX_BITS. For example, in act_wiz.c:
if( value < 0 || value > MAX_BITS )
ch_printf( ch, "Unknown flag: %s\r\n", arg2 );
else
xTOGGLE_BIT( Class->affected, value );
Isn't that wrong? Shouldn't it be:
if( value < 0 || value >= MAX_BITS )
ch_printf( ch, "Unknown flag: %s\r\n", arg2 );
else
xTOGGLE_BIT( Class->affected, value );
Ditto for about 30 other places. |
- Nick Gammon
www.gammon.com.au, www.mushclient.com | | Top |
|
| Posted by
| David Haley
USA (3,881 posts) Bio
|
| Date
| Reply #1 on Wed 18 Jul 2007 06:10 AM (UTC) |
| Message
| It appears to me that you have found a bug. :-)
Looks like somebody either:
1) didn't understand bit math
2) was off by one by mistake and then copy-pasted
3) some combination
Regardless it's clearly incorrect to run xTOGGLE_BIT with a value of 128.
mud.h:#define xTOGGLE_BIT(var, bit) ((var).bits[(bit) >> RSV] ^= 1 << ((bit) & XBM))
128 right-shift 5 is 4. The bits array is of length XBI i.e. 4.
Therefore dereferencing slot 4 is going out of bounds. Oops.
I'll report this on the FUSS forums... |
David Haley aka Ksilyan
Head Programmer,
Legends of the Darkstone
http://david.the-haleys.org | | Top |
|
| Posted by
| Nick Gammon
Australia (23,158 posts) Bio
Forum Administrator |
| Date
| Reply #2 on Wed 18 Jul 2007 06:20 AM (UTC) |
| Message
| Maybe make:
#define MAX_BITS ((XBI * INTBITS) - 1)
Not sure if that helps or hinders. |
- Nick Gammon
www.gammon.com.au, www.mushclient.com | | Top |
|
| Posted by
| Nick Gammon
Australia (23,158 posts) Bio
Forum Administrator |
| Date
| Reply #3 on Wed 18 Jul 2007 06:21 AM (UTC) |
| Message
| | Some places seem to test for >= MAX_BITS. Probably someone need to work out whether MAX_BITS is the highest bit, or the highest bit + 1, and then check each place it is used and make it conform. |
- Nick Gammon
www.gammon.com.au, www.mushclient.com | | Top |
|
| Posted by
| David Haley
USA (3,881 posts) Bio
|
| Date
| Reply #4 on Wed 18 Jul 2007 06:35 AM (UTC) |
| Message
| It seems that a lot of the code uses "MAX" as a length, not as a maximum allowed element. For instance, MAX_TRADE, MAX_FIX, MAX_IFS, MAX_AFFECTED_BY, MAX_ATTACK_TYPE, MAX_DEFENSE_TYPE, and so on.
So for consistency's sake, it seems that MAX_BITS should remain 128. But I would suggest renaming it to NUM_BITS, because that removes the ambiguity of "max".
Regardless, the value of max bit (128) is not the highest allowed bit, because it is out of the bit array bounds (it's allocated with length 4, and using bit 128 brings you to index 4). |
David Haley aka Ksilyan
Head Programmer,
Legends of the Darkstone
http://david.the-haleys.org | | Top |
|
| Posted by
| Gatewaysysop2
USA (146 posts) Bio
|
| Date
| Reply #5 on Thu 19 Jul 2007 04:34 AM (UTC) |
| Message
| Irony of ironies (note the date stamp on the post):
http://www.fussproject.org/index.php?a=topic&t=591
Hope that helps someone to track all these down.
|
"The world of men is dreaming, it has gone mad in its sleep, and a snake is strangling it, but it can't wake up." -D.H. Lawrence | | Top |
|
| Posted by
| Samson
USA (683 posts) Bio
|
| Date
| Reply #6 on Tue 24 Jul 2007 06:10 PM (UTC) |
| Message
| | Yeah, I don't know how that one got by but it's been corrected now. | | Top |
|
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